Commentary

In this issue,[1] a rare case of pontine neurocysticercosis with isolated wall-eyed monocular internuclear ophthalmoplegia (WEMINO) and contraversive ocular tilt reaction (OTR) is presented. In addition to adduction weakness on the side of lesion and monocular nystagmus of the abducting eye, the two essential components of internuclear ophthalmoplegia (INO), there was also marked ipsilateral exotropia along with skew deviation and head tilt directed to the side contralateral to the lesion. Albeit some aspects of the neuro-ophthalmological examination were lacking, the patient illustrated well the roles that medial longitudinal fasciculus (MLF) commonly plays in the ocular motor system.

Adduction defect is such a striking facade of the MLF disorder that some features on the side face could be overlooked by clinicians. The MLF receives projections from the contralateral abducens nucleus, where the neural commands for all horizontal eye movements converge at, taking part not only in saccades from paramedian pontine reticular formation (PPRF) connections, but also in vestibular, pursuit, and optokinetic eye movements by way of vestibular nuclei.

Commonly, INO is associated with skew deviation, which is always associated with ocular torsion and tilts of subjective visual vertical (SVV tilt) toward the undermost eye,[2] reflecting imbalance of vestibular signals in the roll plane.[3] Unilateral INO had been shown to be accompanied by at least one component of OTR: SVV tilt in 96%, ocular torsion in 79%, and skew deviation in 50%. All components were directed to the contralesional side.[4]

Exotropia is another yet habitually disregarded finding associated with the MLF disorder. Contralesional exotropia is probably not as unusual as it was thought in unilateral INO. Postulated as the result of a secondary deviation under fixation with the paretic eye by the hyperactivity of the PPRF contralateral to the damaged MLF,[5] it occurred in 14 of 22 (63.6%) patients with unilateral INO caused by brainstem infarction reported by Kim.[6] Transient exotropia also occurs ipsilesionally in unilateral INO, an extremely rare finding called WEMINO. Johnston and Sharpe described four patients with “WEMINO” but they did not specify the side of exotropia (ipsi- or contralesional).[7] By MRI including diffusion-weighted images, a tiny infarction at the ipsilateral paramedian pontine tegmentum just adjacent to the fourth ventricle, corresponding to the anatomical area of the MLF, was first reported in a case of left WEMINO by Ikeda and Okamoto.[8] An infarction of similar site and size was seen in another case of WEMINO combined with contraversive OTR.[9]

Why the exotropia takes place ipsilaterally to the MLF lesion has not been well explained. Johkura et al.[10] described interesting ocular motor findings in four patients with one-and-a-half syndrome (OHS). All had mild outward deviation in both eyes with fixation prevented by Frenzel goggles. In three patients whose outward eye deviation was greater on the ipsilateral side, the OHS transited to ipsilateral INO (implying the MLF was more damaged), whereas the one with greater outward deviation on the other side transited to ipsilateral gaze palsy (implying the PPRF was more damaged). They suggested that outward deviation of the ipsilateral eye is due to an imbalance of vestibular signals destined for the ipsilateral medial rectus in the MLF and that of the contralateral eye is due to an imbalance of PPRF signals. Drawing an analogy to the theory that OTR reflects imbalance of vestibular signals in the roll plane for unilateral MLF disorder, ipsilateral exotropia in MLF disorders probably mirrors imbalance in the yaw plane that can be applied to the WEMINO.[11]