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. Any ocular misalignment, such that only one eye views the object of regard with the correct vertical orientation, is called “strabismus.” The misalignment may be in any direction—inward, outward, up, down, or torsional. The amount of deviation is the angle by which the deviating eye is misaligned. Strabismus present under binocular viewing conditions is manifest strabismus, heterotropia, or tropia (see Box 12.1 for definitions). A deviation present only after binocular vision has been interrupted (eg, by occlusion of one eye) is called latent strabismus, heterophoria, or phoria.
Strabismus is present in about 4% of children. Treatment should be started as soon as a diagnosis is made in order to ensure the best possible visual acuity and binocular visual function. There is no such thing as “outgrowing” childhood strabismus. Strabismus may also be acquired, due to cranial nerve palsies, orbital masses, orbital fractures, or Graves' ophthalmopathy.
Box 12.1. Definitions
Angle Kappa: The main angle between the visual axis and the central pupillary line. When the eye is fixing a light, if the corneal reflection is centered on the pupil, the visual axis and the central pupillary line coincide and the angle kappa is zero. Ordinarily, the light reflex is 2°–4° nasal to the pupillary center, giving the appearance of slight exotropia (positive angle kappa). A negative angle kappa gives the false impression of esotropia.
Conjugate movement: Movement of the eyes in the same direction at the same time.
Deviation: Magnitude of ocular misalignment, usually measured in prism diopters (see later in the chapter) but sometimes measured in degrees.
- Comitant deviation: Deviation not significantly affected by which eye is fixing or direction of gaze, typically a feature of childhood (nonparetic) strabismus.
- Incomitant deviation: Deviation varies according to which eye is fixing and direction of gaze, usually a feature of recent onset extraocular muscle paresis and other types of acquired strabismus.
- Primary deviation: Incomitant deviation measured with the normal eye fixing (Figure 12–2).
- Secondary deviation: Incomitant deviation measured with the affected eye fixing.
Ductions: (Figure 12–3) Monocular rotations with no consideration of the position of the other eye.
- Adduction: Inward rotation.
- Abduction: Outward rotation.
- Supraduction (elevation): Upward rotation.
- Infraduction (depression): Downward rotation.
Fusion: Formation of one image from the two images seen simultaneously by the two eyes. Fusion has two aspects.
- Motor fusion: Adjustments made by the brain in innervation of extraocular muscles in order to bring both eyes into bifoveal and torsional alignment.
- Sensory fusion: Integration in the visual sensory areas of the brain of images seen with the two eyes into one picture.
Heterophoria (phoria): Latent deviation of the eyes held straight by binocular fusion.
- Esophoria: Tendency for one eye to turn inward.
- Exophoria: Tendency for one eye ...