Pupil: Physiology and Abnormalities

Overview

Pupil

• Opening in the center of the iris
• Space within the eye that allows light to enter and stimulate the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy

Adjacent structures

• Cornea Cornea The transparent anterior portion of the fibrous coat of the eye consisting of five layers: stratified squamous corneal epithelium; bowman membrane; corneal stroma; descemet membrane; and mesenchymal corneal endothelium. It serves as the first refracting medium of the eye. Eye: Anatomy:
  • In front of the pupil 
  • Transparent structure that provides fixed refraction Refraction Refractive Errors to focus light onto the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy
  • Primary site of refraction Refraction Refractive Errors
• Lens Lens A transparent, biconvex structure of the eye, enclosed in a capsule and situated behind the iris and in front of the vitreous humor (vitreous body). It is slightly overlapped at its margin by the ciliary processes. Adaptation by the ciliary body is crucial for ocular accommodation. Eye: Anatomy:
  • Behind the pupil 
  • Transparent structure that provides adjustable refraction Refraction Refractive Errors to focus light onto the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy
• Iris:
  • Surrounding the pupil 
  • Colored portion of the eye, composed of contractile smooth muscle to control the size of the pupil
  • Circular muscle group:
    • Sphincter pupillae for contraction (miosis)
    • Innervation: parasympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification
  • Radial muscle group:
    • Dilator pupillae for dilation ( mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma)
    • Innervation: sympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification

Physiology of the Pupil

Visual pathway Visual pathway The primary visual pathway consists of a relay system, beginning at the retina, whose ganglion cell axons form the optic nerve. The optic nerve fibers from each eye hemidecussate in the optic chiasm (OC), with nasal fibers joining the temporal fibers of the contralateral nerve. Signals are then transmitted to the primary visual cortex of the occipital lobe. The Visual Pathway and Related Disorders

• Afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology: Sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology carry impulses from sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology stimuli to the central nervous system Central nervous system The main information-processing organs of the nervous system, consisting of the brain, spinal cord, and meninges. Nervous System: Anatomy, Structure, and Classification (CNS).
• Efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology: Motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology carry impulses away from the CNS toward the effector organs.

Afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology pathway

From stimuli to the primary visual cortex Primary Visual Cortex The Visual Pathway and Related Disorders:

• Retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy
  • Photoreceptors and bipolar Bipolar Nervous System: Histology cells
  • Light stimulates retinal photoreceptors → signal goes to bipolar Bipolar Nervous System: Histology cells, then reaches ganglion cells Ganglion Cells The Visual Pathway and Related Disorders
• Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions to optic tract Optic Tract Nerve fiber originating from the optic chiasm that connects predominantly to the lateral geniculate bodies. It is the continuation of the visual pathway that conveys the visual information originally from the retina to the optic chiasm via the optic nerves. The Visual Pathway and Related Disorders
  • Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions: Axons Axons Nerve fibers that are capable of rapidly conducting impulses away from the neuron cell body. Nervous System: Histology of the ganglion cells Ganglion Cells The Visual Pathway and Related Disorders converge, forming the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions.
  • Optic chiasm Optic Chiasm The x-shaped structure formed by the meeting of the two optic nerves. At the optic chiasm the fibers from the medial part of each retina cross to project to the other side of the brain while the lateral retinal fibers continue on the same side. As a result each half of the brain receives information about the contralateral visual field from both eyes. The Visual Pathway and Related Disorders: where hemidecussation of optic nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology occurs
  • Optic tract Optic Tract Nerve fiber originating from the optic chiasm that connects predominantly to the lateral geniculate bodies. It is the continuation of the visual pathway that conveys the visual information originally from the retina to the optic chiasm via the optic nerves. The Visual Pathway and Related Disorders:
    • Nasal fibers from left eye Left Eye Refractive Errors decussate to join the temporal fibers of the right eye Right Eye Refractive Errors → right optic tract Optic Tract Nerve fiber originating from the optic chiasm that connects predominantly to the lateral geniculate bodies. It is the continuation of the visual pathway that conveys the visual information originally from the retina to the optic chiasm via the optic nerves. The Visual Pathway and Related Disorders
    • Nasal fibers from right eye Right Eye Refractive Errors decussate to join the temporal fibers of the left eye Left Eye Refractive Errors → left optic tract Optic Tract Nerve fiber originating from the optic chiasm that connects predominantly to the lateral geniculate bodies. It is the continuation of the visual pathway that conveys the visual information originally from the retina to the optic chiasm via the optic nerves. The Visual Pathway and Related Disorders
• Lateral geniculate nucleus Lateral Geniculate Nucleus The Visual Pathway and Related Disorders (LGN): synaptic relay in the diencephalon Diencephalon The paired caudal parts of the prosencephalon from which the thalamus; hypothalamus; epithalamus; and subthalamus are derived. Development of the Nervous System and Face
  • Majority of the fibers pass posteriorly → genico-calcarine tracts → terminate in the primary visual cortex Primary Visual Cortex The Visual Pathway and Related Disorders
  • Minority of fibers pass into the superior colliculus Superior Colliculus The anterior pair of the quadrigeminal bodies which coordinate the general behavioral orienting responses to visual stimuli, such as whole-body turning, and reaching. Cranial Nerve Palsies and Edinger-Westphal nuclei (for parasympathetic innervation of the pupil).

Parasympathetic innervation pathway (pupillary light reflex)

• Light stimuli → starts the afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology pathway from the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy, with impulses going to the pretectal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles (at the level of the superior colliculus Superior Colliculus The anterior pair of the quadrigeminal bodies which coordinate the general behavioral orienting responses to visual stimuli, such as whole-body turning, and reaching. Cranial Nerve Palsies)
• Protection from increased light intensity by diminishing the amount of light entering the eye
• Part of the “rest and digest” response
• Efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology pathway:
  • 1st-order neuron 1st-order neuron Originates in the hypothalamus and descends to the first synapse in the cervical spinal cord, located at levels C8–T2 (also called the ciliospinal center of Budge). Horner Syndrome:
    • Pretectal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles projects bilaterally to the Edinger-Westphal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles, where the efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology limb begins (preganglionic parasympathetic) → CN III parasympathetic fibers → ciliary ganglion 
  • 2nd-order neuron 2nd-order neuron Preganglionic pupillomotor fibers exit the spinal cord at T1, travel through the brachial plexus, over the lung apex, ascending to the superior cervical ganglion located near the angle of the mandible and the bifurcation of the common carotid artery. Horner Syndrome:
    • From the ciliary ganglion (postganglionic) → short ciliary nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission on sphincter pupillae → miosis
• Consensual light reflex Consensual Light Reflex Ophthalmic Exam:
  • Pretectal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles supplies the Edinger-Westphal nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles bilaterally.
  • Shining light in 1 eye → bilateral constriction of the pupils (miosis)

Sympathetic innervation pathway

• Psychosensory or physical stimuli Physical stimuli Act of eliciting a response from a person or organism through physical contact. Angioedema (darkness) → sympathetic response, stimulating the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy and optic tracts to send signal to the hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus
• Part of the “fight or flight” response
• Efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology pathway:
  • 3-neuron chain originating from the hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus
  • 1st-order (central neuron):
    • Neurons Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology from the posterior hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus → spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy at C8–T2 (ciliospinal center of Budge) 
  • 2nd-order (preganglionic neuron):
    • Passes out of the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy → cervical sympathetic chain (through the brachial plexus Brachial Plexus The large network of nerve fibers which distributes the innervation of the upper extremity. The brachial plexus extends from the neck into the axilla. In humans, the nerves of the plexus usually originate from the lower cervical and the first thoracic spinal cord segments (c5-c8 and T1), but variations are not uncommon. Peripheral Nerve Injuries in the Cervicothoracic Region, over the lung apex) 
    • From the sympathetic chain (going superiorly) → superior cervical ganglion 
  • 3rd-order (postganglionic neuron):
    • From the superior cervical ganglion → oculosympathetic pathway joins the ophthalmic division of the trigeminal nerve Trigeminal nerve The 5th and largest cranial nerve. The trigeminal nerve is a mixed motor and sensory nerve. The larger sensory part forms the ophthalmic, mandibular, and maxillary nerves which carry afferents sensitive to external or internal stimuli from the skin, muscles, and joints of the face and mouth and from the teeth. Most of these fibers originate from cells of the trigeminal ganglion and project to the trigeminal nucleus of the brain stem. The smaller motor part arises from the brain stem trigeminal motor nucleus and innervates the muscles of mastication. The 12 Cranial Nerves: Overview and Functions (CN V) 
    • Oculosympathetic fibers synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission on the dilator pupillae ( mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma) and superior tarsal muscle/Müller’s muscle (upper lid elevation).

Near response

Consists of 3 responses:

• Convergence
  • Medial eye movement to track object
  • Oculomotor nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles stimulates contraction of bilateral medial rectus Medial rectus Orbit and Extraocular Muscles: Anatomy muscles.
• Contraction of the ciliary muscles
  • Alteration of lens Lens A transparent, biconvex structure of the eye, enclosed in a capsule and situated behind the iris and in front of the vitreous humor (vitreous body). It is slightly overlapped at its margin by the ciliary processes. Adaptation by the ciliary body is crucial for ocular accommodation. Eye: Anatomy shape to maintain focus on the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy
  • Initially, ciliary muscles are relaxed, with eyes focused on distant objects.
  • With near objects, parasympathetic pathway stimulates the ciliary muscles → lens Lens A transparent, biconvex structure of the eye, enclosed in a capsule and situated behind the iris and in front of the vitreous humor (vitreous body). It is slightly overlapped at its margin by the ciliary processes. Adaptation by the ciliary body is crucial for ocular accommodation. Eye: Anatomy thickening → ↑ refractive power ( lens Lens A transparent, biconvex structure of the eye, enclosed in a capsule and situated behind the iris and in front of the vitreous humor (vitreous body). It is slightly overlapped at its margin by the ciliary processes. Adaptation by the ciliary body is crucial for ocular accommodation. Eye: Anatomy accommodation Accommodation Refractive Errors)
• Constriction of the pupils
  • Adjustment of light to focus on near objects
  • Parasympathetic pathway stimulates the sphincter pupillae.

Examination of the Pupil

Light and near response

• Examination in dim light
  • Direct response: Pupil constricts on direct illumination.
  • Consensual response: constriction on illumination of the contralateral eye
  • Without light or in the dark: Pupils dilate. 
  • Abnormal response: Pupil fails to dilate to the dark or fails to constrict to light or accommodation Accommodation Refractive Errors (near response).
• Note the speed of response:
  • 4 +: brisk response 
  • 3 +: a moderate response
  • 2 +: a small, slowed response
  • 1 +: represents a tiny/just visible response
  • 0: indicates unresponsive pupils

Pupil size

• Normal: 2–4 mm (bright), 4–8 mm (dark)
• Isocoria: same size of pupils
• Anisocoria: difference in size of pupils

Swinging-flashlight test

• In a dark room, the patient fixates on a distant object.
• With light source below the line of sight, confirm pupils are isocoric.
• Shift illumination after 3 seconds from one eye to the other.
• Normal response:
  • Shine light in 1 eye → equal bilateral constriction
  • During transfer, there is dilation.
  • Swing light to other eye → similar equal bilateral constriction 
  • Expected in an intact retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy and/or optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions

Slit-lamp examination Slit-Lamp Examination Blepharitis

• Examines the surface of the iris and pupillary margin
• Irregularities, raised lesion, and pigment abnormalities can be seen.

Pharmacologic tests

• Pharmacologic agents (topical cocaine Cocaine An alkaloid ester extracted from the leaves of plants including coca. It is a local anesthetic and vasoconstrictor and is clinically used for that purpose, particularly in the eye, ear, nose, and throat. It also has powerful central nervous system effects similar to the amphetamines and is a drug of abuse. Cocaine, like amphetamines, acts by multiple mechanisms on brain catecholaminergic neurons; the mechanism of its reinforcing effects is thought to involve inhibition of dopamine uptake. Local Anesthetics, apraclonidine Apraclonidine Glaucoma, pilocarpine Pilocarpine A slowly hydrolyzed muscarinic agonist with no nicotinic effects. Pilocarpine is used as a miotic and in the treatment of glaucoma. Cholinomimetic Drugs or hydroxyamphetamine) utilized to check pupillary reactivity
• Work-up for Horner’s syndrome, tonic pupil Tonic Pupil A pupillary abnormality characterized by a poor pupillary light reaction, reduced accommodation, iris sector palsies, an enhanced pupillary response to near effort that results in a prolonged, ‘tonic’ constriction, and slow pupillary redilation. This condition is associated with injury to the postganglionic parasympathetic innervation to the pupil. Amyloidosis, or pharmacologic mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma

Disorders of the Afferent Pathway

Relative afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology pupillary defect

• Also called Marcus Gunn pupil
• Pathophysiology: caused by unilateral or asymmetrical disease of the optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions or retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy
• Presentation:
  • Impaired direct pupillary reflex in the affected eye and impaired indirect pupillary reflex in the normal eye
• Swinging-flashlight test:
  • Abnormal response: paradoxical dilation or weak constriction when light shines into the affected eye (Marcus Gunn pupil)
• Associated conditions:
  • Optic neuritis Optic neuritis Inflammation of the optic nerve. Commonly associated conditions include autoimmune disorders such as multiple sclerosis, infections, and granulomatous diseases. Clinical features include retro-orbital pain that is aggravated by eye movement, loss of color vision, and contrast sensitivity that may progress to severe visual loss, an afferent pupillary defect (Marcus-Gunn pupil), and in some instances optic disc hyperemia and swelling. Inflammation may occur in the portion of the nerve within the globe (neuropapillitis or anterior optic neuritis) or the portion behind the globe (retrobulbar neuritis or posterior optic neuritis). Cranial Nerve Palsies 
  • Traumatic optic neuropathy Neuropathy Leprosy
  • Ischemic optic disease or retinal disease
  • Glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma causing optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions damage
  • Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment
  • Severe macular degeneration Macular degeneration Age-related macular degeneration (AMD) is visual impairment due to changes in the macula, the area responsible for high-acuity vision. It is marked by central vision loss with peripheral vision relatively spared. Risk factors include advanced age, smoking, family history, and cardiovascular disease. Macular Degeneration
  • Retinitis ( cytomegalovirus Cytomegalovirus CMV is a ubiquitous double-stranded DNA virus belonging to the Herpesviridae family. CMV infections can be transmitted in bodily fluids, such as blood, saliva, urine, semen, and breast milk. The initial infection is usually asymptomatic in the immunocompetent host, or it can present with symptoms of mononucleosis. Cytomegalovirus (CMV), herpes)
  • Optic nerve Optic nerve The 2nd cranial nerve which conveys visual information from the retina to the brain. The nerve carries the axons of the retinal ganglion cells which sort at the optic chiasm and continue via the optic tracts to the brain. The largest projection is to the lateral geniculate nuclei; other targets include the superior colliculi and the suprachiasmatic nuclei. Though known as the second cranial nerve, it is considered part of the central nervous system. The 12 Cranial Nerves: Overview and Functions tumors
  • Other optic neuropathies Neuropathies Chédiak-Higashi Syndrome

Anisocoria

Physiologic anisocoria

• Difference between the pupil size remains the same regardless of illumination.
• Affects 20% of normal individuals 
• Difference in size: typically < 1 mm
• No damage to the sphincter and dilator pupillae innervation
• Normal pupillary reactions to light and darkness 
• Review of old photographs Photographs Ultrasound (Sonography) or driver’s license helps with diagnosis.

Pathologic anisocoria

• Pathophysiology:
  • Neural defect of the efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology pupillomotor pathways innervating the iris muscles in 1 or both eyes Both Eyes Refractive Errors
  • Structural defect of 1 or both irides 
• Presentation:
  • Pathologically small pupil: in darkness, usually remains small while the normal pupil dilates
  • Pathologically large pupil: in bright light, remains large while the normal pupil constricts
  • Difference usually > 1 mm
• Associated conditions:
  • Horner’s syndrome
  • CN III palsy Palsy paralysis of an area of the body, thus incapable of voluntary movement Cranial Nerve Palsies with pupillary involvement
  • Adie tonic pupil Tonic Pupil A pupillary abnormality characterized by a poor pupillary light reaction, reduced accommodation, iris sector palsies, an enhanced pupillary response to near effort that results in a prolonged, ‘tonic’ constriction, and slow pupillary redilation. This condition is associated with injury to the postganglionic parasympathetic innervation to the pupil. Amyloidosis
  • Anterior uveitis Uveitis Uveitis is the inflammation of the uvea, the pigmented middle layer of the eye, which comprises the iris, ciliary body, and choroid. The condition is categorized based on the site of disease; anterior uveitis is the most common. Diseases of the Uvea
  • Acute angle closure glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma
  • Trauma or surgery
  • Congenital defects ( coloboma Coloboma Congenital anomaly in which some of the structures of the eye are absent due to incomplete fusion of the fetal intraocular fissure during gestation. Esophageal Atresia and Tracheoesophageal Fistula or heterochromia iridis)

Disorders of the Efferent Pathway

Adie tonic pupil Tonic Pupil A pupillary abnormality characterized by a poor pupillary light reaction, reduced accommodation, iris sector palsies, an enhanced pupillary response to near effort that results in a prolonged, ‘tonic’ constriction, and slow pupillary redilation. This condition is associated with injury to the postganglionic parasympathetic innervation to the pupil. Amyloidosis

• Large pupil, impaired light reaction
• Presentation:
  • Tonic, usually unilateral mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma 
  • Has light-near dissociation Dissociation Defense Mechanisms:
    • No or poor constriction on illumination 
    • Positive near response (constriction) but slow redilation
  • More common in women 30–40 years of age
  • Holmes-Adie syndrome: loss of deep tendon reflexes Deep Tendon Reflexes Neurological Examination
• Pathophysiology: degeneration of the ciliary ganglion and postganglionic parasympathetic denervation of the affected pupil
• Associated conditions:
  • Idiopathic Idiopathic Dermatomyositis (most cases)
  • Infection (viral or bacterial)
  • Trauma or surgery
  • Tumors
  • Ross syndrome: tonic pupil Tonic Pupil A pupillary abnormality characterized by a poor pupillary light reaction, reduced accommodation, iris sector palsies, an enhanced pupillary response to near effort that results in a prolonged, ‘tonic’ constriction, and slow pupillary redilation. This condition is associated with injury to the postganglionic parasympathetic innervation to the pupil. Amyloidosis, hyporeflexia Hyporeflexia Duchenne Muscular Dystrophy, sudomotor disturbances
• Pilocarpine Pilocarpine A slowly hydrolyzed muscarinic agonist with no nicotinic effects. Pilocarpine is used as a miotic and in the treatment of glaucoma. Cholinomimetic Drugs test:
  • Tonic pupil Tonic Pupil A pupillary abnormality characterized by a poor pupillary light reaction, reduced accommodation, iris sector palsies, an enhanced pupillary response to near effort that results in a prolonged, ‘tonic’ constriction, and slow pupillary redilation. This condition is associated with injury to the postganglionic parasympathetic innervation to the pupil. Amyloidosis:
    • Has cholinergic supersensitivity
    • Will constrict; more miotic response than normal pupil

Argyll Robertson pupil

• Small bilateral pupils, impaired light reaction
• Presentation:
  • Always bilateral, may be asymmetric
  • Light-near dissociation Dissociation Defense Mechanisms:
    • Pupils do not constrict to illumination
    • Positive constriction to accommodation Accommodation Refractive Errors/near response
• Pathophysiology: result of dorsal midbrain Midbrain The middle of the three primitive cerebral vesicles of the embryonic brain. Without further subdivision, midbrain develops into a short, constricted portion connecting the pons and the diencephalon. Midbrain contains two major parts, the dorsal tectum mesencephali and the ventral tegmentum mesencephali, housing components of auditory, visual, and other sensorimotor systems. Brain Stem: Anatomy lesion (damages nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology that control pupillary reflex but spares more ventral nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology that control accommodation Accommodation Refractive Errors reflex)
• Associated with late-stage syphilis Syphilis Syphilis is a bacterial infection caused by the spirochete Treponema pallidum pallidum (T. p. pallidum), which is usually spread through sexual contact. Syphilis has 4 clinical stages: primary, secondary, latent, and tertiary. Syphilis:
  • Other symptoms: sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology ataxia Ataxia Impairment of the ability to perform smoothly coordinated voluntary movements. This condition may affect the limbs, trunk, eyes, pharynx, larynx, and other structures. Ataxia may result from impaired sensory or motor function. Sensory ataxia may result from posterior column injury or peripheral nerve diseases. Motor ataxia may be associated with cerebellar diseases; cerebral cortex diseases; thalamic diseases; basal ganglia diseases; injury to the red nucleus; and other conditions. Ataxia-telangiectasia and lancinating pains in tabes dorsalis Tabes dorsalis Parenchymatous neurosyphilis marked by slowly progressive degeneration of the posterior columns, posterior roots, and ganglia of the spinal cord. The condition tends to present 15 to 20 years after the initial infection and is characterized by lightning-like pains in the lower extremities, urinary incontinence; ataxia; severely impaired position and vibratory sense, abnormal gait, optic atrophy; Argyll-Robertson pupils, hypotonia, hyperreflexia, and trophic joint degeneration. Syphilis
  • Treatment: penicillin Penicillin Rheumatic Fever regimen for neurosyphilis Neurosyphilis Infections of the central nervous system caused by treponema pallidum which present with a variety of clinical syndromes. The initial phase of infection usually causes a mild or asymptomatic meningeal reaction. The meningovascular form may present acutely as brain infarction. The infection may also remain subclinical for several years. Late syndromes include general paresis; tabes dorsalis; meningeal syphilis; syphilitic optic atrophy; and spinal syphilis. General paresis is characterized by progressive dementia; dysarthria; tremor; myoclonus; seizures; and argyll-robertson pupils. Syphilis

Horner’s syndrome

• Small pupil, normal light reaction
• Anisocoria more noted in the dark (abnormal pupil with dilation lag)
• Presentation:
  • Ptosis Ptosis Cranial Nerve Palsies: paresis Paresis A general term referring to a mild to moderate degree of muscular weakness, occasionally used as a synonym for paralysis (severe or complete loss of motor function). In the older literature, paresis often referred specifically to paretic neurosyphilis. ‘general paresis’ and ‘general paralysis’ may still carry that connotation. Bilateral lower extremity paresis is referred to as paraparesis. Spinal Disk Herniation of Müller’s muscle
  • Anhidrosis: damage proximal to fibers mediating sweat and temperature
  • Miosis: loss of innervation to dilator pupillae 
• Pathophysiology: results from disruption of sympathetic chain 
• Associated conditions:
  • 1st-order neuron 1st-order neuron Originates in the hypothalamus and descends to the first synapse in the cervical spinal cord, located at levels C8–T2 (also called the ciliospinal center of Budge). Horner Syndrome: lateral medullary syndrome Lateral Medullary Syndrome Lateral medullary syndrome (also known as Wallenberg syndrome, posterior inferior cerebellar artery (PICA) syndrome, and vertebral artery syndrome) is a neurological constellation of symptoms and signs due to obstruction in vessels supplying the medulla, resulting in brainstem ischemia or infarction. Lateral Medullary Syndrome (Wallenberg Syndrome) (Wallenberg’s syndrome) or tumors or strokes affecting 1st-order sympathetic tract 
  • 2nd-order neuron 2nd-order neuron Preganglionic pupillomotor fibers exit the spinal cord at T1, travel through the brachial plexus, over the lung apex, ascending to the superior cervical ganglion located near the angle of the mandible and the bifurcation of the common carotid artery. Horner Syndrome: trauma or tumors in spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy, thoracic outlet Thoracic Outlet Thoracic Outlet Syndrome, or lung apex ( Pancoast tumor Pancoast Tumor Thoracic Outlet Syndrome)
  • 3rd-order neuron 3rd-order neuron Pupillomotor fibers ascend along the internal carotid artery and enter the cavernous sinus where it is in close relation with the abducens nerve (cranial nerve (CN) VI). These fibers enter the orbit with the ophthalmic branch (V1) of the trigeminal nerve (CN V) via the long ciliary nerves, which innervate the iris dilator and Müller muscles. Horner Syndrome: carotid dissection Carotid dissection Carotid and vertebral artery dissections occur when the integrity of the arterial wall structure fails, usually abruptly, resulting in intramural hematoma formation and a false lumen between the tunica media and the intimal or adventitial layers. This may result in aneurysm, stenosis, or occlusion. Dissection of the Carotid and Vertebral Arteries, thrombosis Thrombosis Formation and development of a thrombus or blood clot in the blood vessel. Epidemic Typhus, cavernous sinus aneurysm Aneurysm An aneurysm is a bulging, weakened area of a blood vessel that causes an abnormal widening of its diameter > 1.5 times the size of the native vessel. Aneurysms occur more often in arteries than in veins and are at risk of dissection and rupture, which can be life-threatening. Thoracic Aortic Aneurysms
• Pharmacologic tests:
  • Cocaine Cocaine An alkaloid ester extracted from the leaves of plants including coca. It is a local anesthetic and vasoconstrictor and is clinically used for that purpose, particularly in the eye, ear, nose, and throat. It also has powerful central nervous system effects similar to the amphetamines and is a drug of abuse. Cocaine, like amphetamines, acts by multiple mechanisms on brain catecholaminergic neurons; the mechanism of its reinforcing effects is thought to involve inhibition of dopamine uptake. Local Anesthetics solution:
    • Blocks reuptake of norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS and dilates the eye with intact sympathetic innervation  
    • Horner’s syndrome: poor-to-no dilation (impaired sympathetic innervation)
  • Apraclonidine Apraclonidine Glaucoma (0.5%–1% solution):
    • A weak alpha-1 agonist and strong alpha-2 agonist
    • Horner’s syndrome (dominant supersensitivity to alpha-1 receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors): Affected pupil dilates. 
  • Localization with topical hydroxyamphetamine:
    • Hydroxyamphetamine releases norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS from intact postganglionic fibers (this test helps to localize the lesion).
    • Horner’s syndrome with intact postganglionic fibers: dilate affected pupil ≥ normal pupil
    • Horner’s syndrome with damaged postganglionic fibers: no pupil dilation

Pharmacology and Pupillary Effects

Differential Diagnosis

• Glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma: an optic neuropathy Neuropathy Leprosy with distinctive changes in the optic cup and visual field Visual Field The Visual Pathway and Related Disorders defect. Glaucoma Glaucoma Glaucoma is an optic neuropathy characterized by typical visual field defects and optic nerve atrophy seen as optic disc cupping on examination. The acute form of glaucoma is a medical emergency. Glaucoma is often, but not always, caused by increased intraocular pressure (IOP). Glaucoma is often associated with increased pressure within the eyeball (commonly within the anterior and posterior chambers), which results in gradual vision Vision Ophthalmic Exam loss. This can occur acutely due to a blockage of the aqueous humor Humor Defense Mechanisms drainage ( acute angle-closure glaucoma Acute Angle-Closure Glaucoma Glaucoma) and presents as a mid-dilated pupil. Diagnosis is critical, as acute angle-closure glaucoma Acute Angle-Closure Glaucoma Glaucoma is an emergency. Gonioscopy Gonioscopy Examination of the angle of the anterior chamber of the eye with a specialized optical instrument (gonioscope) or a contact prism lens. Glaucoma is utilized to visualize the angle. Treatment of choice is peripheral iridotomy.
• Multiple sclerosis Sclerosis A pathological process consisting of hardening or fibrosis of an anatomical structure, often a vessel or a nerve. Wilms Tumor ( MS MS Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease that leads to demyelination of the nerves in the CNS. Young women are more predominantly affected by this most common demyelinating condition. Multiple Sclerosis): a chronic inflammatory autoimmune demyelinating disease of the central nervous system Central nervous system The main information-processing organs of the nervous system, consisting of the brain, spinal cord, and meninges. Nervous System: Anatomy, Structure, and Classification. Multiple sclerosis Sclerosis A pathological process consisting of hardening or fibrosis of an anatomical structure, often a vessel or a nerve. Wilms Tumor causes acute demyelinating optic neuropathy Neuropathy Leprosy. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship report acute-to-subacute vision Vision Ophthalmic Exam loss with pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways on eye movements. Detection of relative afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology pupillary defect (RAPD) is suggestive of optic neuropathy Neuropathy Leprosy. Internuclear ophthalmoplegia Ophthalmoplegia Paralysis of one or more of the ocular muscles due to disorders of the eye muscles, neuromuscular junction, supporting soft tissue, tendons, or innervation to the muscles. Orbital and Preseptal Cellulitis can also result from MS MS Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease that leads to demyelination of the nerves in the CNS. Young women are more predominantly affected by this most common demyelinating condition. Multiple Sclerosis, exhibited as diplopia Diplopia A visual symptom in which a single object is perceived by the visual cortex as two objects rather than one. Disorders associated with this condition include refractive errors; strabismus; oculomotor nerve diseases; trochlear nerve diseases; abducens nerve diseases; and diseases of the brain stem and occipital lobe. Myasthenia Gravis, nystagmus Nystagmus Involuntary movements of the eye that are divided into two types, jerk and pendular. Jerk nystagmus has a slow phase in one direction followed by a corrective fast phase in the opposite direction, and is usually caused by central or peripheral vestibular dysfunction. Pendular nystagmus features oscillations that are of equal velocity in both directions and this condition is often associated with visual loss early in life. Albinism, and loss of depth perception Depth perception Perception of three-dimensionality. Albinism. Magnetic resonance imaging helps detect the demyelinating plaques on the affected nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification. Treatment involves immunomodulation and symptomatic relief.
• 3rd nerve palsy Palsy paralysis of an area of the body, thus incapable of voluntary movement Cranial Nerve Palsies: neurologic deficit characterized by a large, sluggishly reactive to unreactive pupil, ptosis Ptosis Cranial Nerve Palsies, and paralysis of eye movement ( adduction Adduction Examination of the Upper Limbs, elevation, and depression) resulting from lesions along its path from the oculomotor nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles in the midbrain Midbrain The middle of the three primitive cerebral vesicles of the embryonic brain. Without further subdivision, midbrain develops into a short, constricted portion connecting the pons and the diencephalon. Midbrain contains two major parts, the dorsal tectum mesencephali and the ventral tegmentum mesencephali, housing components of auditory, visual, and other sensorimotor systems. Brain Stem: Anatomy to the orbit. Causes include aneurysm Aneurysm An aneurysm is a bulging, weakened area of a blood vessel that causes an abnormal widening of its diameter > 1.5 times the size of the native vessel. Aneurysms occur more often in arteries than in veins and are at risk of dissection and rupture, which can be life-threatening. Thoracic Aortic Aneurysms, ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage, and trauma. Magnetic resonance imaging with angiography Angiography Radiography of blood vessels after injection of a contrast medium. Cardiac Surgery helps evaluate the etiology. Treatment is directed toward the underlying condition causing the palsy Palsy paralysis of an area of the body, thus incapable of voluntary movement Cranial Nerve Palsies.
• Retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment: separation of the retina Retina The ten-layered nervous tissue membrane of the eye. It is continuous with the optic nerve and receives images of external objects and transmits visual impulses to the brain. Its outer surface is in contact with the choroid and the inner surface with the vitreous body. The outermost layer is pigmented, whereas the inner nine layers are transparent. Eye: Anatomy from the retinal pigment epithelium Retinal Pigment Epithelium The single layer of pigment-containing epithelial cells in the retina, situated closely to the tips (outer segments) of the retinal photoreceptor cells. These epithelial cells are macroglia that perform essential functions for the photoreceptor cells, such as in nutrient transport, phagocytosis of the shed photoreceptor membranes, and ensuring retinal attachment. Hypertensive Retinopathy resulting in rapid photoreceptor damage. Symptoms include painless vision Vision Ophthalmic Exam changes such as sudden flashes of light, floaters Floaters Chorioretinitis, worsening peripheral vision Vision Ophthalmic Exam, or having a shadow in the field of vision Vision Ophthalmic Exam. Vision Vision Ophthalmic Exam is described as curtain-like. Macular involvement would present as rapid afferent Afferent Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology pupillary defect due to photoreceptor degeneration caused by the detachment. Acute visual loss from retinal detachment Retinal detachment Retinal detachment is the separation of the neurosensory retina from the retinal pigmented epithelium and choroid. Rhegmatogenous retinal detachment, the most common type, stems from a break in the retina, allowing fluid to accumulate in the subretinal space. Retinal Detachment is an emergency requiring corrective surgery.
• Hutchinson’s pupil: dilated pupil that occurs due to compression Compression Blunt Chest Trauma of the ipsilateral 3rd cranial nerve arising from trauma, tumor Tumor Inflammation, or aneurysm Aneurysm An aneurysm is a bulging, weakened area of a blood vessel that causes an abnormal widening of its diameter > 1.5 times the size of the native vessel. Aneurysms occur more often in arteries than in veins and are at risk of dissection and rupture, which can be life-threatening. Thoracic Aortic Aneurysms. Irritation on the nerve initially leads to pupillary constriction of the affected eye. As the nerve is mechanically compressed, the superficially located parasympathetic nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology on the oculomotor nerve Oculomotor nerve The 3D cranial nerve. The oculomotor nerve sends motor fibers to the levator muscles of the eyelid and to the superior rectus, inferior rectus, and inferior oblique muscles of the eye. It also sends parasympathetic efferents (via the ciliary ganglion) to the muscles controlling pupillary constriction and accommodation. The motor fibers originate in the oculomotor nuclei of the midbrain. The 12 Cranial Nerves: Overview and Functions are paralyzed, so isolated mydriasis Mydriasis Dilation of pupils to greater than 6 mm combined with failure of the pupils to constrict when stimulated with light. This condition may occur due to injury of the pupillary fibers in the oculomotor nerve, in acute angle-closure glaucoma, and in adie syndrome. Glaucoma follows. Further increases in intracranial pressure Intracranial Pressure Idiopathic Intracranial Hypertension leads to bilateral pupillary dilation. This dilation is associated with uncal herniation Uncal Herniation Increased Intracranial Pressure (ICP). Brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification imaging is necessary to determine etiology.