One of the most extreme instances of saccadic dysfunction is the complete inability to volitionally initiate saccades, which is termed ocular motor apraxia. (An apraxia is an inability to voluntarily initiate a movement that can be initiated by some other means, usually via a reflex, which reveals that a paralysis is not present.) Patients with congenital ocular motor apraxia characteristically use horizontal head thrusts past the point of interest, employing the VOR to move the eyes into extreme contraversion until foveation on the target is possible; this is followed by slower head rotation in the opposite direction to primary position while the eyes maintain fixation on the target. Nonvolitional saccades that occur as a reflex to a moving object or sound and vertical eye movements are normal. The location of the lesion that causes congenital ocular motor apraxia is not known but probably is above the brainstem centers that drive volitional saccades. Patients may have other neurologic abnormalities, including delayed development. Also, ocular motor apraxia is associated with several diseases, including ataxia telangectasia, Pelizaeus-Merzbacher disease, Niemann-Pick type C, Gaucher disease, Tay-Sachs disease, Joubert syndrome, abetalipoproteinemia (vitamin E deficiency), and Wilson disease.
Acquired ocular motor apraxia results from bilateral lesions of the supranuclear gaze pathways of the frontal and parietal lobes, usually from bilateral strokes, often as part of an anoxic encephalopathy following cardiac arrest, or post–coronary artery bypass grafting. Patients often blink to break the fixation and then turn their head toward a new point of interest. Bilateral lesions at the parieto-occipital junction may impair the guidance of volitional saccades. Such inaccurate saccades, together with inaccurate arm pointing (ie, a patient may misdirect his or her hand when attempting to shake yours, despite being able to see your hand) and simultanagnosia (disordered visual attention that makes it difficult for a patient to perceive all the major features of a visual scene at once), are known as the Balint syndrome; this syndrome is often associated with cognitive dysfunction (see Chapter 6).
. Congenital ocular motor apraxia.1966;1(4):253–260.