Anatomy

The surgeon needs to be aware of the important structures at the site where the surgery occurs, being especially careful not to damage delicate neurovascular structures.

Bones of the cranial dome

• Frontal
• Parietal
• Temporal
• Occipital

Bones of the cranial floor

• Sphenoid
• Ethmoid

Meninges Meninges The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. Meninges

Table: Meninges Meninges The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. Meninges

Layer	Characteristics
Epidural space	Potential space between the dura mater and skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull/ vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column Contains blood vessels and fat Site of blood collection in cases of middle meningeal artery injury → epidural hematoma
Dura mater	Divided into 2 layers: Superficial periosteal layer Inner meningeal layer Grows adhered to the periosteum of the calvaria Blood supply: middle meningeal artery Nerve supply: Trigeminal nerve branches: innervate supratentorial structures Cervical nerves (C2 and C3): innervate infratentorial structures
Subdural space	Potential space between the arachnoid mater and the dura mater Site of blood collection in cases of injury to bridging veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins → subdural hematoma
Arachnoid mater (leptomeninges)	Outer layer of the subarachnoid space Avascular Arachnoid trabeculae: web-like strands that separate the arachnoid and pia mater
Subarachnoid space	Arachnoid/pacchionian granulations: allow CSF to enter from the subarachnoid space into the venous system CSF: produced by the choroid plexus and contained in the subarachnoid space Site of blood collection in cases of saccular 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. Extremity and Visceral Aneurysms rupture → subarachnoid hemorrhage Subarachnoid Hemorrhage Subarachnoid hemorrhage (SAH) is a type of cerebrovascular accident (stroke) resulting from intracranial hemorrhage into the subarachnoid space between the arachnoid and the pia mater layers of the meninges surrounding the brain. Most SAHs originate from a saccular aneurysm in the circle of Willis but may also occur as a result of trauma, uncontrolled hypertension, vasculitis, anticoagulant use, or stimulant use. Subarachnoid Hemorrhage (SAH)
Pia mater (leptomeninges)	Inner layer of the subarachnoid space Adherent to the brain Confers shiny appearance Highly vascularized

Ventricular system Ventricular System The ventricular system is an extension of the subarachnoid space into the brain consisting of a series of interconnecting spaces and channels. Four chambers are filled with cerebrospinal fluid (CSF): the paired lateral ventricles, the unpaired 3rd ventricle, and the unpaired 4th ventricle. Ventricular System

The ventricular system is composed of the following structures:

• Lateral ventricles:
  • Body:
    • Spans the frontal, parietal, temporal, and occipital lobes 
    • Extends from the interventricular foramen of Monro to the splenium of the corpus callosum.
  • Anterior horn: in the frontal lobe
  • Posterior horn: curves posteromedially into the occipital lobe
  • Inferior horn
    • Largest compartment of the lateral ventricle
    • Extends forward into the temporal lobe
• 3rd ventricle: midline, slit-like cavity
• Cerebral aqueduct: small tube extending throughout the dorsal quarter of 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 in the midline and surrounded by the periaqueductal gray
• 4th ventricle: between the brain stem Brain Stem The brain stem is a stalk-like structure that connects the cerebrum with the spinal cord and consists of the midbrain, pons, and medulla oblongata. It also plays a critical role in the control of cardiovascular and respiratory function, consciousness, and the sleep-wake cycle. Brain Stem and the cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum

Arterial supply of the head

The arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries that supply the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull and its contents can be divided into 2 large groups.

• The external group is made up of the branches of the external carotid artery: 
  • Occipital artery
  • Posterior auricular artery
  • Superficial temporal artery
• The internal group is made up of branches of the circle of Willis:
  • Anterior cerebral artery
  • Middle cerebral artery
  • Posterior cerebral artery

Dural venous sinuses

The venous drainage system of the brain is located between the endosteal and meningeal layers of dura mater. These vessels are lined by endothelium and have no valves or smooth muscle cells in their walls. 

The following sinuses are in contact with the bones of the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull:

• Superior sagittal sinus
• Transverse sinus
• Sigmoid sinus

Vertebral column

• 33 vertebrae placed in series and connected by intervertebral disks and ligaments
• Vertebral column segmentation
  • Cervical: 7
  • Thoracic: 12
  • Lumbar: 5
  • Sacral: 5
  • Coccygeal: 4 (3–5)
• The vertebrae form the spinal canal, which houses 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 and spinal nerves 
• Intervertebral foramina (neuroforamen or neural foramen): foramen for the spinal nerves exiting the vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column  between 2 spinal vertebrae

Vertebrae

• Components:
  • Body (anterior)
  • Vertebral arch (lateral and posterior) formed by the vertebral pedicles, laminae, and spinous processes
  • Spinous processes: 
    • Transverse (lateral)
    • Articular (superior and inferior, form the facets)
    • Spinous (posterior) 
• Foramina:
  • Vertebral: Large central openings in vertebrae that collectively form the vertebral, or spinal, canal, which contains:
    • Spinal cord
    • Nerve roots
    • Blood vessels
  • Intervertebral foramina (neuroforamen or neural foramen): opening for the spinal nerves 
• Intervertebral disks:
  • Components:
    • Annulus fibrosus
    • Nucleus pulposus
    • Cartilaginous end plates anchor the disks to the adjacent vertebrae

Craniotomy/Craniectomy

Definition

• Craniotomy: surgical procedure that aims to access the cranial cavity and operate directly on the cerebral parenchyma by removing a bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap from the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull.
• Craniectomy: a craniotomy in which the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap is used as a template for a titanium or acrylic plate or is stored for later reimplantation. 
• Cranioplasty: surgical procedure to reconstruct the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull by placing the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap, or a synthetic replacement, into position during a 2nd intervention.

Classification

• Hemicraniectomy: removal of bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones in a single hemisphere 
• Bilateral craniectomy: removal of a single bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap from both hemispheres

Depending on the location, a craniotomy is:

• Supratentorial
• Infratentorial

Indications

• Craniotomy:
  • Brain mass/tumor removal
  • Blood clot treatment
  • Removing a foreign body
  • Implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week of a device with therapeutic intent (e.g., deep brain stimulation in the treatment of Parkinson disease)
• Craniectomy:
  • Elevated intracranial pressure ( ICP ICP Normal intracranial pressure (ICP) is defined as < 15 mm Hg, whereas pathologically increased ICP is any pressure ≥ 20 mm Hg. Increased ICP may result from several etiologies, including trauma, intracranial hemorrhage, mass lesions, cerebral edema, increased CSF production, and decreased CSF absorption. Increased Intracranial Pressure (ICP)) refractory to medical intervention
  • Traumatic brain injury
  • Stroke
  • Chiari malformation
  • Brain herniation: pathologic displacement of intracranial structures due to an increased pressure gradient between the intracranial compartments

Contraindications

• Severe cardiopulmonary disease
• Severe systemic collapse (e.g., sepsis Sepsis Organ dysfunction resulting from a dysregulated systemic host response to infection separates sepsis from uncomplicated infection. The etiology is mainly bacterial and pneumonia is the most common known source. Patients commonly present with fever, tachycardia, tachypnea, hypotension, and/or altered mentation. Sepsis and Septic Shock, adrenal crisis, massive hemorrhage)
• Abnormal coagulation parameters

Operative care

When used for decompression, the craniotomy/craniectomy procedure turns the intracranial compartment from a semirigid shape into an elastic container.

1. An incision is made on the scalp. A local anesthetic with epinephrine may be administered at the incision site to prevent heavy bleeding of the scalp.
2. Subcutaneous tissue and muscles are dissected to expose the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull.
3. Retractors are placed on the edges of the incision to expose the work area.
4. Burr holes are made in the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull to mark the corners of the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull segment to be removed. 
5. The burr holes are linked using a craniotome, creating a bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap.
6. The dura is separated from the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones.
7. After separating the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap from the dura, it is elevated and then removed. At this time, the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap can be:
   • Kept on the surgical table until reimplantation
   • Discarded (requires immediate intraoperative reconstruction with synthetic materials)
   • Placed within an abdominal subcutaneous incision
   • Preserved in a tissue bank
8. The dura is cut and retracted to expose the brain to perform the specific intervention.
9. The intended procedure is performed on the brain parenchyma.
10. The bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap is reattached to the rest of the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull using plates and screws.
11. The incision on the scalp is closed by layers using absorbable sutures.
    • The surgeon is careful to ensure adequate homeostasis before closing the scalp. 
    • The surgeon can decide to leave a subdural or subgaleal drain in place.
12. The skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin is closed using nonabsorbable sutures. 
13. The skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin is cleansed of any residue (e.g., blood, adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or "brite" adipose tissue, which is a transitional form. Adipose Tissue)
14. A sterile gauze and skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin dressing are placed over the surgical wound. 

Cranioplasty

• The timing for cranioplasty depends on the individual’s clinical status and is left to the surgeon’s discretion (at least 6 weeks to 3 months after injury).
• Some surgeons wait up to 6 months, depending on the individual’s underlying medical issues.
• Autologous bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones should be used whenever possible (cheaper than synthetic materials), especially in young individuals owing to skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull growth. 
• Rehabilitation can be started and continued while the individual is wearing a helmet. 

Prognosis

• Depends on the underlying condition
• When performed for decompression, neurologic recovery has been reported in the weeks following the procedure (e.g., improved motor strength and language function)

Complications

• Intracranial hematomas (subdural, epidural, subarachnoid)
• Heavy bleeding
• Dural sinus perforation
• Postcraniotomy headache
• Neurologic deficit
• Hydrocephalus: potentially life-threatening condition caused by the excess accumulation of CSF within the ventricular system. Hydrocephalus can be classified as communicating, which is caused by either impaired CSF absorption Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion and Absorption or excess CSF production, or noncommunicating, which is caused by a structural blockage in CSF flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure
• Seizures Seizures A seizure is abnormal electrical activity of the neurons in the cerebral cortex that can manifest in numerous ways depending on the region of the brain affected. Seizures consist of a sudden imbalance that occurs between the excitatory and inhibitory signals in cortical neurons, creating a net excitation. The 2 major classes of seizures are focal and generalized. Seizures: abnormal, excessive and hypersynchronous firing of neurons. Seizures Seizures A seizure is abnormal electrical activity of the neurons in the cerebral cortex that can manifest in numerous ways depending on the region of the brain affected. Seizures consist of a sudden imbalance that occurs between the excitatory and inhibitory signals in cortical neurons, creating a net excitation. The 2 major classes of seizures are focal and generalized. Seizures can be generalized (involving both hemispheres and compromising awareness) or focal (involving a single area of the brain and not compromising awareness)
• Stroke: refers to the injury undergone by brain tissue after interruption of blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure ( ischemic stroke Ischemic Stroke An ischemic stroke (also known as cerebrovascular accident) is an acute neurologic injury that occurs as a result of brain ischemia; this condition may be due to cerebral blood vessel occlusion by thrombosis or embolism, or rarely due to systemic hypoperfusion. Ischemic Stroke) or active hemorrhage (hemorrhagic stroke), which has characteristic neurologic clinical features
• Coma Coma Coma is defined as a deep state of unarousable unresponsiveness, characterized by a score of 3 points on the GCS. A comatose state can be caused by a multitude of conditions, making the precise epidemiology and prognosis of coma difficult to determine. Coma: clinical state characterized by unarousability and unresponsiveness to external stimuli
• Surgical site infection
• Osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis of the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones flap: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones due to infection (most commonly by bacterial agents)
• Bacterial, viral, fungal meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the leptomeninges due to an infectious agent
• Air embolism: embolization of gas bubbles within the circulation leading to blockage of arterial or venous blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure