Comparing Ocular Responses to Caloric Irrigation and Electrical Vestibular Stimulation in Vestibular Schwannoma

Abstract

Electrical Vestibular Stimulation (EVS) is a non-invasive technique for activating the vestibular-ocular reflex, evoking mainly a torsional eye movement response. We have previously demonstrated that this response can be used to detect vestibular asymmetry in patients with vestibular schwannoma (VS). Here we perform a direct comparison of EVS with caloric irrigation in this patient group. We studied 30 patients with unilateral VS, alongside an equal number of aged-matched healthy control subjects. EVS current was delivered to the mastoid process in a monaural configuration using a sinusoidal stimulus (2 Hz; ± 2 mA; 10 s), with an electrode placed over the spinous C7 process. Evoked eye movements were recorded from the right eye in darkness using an infra-red sensitive camera while the subject sat relaxed with their head on a chinrest. Ocular torsion was subsequently tracked off-line using iris striations. Each subject separately underwent water caloric irrigation, in accordance with the British Society of Audiology guidelines. For the caloric test, eye movement was recorded in the yaw axis using electro-oculography. For both EVS and calorics, inter-aural response asymmetry was calculated to determine the extent of canal paresis. Both tests revealed impaired vestibular function in the ipsilesional ear of VS patients, with a mean asymmetry ratio of 15 ± 17% and 18 ± 16% for EVS and calorics, respectively. Overall, the caloric test results discriminated controls from patients slightly more effectively than EVS (Cohen's D effect size = 1.44 vs. 1.19). Importantly, there was a significant moderate correlation between the AR values produced by EVS and calorics (r = 0.53, p < 0.01), and no significant difference between mean AR estimates. When questioned, ≥85% of participants subjectively preferred the EVS experience, in terms of comfort. Moreover, it took ~15 min to complete, vs. ~1 h for caloric. These results confirm that the results of the EVS test broadly agree with those of caloric irrigation, in terms of detecting vestibular asymmetry. Furthermore, they suggest a higher degree of convenience and patient comfort.

Keywords: asymmetry ratio; caloric irrigation; electrical vestibular stimulation; ocular torsion; vestibular schwannoma.

Figure 1

Analysis of EVS-evoked ocular responses. Adapted from Mackenzie and Reynolds (18). (A) Subjects sat in darkness with the head fixed during monaural stimulation (10 s, 2 Hz, ±2 mA). (B) 3D eye movements were recorded using an infrared camera and then tracked off-line. (C) An eye acceleration threshold was used to detect fast phase movements which were then removed using a compensatory inverse nystagmus algorithm. (D) Response gain was determined by the ratio of the peak EVE-eye cross correlation to the peak EVE-EVS auto correlation.

Figure 2

Analysis of caloric-evoked ocular responses. (A) Eye position was recorded using EOG in complete darkness following the cessation of the water irrigation. High frequency noise was removed using a Savitzky-Golay filter. (B) Slow and fast phase components were identified using the k-means clustering technique in Matlab. (C) Slow phase segments were concatenated, and a curve was fitted to this position signal. (D) Peak SPV was identified from the differentiated position signal.

Figure 3

Torsional eye movements evoked by EVS stimulation. EVS induces a sensation of head roll about the naso-occiptal axis, evoking a torsional eye response. A control subject's torsional eye response to right and left ear stimulation were similar in magnitude. However, the vestibular schwannoma patients show a reduced response magnitude during ipsilesional stimulation.

Figure 4

EVS-evoked torsional eye movement response and asymmetry ratio. (A) Response gains for controls left and right ear stimulation (grey) and patients contralesional (grey) and ipsilesional ear (black). (B) Asymmetry ratio for controls (grey) and patients (black). The grey region depicts the range of a healthy response (±2 SD of control data). Mean and SD presented.

Figure 5

Horizontal nystagmus evoked by caloric stimulation. The direction (down; rightwards, up; leftwards) of the slow phase is dependent on the ear and temperature. A controls subject's responses produced similar slow phase velocity during left and right ear stimulation. However, vestibular schwannoma patients show an attenuated response during ipsilesional stimulation.

Figure 6

Caloric-evoked peak SPV and asymmetry ratio. (A) Peak SPV for controls left and right ears (grey). Patients contralesional (grey) and ipsilesional (black) SPV. (B) Asymmetry ratio for controls (grey) and patients (black). The grey region depicts the range of a healthy response (±2 SD of control data). Mean and SD presented.

Figure 7

Experimental comparison. Both EVS and caloric tests produced similar asymmetry ratios, resulting in a significant positive correlation for controls (grey) and patients (black). The grey region depicts the range of healthy responses for both tests (±2 SD of control data).

Figure 8

Bland-Altman comparison of EVS and Caloric asymmetry estimates. The difference between VS asymmetry ratios has been plotted against the mean value of both tests. Horizontal dashed lines show the mean value of the difference along with ± 1.96 standard deviations.