6

Auditory Nerve Disorders

INTRODUCTION

This chapter covers three main topics, including acoustic neuromas, auditory neuropathy spectrum disorder, and vascular loops. These three topics are not the only disorders that can affect the auditory nerve, but they represent relatively common occurrences and are well documented in the literature. These topics provide a snapshot of the spectrum of dysfunction of the auditory nerve and the great importance it plays in hearing. It was once commented at a conference that the auditory nerve was a “bottleneck” for all auditory information coming from the cochlea. This was an astute comment as all auditory information processed in the cochlea is passed to the central auditory nervous system via the auditory nerve, making it a critical structure within the auditory system.

In the adult human, the auditory nerve is about 22 mm in length and is composed of approximately 30,000 fibers. It consists of type I fibers (90%) that connect to inner hair cells and type II fibers that connect to outer hair cells (10%). The more distal segment (fibers exiting the cochlea) is unmyelinated, with the more proximal portion (fibers projecting to the brainstem) myelinated in reference to type I fibers (i.e., the afferent fibers). The auditory nerve fibers project from the terminal buttons on the hair cells through the habenula perforata to Rosenthal’s canal, which accommodates the spiral ganglion cells. These fibers form a trunk in the modiolus and then course through the internal auditory meatus before entering the cerebellopontine angle (CPA). The central projection of auditory nerve fibers is to the root entry zone in the brainstem between the anterior and posterior ventral cochlear nuclei. (For more information on the anatomy and physiology of the auditory nerve, see Musiek & Baran, 2020, and Chapter 2, “Structure and Function of the Auditory and Vestibular Systems.”)

ACOUSTIC NEUROMA

Introduction

Although historically referred to as acoustic neuromas, these benign tumors of the eighth cranial nerve originating from Schwann cells are more accurately referred to as vestibular schwannomas. However, both terms remain in common use and are used interchangeably in this text. These tumors occur, almost exclusively, on the vestibular branches of the vestibulocochlear nerve. Schwann cells produce myelin that insulates the nerve fibers subserving hearing and balance (Musiek & Baran, 2020). Although these tumors typically cause tinnitus, unilateral hearing loss, and disequilibrium, they may grow to considerable sizes and compress the brainstem, leading to stroke, hydrocephalus, and even death. There has been a steady improvement in the early identification and surgical management of acoustic neuromas over the past several years as audiologists and otologists have worked in concert to improve the outcome for patients with acoustic neuromas.

Symptoms

Tinnitus is the most common symptom in patients with vestibular schwannomas; however, hearing loss is often the presenting complaint in most patients. The hearing loss is classically a unilateral sensorineural hearing loss, the presence of which should signal the need for a workup for acoustic neuroma. The hearing loss in patients with acoustic tumors tends to be gradually progressive, with a decrease in speech recognition performance that is out of proportion to the loss of pure-tone hearing sensitivity. Asymmetric sensorineural hearing loss reportedly occurs in 85% of confirmed tumor cases (see Tucci, 1997). However, others have reported that the presenting complaint for 75% of patients with acoustic tumors is hearing loss, but that when all patients with confirmed acoustic tumors are formally evaluated, that 95% of the patients actually are found to have hearing loss (Angeli & Jackson, 1997). The high incidence of hearing loss in this population of patients is most likely the reason why the terms acoustic neuroma and acoustic tumors remain in common use.

Tucci (1997) reviewed the incidence of hearing loss in patients with vestibular schwannomas and found that 95% of patients with tumors larger than 3 cm had hearing loss, whereas patients with tumors 1 to 3 cm in size and those with tumors less than 1 cm in size had incidences of hearing loss of 88% and 77%, respectively.

Sudden hearing loss can occur in some patients with acoustic tumors, and some may even recover partially following treatment with steroids. Pensak and colleagues reported that 15% of their patients with acoustic tumors had experienced a sudden loss of hearing (Pensak, Glasscock, Josey, Jackson, & Gulya, 1985). However, less than 10% of patients with sudden sensorineural hearing loss will turn out to have acoustic neuromas on magnetic resonance imaging (MRI) testing (Ramos et al., 2005). In a large-scale study, Saunders and colleagues found that 3 out of every 100 patients with sudden hearing loss were diagnosed with an acoustic neuroma (Saunders, Luxford, Devgan, & Fetterman, 1995), a somewhat smaller percentage than that noted by Ramos et al. (2005). Based on their findings, Saunders et al. (1995) recommended that either ABR or MRI with contrast be performed for patients who present with sudden hearing loss. They also suggest that patients who present with pain, facial paresthesia, or unilateral tinnitus also be evaluated with MRI with contrast to evaluate for an acoustic neuroma. Sudden hearing loss, when it does occur in patients with acoustic tumors, rarely improves even after treatment. In fact, the most common outcome is that the hearing loss progresses regardless of the chosen treatment.

Incidence and Prevalence

Acoustic neuromas are the most common tumors of the CPA and they are also among the most common types of benign intracranial tumors, accounting for approximately 8% of all intracranial tumors (Gruskin, Carberry, & Chandrasekhar, 1997; Zülch, 1957). Nevertheless, their incidence is low, about 1 to 2 in 100,000 individuals. This equates to about 3,000 new cases of acoustic neuroma in the United States each year, of which only about 5% are bilateral (Pool, Palva, & Greenfield, 1970; Stangerup, Tos, Thomsen, & Caye-Thomasen, 2010).

Etiology and Pathology

A great deal has been learned about the etiology of acoustic neuromas in the last several years. As mentioned earlier, in most cases, the “neuroma” is really a schwannoma that results in a proliferation of vestibular Schwann cells. The Schwann cells are the supporting cells that form a wrapping around the neurons and are important for conduction of neural signals. Schwann cell proliferation is regulated by a gene called neurofibromin 2 (NF2), which encodes a tumor suppressor protein called merlin or schwannomin. This suppressor gene resides on chromosome 22q12. A variety of mutations may occur in this gene, and the type of mutation present has been linked to the severity of disease. Loss of myelin function results in Schwann cell proliferation and tumor development. This may occur when both NF2 gene copies contain mutations, which has been referred to as the “two-hit hypothesis” (see Scarivilli, 2003).

The vast majority of acoustic neuromas (>95%) occur sporadically, but 2% to 5% of tumors occur as part of a genetic hereditary syndrome referred to as neurofibromatosis type 2 (NF2), where multiple tumors are common and not necessarily limited to the eighth nerve. Neurofibromatosis type 2 is believed to be inherited through an autosomal dominant pattern of inheritance with incomplete penetrance. (See Chapter 9, “Hereditary and Congenital Hearing Loss,” for additional information on genetics and inherited hearing loss). However, unlike the majority of other autosomal dominant conditions, where only a single copy of the affected gene is needed to result in the disorder, it appears that two copies of the NF2 gene must be mutated for tumor formation to occur (U.S. National Library of Medicine, 2019). Most affected individuals inherit one defective copy of the tumor suppressor gene from one of their parents and a second copy is injured or mutated at some point during the individual’s lifetime. Many NF2 patients may also develop spontaneous NF2 mutations with no other affected family members. There have been several classifications systems used in the diagnosis of NF2, including the National Institutes of Health (NIH Health Consensus Development Conference, 1988) and Manchester (Evans et al., 1992) criteria. Baser and colleagues (2002) proposed that these diagnostic criteria are inadequate in diagnosing those without bilateral acoustic neuromas as having NF2. In 2011, Baser et al. proposed a new diagnostic system that demonstrates 79% sensitivity and 100% specificity. The new criteria include bilateral acoustic neuromas/vestibular schwannomas with tumors documented before the age of 30 or a “constitutional nonmosaic pathogenic mutation of the NF2 gene” (Baser et al., 2011).

Individuals with NF2 usually develop acoustic neuromas early in life and are prone to having multiple cranial nerve schwannomas, neuromas, spinal tumors, meningiomas, and other brain tumors as well. Additionally, they may present with clouding of the lens of the eye (juvenile posterior subcapsular lenticular opacities) and may develop early-onset cataracts (Baser et al., 2002). Patients diagnosed with NF2 typically require multiple surgical procedures and often end up with bilateral deafness, which usually cannot be corrected by cochlear implantation. Genetic counseling is of paramount importance in these patients because the gene mutation can be passed on by the affected parent to his or her offspring in an autosomal dominant fashion (see Pensak et al., 1985, and Scarivilli, 2003).

Site of Lesion

Acoustic neuroma is perhaps one of the most common and certainly the most heralded retrocochlear lesion that is encountered in the clinical setting. As such, site-of-lesion tests that differentiate retrocochlear from cochlear conditions are used in the audiologic diagnosis of acoustic neuroma. Acoustic tumors arise from the vestibular portion of the eighth cranial nerve about 90% of the time. These tumors typically originate in the internal auditory canal and as they grow, they extend medially into the CPA and/or erode the bone of the internal auditory canal (Gruskin et al., 1997). However, as mentioned earlier, the vast majority of acoustic neuromas reside in the CPA.

It is important to note that tumors greater than 2 cm in maximum diameter are likely to eventually contact and compress the brainstem and cerebellum (Musiek & Kibbe, 1986). In this situation, the central auditory nervous system becomes involved and additional auditory and/or vestibular symptoms may appear.

Audiology

Although the auditory brainstem response (ABR) is typically recognized as the standard test protocol for use in the assessment of auditory nerve function, the audiology of acoustic neuromas includes more than just the ABR. There is valuable information to be gained from the puretone audiogram and word recognition test scores. In addition, acoustic reflexes and otoacoustic emissions (OAEs) can be of help. Therefore, each of these tests is discussed along with the various permutations of the ABR that may be observed in patients with acoustic neuromas.

Pure-Tone Threshold and Speech Recognition Tests

It has been reported, and mentioned earlier, that more than 90% of patients with acoustic tumors present with unilateral sensorineural loss (Lustig & Jackler, 1997). However, approximately 1% to 7.5% of individuals with asymmetric hearing loss will actually have an acoustic neuroma (Fortnum et al., 2009). Therefore, to use unilateral sensorineural loss as a key criterion for referral can be problematic. The most common audiometric configuration for acoustic neuromas is a sloping high-frequency loss, which occurs almost two-thirds of the time (Johnson, 1977). Speech recognition scores vary widely, but interestingly, in one large study, 35% of the patients studied had scores of 0% (Johnson, 1977). The classic audiologic signs of unilateral sensorineural hearing loss and reduced speech recognition scores remain important warning signs for a potential acoustic neuroma; however, the sensitivity and specificity of these measures do not permit them to be highly diagnostic.

An important issue regarding puretone thresholds is what criteria should be used to determine if further workup is indicated. The issue here centers around the degree of asymmetry that should exist between the two ears before a referral for further testing is made. There are a variety of criteria that have been proposed for this purpose. Obholzer, Rea, and Harcourt (2004) reviewed six different criteria that have been used and offered yet another set of criteria. Their criteria require that in cases of unilateral hearing loss, interaural differences at two adjacent frequencies be greater than 15 dB. A criteria of 20 dB was recommended in cases of bilateral hearing loss. Application of these criteria revealed a high sensitivity (>90%) but a moderately poor specificity, which is typical of all such criteria. However, in spite of the moderately poor specificity, the Obholzer et al. criteria seem to be of some value when determining the need for follow-up in patients presenting with asymmetric sensorineural hearing losses.

Clearly the best audiologic test for acoustic neuromas is the ABR. In fact, it has been shown that for acoustic neuromas, the best procedure is the ABR alone. When it is combined with other tests, overall test efficiency often suffers (Turner, Frazer, & Shepard, 1984). The ABR indices that are most telling in acoustic neuroma patients are the I–III and the I–V interwave intervals and the interaural latency difference (ILD). These indices individually or in combination will yield hit rates of approximately 90% or better and false positive rates of usually less than 15 % (Musiek, Gonzalez, & Baran, 2015; Musiek, Shinn, & Jirsa, 2007). In many cases of acoustic neuromas, the ABR is totally absent even when the audiogram reveals reasonably good hearing sensitivity. Another diagnostic protocol that has started to emerge in the audiologic assessment of the patient considered to be at risk for an acoustic neuroma is the comparison of the puretone thresholds with the ABR thresholds. If there is a large discrepancy between these measures (i.e., if behavioral thresholds are considerably better than ABR thresholds), then retrocochlear involvement should be considered (Bush, Jones, & Shinn, 2008).

It is important to consider utilization of the ABR, with its impressive record of high sensitivity for acoustic tumors in the assessment of patients who may be considered to be at risk for acoustic tumors. Its use may help to reduce the number of over-referrals for MRIs that are made based on findings of asymmetric hearing loss alone. The ABR test can also be used in cross-checking radiologic results in that it has now been shown that MRIs are not without flaws. An interesting report highlights a false positive result for acoustic neuroma based on MRI where ABR results were normal (House, Bassim, & Schwartz, 2008). This report adds to a growing number of reports revealing false positive MRI results in patients with asymmetric sensorineural hearing losses. These false positive MRI cases involve very small lesions (1–2 mm) that mimic vestibular schwannomas and may even have unilateral auditory symptoms; however, they lack a discrete tumor during surgery. Also, there are many individuals who cannot undergo MRIs or CTs for a number of reasons and, in these cases, the application of ABR test procedures would be essential. In addition, the ABR provides information regarding the auditory nerve function that is not provided by MRI as the ABR is a physiologic measure, whereas the MRI is not. There are in fact many cases of auditory nerve dysfunction that show nothing significant on MRI, as this diagnostic procedure does not assess physiologic function. Auditory brainstem response testing is therefore considered to be an important adjunct to imaging procedures because physiologic dysfunction must be confirmed and quantified to effectively manage the patient with eighth nerve involvement.

Videonystagmography (VNG) can be of value in quantifying vestibular function in cases of acoustic neuroma. One of the important findings is that of unilateral paresis on the involved side. This is especially noteworthy if the patient has minimal vestibular symptoms, which may be the case with acoustic neuromas. The status of the function of the vestibular system is helpful to know in acoustic neuroma cases in regard to predicting how stable the patient may be after surgery. To elaborate, individuals with good preoperative vestibular function who undergo sectioning of the vestibular nerve may have a longer adjustment period postoperatively than those who have poor vestibular function before surgery. This is likely due to the fact that those with intact vestibular function preoperatively are likely to require more time for central vestibular compensation to occur following surgery; however, the extent and rate of this central compensation also may be influenced by patient age and other factors. Sharing this information with the patient prior to surgery can help the patient establish realistic expectations for postsurgical adjustment and recovery and result in a better outcome for the patient. Finally, central vestibular abnormalities may also be observed on eye tracking tests in some patients with acoustic neuromas if there is secondary brainstem involvement (i.e., compression effects) due to the locus or size of the peripheral lesion.

Medical Examination

The physical examination should include examination of the ears, nose, throat, and cranial nerves, as well as the patient’s balance and gait. The ENT exam is usually normal. The cranial nerve exam, however, will generally indicate a hearing loss on the side of the tumor which should be confirmed audiometrically. Extraocular movements are generally normal in patients with acoustic neuromas, and double vision or impairment of extraocular movement should signify another type of tumor, such as a meningioma. The fifth cranial nerve (or trigeminal nerve) is examined by light touch and pinprick over the three divisions of the face. Loss of sensation over part of the face is sign of a large tumor pressing on the trigeminal nerve. The corneal reflex may be absent on the affected side as well. Facial nerve function is usually intact. The presence of facial nerve weakness usually signifies facial nerve tumor rather than acoustic neuroma. A careful search for muscle fasciculations, especially around the upper and lower eyelids, should also be made. The facial nerve has sensory fibers as well as motor fibers. Loss of sensation to light touch around the posterior edge of the external auditory meatus (“Hitselberger’s sign”) is a sensitive early sign of acoustic neuroma (Hitselberger & House, 1966). Eighth nerve function involves balance as well as hearing. Nystagmus, a beating eye movement, is the cardinal sign of a balance system impairment. However, because balance deficits are usually well compensated, nystagmus is a rare finding. Brun’s nystagmus, which can occur in patients with cerebellar involvement, is an asymmetric nystagmus in which there is little or no spontaneous nystagmus in the primary position, but an asymmetry exists at the extremes of lateral gaze (Robinson, Zee, Hain, Holmes, & Rosenberg, 1984). This could be a sign of a very large tumor that is exerting pressure on the cerebellum. Gait ataxia or inability to walk arises from pressure on the cerebellum and brainstem. The lower cranial nerves—which are responsible for swallowing, the gag reflex, vocal fold function, and tongue function—are rarely involved.

Audiologic Management

Audiologic management of patients with acoustic neuromas depends to a great extent on the symptoms of the patient after surgery. This could include auditory and/or vestibular management. If hearing has been partially or totally lost after surgery, then appropriate audiologic steps should be pursued. This may include a hearing aid fitting and counseling. Also, auditory training may be useful and appropriate in some cases.

If the patient has balance difficulties after surgery, then vestibular rehabilitation approaches may be initiated. Vestibular exercises can be of value in helping to stabilize the patient. It is also useful to help the patient connect with others who have had this surgery for support. Patient groups across the country have been formed and provide valuable assistance in this regard.

Medical Management

The treatment options for acoustic neuroma include surgical removal, stereotactic radiation therapy, or serial observation (see Luxford, 1997). The choice of treatment depends on the size of the tumor, the age and general health of the patient, the patient’s hearing status, and (increasingly) the patient’s preference.

Surgical removal of an acoustic neuroma is usually performed as a team approach between a neurotologist and a neurosurgeon. The surgery can be done by one of three approaches: translabyrinthine, retrosigmoid (or suboccipital), or middle cranial fossa. The choice of approach depends on the size of the tumor, the desire to preserve hearing, and the surgeon’s preference.

The translabyrinthine approach is done through a postauricular incision. Following a complete mastoidectomy, the drilling is extended through the semicircular canals and the posterior petrous bone to access the CPA and the internal auditory canal. Hearing is always lost during this approach. The main virtue of this approach is that the facial nerve is identified early as it exits the internal auditory canal and the tumor can be separated easily from the facial nerve in this location, ensuring safety to the nerve. Tumors of almost any size and location can be removed through this approach.

Surgery carries certain risks in addition to facial paralysis and hearing loss. Patients will routinely experience unilateral loss of vestibular function, which may result in vertigo and nausea with head movement in the first 3 to 7 days postoperatively, and imbalance lasting for several weeks following surgery. As mentioned earlier, larger tumors have often already resulted in considerable loss of vestibular function before surgery and result in preoperative compensation. This mitigates the vertigo and imbalance to a large degree in many patients. Some patients, especially elderly patients, will require more time to recover and may need vestibular rehabilitation therapy. Most will regain their balance function to near-normal levels with time and can generally return to work in about a six-week time frame. However, some patients will continue to experience transient loss of balance with rapid head movement, but these patients tend to learn to compensate for this deficit without much modification of daily activities of life.

Additional Information

Acoustic neuromas constitute about 85% of tumors occurring in the CPA and about 90% of tumors involving the internal auditory canal (see Hain, 2019, for review). The differential diagnosis of lesions occurring in these locations includes meningiomas, epidermoid tumors, lipomas, schwannomas of other cranial nerves (facial, trigeminal), and arachnoid cysts. Other rare lesions may occur, including malignancies (primary or metastatic), chordomas, vascular tumors, or inflammatory conditions. Meningiomas and other benign tumors are treated in a similar fashion to acoustic neuromas (i.e., surgical excision with attempted preservation of adjacent neural structures). Smaller lesions can sometimes be simply observed if they are asymptomatic. Arachnoid cysts are generally treated conservatively unless they become very large and cause compressive symptoms. Malignant lesions are usually managed with the assistance of an oncologist and involve multiple modes of treatment, such as surgery, radiation, and/or chemotherapy. The selection of treatment is individualized according to tumor type, grade, stage, and patient status (Luxford, 1997).

Case 6–1: Acoustic Neuroma

History

A 44-year-old female presented with problems of imbalance, light-headedness, and vertigo. These symptoms reportedly were first experienced following a sailing trip 2 months prior to the current evaluation. The patient’s husband reported concerns regarding a decrease in his wife’s hearing sensitivity, and the patient reported some hypersensitivity to sound. She additionally reported severe headaches and unilateral left-sided tinnitus.

Audiology

An otoscopic check was unremarkable bilaterally. A comprehensive audiologic evaluation indicated essentially normal peripheral hearing sensitivity and normal word recognition scores bilaterally (Figure 6–1A). Tympanograms yielded normal pressure, volume, and compliance bilaterally, and speech recognition thresholds were in good agreement with puretone averages for both ears. A routine ABR test was completed with test results revealing normal absolute and interwave latencies for both ears. However, when a threshold ABR was obtained (Figure 6–1B) and compared to behavioral thresholds for the click stimulus, the right ear demonstrated good agreement between the behavioral and electrophysiologic thresholds, whereas the electrophysiologic threshold for the left ear was found to be significantly elevated (30 dB) above the behavioral threshold for the same ear, suggesting retrocochlear involvement. A VNG was also performed and was unremarkable.

Medical Examination

The otolaryngologic exam revealed an abnormal Romberg examination with the patient leaning significantly to the left. Given the patient’s reported symptoms and her audiologic test results, an MRI with contrast was performed to rule out retrocochlear involvement. However, test results revealed a small left-sided acoustic tumor (Figure 6–1C).

Impression

Small left-sided acoustic neuroma.

Audiologic Recommendations and Management

Continue to monitor hearing sensitivity.

Medical Recommendations and Management

Given the small size of the acoustic neuroma, the decision was made in consultation with the patient to continue to monitor for any progression of the tumor. The patient was to be seen for routine audiologic evaluations and MRIs to monitor for any changes in the tumor and to reevaluate treatment options in 9 months.

Stay updated, free articles. Join our Telegram channel

Join