Ocular motility disorders and strabismus

Strabismus is a condition characterized by misalignment of the eyes when looking at an object. One eye deviates (either constantly or intermittently) from the normal visual axis, which results in the inability of the brain to fuse together the images from the right and left eye. Strabismus is classified as either concomitant (nonparalytic) or paralytic. Concomitant strabismus primarily occurs in early childhood and manifests with a constant angle of deviation, in which the misaligned eye follows the unaffected eye. Paralytic strabismus is frequently acquired and is due to the functional weakness of individual extraocular muscles, which alter the angle of deviation depending on the direction of view. Further typical features include double as well as decreased vision. Treatment at an early stage (e.g., via occlusion treatment or surgery) is essential to prevent complications such as amblyopia (decreased vision in an eye with no apparent structural abnormality) and loss of binocular vision. Further complex ocular motility disorders can result from central nervous pathologies. Internuclear ophthalmoplegia, for example, is caused by a lesion of the medial longitudinal fasciculus and causes disturbances in horizontal eye movements.

• Strabismus: abnormal alignment of the eyes; the visual axes of the eyes are not parallel (crossed-eyes)
  • Heterotropia: manifest strabismus
    • Esotropia: inward misalignment
    • Exotropia: outward misalignment
    • Hypertropia: upward deviation of one eye
    • Hypotropia: downward deviation of one eye
    • Cyclotropia: rotation of one eye around an anterior-posterior axis
  • Heterophoria: latent strabismus ; presents with the same (latent) misalignments seen in heterotropia.
    • Esophoria: a type of heterophoria in which one eye has a tendency to converge towards the nose when the other eye is shut; the opposite of exophoria
    • Exophoria: a type of heterophoria in which one eye has a tendency to deviate away from the nose when the other eye is shut; the opposite of esophoria

References:^[1][2]

See extraocular muscles in eye and orbit.

Definition

• Strabismus in which the degree of deviation (angle between the visual axes of both the eyes) remains constant in all directions of gaze

Etiology

• Genetic
• Uncorrected refractive error (particularly hyperopia and anisometropia)
• Unilateral visual impairment/amblyopia (e.g., organic causes, retinal disease, retinoblastoma)
• Other: perinatal lesions (e.g., preterm birth, asphyxia), cerebral damage (e.g., trauma, encephalitis)

Types

• Congenital or infantile concomitant strabismus
  • Evident at birth or onset within 6 months of age ; does not resolve spontaneously
  • May manifest as esotropia or exotropia
• Microstrabismus: angle of deviation < 5°; therefore frequently a late diagnosis with high risk of amblyopia
• Latent strabismus: usually no clinical significance; the deviation is compensated by fixation (fusion); decompensation and manifestation occur in situations of physical stress.

Diagnostics

• Hirschberg test; : a test for determining if the eyes are in alignment. A light is shone at the eyes and the location of the light reflex on the cornea is observed in reference to the pupil. Asymmetrical corneal reflections on examination of the eyes indicate that the visual axes are not aligned (strabismus).
• Cover tests (see “Cover tests” in “Examination of the eyes”
• Measurement of the angle of deviation: if necessary with the help of a tangent screen

Therapeutic options

The main goals in strabismus management are to optimize visual acuity and achieve binocularity.

• Correction of refractive errors
• Visual training therapy
  • Training to correct eye movements (e.g., in strabismus) and visual-motor deficiencies
  • Uses specialized computer and optical devices (e.g., lenses and prisms)
• Occlusion treatment
  • Initiate as early as possible!
  • Cover the unaffected eye using a patch (occlusion) → training of the weaker eye
  • Duration of coverage depends on the child's age
• Penalization therapy (cyclopentolate drop therapy): : apply cyclopentolate drops ; to the unaffected eye → blurs vision → encourages monocular use of the affected eye
• Botulinum toxin therapy : toxin injection into the stronger muscle → temporary and partial paralysis → weaker muscle forced to contract → long-lasting alteration in ocular alignment
• Strabismus surgery
  • Transposition or repositioning of muscles
  • Tucking or advancement to tighten muscles
  • Myectomy or tenectomy to loosen extraocular muscles

Complications

• Disturbances of binocular vision
• Amblyopia
  • Definition: visual decrease in one or both eyes (functional visual impairment) due to a developmental vision disorder during early childhood
  • Pathophysiology: one or both eyes convey poor or mismatched visual information to the brain → brain suppresses information from one or both eyes → disuse of the eye → lacking visual stimuli with partial underdevelopment of the visual cortex
  • Forms
    • Deprivation amblyopia (e.g., via ptosis, cataract, occlusion)
    • Refractive amblyopia
    • Strabismus amblyopia

Failure to detect or adequately treat strabismus may result in irreversible amblyopia!

A serious underlying condition (e.g., brain tumor) should be suspected in infants with strabismus, especially in the presence of additional ocular findings like leukocoria!

References:^{[3][4][5][6][7][8]}

Definition

• Strabismus caused by paresis (partial failure of action) or paralysis (total failure of action) of one or more extraocular muscles (ophthalmoplegia)
• The angle of deviation alters depending on the direction of gaze (incomitant strabismus)

Etiology

• Neuropathies: 3^rd, 4^th, and 6^thcranial nerve palsies (see cranial nerve disorders)
• Demyelinating disease (e.g., multiple sclerosis)
• Myopathies (e.g., myasthenia gravis, ocular myopathies, restrictive thyroid myopathy)
• Microangiopathic vasculopathy (often related to diabetes and hypertension)
• Trauma
• Intracranial or intraorbital masses
• Congenital in rare cases (e.g., overaction of the inferior oblique muscles)

Clinical features

• Diplopia (double vision): most pronounced when looking in the direction usually enabled by the paralyzed muscle
• Often compensatory head posture
• Impaired extraocular muscle function

Patients with poor visual acuity may not notice diplopia. Therefore, complete optical (refractory) correction must be achieved before testing for strabismus!

Diagnosis

• Horizontal paralytic strabismus
  • Exotropia → medial rectus weakness → oculomotor nerve palsy
  • Esotropia → lateral rectus weakness → abducens nerve palsy
• Vertical paralytic strabismus: perform a 3-step Park-Bielchowsky test

Treatment

• Correction of refractive errors
• Prismatic glasses
• Treatment of underlying cause
• Strabismus surgery

References:^[9][10]

• Definition:
  • Damage to the medial longitudinal fasciculus (the connection between the abducens nucleus, CN VI, on one side and the oculomotor nucleus, CN III, on the other), which leads to impaired lateral gaze.
  • Manifests primarily with impaired adduction of the eye ipsilateral to the lesion (ipsilateral to the medial longitudinal fasciculus lesion)
  • Depending on which eye is affected, INO is classified as left, right, or bilateral
• Etiology
  • Multiple sclerosis (MS)
    • INO is common in individuals with MS between 20–50 years of age ^[11]
    • Typically bilateral
  • Hemorrhage (common cause in older patients)
  • Rare causes: brain tumors, chronic alcohol and recreational drug use, encephalitis, metabolic disorders
• Pathophysiology
  • Normally, CN VI receives a signal from the ipsilateral paramedian pontine reticular formation and sends a signal to the contralateral CN III via the medial longitudinal fasciculus.
    • Activation of the CN VI ipsilateral to the lesion → activation of the ipsilateral lateral rectus → abduction of the ipsilateral eye
    • Activation of the CN III contralateral to the lesion → activation of the contralateral medial rectus → adduction of the contralateral eye
  • Disruption of the medial longitudinal fasciculus fibers linking the CN VI ipsilateral and the CN III contralateral to the lesion → failure of signal transmission from CN VI to CN III → the ipsilateral lateral rectus is activated while the contralateral medial rectus is not → abduction of the ipsilateral eye, no adduction of contralateral eye
  • Firing from CN VI which fails to be transmitted to CN III is instead partially transmitted to the lateral rectus ipsilateral to the lesion → nystagmus of the ipsilateral abducting eye
• Clinical findings
  • Adduction limited in horizontal eye movements
  • Adduction is retained in convergence reaction
  • Dissociated nystagmus: gaze to the opposite side → nystagmus of the abducted contralateral eye
  • In bilateral INO: possible vertical nystagmus

Remember that internuclear ophthalmoplegia (INO) is characterized by Impaired adduction of the eye ipsilateral to the lesion and Nystagmus on the Opposite side!

References:^[12]