Brady Workman is back this week with an article about vibration induced nystagmus, and how they can be helpful in vestibular function testing:
In the area of vestibular assessment, our test findings can sometimes be a bit unclear, often leaving us with as many questions as answers. This is especially true in cases where patients’ vestibular function tests do not align with one another; or when all test measures are unable to be implemented, in which scenario, a cross check measure would be gladly welcomed. I was recently reading an article by Dumas et al that purported the skull vibration induced nystagmus test (SVINT) as a “vestibular Weber test”, the auditory version of which is frequently utilized to differentiate between unilateral sensorineural (inner ear or hearing nerve) vs. conductive (problem with ear canal or middle ear) hearing loss. So lets take a deeper look at the author’s review of the literature, description of the test and personal findings.
A bone-conducted vibration applied to the skull can evoke nystagmus (jerking eye movements) in patients with a unilateral peripheral vestibular loss (reduction in inner ear balance input on one side). This nystagmus is not present in individuals with normal vestibular function or in cases of bilateral vestibular loss (reduced inner ear balance function on both sides). Vibration of the skull simultaneously stimulates both ears by setting the inner ear fluids into motion. This fluid motion equally stimulates both inner ears in the case of normal, symmetric vestibular function. When an individual has reduced or altered vestibular input on one side, stimulation through bone conduction does not equally stimulate each ear and nystagmus can occur, as there is asymmetric input from the labyrinths being sent to the brain. The nystagmus, when evoked, is typically of low degree and can be observed utilizing frenzel or videonystamography goggles.
In cases of unilateral peripheral vestibular loss, skull vibration induced nystagmus ranges from 75%-98% sensitivity with a specificity of 94%. Skull vibration induced nystagmus is more often present when larger degrees of asymmetry exist between labyrinths (caloric hypofunction of >50%). It is also noteworthy that skull vibration induced nystagmus is much more sensitive at detecting peripheral vestibular disease than brainstem lesions (better for detecting ear abnormalities than brain abnormalities).
The observed nystagmus is primarily horizontal in nature, but may also exhibit vertical and torsional (twisting) components. With superior semicircular canal dehiscence, the nystagmus may exhibit a larger torsional component. In cases of unilateral peripheral vestibular loss the fast phase of the nystagmus tends to beat (jerk) toward the healthy labyrinth or away from the affected ear. This pattern is observed in 91-98% of patients with unilateral peripheral vestibular loss. Skull vibration induced nystagmus can also be found in 82 to 100% of cases of unilateral superior semicircular canal dehiscence, most frequently with fast phases beating (jerking) toward the affected ear, or away from the intact labyrinth. In superior semicircular canal dehiscence a labyrinthine hypofunction is not the cause of the nystagmus, instead the fluid dynamics of the vestibular system have been altered due to the dehiscent canal, leading to increased stimulation of the ear with the dehiscence and asymmetric labyrinthine input to the brain.
The necessary skull vibrations can be induced with a number of vibratory sources ranging from relatively affordable to very expensive, making this test a viable measure for many clinics. Lists of vibrators that can be utilized are available in the linked article above. With this measure, there are few adverse side effects and the test is typically well tolerated by patients, with only some instances of reported lateropulsion (feeling of being pulled to the side) and/or nausea. However, the authors do caution against this test’s use in certain patient groups such as: recently operated otosclerosis, retinal detachment, recent cerebral hematoma and/or those with poorly controlled anticoagulant therapy.
The skull vibration induced nystagmus test is a minimally invasive and easily administered test that may be useful in cases of vestibular neuritis/labyrinthitis (inner ear and/or nerve inflammation), vestibular schwannoma (tumor of hearing/balance nerve), Meniere’s disease (fluctuating inner ear fluid abnormality), and in cases where caloric irrigations and/or head impulses cannot be performed. This measure may also be useful in differentiating otosclerosis (abnormal growth of middle ear bones) or other conductive hearing loss from superior semicircular canal dehiscence, as a conductive hearing loss alone should not cause one to have vibration induced nystagmus, whereas a dehiscent superior semicircular canal is likely to create a nystagmus pattern with fast phases beating toward the affected labyrinth. Therefore, the skull vibration induced nystagmus test is likely a useful complementary measure to include in ones vestibular assessment test battery. Readers interested in learning more or in the implementation of this test clinically are referred to the above linked text for more detailed information.
Photo courtesy of Frontiers in Neurology