Head shaking after nystagmus in patients with unilateral cerebral lesion.

The head-shaking test (HST) and the caloric test were used in the examination of 31 patients with unilateral internal capsule vascular lesions demonstrated by CT scan.Monophasic, biphasic and triphasic types of HSAN were observed in 13 patients (42%). In subtype A (3 patients, 10%) the first phase of nystagmus was to the side contralateral to that of the lesion, and in subtype B (10 patients, 32%) it was to the same side. The DP type of HSAN, which becomes slightly weaker and continues for at least 30 seconds, was observed in 14 patients (45%), 12 subtype A (39%) and 2 patients subtype B (6%). The first phase inhibited type of HSAN was seen in 1 patient subtype B (3%). No HSAN was observed in 3 patients (10%).Slow-phase velocity (SPV) could be determined in 30 patients; in 16 the caloric test caused DP nystagmus to the side of the lesion and in 14 to the contralateral side. There was no relationship between the direction or the SPV of the first nystagmus in the monophasic, biphasic or triphasic type of HSAN and the direction or the value of DP (%) in the caloric nystagmus. There was also no relationship between the direction of the DP type of HSAN and the direction or the value of DP (%) in the caloric nystagmus.Our results suggest that HSAN in patients with unilateral internal capsule lesions is due to a tonic imbalance, caused by the lesion, of the cortico-fugal tonic fibers ipsilaterally to the vestibulo-ocular system with eyes open in darkness.