Strabismus is the term used to describe any
ocular misalignment. The term originates from the Greek strabismus,
meaning to “look askance” or the “evil
eye.” Ocular misalignment that is constantly present and
not controlled by fusional mechanisms is termed a tropia.
A phoria is defined as a latent deviation that
is controlled by fusional mechanisms and only present when one eye
is blocked or covered. An intermittent tropia is
a deviation that may sometimes be latent and controlled by fusional
mechanisms (ie, a phoria) but may at other times be spontaneously
manifest, often with illness or fatigue. Most children with strabismus
develop a deviation that manifests mostly in one eye (the nondominant
eye). Some strabismic children are able to switch fixation, using
one eye at times and the other eye at other times, so that the strabismus
will appear to shift from one eye to the other. This is termed alternating
strabismus. In some children, strabismus develops as a
result of poor vision in one eye and is termed sensory strabismus.
Strabismus may also be the presenting sign of a life-threatening
disease (eg, brain tumors with cranial nerve palsy) or vision-threatening
conditions such as retinoblastoma or cataract.
Strabismus affects between 1% and 4% of children.
The two most common types of strabismus are esotropia (cross-eyes,
convergent squint, inwardly turned eyes) and exotropia (walleyes,
divergent squint, outwardly turned eyes). Strabismus has been shown
to occur in families and is noted more frequently among those children
who have other neurological or medical conditions, including those
affected by prematurity, low birth weight, cerebral palsy, intrauterine
infection or drug exposure, craniofacial abnormalities, Down syndrome,
and other genetic disorders.
Amblyopia develops in as many as 50% of children with
strabismus.1 Early diagnosis and treatment of strabismus
is therefore crucial to developing optimal, binocular visual function.
Brief periods of intermittent esotropia or exotropia may be commonly
observed during the first 2 to 3 months of life. This does not require
referral to an ophthalmologist. However, any ocular misalignment
after 3 months of age should be referred to an ophthalmologist for
further evaluation. Likewise, any large or constant deviation should
never be considered normal.
In addition to defining strabismus by the direction of the misalignment,
it can also be categorized by the consistency of the amount of misalignment
in different positions of gaze. A comitant strabismus is one in
which the ocular misalignment is the same magnitude (measured by
the ophthalmologist in units called prism diopters)
in primary (straight-ahead gaze), left, right, up, and down gaze.
All of the esotropias and exotropias discussed below are comitant.
When one or more muscle(s) is paretic or restricted, the amount
of strabismus will differ depending on the position of gaze. This
is called an incomitant strabismus. Although nearly
every paretic vertical strabismus (most commonly a superior oblique [cranial
nerve IV] palsy) starts out incomitant (different in different
gazes), it often becomes comitant over ...