MLF (medial longitudinal fasciculus) syndrome and one-and-a-half syndrome

Abstract

　A lesion in the medial longitudinal fasciculus (MLF) results in the MLF syndrome or internuclear ophthalmoplegia (INO), which is classically defined by the triad of lack of adduction of the ipsilateral eye, dissociated nystagmus in the contralateral eye during abduction, and preserved vergence. However, the clinical manifestations of lesions in the MLF may be more complex and variable, because not only fibers concerned with horizontal eye movements, but also those concerned with vertical eye movements, such as vertical VOR or smooth pursuit, pass through the MLF. Accordingly, in addition to the above triad, a skew deviation, that is, dissociated vertical-torsional nystagmus, with the eye beating down on the side of the lesion may also occur. The most common causes of this syndrome are brainstem infarction and multiple sclerosis (MS). The adduction weakness can be explained by the interruption caused by lesions of the abducens internuclear neurons, which project via the MLF to the contralateral medial rectus subgroups of the oculomotor nucleus. However, the mechanism of the abduction nystagmus in the contralateral eye is still unknown. Although several hypotheses have been put forward, the most plausible one appears to be an adaptive mechanism: In order to overcome the weakness of the adducting eye, there is an increasing change in the innervation to the adducting eye, and according to Hering's law of equal innervation, it must be accompanied by compensatory changes in reinforcing innervation to the abducting eye. This could cause abnormal overshooting saccades followed by backward postsaccadic drift, manifesting clinically as abduction nystagmus. Presumably, extension of the lesion into structures near but outside the MLF, or the involvement of cell bodies intermixed with MLF fibers, may also be involved in the pathogenesis of abduction nystagmus and occasional slowing of abducting saccades. Another potential mechanism of the abduction nystagmus is the presence of a superimposed gaze-evoked nystagmus, coincidentally involving extensive lesions of structures near but outside the MLF, such as the medial vestibular nuclei, prepositus hypoglossal nuclei or the cerebellar flocculus/paraflocculus.

　One-and-a-half syndrome refers to horizontal gaze palsy in one eye and adduction palsy (MLF syndrome) in the contralateral eye. This occurs due to lesions in the dorsal pontine tegmentum that affects the ipsilateral paramedian pontine reticular formation (PPRF) or abducens nucleus and ipsilateral MLF.

　Occasionally, vertical one-and-a-half syndrome is also encountered, which consists of vertical gaze palsy in one eye and upward or downward palsy in the contralateral eye. This syndrome is mainly caused by paramedian thalamo-mesencephalic lesions.

　This review provides an overview of the history, etiology, clinical features, and pathophysiology of both MLF syndrome and one-and-a-half syndrome.