Ocular torsion and perceived vertical in oculomotor, trochlear and abducens nerve palsies.

Ocular torsion (OT) and subjective visual vertical (SVV) were determined in acute and chronic oculomotor (n = 6), trochlear (n = 21) and abducens (n = 7) palsies separately for each eye in the primary position with the head upright. Ocular torsion measured by fundus photographs was not only within normal range in all abducens palsies, but unexpectedly also in 68% of third and fourth nerve palsies which involve oblique eye muscles. Pathological OT, when measurable, was slight (2 degrees - 8 degrees), monocular and occurred either in the paretic or in the nonparetic eye. Subjective visual vertical tilts were more frequent (67% of third and fourth nerve palsies) although mostly small in amplitude (1 degree - 6 degrees). They were confined either to the paretic or the nonparetic eye depending on the duration of the palsy. Determinations of SVV were always normal under binocular viewing conditions. The dissociated occurrence of OT and SVV tilts in the paretic or the nonparetic eye was dependent on the acuteness of the palsy and reflected sensory and/or motor compensation mechanisms. Third and fourth nerve palsies cause only minor and unpredictable monocular OT and SVV tilts as distinct from the frequent binocular and conjugate tilts seen in patients with acute unilateral brainstem lesions.