With a complete lesion of the left MLF, the left ipsilateral eye fails to adduct when the patient looks to the right; this condition is referred to as internuclear ophthalmoplegia (INO; Fig. 13-1). Reciprocally, with a lesion of the right MLF, the right eye fails to adduct when the patient looks to the left and the patient has a right internuclear ophthalmoplegia. Quite often, rather than a complete paralysis of adduction, there are only slowed adducting saccades in the affected eye while the other eye quickly arrives at its fully abducted position. This slowing can be observed by having the patient make large side-to-side refixation movements between two targets or by observing the slowed corrective saccades induced by optokinetic stimulation. Typically, the affected eye at rest does not lie in an abducted position, but there are exceptions and in most cases the absence of exotropia most dependably differentiates INO from a partial third-nerve palsy with weakness of the medial rectus muscle. The exception is the WEBINO syndrome noted below.
A second component of INO is nystagmus that is limited to, or most prominent in, the opposite (abducting) eye. The intensity of nystagmus varies greatly from case to case. Several explanations have been offered to account for this dissociated nystagmus, all of them speculative. The favored one invokes the Hering law in which activated pairs of yoked muscles receive equal and simultaneous innervation; because of an adaptive increase in innervation of the weak adductor, there is a commensurate increase in innervation to the strong abductor. A mismatch in the generated pulse and step signals results in dissociated nystagmus manifesting in that eye. Zee and colleagues evaluated this concept by assessing whether short-term patching of one eye altered the central adaptive response and therefore modulated the extent of abducting nystagmus in patients with INO. In several cases, they found that patching the affected eye for several days caused the abducting nystagmus in the fellow eye to diminish. Conversely, patching the unaffected eye lead to increased abducting nystagmus in the fellow eye.
The MLF also contains axons that originate in the vestibular nuclei and govern vertical eye position, for which reason an INO may also cause a skew (vertical deviation of one eye). Vertical nystagmus and impaired vertical pursuit are other common features, especially with bilateral INO.
The two medial longitudinal fasciculi lie close together, each being situated adjacent to the midline, so that they are frequently affected together, yielding a bilateral internuclear ophthalmoplegia. When the MLF is affected by a lesion in the pons, convergence is spared and the alignment of the eyes in primary gaze is normal. In some cases, both eyes take an abducted position, giving rise to the “wall-eyed bilateral INO,” or WEBINO syndrome. Lesions involving the MLF in the high midbrain impair convergence and also cause an exotropia because of proximity to the medial rectus subnucleus. Abducting nystagmus tends to be slight in this mesencephalic type.
The terms “anterior” and “posterior” INO have also been applied but their meaning has been taken differently by various authors thus making them less useful. Cogan categorized INO as anterior with convergence was impaired, and posterior convergence was spared but abduction or horizontal gaze were partly affected. In contrast, the term “posterior INO of Lutz” refers to abduction paresis that can be overcome by vestibular stimulation. The responsible lesion is proposed to be between the PPRF and the sixth nerve nucleus.