Vertigo primarily results from disorders of the vestibular system, which includes the vestibular labyrinth, vestibular nerve, vestibular nuclei in the brain stem, vestibular portions of the cerebellum, connections between these structures, and only rarely higher in the cerebrum. The vestibular labyrinth, located in the temporal bones, is composed of the three orthogonally oriented semicircular canals (anterior, posterior, and lateral) and the vestibule, which contains the otolith organs, the utricle and saccule, which are also angled at approximately 90 degrees to each other. The former responds to angular acceleration and the latter to linear acceleration including translation or tilt. When the head is rotated, endolymphatic fluid in the semicircular canals lags behind, leading to a deflection of the gelatinous cupula within the canal, which activates or inhibits the firing of hair cells. Activation on one side is paired with inhibition in the complementary canal on the other. The otolith organs, the utricle and saccule, contain hair cells on which calcium carbonate crystals, the otoconia, rest. Translational motion or tilt (via gravity) will activate or inhibit these cells. From the vestibular labyrinth, neurons travel centrally through the vestibular portion of the eighth cranial nerve into the brain stem to the vestibular nuclei and then project on to the cerebellum, ocular motor nuclei, spinal cord, and, via some less well understood pathways, to the
cerebrum. Integration of the combinations of activations and inhibitions of the various components of the vestibular system of both ears, along with visual input and proprioceptive input, detects motion, rotation, translation, and tilt and affects eye movements and posture.
Vertigo can result from disorders of the peripheral vestibular system (labyrinth or nerve) or central vestibular system (brain stem, cerebellum, connections, and rarely, cerebrum) and this localization is the natural next step in the evaluation of vertigo. Table 4.2
lists some differentiating features.