Vestibulo-ocular reflex (VOR) preserved in bilateral severe vestibular malformations with internal auditory canal stenosis

Changes in Vestibulo-Ocular Reflex Gain After Surgical Plugging of Superior Semicircular Canal Dehiscence

Superior semicircular canal dehiscence (SCD), which is characterized by a “third mobile window” in the inner ear, causes various vestibular and auditory symptoms and signs. Surgical plugging of the superior semicircular canal (SC) can eliminate the symptoms associated with increased perilymph mobility due to the presence of the third window. However, the natural course of vestibular function after surgical plugging remains unknown. Therefore, we explored longitudinal vestibular function after surgery in 11 subjects with SCD who underwent SC plugging using the middle cranial fossa approach. Changes in vestibulo-ocular reflex (VOR) gain in all planes were measured over 1 year with the video head impulse test. We also evaluated surgical outcomes, including changes in symptoms, audiometric results, and electrophysiological tests, to assess whether plugging eliminated third mobile window effects. The mean VOR gain for the plugged SC decreased from 0.81 ± 0.05 before surgery to 0.65 ± 0.08 on examinations performed within 1 week after surgery but normalized thereafter. Four of seven subjects who were able to perform both VOR tests before surgery and immediately after surgery had pathologic values (SC VOR gain < 0.70). Conversely, the mean VOR gain in the other canals remained unchanged over 1 year. The majority of symptoms and signs were absent or markedly decreased at the last follow-up evaluation, and no complications associated with the surgery were reported. Surgical plugging significantly attenuated the air-bone gap, in particular at low frequencies, because of increased bone conduction thresholds and deceased air conduction thresholds. Moreover, surgical plugging significantly increased vestibular-evoked myogenic potential thresholds and decreased the ratio of summating potential to action potential in plugged ears. Postoperative heavily T2-weighted images were available for two subjects and showed complete obliteration of the T2-bright signal intensity in the patent SC lumen in preoperative imaging based on filling defect at the site of plugging. Our results suggest that successful plugging of dehiscent SCs is closely associated with a transient, rather than persistent, disturbance of labyrinthine activity exclusively involved in plugged SCs, which may have clinical implications for timely and individualized vestibular rehabilitation.

Visual vertigo: Vertigo of oculomotor origin

Since Róbert Bárány proposed his hypothesis on vestibulo-ocular reflex (VOR), dizziness associated with vertigo has been interpreted as being vestibular in origin. However, there have been many contradictory findings showing modulations of VOR, which have caused confusion as to VOR's role and accuracy. Further, there seems to be an influence of VOR when the anatomical inner ear structures are congenitally absent. Many people report vertiginous symptoms when they are exposed to visually challenging situations. These people with visually induced vertigo are usually found to have only mildly abnormal labyrinthine findings. Accurate visual information via binocular vision in animals, including humans, is important for the survival. Understanding how visual information is used in balance can help us to apply a different approach to the mechanism of vertigo. This article will review how accurate binocular viewing is possible for precise images through a complex oculomotor system and the proprioceptive senses of the external ocular muscles (EOMs). The proprioceptive senses from EOMs appear to affect motor efferents of the body. Oculomotor activities during viewing are important not just for learning but also for executing whole body motor responses. An error in the oculomotor afferents will cause a reaction to the error signal. This can be troubling for proper balancing during movement. Especially, common oculomotor causes (including fatigue of EOMs which is common in today's lifestyle) can contribute to many vertiginous conditions.

Preservation of vestibular function after scala vestibuli cochlear implantation

A 58-year-old man, in whom the cochlear implant (CI) had been inserted into the left ear, had right middle-ear cancer. The CI was removed immediately before receiving subtotal removal of right temporal bone. Four months later, the CI was again inserted in his left cochlea. Because of obliterated scala tympani, the 22 active electrodes of the CI were placed into the scala vestibuli. After the surgery, the patient complained that he experienced rotary vertigo and "jumbling of vertical direction" of objects on walking. Using rotation test, we evaluated vestibular function of remaining left ear. Numerous horizontal nystagmus beats were induced during earth-vertical axis rotation, whereas vertical downbeat nystagmus was scarcely induced during off-vertical axis rotation. The horizontal vestibulo-ocular reflex (VOR) was almost normally induced by sinusoidal stimulation at 0.8Hz. These data suggest that the scala vestibuli insertion of CI would be not so invasive against the lateral semicircular canal.