- Clinical science
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.
Strabismus: abnormal alignment of the eyes; the visual axes of the eyes are not parallel (crossed-eyes)
- Heterotropia: manifest strabismus
- Heterophoria: latent strabismus ; presents with the same (latent) misalignments seen in heterotropia.
- Strabismus in which the degree of deviation (angle between the visual axes of both the eyes) remains constant in all directions of gaze
- 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)
- Congenital or infantile concomitant strabismus
- Latent strabismus: usually no clinical significance; the deviation is compensated by fixation (fusion); decompensation and manifestation occur in situations of physical stress.
- 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|
- Measurement of the angle of deviation, if necessary with the help of a tangent screen
The main goals in strabismus management are to optimize visual acuity and achieve binocularity.
- Correction of refractive errors
- Visual training therapy
- Occlusion treatment
- Penalization therapy (cyclopentolate drop therapy): : apply cyclopentolate drops to the unaffected eye → blurs vision → encourages monocular use of the affected eye
- Transposition or repositioning of muscles
- Tucking or advancement to tighten muscles
- Myectomy or tenectomy to loosen extraocular muscles
- Disturbances of binocular vision
- 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)
- 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)
- Neuropathies: 3rd, 4th, and 6th cranial nerve palsies (see )
- Demyelinating disease (e.g., )
- Myopathies (e.g., , ocular myopathies, restrictive thyroid myopathy)
- Microangiopathic vasculopathy (often related to diabetes and hypertension)
- Intracranial or intraorbital masses
- Congenital in rare cases (e.g., overaction of the inferior oblique muscles)
- Diplopia (double vision): most pronounced when looking in the direction usually enabled by the paralyzed muscle
- Often compensatory head posture
- Impaired extraocular muscle function
- Horizontal paralytic strabismus
- Vertical paralytic strabimsus: perform a 3-step Park-Bielchowsky test
|Step 1: Determine which eye is hypertropic in primary gaze.|
|Step 2: Determine whether hypertropia increases on the right or left gaze.|| |
|Step 3: Determine whether hypertropia increases on right or left head tilt.|
|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|
- Correction of refractive errors
- Prismatic glasses
- Treatment of underlying cause
- 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
- 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.
- 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