Cranial Nerve VIII: Auditory and Vestibular

History

Hearing loss can result from pathologic conditions anywhere along the anatomic pathway for hearing. It may spare the auditory nerve, such as in middle ear pathology (e.g., serous otitis media), or involve the auditory nerve (e.g., acoustic tumors). SNHL is a hearing deficit from dysfunction of the cochlea (sensory), the auditory nerve (neural), or any part of the central auditory pathway. Auditory nerve dysfunction usually results in tinnitus, SNHL, or both. A targeted history and physical examination narrow the diagnosis. The temporal profile of symptom onset (i.e., sudden, progressive, fluctuating, or stable) is critical.

Tinnitus presents with or without concomitant SNHL and is classified into two groups. Subjective tinnitus, the most common, is heard solely by the patient. It can range from soft fluctuating ringing noise to loud constant debilitating roar. The cause of subjective tinnitus is usually unknown but is most often associated with SNHL or it can often be associated with exposure to loud noise, ototoxic drugs (such as aspirin, cisplatinum, and aminoglycosides), acoustic tumors, Meniere disease, and cochlear otosclerosis. Objective tinnitus is heard by the patient and the examiner and is usually not a sign of auditory nerve dysfunction. Pulsatile tinnitus is usually secondary to vascular causes, such as arteriovenous malformations or glomus tumors. Middle ear effusions, as in serous otitis media, can magnify vascular pulsations from the nearby internal carotid artery and produce vascular tinnitus. Pulsatile tinnitus may also occur with exposure to CNS pulsations or bone density changes allowing easier transmission of vascular flow sounds through the temporal bone to the cochlea. Clicking tinnitus is secondary to temporomandibular joint disease, palatal myoclonus, or spontaneous contraction of the middle ear muscles.

Determining the laterality of hearing loss is essential. Bilateral deficits occur in processes such as ototoxicity, noise exposure, and hearing loss related to aging (presbycusis). Unilateral hearing loss raises the concern of neoplastic, vascular, neurologic, or infectious etiologies. Fluctuation of hearing is seen in Meniere disease, autoimmune inner ear disease. Progressive SNHL is usually seen with aging or tumors, whereas sudden SNHL occurs with viral neuritis or vascular processes.

Whether the hearing loss involves a process in the external or middle ear versus the inner ear must be determined. An audiogram should be performed on any patient with a complaint of a hearing disorder. The audiogram along with the history and physical examination will then lead to next steps of evaluation or diagnosis by the physician.

Only a few processes, such as otosclerosis and otitic meningitis, involve both the middle ear and the inner ear. Typically, tinnitus and vertigo are inner ear symptoms and indicate involvement of the cochlea, vestibular labyrinth, auditory nerve, or a combination of these structures. However, tinnitus, associated with SNHL, is now known to be a central event, based upon functional MRI studies, much like phantom limb syndrome.

Hearing loss associated with otalgia, otorrhea, headache, and aural fullness is most likely inflammatory and can be confirmed by physical examination. Concomitant tinnitus, vertigo, or both suggest the ominous extension of the inflammatory process to the inner ear or beyond. In this setting, a formal audiogram is indicated to determine whether the perceived hearing loss is secondary to a middle ear effusion or an additional sensorineural component. The latter is an otolaryngologic emergency.

With ototoxicity (aminoglycosides, salicylates, or loop diuretics), migraine headache disorders, and Meniere disease, vestibular symptoms, tinnitus, aural fullness, or a combination of these symptoms may accompany hearing loss. In conditions such as presbycusis and noise-induced hearing loss, vestibular symptoms are less likely to be part of the presentation.

Neurologic or ophthalmologic manifestations accompanying primary otologic symptoms occur with diseases such as multiple sclerosis or expanding neoplastic lesions, which may lead to combined facial nerve, trigeminal nerve, or ophthalmologic symptoms.

Trauma to the temporal bone, resulting in labyrinthine or auditory nerve injury, can result in auditory nerve dysfunction. Diving and flying may cause barotrauma, leading to rupture of the cochlear membranes, with subsequent SNHL. Occupational and recreational noise exposure damages the cochlea's outer hair cells, creating high-frequency SNHL. A family history of hearing loss is important to establish because this can be an important mechanism or predisposing factor for SNHL.

Physical Examination

Cerumen impaction or foreign bodies are easily identified on inspection of the external auditory canal. Otoscopic examination allows the physician to inspect the ear canal, tympanic membrane, and middle ear. This is important for identifying various conditions that can contribute to conductive hearing loss. It allows for the identification of fluid in the middle ear, cholesteatoma, or masses. Pneumatic otoscopy is used to assess the mobility of the tympanic membrane.

Tuning fork tests assess whether the hearing loss is conductive or sensorineural ( Fig. 9.3 ). During the head and neck examination, a complete cranial nerve examination must also be performed to assess other potential cranial nerve abnormalities. Facial nerve weakness may be attributed to viral infections, such as herpes zoster oticus, or expanding neoplasms in the internal auditory canal or cerebellopontine angle, such as acoustic neuromas, meningiomas, or facial neuromas. Auscultation of the areas around the orbit and ear may detect objective pulsatile tinnitus. The pattern of SNHL can sometimes assist in the diagnosis of the etiology of hearing loss. The typical pattern for noise-induced SNHL is a drop in the 3–6 kHz frequency range which then improves in higher frequencies. Noise-induced SNHL may also be unilateral if the affected ear is closer to the noise source. Meniere disease and migraine disorders typically manifest with a low-frequency SNHL. Acoustic neuromas typically cause a unilateral SNHL. Speech discrimination significantly worse than would be expected on audiogram results may point to a central auditory processing disorder. Finally, a condition known as auditory neuropathy is manifested by normal auditory brainstem response testing and absence of otoacoustic emissions.

Meniere Disease

Meniere disease is an idiopathic condition characterized by a combination of episodic vertigo, fluctuating SNHL, tinnitus, and aural fullness. These episodes typically last hours, and once the episode subsides the symptoms resolve. Over the course of time the hearing loss may become permanent, typically starting in the low-frequency tones before involving the mid- and high-frequency tones. A condition known as “cochlear Meniere disease” typically has the fluctuating hearing loss (maybe tinnitus and fullness) but no vertigo. Meniere disease is often a diagnosis of exclusion, so a full evaluation including MRI scan of the brain with contrast is warranted.

Neoplasms

In any case of sudden, unilateral SNHL, neoplastic lesions, although rare, should be considered in the differential until excluded by diagnostic and radiologic testing. Vestibular schwannomas (also known as acoustic neuromas) are benign tumors arising from the Schwann cells of CN-VIII and account for 6% of all intracranial tumors ( Fig. 9.4 ). These occur on the vestibular portion of CN-VIII and involve the adjacent cochlear division by compression against the bony walls of the internal auditory canal. Less commonly, neuromas can also arise directly from the cochlear nerve.

Hearing loss is unilateral and is the most commonly reported symptom, occurring at some point in approximately 95% of patients with vestibular schwannoma. Progressive SNHL generally results from stretching or compression of the cochlear nerve as the tumor grows. In contrast, when hearing loss is precipitous, it is thought to be secondary to occlusion of the internal auditory artery supplying the cochlea. Tinnitus with acoustic neuromas is typically high pitched, continuous, and unilateral. Paradoxically, vestibular symptoms are less frequently reported with vestibular nerve schwannomas because as these lesions grow, the central vestibular system gradually compensates, often limiting any significant or longstanding vestibular symptomatology. Larger tumors occasionally lead to facial or trigeminal nerve involvement with symptoms of facial paralysis or paresthesias, respectively.

Before MRI, BAERs were the diagnostic test of choice for acoustic neuromas, with a sensitivity of 93%–98%. The sensitivity is significantly lower with tumors less than 1 cm (58%). Gadolinium-enhanced MRI scan of the brain (with particular attention to the internal auditory canals) will detect smaller tumors in patients who have had normal BAERs.