The vestibular labyrinth, situated in the inner ear, consists of the sensory organs that provide the dominant input for perception of head movement, perception of spatial orientation, and generation of gaze- and posture-stabilizing reflexes. In each ear, there are three roughly orthogonal semicircular canals which detect rotational head movements, as well as the otolithic endorgans, which transduce linear acceleration. A healthy vestibular system is essential for maintenance of normal vision, balance, and head orientation because the vestibular system mediates the vestibulo-ocular reflex (VOR), which stabilizes the eyes with respect to space during head movement, permitting clear and steady vision even during high frequency, high acceleration involuntary perturbations of the head and body.
A challenge for health care providers is to obtain a quick and accurate diagnosis of vestibular function in a manner that minimizes potential discomfort for the patient. Current methods of measuring vestibular sensory function are suboptimal with regard to size, expense, ease of use, patient acceptance, portability, accuracy, and the need for highly skilled operators.
One of the current standard methods of vestibular function assessment is the caloric test. This test involves irrigating the patient's ear with water at a higher or lower temperature relative to body temperature, creating convection currents in the ipsilateral horizontal semicircular canal, which in turn elicit measurable eye movements. Eye movement responses are typically measured using electro-oculography, which only provides an approximation to horizontal eye angular position. This test is often uncomfortable for the patient, and therefore can fail due to patient intolerance.
The caloric test can only provide information regarding the function of one portion of the inner ear (i.e., the horizontal semicircular canal) and cannot provide information regarding the function of other inner ear labyrinth sensors (i.e., the anterior semicircular canals, posterior semicircular canals, utricle, or saccule). The caloric test is limited to testing only low-frequency VOR performance and therefore cannot accurately assess VOR performance of the high acceleration, high frequency transient head movements for which the VOR is most important to stabilize gaze. The apparatus for caloric testing requires a highly skilled operator.
Another current standard method of vestibular function assessment is the rotary chair test. The apparatus for this test comprises a rotating motor atop which the patient is seated in a chair. The motor generates whole-body rotations and a means for measuring eye movement responses used to assess VOR function. This allows examination of vestibular responses to higher frequency head movements than the caloric test, but it is still limited to head accelerations and frequencies lower than those for which the VOR typically stabilizes gaze in healthy subjects. As the chair must move the subject's entire body, high torque, high power motors are required, making the rotary chair test apparatus very expensive to build, install, and maintain, as well as large and potentially dangerous. A highly skilled technician is required to perform rotary chair testing. Despite the cost, size, and complexity, a typical rotary chair apparatus only measures function of the horizontal semicircular canals, providing no information about the other sensors within the inner ear labyrinth.
Attempts to overcome the shortcomings of caloric and rotary chair tests led to the use of head-on-body rotations to impart high acceleration, high frequency head movement stimuli to probe VOR performance without the expense and space required for a rotary chair. Manual head thrusts (quick, transient, small amplitude head-on-body rotations) can be administered to the patient by a trained examiner. This involves the examiner gripping the patient by the head and turning the head at a high acceleration to evoke the VOR. Eye movements are typically recorded using a magnetic, electrical, or video system. Manually applied head rotations generated by a human examiner are highly variable from trial to trial, reducing the yield of this assessment method.
Accordingly, there is a need for a device that enables health care providers of various training to uniformly administer head impulse testing without the need for substantial commitment of capital and floor space.