In recent years, audiologists have gained valuable new tools and procedures to assess a patient's vestibular functioning. Scientific and practical breakthroughs have been achieved-both in equipment available and testing that can be undertaken-that have helped audiologists improve their ability to accurately diagnose a range of balance and equilibrium conditions.

However, while testing methods such as MRIs, CT scans, audiograms, VEMP, and caloric and rotational tests certainly provide valuable information for the assessment process, they also have important shortcomings. No single existing test or device provides adequate, quality information to assess semicircular canal function in patients with balance disorders.

Consequently, there is need for ongoing improvement. Fortunately, as the new decade gets underway, exciting and more effective new diagnostic methods are in the works. As a prelude to appreciating what is in store for audiologists, let's look at the limitations of our current vestibulo-ocular reflex (VOR) tests:

Caloric-The thermal stimulus that reaches the inner ear depends upon anatomical and technical factors. The end result is that test-retest reliability can be suspect, making calorics a poor choice for tracking changes in vestibular function. The identification of bilateral vestibular loss (BVL) is also uncertain due to wide variations in response amplitudes in healthy patients. And the unusual nature of the stimulus (evoking sensations of sustained rotation in a supine position) often provokes nausea.

Sinusoidal Harmonic Acceleration-The chief disadvantage of the SHA, and indeed all traditional rotation tests, is that they often do not provide ear-specific information in patients with unilateral (UVL) or bilateral vestibular loss. This is because the high resting discharge rates of vestibular afferents that allow each ear to encode

bidirectional head rotations do not reach zero during any portion of the stimulus, and the central nervous system often compensates for acute UVL. Once compensation is achieved, only minor signs of vestibular dysfunction can be recognized in conventional rotation test results.

Autorotation-After years of clinical experience, there is no consensus regarding the utility and reliability of the autorotation test. Factors contributing to poor reliability include eye-recording artifacts from high-head accelerations; imprecise monitoring of head motion; patients' inconsistent ability to achieve regular high frequency oscillations; and analysis artifacts from poor tracking of fast-phase eye movements. 

Head Impulse-As for the head impulse test, it too has poor repeatability and, importantly, has poor sensitivity in cases of partial canal paresis. Additionally, patients with neck injury or other limitations in neck mobility cannot undergo rapid head rotations.

Looking Ahead

There are two promising new rotation tests under development, both of which incorporate new and advanced technologies that appear to efficiently and effectively overcome current shortcomings.

The first of these new tests is the pulse-step-sine test (PSST) and the other is the computerized head thrust (CHT).

Building on pioneering work by Robert Peterka, PhD, at the Oregon Health and Sciences University, the PSST is based on a rotational stimulus that takes advantage of the 3D arrangement of the three pairs of semicircular canals in the two ears and the physiological properties of eighth nerve afferents innervating each semicircular canal. Dr. Peterka has shown in his studies that the test can identify UVL, BVL and the side-of-lesion in subjects with well-compensated UVL. These results cannot be identified in standard testing.

Also on the near horizon is the CHT. By using a precision high-torque motor along with accurate and high-speed eye tracking found in advanced neuro-otologic testing device systems, it appears that this new head thrust test is an accurate, reliable and repeatable VOR measure superior to other techniques.

Much work remains to be done in perfecting and validating the early findings of the PSST and CHT. But even at this early stage, one thing is clear: audiologists and other specialists will have new and better tests in their arsenal to help them deliver more cost-effective diagnoses. Stay tuned.

Alex Kiderman, PhD, is chief technology officer at Neuro Kinetics Inc. Contact him at akiderman@neuro-kinetics.com.