++
Under normal binocular viewing conditions, the image of the object of regard falls simultaneously on the fovea of each eye (bifoveal fixation), and the vertical retinal meridians are both upright. Strabismus is any ocular misalignment in which only one eye fixates with the fovea on the object of regard. (In everyday language, squint means partial closure of the eye to see more clearly, but sometimes it is used to mean strabismus.) Misalignment of the eyes may be in any direction—inward (eso-), outward (exo-), up (hyper-), down (hypo-), or torsional. The amount of deviation is the angle by which the deviating eye is misaligned. Tropia (manifest strabismus, heterotropia; Box 12–1) is strabismus present under binocular viewing conditions. Phoria (latent strabismus, heterophoria) is a deviation present only after binocular vision has been interrupted by occlusion of one eye.
++
Strabismus is present in about 4% of children, and treatment should be started as soon as it is identified to ensure the best possible visual acuity and binocular visual function. Strabismus may also be acquired due to cranial nerve palsies brainstem disease or orbital disease including masses, fractures, and thyroid eye disease.
++
+++
Individual Muscle Functions
++
All six (four rectus and two oblique) extraocular muscles contribute to positioning the globe on its three axes of rotation (Table 12–1). Each muscle has principal (primary) and lesser (secondary and tertiary) actions, and these are determined by its site of attachment to the globe, its site of origin, the orbital connective tissues that regulate its direction of action by acting as its functional mechanical origin (active pulley hypothesis), and the varying orientation of the globe in the orbit.
++++
Even when stationary, the position of the eye is determined by the overall effect of the combined activity of all six extraocular muscles. The primary position of gaze is when the eyes are looking straight ahead with the head and body erect. To move the eye into another direction of gaze, the activity of the agonist muscles for the required movement increases and the activity of the antagonist muscles decreases. The field of action of a muscle is the direction of rotation of the globe when it contracts (agonist effect). The medial and lateral rectus muscles adduct and abduct the eye, respectively, with little effect on elevation or torsion. The vertical rectus and oblique muscles have both vertical and ...