Strabismus occurs when the visual axes of the eyes are misaligned.
It is one of the most common disorders encountered in pediatric
ophthalmology, estimated to affect approximately 3% to
5% of children. The type of strabismus is defined by the
direction of misalignment and whether the deviation is latent, intermittent,
or constant. A phoria describes a latent
strabismus that is present when one eye is covered. The eyes return
to normal alignment when the eye is uncovered and the patient views
under normal binocular viewing conditions. A tropia describes
strabismus that is present when both eyes are viewing. Intermittent strabismus is present
when the eyes vary between being misaligned and straight.
When the eyes are horizontally misaligned, they are esotropic if they are turned toward
each other (“cross-eyed”) and exotropic if
they are directed away from each other (“wall-eyed”).
If the eyes are vertically misaligned, the type of strabismus is
described by the deviating eye. If the deviating eye is lower than
the straight eye, it is hypotropic.
If it is higher than the straight eye, it is hypertropic.
Vertical strabismus is much less common than horizontal strabismus
If the eyes are aligned normally, they are orthophoric.
A deviation is comitant if the amount
of misalignment does not change in different positions of gaze.
This is the most common situation in primary strabismus. A deviation
is incomitant if the amount of misalignment
changes as the eyes move in various directions. This occurs in patients
with cranial nerve palsies or other forms of strabismus with limited
extraocular movements. Horizontal eye movements are called adduction when the moves in an inward
direction and abduction when it moves outward. Pseudostrabismus is
present when the eyes appear misaligned, but they are optically
Eye movements result from contraction of the extraocular muscles.
Horizontal movements are produced by the medial and lateral rectus
muscles. Vertical movements are produced by the inferior and superior
rectus muscles. The superior oblique muscle causes downward and
intorsional movements. The inferior oblique muscle causes upward
and extorsional movements (Figure 34–1).
The extraocular muscles. Top: View of right eye from
above. Bottom: Frontal view of left eye.
Movements of the extraocular muscles are controlled by inputs
from the third, fourth, and sixth cranial nerves. The third cranial
nerve controls 4 of the 6 extraocular muscles (medial rectus, inferior
rectus, superior rectus, and inferior oblique muscles), in addition
to the sphincter muscle of the iris and the levator muscle of the
eyelid. It begins in the rostral midbrain at the level of the superior
colliculus, where it is composed of separate subnuclei that subserve
the different muscles controlled by the nerve. The fourth cranial
nerve controls the superior oblique ...