Strabismus

Strabismus is the misalignment of the eyes while fixating the gaze on an object. Strabismus can be idiopathic, but it may also be caused by cerebral palsy, uncorrected refractive errors, and extraocular muscle or cranial nerve dysfunction. Clinical presentation may include vertical or horizontal diplopia (double vision). Diagnosis of strabismus is established clinically with the eye cover–uncover test. Management includes therapy with cycloplegic eye drops, eye patching, or surgery. Untreated strabismus can lead to amblyopia, which is cortical blindness with no structural defect.

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Overview

Definition

Strabismus is the misalignment of the eyes in any direction (vertical, horizontal). Colloquially, it is referred to as “crossed eyes.”

Classification

Strabismus can be classified as:

  • Infantile
  • Acquired

Medical terminology describes the direction of eye deviation:

  • Horizontal strabismus:
    • Esotropia: inward (nasal) deviation of the eye
    • Exotropia: outward (temporal) deviation of the eye
  • Vertical strabismus:
    • Hypertropia: upward deviation of the eye
    • Hypotropia: downward deviation of the eye

Other considerations:

  • A “-tropia” is detectable with both eyes open:
    • May be constant or intermittent
    • Can affect one or both eyes
  • A “-phoria” describes latent strabismus that is present only when 1 eye is covered (e.g., esophoria).

Epidemiology

  • Prevalence in the United States: 
    • 2%–5% of the population
    • 5 million–15 million people
  • Affects boys and girls equally
  • Age:
    • Infantile esotropia typically starts at < 6 months of age. 
    • Intermittent esotropia can present at any age. 

Etiology

Most strabismus results from an abnormality of the neuromuscular control of eye movement. Less commonly, there is a problem with the extraocular muscles.

  • Infantile strabismus:
    • Familial in approximately 30% of children
    • Idiopathic
    • Preterm birth and/or low birth weight
    • Family history of strabismus
    • Genetic disorders (e.g., Down syndrome)
    • Prenatal drug exposure
    • Congenital eye defects
  • Acquired strabismus:
    • Uncorrected refractive errors
    • Cranial nerve palsy:
      • 3rd cranial nerve (oculomotor palsy)
      • 4th cranial nerve (superior oblique palsy)
      • 6th cranial nerve (lateral rectus palsy)
    • Neurologic conditions:
      • Cerebral palsy
      • Spina bifida
      • Hydrocephalus
      • Encephalitis/meningitis
      • Brain tumors
      • Stroke (the leading cause of adult strabismus)
      • Multiple sclerosis
    • Can be a presenting sign of retinoblastoma
    • Head injuries
    • Graves’ disease

Pathophysiology

  • During forward gaze, both eyes normally fixate on an object together.
  • Regulated by the 6 extraocular muscles 
  • These 6 extraocular muscles operate under: 
    • Hering’s law of equal innervation: equal innervation occurs through synergistic agonist muscles in both eyes.
    • Sherrington’s law of reciprocal innervation: ↑ innervation to a muscle is accompanied by an equal ↓ in innervation to the antagonist muscle.
  • Any process that results in a deviation from these results in strabismus.

Clinical Presentation

Strabismus presents as:

  • Misalignment of the eyes: 
    • Unilateral or bilateral 
    • Intermittent or constant
  • Manifest strabismus: evident without covering the eye
  • Latent strabismus: evident only with the eye-cover test
  • Diplopia:
    • Vertical (present in vertical strabismus) 
    • Horizontal (present in horizontal strabismus) 
  • Amblyopia (↓ vision in one eye) can develop.
  • Patients may alter head positioning to compensate.
Esotropia

Esotropia (inward deviation) of the right eye:
The light reflex is central in the left eye (the nondeviated eye) but over the iris in the right eye (the deviated eye).

Image: “Patient IV:1 – infantile esotropia with convergence excess” by Khan AO, Shinwari J, Abu Dhaim N, Khalil D, Al Sharif L, Al Tassan N. License: CC BY 3.0

Diagnosis

There are several historical factors to consider in determining the cause and treatment of strabismus.

History

  • Onset:
    • Age at onset
    • History of birth trauma
    • Sudden or gradual onset
  • Family history of strabismus may be a contributing factor.
  • Exposure to toxins
  • Symptoms: 
    • May have no subjective symptoms
    • Diplopia
    • Hyperopia (farsightedness)
    • Difficulty reading
    • Eyestrain
    • Headaches

Physical exam

  • In infants, look for: 
    • Closing or covering 1 eye when looking at a near object
    • Closing 1 eye when viewing faraway objects
    • Tilting or turning the head
  • Assess visual acuity in children or adults.
  • Note clinical features:
    • Strabismus in 1 or alternating between eyes
    • Severity
    • Constant or intermittent
    • Asymmetric red reflex

Other diagnostic testing

  • Asymmetric corneal light reflex
    • Initial screen for strabismus
    • Hold an object several feet in front of a child’s face with a penlight next to it.
    • If ocular alignment is normal, the light reflex is positioned centrally and symmetrically in each eye.
    • With strabismus, the reflection of the corneal light reflex will show ocular misalignment.
  • Cover test:
    • Cover 1 of the patient’s eyes.
    • Observe the opposite eye for movement.
  • Cover/uncover test:
    • Used to detect latent strabismus
    • Each eye is sequentially covered.
    • Refixation movements are observed in the eye affected with strabismus.
    • Small, barely perceptible phorias are common and not pathologic.
  • Neuroimaging studies are rarely needed, but can be considered in children with:
    • Craniofacial malformations
    • Abnormal neurologic exam
    • Head or orbit trauma

Management

Management

Abnormal eye examination or strabismus noted on testing in a child should be referred to a pediatric ophthalmologist.

  • Correction of refractive errors (e.g., myopia, hyperopia):
    • Glasses
    • Contacts
  • Amblyopia management:
    • Penalization therapy:
      • Atropine drops causes pupil dilation → blurring the vision in normal eye
      • Eye with strabismus is forced to receive light signals and transmit them to the brain to improve vision.
    • Occlusion therapy:
      • Patch the normal eye
      • Eye with strabismus is forced to receive light signals and transmit them to the brain.
  • Surgery to adjust the extraocular muscles

Complications

  • Amblyopia: 
    • Permanently reduced vision in 1 eye caused by failure of the brain to process visual input
    • Over time, only 1 eye is used for vision.
  • Diplopia (with acquired strabismus in children > 9 years of age)
  • Secondary contracture of the extraocular muscles
  • Adverse psychosocial and vocational consequences

Differential Diagnosis

  • Ocular instability of infancy: physiologic intermittent misalignment of the eyes during the first few months of life. This instability occurs because of normal initial weakness of infantile extraocular muscles. Ocular instability of infancy may present either with exotropia or esotropia. Diagnosis is clinical. Management is to reassure the parents. In contrast, eye misalignment that is constant or lasts > 4–6 months is pathologic and warrants further investigation.
  • Pseudostrabismus: appearance of misalignment of eyes in infants due to epicanthal skin fold. An epicanthal skin fold covering the medial sclera may make the eye appear deviated; however, corneal light reflex testing showing a light reflection on the corneal center excludes true strabismus. Diagnosis is clinical. Management is to reassure the parents. 
  • Internuclear ophthalmoplegia (INO): inability to adduct the eye during voluntary horizontal gaze. Internuclear ophthalmoplegia is caused by a lesion of the medial longitudinal fasciculus. Bilateral INO is commonly seen in multiple sclerosis and is a clinical diagnosis. However, investigations involving neuroimaging, especially MRI, help establish the diagnosis. Management of INO varies depending on the cause. Prognosis mainly depends on the etiology.
  • Opsoclonus-myoclonus syndrome: rare ocular motility disorder characterized by involuntary, repetitive, rapid conjugate eye movements occurring in all directions of gaze. This syndrome is more common in children than in adults and can be idiopathic or a paraneoplastic disorder. Opsoclonus-myoclonus syndrome presents with other neurologic symptoms, such as ataxia and movement disorders, and diagnosis includes ruling out neuroblastoma as an underlying cause. There is no definitive treatment, but steroids and immunologic agents have been used. 

References

  1. Coats, D.K., Paysse, E.A. (2019). Evaluation and management of strabismus in children. UpToDate. Retrieved June 14, 2021, from https://www.uptodate.com/contents/evaluation-and-management-of-strabismus-in-children
  2. Coats, D.K., Paysse, E.A. (2021). Causes of horizontal strabismus in children. UpToDate. Retrieved June 14, 2021, from https://www.uptodate.com/contents/causes-of-horizontal-strabismus-in-children
  3. Fioretto, M. (2020). Acquired esotropia: background, pathophysiology, epidemiology. MedScape. Retrieved June 14, 2021, from https://emedicine.medscape.com/article/1198784-overview
  4. Kanukollu, V.M., Sood, G. (2020). Strabismus. StatPearls. Retrieved June 16, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK560782/
  5. Khazaeni, L.M. (2020). Strabismus. MSD Manual Professional Version. Retrieved June 16, 2021, from https://www.merckmanuals.com/professional/pediatrics/eye-defects-and-conditions-in-children/strabismus

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