CHAPTER 6: Postural Abnormalities in Vestibular Disorders

Introduction

When this chapter was written for the first edition of this book, the vestibular system was considered to play its primary role in the control of posture and balance. In recent years it has been acknowledged that rather than initiate automatic postural reactions, the vestibular system is responsible for governing orientation in space.^1–3 Whereas the somatosensory system provides information about the position and motion of the body with respect to the support surface and the body segments with respect to each other, the vestibular system provides information with respect to gravity and other inertial forces. The central nervous system adapts quickly to the loss of peripheral vestibular inputs from the labyrinths so that it is sometimes difficult to objectively identify symptoms of vestibular deficit. Identification of the role of the vestibular system in posture and orientation has relied on findings of postural and orientation disorders in patients and animals with vestibular abnormalities.^4–8 Most clinical diagnoses are based on subjective complaints, and patient descriptions of a symptom might differ. One might experience a perception of the world spinning about, while another complains of imbalance and falling, yet both could have the same vestibular pathology.⁹ Since the process of central nervous compensation proceeds over a lengthy period of time, patients can also have different symptoms when they finally arrive at a clinic, although suffering a similar deficit. Both clinical and experimental observations have shown that symptoms of vertigo, past pointing, nystagmus, and equilibrium disturbances are the major complaints of patients with partial or total destruction of the vestibular labyrinths.¹⁰ Despite these fairly consistent symptoms, examination of any one patient with postural abnormalities arising from damage to the vestibular system could yield an uncertain diagnosis.^9,10

The question for the clinician and the clinical investigator is whether any one compensatory strategy would prove most efficient or effective for the population of patients with a vestibular deficit. If one compensatory strategy is best, then a systematic approach to treatment could be followed. But to determine the effectiveness of the compensation, we must first determine how to reliably indicate whether the patient suffers from postural dyscontrol, and whether it is vestibular dysfunction that is responsible for the symptoms. Although standard clinical tests of the vestibular system have not changed much, technologies such as virtual reality (VR) and vestibular evoked myogenic potentials (VEMP) allow us to explore vestibular function when combined with other sensory signals (VR) or to specifically identify what part of the labyrinths or vestibular pathways may be affected (VEMP).^11,12

In this chapter, methods available for testing postural disorders that are associated with vestibular pathology are briefly discussed. Then, postural behaviors that have been quantified and associated with specific vestibular pathologies are described. Finally, the issue of how the postural system compensates for loss or damage to vestibular signals, including the changes that occur with natural ...