Brady Workman is back this week with another blog post. This weeks post will focus on comprehensive vestibular assessment and what that entails.
Recently, I had the opportunity to observe with staff members in the Balance Clinic at the Vanderbilt University Bill Wilkerson Center. Vanderbilt University is a prestigious university with an exemplary Balance Clinic, known worldwide as a leader in the area of vestibular assessment. This was an excellent learning experience for myself and led me to think about vestibular assessment protocols in a university hospital setting compared to what many other clinics routinely perform.
But first a quick review of vestibular system function for clarity
For those unfamiliar, the vestibular system (inner ear balance system) consists of three semicircular canals and two otolith organs. The semicircular canals are the head-turn sensors of the inner ear, while the otolith organs are the linear sensors. The primary role of the vestibular system is to maintain visual clarity with head movements, which it performs by providing reflexive eye movement in response to head movement. The vestibular system also allows for maintenance of upright posture through the vestibular spinal and vestibular collic reflexes, which send reflexive signals to the lower extremities for postural adjustment.
What does vestibular assessment entail?
In a university hospital such as Vanderbilt, vestibular assessment protocols are thorough and include the measurement of all five peripheral vestibular end organs within each inner ear. This may be shocking to those not accustomed with vestibular assessment, as it would seem hard to form a professional opinion on the function of ones vestibular system without first measuring all of the vestibular end organs.
However, it has been only in recent years that new assessment techniques such as the vestibular evoked myogenic potential (VEMP) and the video head impulse test (vHIT) have allowed for the assessment of all peripheral vestibular end organs.
Traditionally, vestibular testing has focused on assessment through a grouping of tests known as a videonystagmography (VNG) or electronystagmograpy (ENG). The VNG utilizes infrared video goggles while the ENG utilizes surface skin electrodes; however, both of these recording techniques monitor eye movements. It is key to understand that inner ear disease and some central (brain) dysfunctions generate abnormal patterns of eye movement, which leads to symptoms of dizziness. These eye movements (nystagmus) may be observed with the goggles or electrodes and can then be interpreted.
A VNG/ENG includes measures of ocular motor abilities (eye movement tasks), position change and lastly the vestibular system response to temperature change through caloric irrigation. Caloric irrigations are a measure of the horizontal semicircular canal (senses horizontal head turns) in response to temperature change introduced through air or water into the ear canal. The response for each ear is obtained and compared to one another to determine strength and symmetry of the response. There are inherent flaws to this assessment technique including the potential to miss pathologies affecting any of the other vestibular end organs. Also, the vestibular ocular reflex (VOR) response to temperature change with caloric irrigation samples this reflex at an extremely slow speed, rather than speeds typically encountered in daily life.
There is a need for more
Comprehensive vestibular evaluations, which are typically only performed within a university or medical center setting such as Vanderbilt or here at Wake Baptist, include measures of the VOR across a wide range of speeds and in multiple planes of head rotation, as well as assessment of the otolith organs through VEMP testing. Two other frequently utilized measures of the VOR in this setting include rotational chair and vHIT. These allow for measurement of the VOR in response to slow to moderate speed movements, in the case of rotational chair, and for fast movements, as with vHIT. Both of these measures are primarily used in the lateral plane, once again assessing the horizontal semicircular canal.
However, the use of the vHIT allows for the measurement of the vestibular ocular reflex in all planes, including the VOR response to vertical head movements. Both of these measures are sensitive to vestibular pathology and can give the examiner an objective measure of compensation (central recalibration) from vestibular injury. A basic tutorial of vHIT can be found here and more on rotational chair testing can be found here.
The use of VEMP testing has allowed for more clinically feasible means of otolith organ assessment. The main two VEMPs utilized clinically include the cervical VEMP, which is a measure of saccular function (up & down sensor), and the ocular VEMP, a measure of utricle function (front & back sensor). VEMP testing does not include the use of video goggles, but instead measures muscle activity in response to auditory or vibratory stimulation. More on the specifics of VEMP testing can be found here.
Comprehensive or Incomplete?
In order to detect deficits of the vestibular system and to best promote recovery from vestibular injury, proper detection of all vestibular injuries must occur. This can only happen with sensitive vestibular test protocols that adequately assess all of the vestibular end organs, as well as assess the VOR in response to a wide range of head speeds.
Without the use of measures such as rotational chair, vHIT, and VEMP testing, significant vestibular pathology may not be detected. That is not to say the VNG/ENG is useless, rather VNG is only a partial measure of vestibular function and is most effective when utilized in conjunction with the other aforementioned measures. If a VNG/ENG detects vestibular damage, then a vestibular cause of symptoms may be identified, but one cannot rule out vestibular dysfunction based on a normal VNG/ENG.
Photo courtesy of UGA Admissions Blog – Blogspot