Auditory Nerve

Clinical Vignette

A 68-year-old man presented with sudden onset of unilateral right-sided hearing loss. He stated that this was preceded by several months of constant ringing in his right ear. He had a history of hypertension and type 2 diabetes mellitus that was well controlled by oral hypoglycemic agents. He had no recent head trauma or previous surgeries. His only medications included atenolol and glyburide. There was no family history of hearing loss. He did not have a history of excess noise exposure or recent travel. No other otolaryngologic symptoms were reported.

On examination, his tympanic membranes (TMs) were normal. The Weber test lateralized the tuning fork to the side opposite the hearing loss. Rinne’s test results were normal. The rest of the head and neck examination was unremarkable.

Complete blood count was normal, and a fluorescent treponemal antibody absorption blood test was negative. The patient received a baseline audiogram demonstrating a right ear sensorineural hearing deficit predominantly in the higher frequencies. Brain MRI failed to demonstrate any mass lesion. A brainstem auditory evoked response (BAER) study was performed and revealed a prolonged interaural wave I–III latency, compatible with a retrocochlear process.

Sudden unilateral hearing loss, such as in the above vignette, in the absence of a tumor or lesion involving the vestibular nerve and with a history of diabetes is often attributable to microvascular infarction of the auditory nerve. However, sudden unprovoked hearing loss is not uncommon in patients without microvascular risk factors and the cause, in many cases, remains unknown.

Anatomy

The eighth cranial nerve (CN-VIII) is actually composed of two separate portions: the vestibular and cochlear nerves (vestibulocochlear nerve). The vestibular nerve is responsible for efferent and afferent fibers that control balance and equilibrium (see next section). The cochlear nerve, also called the auditory nerve, carries efferent and afferent fibers for hearing. To understand dysfunction of the auditory nerve, a brief description of the human hearing mechanism is required.

Sound waves travel through the external auditory canal and vibrate the TM, which in turn produces motion of the middle ear ossicles (incus, malleus, and stapes). The vibrations are transmitted through the oval window at the footplate of the stapes, causing a wave to travel through the endolymphatic fluid of the cochlea of the inner ear. The fluid waves vibrate the organ of Corti’s basilar membrane, stimulating inner and outer hair cells (Fig. 8-1). Hair cells, receptors of the sensorineural system, transmit action potentials to bipolar neurons, the bodies of which are in the spiral ganglion.

Figure 8-1 Pathway of Sound Reception.

Afferent fibers projecting toward the CNS comprise the auditory nerve (Fig. 8-2). They travel to the dorsal and ventral cochlear nuclei located in the caudolateral pons. Most of the secondary neurons project contralaterally across the midline to the superior olivary nucleus and then travel up the lateral lemniscus into the inferior colliculus of the midbrain. Decussating fibers from the cochlear nucleus to the superior olivary nucleus are located in the trapezoid bodies and also in the base of the pons. Fibers from the inferior colliculus continue to travel rostrally to the medial geniculate body of the thalamus and then terminate in the auditory cortex located in the transverse temporal gyri of Heschl.

Figure 8-2 Afferent Auditory Pathways.

Clinical Presentation

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). Sensorineural hearing loss (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 it can often be associated with exposure to loud noise, ototoxic drugs (such as aspirin, cisplatinum, and aminoglycosides), acoustic tumors, Ménière 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. 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 Ménière disease, and progressive loss with acoustic neuromas, whereas sudden hearing loss 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. Only a few processes, such as otosclerosis and otitic meningitis, involve both areas. Typically, tinnitus and vertigo are inner ear symptoms and indicate involvement of the cochlea, vestibular labyrinth, auditory nerve, or a combination of these structures.

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 both ototoxicity (aminoglycosides, salicylates, or loop diuretics) and Ménière disease, concomitant 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 (MS) or expanding neoplastic lesions that may lead to combined facial nerve, trigeminal nerve, or ophthalmologic symptoms.

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

Physical Examination

Cerumen impaction or foreign bodies are easily identified on inspection of the external auditory canal. Pneumatic otoscopy is used to assess quality (color, lucency, and mobility) of the TM and defines conductive hearing loss as a reason for hearing deficit. Decreased TM mobility can be attributed to ossicular fixation, such as in otosclerosis, or middle ear effusion, as in otitis media. Middle ear or expanding jugular foramen tumors can present as a mass behind the TM causing conductive hearing loss.

Tuning fork tests assess whether the hearing loss is conductive or sensorineural (Fig. 8-3). 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 cerebello-pontine angle, such as meningiomas or facial neuromas. Auscultation of the areas around the orbit and ear may detect pulsatile tinnitus. The type of SNHL can assist in the localization of the lesion. Ototoxic drugs, excess noise exposure, and autoimmune diseases affect the hair cells within the cochlea, the primary sensory organ of hearing, and lead to hearing loss usually described as decreased sensitivity to pure tones but preserved speech discrimination. Hearing loss caused by retrocochlear lesions of the nerve fibers of CN-VIII or its central auditory projections begins as decreased speech discrimination with relatively normal pure-tone sensitivity. However, decreased speech discrimination is not exclusive to retrocochlear lesions; it is also observed with extensive hair cell damage.