CHAPTER 12: Otolith Function Tests

Introduction

The balance organs in the inner ear—the vestibular sensory regions—are the gyros of the human body, and unnoticed, they function continuously in an almost perfect fashion, providing the brain with information about head position and head movement. Until these organs fail, no one really appreciates their significance in daily life, and anyone who has experienced an attack of vertigo will readily verify their importance. The accelerations involved in movements generate forces, and the biological gyros of the inner ear detect these forces: both the force imposed by gravity and the forces generated when we move. The vestibular sensors are constructed in such away that different regions detect rotational forces and linear forces in any direction. The brain synthesizes the information from these separate force detectors to provide a global integrated summary of where a person is and how the person is moving. This realization is of clinical importance, because it implies that disease affecting only one isolated region of the inner ear has consequences for the overall integration of all the vestibular sensory input. In this chapter, we deal only with the otoliths, the structures that sense linear forces, such as the force of gravity or the “straight line” acceleration experienced in a vehicle accelerating from a stop.

The peripheral vestibular system is sensitive to both linear and angular accelerations: the semicircular canals (SCCs) sense angular acceleration, whereas the otoliths— the saccule and the utricle—sense linear accelerations. Many different ways of testing otolith function have been proposed, including the measurement of horizontal, vertical, and torsional eye movements as well as psychophysical settings in response to linear accelerations produced on swings,¹ sleds,² centrifuges,³ tilt-chairs,⁴ and barbeque spits.⁵ For an otolith function test to be clinically useful, it must be safe, practical, robust, and reproducible. The test also needs to be specific for, and sensitive to, otolith dysfunction, particularly unilateral otolith hypofunction. In our view, only two means of testing otolith function— the subjective visual horizontal or vertical (SVH or SVV) and the vestibular evoked myogenic potentials—fulfill these requirements. Both have been regularly used in our clinical laboratories for many years, and what follows is in part a distillation of our own experience with these tests and of their scientific basis. The SVV and SVH are perceptual tests that are well known. The newer VEMP tests have been the subject of intense recent interest, and so we devote detailed discussion of them below. Before considering these tests, however, one must have some familiarity with the structure and function of the otoliths.

Anatomy

The basic element of all vestibular transducers is the receptor hair cell, which is similar in both the angular and the linear force-sensing systems (Fig. 12.1A to E). The SCCs and the otoliths detect these two different forces, not because of any differences in the intrinsic properties of the hair cells themselves ...