The vestibular system: Unsteadiness, dizziness, and vertigo.

This chapter provides a detailed overview of the vestibular system's anatomy, physiology, and clinical relevance, emphasizing how inner-ear sensory structures and brainstem pathways interact to maintain balance, gaze stability, and spatial orientation. It begins with the peripheral vestibular apparatus, describing the semicircular canals and otolith organs in detecting angular and linear accelerations. These signals are transmitted via the vestibulocochlear nerve to the vestibular nuclei at the pontomedullary junction, where they integrate with visual and somatosensory inputs to drive coordinated reflexes - most notably the vestibulo-ocular (VOR) and vestibulo-spinal reflexes - that stabilize vision and posture. We then examine central vestibular pathways, including reciprocal connections between the brainstem, cerebellum, and cortex. These circuits enable adaptive mechanisms like VOR gain recalibration and support functions such as self-motion perception. A range of common vestibular disorders is discussed - benign paroxysmal positional vertigo (BPPV), vestibular neuritis, Ménière's disease, vestibular migraine, and persistent postural-perceptual dizziness - covering their pathophysiology and clinical features. Diagnostic tools include bedside exams (e.g., head impulse test), imaging, and objective measures like caloric testing, video head impulse testing, and vestibular-evoked myogenic potentials. Treatment strategies are reviewed, highlighting the effectiveness of repositioning maneuvers for BPPV and vestibular rehabilitation for compensation after vestibular loss. The chapter concludes with emerging advances, including vestibular prosthetics and high-resolution imaging, which are redefining diagnosis and treatment. These innovations underscore the vestibular system's vital role in equilibrium and point to promising therapies for chronic dizziness and imbalance.