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Ocular motility disorders and strabismus

Last updated: September 23, 2021

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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.

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

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

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

Steps Inference Underlying principle
Step 1: Determine which eye is hypertropic in primary gaze.
  • Hypertropic right eye weakness of right eye depressors or left eye elevators
  • Hypertropic left eye weakness of right eye elevators or left eye depressors
Step 2: Determine whether hypertropia increases on the right or left gaze.
Step 3: Determine whether hypertropia increases on right or left head tilt.
  • Vertical strabismus increases on tilting the head towards the right shoulder → weakness of a right eye intorter or a left eye extorter
  • Vertical strabismus increases on lilting the head towards the left shoulder → weakness of a left eye intorter or right eye extorter
Weak extraocular muscle Step 1: Which eye is hypertropic? Step 2: Vertical strabismus increases with lateral gaze in this directon Step 3: Vertical strabismus increases with head-tilt towards this shoulder
Right superior oblique Right eye Left lateral gaze Right shoulder
Left superior oblique Left eye Right lateral gaze Left shoulder

Treatment

References:[9][10]

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]

  1. Khaled Mohamed MohamedKoriem. Multiple sclerosis: New insights and trends. Khaled Mohamed MohamedKoriem. 2016 .
  2. Internuclear Ophthalmoplegia (INO). https://webeye.ophth.uiowa.edu/eyeforum/atlas/pages/INO/index.htm. Updated: January 11, 2015. Accessed: June 22, 2018.
  3. Fricke L. Diagnosis and management of accommodative esotropia. Clin Exp Optom. 2006; 89 (5): p.325-331. doi: 10.1111/j.1444-0938.2006.00059.x . | Open in Read by QxMD
  4. Rutstein RP. Update on accommodative esotropia. Optometry. 2008; 79 (8): p.422-431. doi: 10.1016/j.optm.2007.11.011 . | Open in Read by QxMD
  5. Engle EC. Genetic basis of congenital strabismus. Arch Ophthalmol. 2007; 125 (2): p.189-195. doi: 10.1001/archopht.125.2.189 . | Open in Read by QxMD
  6. Concomitant Strabismus Definition. http://www.alpfmedical.info/visual-acuity/concomitant-strabismus-definition.html. Updated: January 21, 2017. Accessed: March 15, 2017.
  7. Optometric Clinical Practice Guideline, Care of the Patient with Strabismus: Esotropia and Exotropia. http://www.aoa.org/documents/optometrists/CPG-12.pdf. Updated: January 1, 2010. Accessed: March 15, 2017.
  8. Wright KW, Spiegel PH . Pediatric Ophthalmology and Strabismus. Springer ; 2003
  9. Martinez-Thompson JM, Diehl NN, Holmes JM, Mohney BG. Incidence, types, and lifetime risk of adult-onset strabismus. Ophthalmology. 2014; 121 (4): p.877-882. doi: 10.1016/j.ophtha.2013.10.030 . | Open in Read by QxMD
  10. Karlsson VC. A Systematic Approach to Strabismus. SLACK Incorporated ; 2009
  11. Helveston EM. Understanding, detecting, and managing strabismus.. Community Eye Health. 2010; 23 (72): p.12-4.
  12. Babinsky E, Sreenivasan V, Candy TR. Near heterophoria in early childhood. Invest Ophthalmol Vis Sci. 2015; 56 (2): p.1406-1415. doi: 10.1167/iovs.14-14649 . | Open in Read by QxMD
  13. 1.3 Cover test FBC3E64F-CA9C-489A-ADDD-758BF5B068.
  14. 1.4 Alternating Cover Test.
  15. Kaplan. USMLE Step 1 Anatomy Lecture Notes 2018. Kaplan