• Clinical science

Ocular motility disorders and strabismus

Abstract

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. Intranuclear ophthalmoplegia, for example, is caused by a lesion of the medial longitudinal fasciculus and causes disturbances in horizontal eye movements.

Basic terms

  • 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]

Muscles of the eye

The orbit contains 6 muscles that are attached to the eyeball. There is an additional muscle in the orbit that attaches to the upper eyelid, the levator palpebrae superioris, which functions to elevate the eyelid.

Muscle Function Attachments Innervation
Superior rectus muscle
  • Elevation, intorsion, and adduction of the eyeball
  • Origin: common tendinous ring
  • Insertion: upper globe
Inferior rectus muscle
  • Depression and extorsion of the eyeball
  • Origin: common tendinous ring
  • Insertion: lower globe
Medial rectus muscle
  • Adduction of the eyeball
  • Origin: common tendinous ring
  • Insertion: medial globe
Lateral rectus muscle
  • Abduction of the eyeball
  • Origin: common tendinous ring
  • Insertion: lateral globe
Superior oblique muscle
  • Abduction, depression, and intorsion of the eyeball
Inferior oblique muscle
  • Extorsion, elevation, and abduction of the eyeball
  • Origin: lateral to the lacrimal groove; on the floor of the orbit
  • Insertion: lower posterior globe
Levator palpebrae superioris muscle
  • Elevation of the upper eyelid
  • Origin: lesser wing of the sphenoid bone
  • Insertion: superior tarsus

References:[3][8241;247]

Concomitant strabismus

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
Single cover test for heterotropia Cover-uncover test for heterophoria Alternating cover test
Method
  • Cover one eye for 1–2 seconds
  • Observe uncovered eye for shift in fixation
  • Uncover eye and observe both eyes for refixation movements
  • Same procedure as in single cover test
  • Observe the occluded eye when uncovering it
  • Cover one eye for several seconds , then rapidly alternate between covering both eyes.
  • Continually observe the initially covered eye.
Interpretation
  • No shift in fixation in either eye no misalignment
  • Fixation shift in the initially uncovered eye misaligned eye
  • Fixation shift in the covered eye on uncovering it → misaligned eye
  • Refixation movement of the occluded eye on being uncovered → heterophoria
  • Determines extent and direction of deviation
  • 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.

  1. Correction of refractive errors
  2. Visual training therapy
  3. 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
  4. Penalization therapy (cyclopentolate drop therapy): : apply cyclopentolate drops to the unaffected eye blurs vision → encourages monocular use of the affected eye
  5. Botulinum toxin therapy : toxin injection into the stronger muscle → temporary and partial paralysis → weaker muscle forced to contract → long-lasting alteration in ocular alignment
  6. 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 (cortical blindness)
    • Forms

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:[4][5][6][7][8][9][10]

Paralytic strabismus

Definition

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

Etiology

Clinical features

  • Diplopia: 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.
  • Vertical strabismus increases on looking right weakness of right superior rectus, right inferior rectus, left superior obliquus, or left inferior obliquus
  • Vertical strabismus increases on looking left → weakness of right superior obliquus, right inferior obliquus, left superior rectus, or left inferior rectus
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 rectus Left Right Right
Right inferior rectus Right Right Left
Right superior oblique Right eye Left lateral gaze Right shoulder
Right inferior oblique Left Left Left
Left superior rectus Right Left Left
Left inferior rectus Left Left Right
Left superior oblique Left eye Right lateral gaze Left shoulder
Left inferior oblique Right Right Right

Treatment

References:[11][12]

Internuclear ophthalmoplegia

  • Definition: damage to the medial longitudinal fasciculus (connection between the abducens nucleus, CN VI, on one side and the oculomotor nucleus, CN III, on the other); leads to a complex disturbance of horizontal eye movements; primarily affects adduction of the ipsilateral eye
  • Etiology
    • Multiple sclerosis (common cause in young patients)
    • Hemorrhage (common cause in older patients)
    • Rare causes: brain tumors, intoxication, encephalitis, metabolic disorders
  • 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

References:[3][13]

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last updated 11/08/2018
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