Study of pursuit and vestibulo-ocular cancellation

Impact of the sensory environment on balance in children with bilateral cochleovestibular loss

BACKGROUND

The aim of the present study was to determine the role of auditory and visual sensory input on balance in children with bilateral cochlevestibular loss. The prevalence of vestibular impairment, and specifically bilateral vestibular loss (BVL) in children with sensorineural hearing loss (SNHL) is high and children with profound cochleovestibular loss (SNHL-BVL) have impaired balance (Suarez et al., 2007; Suarez et al., 2019). Given that both hearing and vestibular impairments are often congenital or acquired in early life, it remains difficult to tease out the individual developmental impact of either one on balance and spatial awareness in children who experience both of these sensory deficits. While cochlear implants (CI) can provide or restore access to sound in children with SNHL-BVL, there is currently no vestibular prosthetic available for clinical use in this population. These children may also use their intact sensory inputs (i.e. vision) to a greater extent to support balance. Alternately, restoring or providing access to sound may, on its own, help these children to balance better. We hypothesized that balance in children with SNHL-BVL who use bilateral CIs is: 1) improved in the presence of directional sound and 2) impaired when visual cues are dynamic (moving) rather than static.

METHODS

Balance was assessed in 18 children with SNHL-BVL and 34 typically developing children with intact vestibular function and normal hearing by performing the Bruininks-Oseretsky Test of Motor Proficiency-2 (BOT-2) balance subtest in a virtual-reality simulator under 4 sensory conditions of graded complexity. Randomized conditions combined 2 auditory (moving directional street sounds vs. directionless static white noise) and 2 visual (dynamic street scene vs. stationary street scene) stimuli designed to recreate a "real-world" busy downtown street. Balance ability in children with SNHL-BVL was also compared with CI on and off.

RESULTS

As expected and similar to previous work, balance was significantly worse in the children with SNHL-BVL compared to typically developing children in all sensory conditions (p<0.0001). As a group, the mean balance skills of the children with SNHL-BVL were equivalent to that of a 4.4-year-old child despite being much older (mean age =13.8 years). Balance ability improved slightly but significantly when children with SNHL-BVL had access to any sound through their CI (p=0.047) and was positively correlated with duration of implant use (p=0.02). Balance ability did not change further in the presence of moving directional sounds compared to static white noise (p=0.42), or when coupled to a moving visual environment (p=0.32) in children with SNHL-BVL, however opposite to what was hypothesized, in the typically developing group, there was a decrement in performance that occurred in the presence of moving directional sound compared to directionless, static white noise (p=0.02).

CONCLUSIONS

Balance ability in children with SNHL-BVL who use bilateral CI was, as expected, poorer than their typically developing peers in all sensory conditions but improved slightly when they had access to any sound through their implants, with this benefit increasing as duration of implant use increased. This suggests that providing sound inputs through bilateral CIs positively affects balance in children with SNHL-BVL where vestibular and/or auditory inputs are compromised. This benefit was achieved even with auditory inputs that were devoid of moving directional cues (i.e. directionless static white noise) and is consistent with poor spatial hearing in children using bilateral CI.

Abnormal vestibuloocular reflex cancellation in multiple system atrophy and progressive supranuclear palsy but not in Parkinson's disease

We have measured the gain of the vestibuloocular reflex (VOR) in darkness and its cancellation by fixation in 37 patients with Parkinson's disease (PD), 26 patients with multiple system atrophy (MSA), 11 patients with progressive supranuclear palsy (PSP), and 19 normal volunteers. The capacity to cancel the VOR by fixation (VOR cancellation) was significantly reduced in the MSA and PSP patients compared with the PD and normal subjects (p < 10(-4)). A VOR cancellation < 90% (i.e., the mean VOR cancellation of the normals--2 SD) was present in four PD patients, 23 MSA patients, and 11 PSP patients. This criteria distinguished PD and MSA with a 89% sensitivity and a 89% specificity. Our results demonstrate that the VOR cancellation is impaired in most patients with MSA and PSP but not with PD. In MSA patients, the abnormal VOR cancellation is probably not related to the nigrostriatal dopaminergic deficit and more likely reflects a cerebellar dysfunction. Impaired VOR cancellation is a clinical criteria to differentiate MSA and PSP from PD.

An investigation of horizontal combined eye-head tracking in patients with abnormal vestibular and smooth pursuit eye movements

We investigated the interaction of smooth ocular pursuit (SP) and the vestibulo-ocular reflex (VOR) during horizontal, combined eye-head tracking (CEHT) in patients with abnormalities of either the VOR or SP movements. Our strategy was to apply transient stimuli that capitalized on the different latencies to onset of SP and the VOR. During CEHT of a target moving at 15 deg/sec, normal subjects and patients with VOR deficits all tracked the target with a gain close to 1.0. When the heads of normal subjects were suddenly and unexpectedly braked to a halt during CEHT, the eye promptly began to move in the orbit to track the target, but eye-in-orbit velocity transiently fell to about 60-70% of target velocity. In patients with deficient labyrinthine function, following the onset of the head brake, eye movements to track the target were absent, and SP movements were not generated until about 100 msec later. In patients with deficient SP, CEHT was superior to SP tracking with the head stationary; after the onset of the head brake, tracking eye movements were initiated promptly, but eye velocity was less than 50% of target velocity and increased only slightly thereafter. These results indicate that at least two mechanisms operate to overcome the VOR and allow gaze to track the target during CEHT: (1) the SP system provides a signal to cancel a normally-operating VOR (this cancellation signal is not needed by labyrinthine-deficient patients who have no VOR to cancel), and (2) a reduction of the gain of the VOR is achieved, an ability that is preserved even in patients with cerebral lesions that impair SP.

Effects of intravenous diazepam and thiopental on the vestibulo-ocular reflex in man

The effects of i.v. injection of diazepam 0.3 mg/kg and thiopental 6.0 mg/kg on vestibulo-ocular reflex (VOR) were studied in 9 healthy volunteers. One hour after injection of both diazepam and thiopental VOR gain was reduced and time constant shortened. Four hours after intravenous injection of diazepam the VOR gain was still significantly reduced and had not yet returned to control 8 h after injection. Although both drugs were detected in blood still 8 h after the injection, there were no longer significant effects on VOR. Moreover, individual values of VOR gain and time constant did not correlate with the blood concentrations of the drugs. Decrease of VOR gain and shortening of time constant after i.v. given diazepam and thiopental in man are interpreted as resulting from both specific effects on VOR and non-specific reduction of alertness.

Vestibulo-ocular (VOR) abnormalities at high rotational frequencies in patients with Menière's disease

Although visual feedback is required to maintain gaze stability during low-frequency rotations (below 1 Hz) because of suboptimal VOR gain in this frequency range, such behavior is not as evident at higher frequencies. Benson and Hydén et al. noted a steady increase in VOR gain in the higher-frequency range (2 to 5 Hz), where visual feedback has little effect. Similar behavior has also been reported in the monkey models. Eleven patients with diagnoses of Meniere's disease had tests of VOR and VOR cancellations performed with the use of pseudorandom oscillations as high as 5 Hz. The responses at various frequencies were compared with normal data from 17 subjects. The VOR gain in patients exhibited a more rapid rise at high frequencies than that observed in normal subjects. For example, at 3.5 Hz the normal gain was 1.09, whereas patients exhibited a gain of 1.35 (mean of 11 subjects). When the performance during VOR cancellation tasks was compared, Meniere's patients appeared to be less able to perform these tasks; however, when the values were compared by use of a cancellation index that compensates for any difference in VOR gain, this apparent difference disappeared.