Cranial Nerve VIII

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8 Cranial Nerve VIII

Auditory and Vestibular

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.

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.

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.

Diagnostic Approach

Standard laboratory blood tests are not routinely obtained for hearing loss unless a particular cause is suspected by history and physical examination. However, the fluorescent treponemal antibody absorption blood test or the microhemagglutination test for Treponema pallidum or both are usually ordered, as syphilis may often be relatively asymptomatic and is a treatable cause of SNHL.

A basic audiogram with pure tones and speech discrimination evaluation determines the type and amount of hearing loss. Unilateral decrease or asymmetry in speech discrimination, high-frequency hearing loss, or acoustic reflex abnormalities suggests a retrocochlear lesion, warranting further testing.

Gadolinium-enhanced MRI is specifically indicated when history, symptoms, and audiometric tests strongly suggest retrocochlear disease. MRI is the diagnostic “gold standard” for tumors causing hearing loss. For patients presenting with asymmetric hearing loss—especially if sudden—MRI is warranted to exclude acoustic neuromas or other cerebellopontine tumors. MRI with gadolinium can detect 2- to 3-mm tumors within the temporal bone. It is also sensitive for acute and chronic vascular disease or infarction as well as demyelinating lesions.

Brainstem auditory evoked response (BAER) is a useful objective and quantitative test when a retrocochlear deficit is suspected. It can suggest the site of lesion from the cochlea to the inferior colliculus at the pontine mesencephalic junction. BAER studies were initially considered highly sensitive for retrocochlear causes; however, as with most tests, false-negative and false-positive results are possible. The BAER uses electrodes attached to the patient’s head and clicking sounds emitted through earphones. The sounds elicit action potentials through the peripheral and central auditory pathways, and the EEG activity is measured and averaged by a computer. Right and left ear waveform morphologic appearance and latencies are compared. Interaural differences suggest pathologic conditions. Five wave peaks characterize the BAER, corresponding to specific anatomic points within the auditory pathway: I, CN-VIII action potential; II, cochlear nucleus; III, olivary complex; IV, lateral lemniscus; and V, inferior colliculus. A change in peak morphology and latency helps localize the pathologic condition.

Differential Diagnosis

Idiopathic, sudden SNHL is generally defined as loss that develops over 12 hours or less. However, a broad differential of sudden SNHL includes Ménière disease, neoplasms, vascular disorders, viral infections, MS, and rarely, hematologic disorders.

Neoplasms

In any case of sudden, unilateral hearing loss, neoplastic lesions, although rare, should be considered in the differential until excluded by diagnostic or 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. 8-4). 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 the most commonly reported symptom, occurring at some point in approximately 95% of patients with vestibular schwannoma. Progressive hearing loss 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 not frequently seen with vestibular nerve schwannomas because as these lesions grow, the contralateral vestibular system gradually adjusts to the imbalance, preventing 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%). MRI will detect smaller tumors in patients who have had normal BAERs.

Vascular Etiologies

Vertebrobasilar stroke is another cause of sudden, unilateral SNHL with potentially devastating effects. Distinguishing whether hearing loss results from microvascular disease or a brainstem infarct is vital. The anterior inferior cerebellar artery supplies blood to the inferolateral portion of the pons, CN-VII, the spinal trigeminal tract, and the inferior cerebellum. A stroke from occlusion of this artery causes an infarct of the ipsilateral pons, creating a myriad of symptoms: ipsilateral hearing loss and vestibular symptoms, gait ataxia, conjugate gaze palsy, ipsilateral facial paralysis and often contralateral loss of pain and temperature sensation in the extremities (see Chapter 55).

Computed tomography is usually the initial imaging study and excludes hemorrhagic infarction within the cerebellum and brainstem. MRI and MR angiography, however, provides better definition when available and concomitant imaging of the major vessels of the circle of Willis.

Unilateral hearing loss also occurs secondary to occlusion of the cochlear blood supply from the internal auditory artery, a terminal branch of the anterior inferior cerebellar artery, or the basilar artery. This usually occurs secondary to compression by an acoustic neuroma in the internal auditory canal, but a thrombotic, vasculitis, or rarely embolic event can also be the cause.

Microvascular disease due to diabetes and hypertension is linked to sudden, unilateral hearing loss, and the mechanism is thought to be similar to other diabetic cranial neuropathies with involvement of the vaso nervosum and nerve microinfarction.

Multiple Sclerosis

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