[Oculomotor syndromes resulting from mesencephalic lesions in man]

Midbrain lesions may give rise to the most complex eye movement disorders observed in clinical neurology. Three main types of dysfunction may be delineated, which may be combined: 1) intra-axial fascicular syndromes of the third and fourth cranial nerves; 2) nuclear syndromes of the third and fourth cranial nerves; 3) prenuclear and supranuclear syndromes, in which vertical gaze dysfunction is the main abnormality, but frequently with associated horizontal gaze dysfunction. The fascicular syndromes of the third nerve may be diagnosed when the oculomotor palsy is associated with some specific neurologic disturbances (syndromes of Weber, Benedikt, Claude-Nothnagel-Raymond-Cestan, Achard-Levi). The nuclear syndrome of the third nerve Is also quite suggestive, but it may be more difficult to diagnose an intra-axial fascicular or nuclear involvement of the fourth nerve. Supranuclear vertical gaze dysfunction are usually conjugate (upgaze/downgaze/vertical gaze palsies) and correspond to specific anatomical correlates. Dysconjugate vertical gaze syndromes are not as well known, are often less specific, and their topographic correlation is usually less precise; they include skew deviation and variants, slowly alternating skew deviation, see-saw nystagmus, monocular upgaze palsy, ocular tilt reaction, vertical one-and-a-half syndrome, and "V" pattern pseudobobbing. Some of these eye movement disorders are not only specific for the location of the lesion, but also for a definite etiology (for instance, acute downgaze palsy suggests bilateral infarction of the posterior thalamo-subthalamic paramedian territory).