vHIT is also helping researchers and clinicians identify disease states they were previously unable to detect. In a paper recently published in the American Journal of Audiology, Dr. McCaslin reports that some patients with Meniere’s disease have an abnormal caloric test but normal vHIT; as the disease progresses, both tests may become abnormal (Am J Audiol [Published online ahead of print November 7, 2014]; doi: 10.1044/2014_AJA-14-0040). Another study of 26 patients suggests that vHIT is helpful in differentiating vestibular neuritis from stroke (Otol Neurotol [Published online ahead of print October 15, 2014]; doi: 10.1097/MAO.0000000000000638).
Explore this issue:January 2015
While it appears to be a very promising test that will likely complement current vestibular assessments, it’s important to note that uncooperative patients and patients with neck injuries may not be able to undergo vHIT.
Vestibular Evoked Myogenic Potential (VEMP)
VEMP testing allows clinicians to gather information about the function of the otolith organs, part of the vestibular system that was previously beyond clinicians’ reach. The test uses sound or vibration to stimulate the inner ear; that stimulus triggers reflexive muscle movement, which is measured via electrodes placed on the neck (in the case of cervical VEMP) or face (in the case of ocular VEMP).
Cervical VEMP primarily assesses the saccule and the inferior vestibular nerve. To assess cervical VEMP, electrodes are placed on the neck to record the muscle potentials of the sternocleidomastoid muscle. When sound is introduced to the ear, the electrodes record the degree of ipsilateral muscle relaxation.
Ocular VEMP is believed to assess the utricle and superior division of the vestibular nerve. Assessment of ocular VEMP involves a sound or vibratory stimulus and electrodes placed just below the opposite eye. When the stimulus is introduced to the ear, the vestibulo-ocular reflex excites the eye muscles on the opposite side; ocular VEMP measures the muscle potential of the inferior oblique muscle.
VEMP testing can be used to assess the presence or absence of a VEMP response, as well as to measure thresholds and amplitude. “You can compare the amplitude between the two sides to see if there’s a lower amplitude on one side versus the other, suggesting it’s abnormal. You can also look at threshold—how loud you have to make the sound for the muscle to respond. More recently, people have been looking at the tuning characteristics of VEMP, meaning what frequency elicits the best stimulus response in a patient,” Dr. Adams said.