Strabismus

Strabismus - What is Strabismus?

Strabismus is a condition in which the eyes are not properly aligned with each other.

It typically involves a lack of coordination between the extraocular muscles that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception.

Strabismus can be either a disorder of the brain in coordinating the eyes, or of one or more of the relevant muscles' power or direction of motion.

Difficult strabismus problems are usually co-managed between orthoptists and ophthalmologists.

Paralytic strabismus

Forms of paralytic strabismus include

  • Third
  • Fourth
    • Congenital fourth nerve palsy
    • Sixth
    • Total (external) ophthalmoplegia
    • Progressive external ophthalmoplegia
    • Other
      • Kearns-Sayre syndrome

Other strabismus

Other forms of strabismus include:

  • Convergent concomitant/Divergent concomitant
    • Esotropia
    • Exotropia
    • Vertical strabismus
      • Hypertropia
      • Hypotropia
      • Other and unspecified heterotropia
        • Microtropia
        • Monofixation syndrome
        • Heterophoria
          • Esophoria
          • Exophoria
          • Mechanical strabismus
            • Brown's sheath syndrome
            • Other
              • Duane syndrome

Strabismus can be an indication that a cranial nerve has a lesion. Particularly Cranial Nerve III (Occulomotor), Cranial Nerve IV (Trochlear) or Cranial Nerve VI (Abducens).

A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position.

A strabismus may be a sign of increased intracranial pressure, as CN VI is particularly vulnerable to damage from brain swelling, as it runs between the clivus and brain stem.

More commonly however, squints are termed concominant (i.e. non paralytic).

This means the squint is not caused by a lesion reducing innervation. The squint in this example, is caused by a refractive error in one or both eyes.

This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.

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Strabismus Diagnosis

During eye examinations, orthoptists, ophthalmologists and optometrists typically use a cover test to aid in the diagnosis of strabismus.

If the eye being tested is the strabismic eye, then it will fixate on the object after the "straight" eye is covered, as long as the vision in this eye is good enough.

If the "straight" eye is being tested, there will be no change in fixation, as it is already fixated. Depending on the direction that the strabismic eye deviates, the direction of deviation may be assessed.

''Exotropic'' is outwards (away from the midline) and ''esotropic'' is inwards (towards the nose); these are types of horizontal strabismus. "Hypertropia" is upward, and "Hypotropia" is downward; these are types of vertical strabismus, which are less common.

A simple screening test for strabismus is the Hirschberg test. A flashlight is shone in the patient's eye. When the patient is looking at the light, a reflection can be seen on the front surface of the pupil.

If the eyes are properly aligned with one another, then the reflection will be in the same spot of each eye. Therefore, if the reflection is not in the same place in each eye, then the eyes aren't properly aligned.

Laterality

Strabismus may be classified as unilateral if the same eye consistently 'wanders', or alternating if either of the eyes can be seen to 'wander'.

Alternation of the strabismus may occur spontaneously, with or without subjective awareness of the alternation.

Alternation may also be seen following the cover test, with the previously 'wandering' eye remaining straight while the previously straight eye is now seen to be 'wandering' on removal of the cover. The cover-uncover test is used to diagnose the type of strabismus (also known as tropia) present.

Differential diagnosis

Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of strabismus.

With age, the bridge of the child's nose narrows and the folds in the corner of the eyes go away. To detect the difference between pseudostrabismus and strabismus, a Hirschberg test may be used.

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Strabismus Symptoms

One eye moves normally, while the other points in (esotropia or "crossed eyes"), out (exotropia), up (hypertropia) or down (hypotropia).

Strabismus is often referred to as "lazy eye". It is also referred to as "squint", "crossed eye", "cock eye", "codeye" and "wall eye".

"Cross-eyed" means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle less than zero, that is the optic axes overconverge.

"Wall-eyed" means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle greater than zero, that is the optic axes diverge from parallel.

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Strabismus Treatment

As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze.

Whereas amblyopia, if minor and detected early, can often be corrected with use of an eyepatch on the dominant eye and/or vision therapy, the use of eyepatches is unlikely to change the angle of strabismus.

Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment.

Surgery does not change the vision; it attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement.

Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.

Early treatment of strabismus and/or amblyopia in infancy can reduce the chance of developing amblyopia and depth perception problems. Most children eventually recover from amblyopia by around age 10, if they have had the benefit of patches and corrective glasses.

This article is licensed under the Creative Commons Attribution-ShareAlike License. It uses material from the Wikipedia article on "Strabismus" All material adapted used from Wikipedia is available under the terms of the Creative Commons Attribution-ShareAlike License. Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.