Etiology of INO The main cause of unilateral INO is a small paramedian pontine infarction. Other common lesions are lateral medullary infarction (where skew deviation is often a component), a demyelinating plaque of multiple sclerosis (more common as a cause of bilateral INO, as noted below), and infiltrative tumors of the brainstem and fourth ventricular region. Occasionally, an INO is an unexplained finding after mild head injury or with subdural hematoma or hydrocephalus. Some of the more unusual causes are given in the experience of Keane (2005). In addition, adductor weakness from myasthenia gravis can simulate an INO, even to the point of showing nystagmus in the abducting eye.
Bilateral INO is most often the result of a demyelinating lesion (multiple sclerosis) in the posterior part of the midpontine tegmentum. Pontine myelinolysis, pontine infarction from basilar artery occlusion, Wernicke disease, or infiltrating tumors are other causes. Brainstem damage following compression by a large cerebral mass has on occasion produced the syndrome.
An ipsilateral gaze palsy is the simplest oculomotor disturbance that results from a lesion in the paramedian tegmentum. More complex is the one-and-a-half syndrome that involves the pontine center for gaze plus the adjacent ipsilateral MLF on one side that combines a horizontal gaze palsy and INO on the same side. It is usually of vascular or, less often, demyelinative cause. The gaze palsy is, of course, on the side of the lesion and the eyes are deviated contrawise. As a result, one eye lies fixed in the midline for all horizontal movements; the other eye makes only abducting movements and may demonstrate horizontal abducting nystagmus (see Fisher; also Wall and Wray). Unlike the situation of an INO alone, the mobile eye rests abducted because of the gaze palsy, a sign that has been termed “paralytic pontine exotropia.” In some cases, the patient is able to adduct the eye (“nonparalytic exotropia,” a condition which has other causes). An incomplete version of the one-and-a-half syndrome displays only bilateral nystagmus on gaze in one direction (due to paresis of gaze) and nystagmus only in the abducting eye with gaze directed to the other side (due to the lesion in the MLF on the same side). Thrombotic occlusion of the upper part of the basilar artery (“top of the basilar” syndromes) causes a variety of important eye movement abnormalities. These include upgaze or complete vertical gaze palsy, skew deviation, and so-called pseudoabducens palsy, mentioned earlier. Caplan has summarized these features in detail.
Skew deviation Skew deviation is a disorder in which there is vertical deviation of one eye above the other that is caused by an imbalance of the supranuclear vestibular inputs to the ocular motor system. Unlike fourth nerve palsy, where the separation of images is most pronounced when the affected eye is adducted and turned down, skew deviation is typically comitant, meaning that the amount of ocular misalignment is relatively similar in all directions of gaze. Skew deviation does not have precise localizing value but is associated with a variety of lesions of the cerebellum and the brainstem, particularly those involving the MLF. With skew deviation due to cerebellar disease, the eye on the side of the lesion usually rests lower (in a ratio of 2:1 in Keane’s series), but sometimes it is higher than the other eye.