The Torsion Swing Chair test: a ratio method for quantifying nystagmus.
In a Torsion Swing Chair (TSC) test for nystagmus, S is oscillated sinusoidally around the vertical axis with successive cycles diminishing in extent. Channel II of a standard recording ENG traces the chair's movement (differing among Ss according to their weight) while Channel I records S's ENG. A method was devised to quantify S's nystagmus by calculating, for each pair of succeeding cycles, the ratio of the decline in amplitude of the slow phase nystagmic velocity to the decline in amplitude of chair movement. This ratio, relatively constant across cycles, is greater with labyrinthine pathology. Ratios were established with 15 normal young adults as cut-offs for normalcy. The method was applied to 30 patients with peripheral hearing loss whose only complaint was slight vertigo following an acute stage of vertigo. The TSC test was validated by comparing it to the severity of vertigo and to the presence/absence of abnormal results on a standard clinical caloric test in ENG and on standard clinical ECoG. All but 1 of the 30 pts exhibited abnormal results on the TSC test, as compared with 18 for the caloric and 16 for the ECoG. Furthermore, abnormality on the TSC test was more likely to be associated with abnormal results on one or both of the other two tests than were either of the other two tests. It can be concluded that the TSC test is a valid and objective tool useful in ENG practice.