The vestibular system is a complex, highly integrated component of the nervous system that usually goes unnoticed until it is disturbed. Consider the person who goes on a ride at an amusement park feeling well but exits experiencing dizziness, unsteadiness, nausea, and difficulty with vision, concentration, and walking. In this instance, we intuitively identify that the person is experiencing "motion sickness" due to excessive stimulation from the ride. However, these are the symptoms that people with vestibular disorders experience. The vestibular system makes up one the seven special senses and is responsible for the awareness of body position in space, maintaining postural control against gravity, and coordinating head and eye movements. Because of the integral role that the vestibular system plays in postural control, the physical therapist must be able to recognize vestibular system involvement to remediate balance impairments. This chapter presents an overview of the vestibular system and introduces entry-level interventions that address limitations and restrictions based on the clinical presentation of the patient.
Overview of the Peripheral Vestibular System
The peripheral vestibular apparatus originates in the inner ear, adjacent to the cochlea, and lies deep within the temporal bone. It comprises the otoliths (the utricle and saccule), three semicircular canals, and the vestibular portion of the eighth cranial nerve (vestibulocochlear nerve). Each ear contains one set of these structures that work as a team to convey information regarding head position to the central nervous system (CNS). A bony outer labyrinth covers a fluid-filled inner membranous labyrinth that contains the five vestibular sensory organs: the utricle, which detects lateral head tilt and linear translation of the head in the horizontal plane; the saccule, which detects linear translation of the head in the vertical plane; and the three semicircular canals, which detect angular acceleration of the head (rotation).1 Afferent information from the sensory receptors synapse at Scarpa's ganglion then travels to the periphery on the vestibular portion of the vestibulocochlear nerve (Fig. 13.1).
Anatomy of the vestibular labyrinth Structures include the utricle, sacculus, superior semicircular canal, posterior semicircular canal, and the horizontal semicircular canal. The three semicircular canals (SCCs) are orthogonal with each other. Note the superior vestibular nerve innervating the superior (anterior) and horizontal semicircular canals as well as the utricle. The inferior vestibular nerve innervates the posterior semicircular canal and the saccule. The cell bodies of the vestibular nerves are located in Scarpa's ganglion. Also note that the semicircular canals enlarge at one end to form the ampulla. From Schubert, p. 966,2 with permission.
The distinct arrangement of the hair cells in the utricle and saccule provide the system with precise information regarding the head position in relation to gravity. Both the utricle and saccule contain a macula, a patch of specialized hair cells, on top of which sits ...