28 Sudden Sensorineural Hearing Loss

Sudden sensorineural hearing loss (SSNHL) is a syndrome and not a diagnosis. Most commonly, the syndrome has been referred to as idiopathic sudden hearing loss or sudden deafness.1 The definition of sudden hearing loss is widely accepted as a sensorineural hearing loss (SNHL) of 30 dB or more, over at least three contiguous audiometric frequencies, that develops over 3 days or less and can be either abrupt or rapidly progressive. Hearing loss must either be newly occurring in an ear with previously normal hearing or an incremental deterioration in an ear with preexisting loss.

National surveys have estimated the incidence of sudden SSNHL at between 5 and 30 cases per 100,000 per year.^2–4 However, a study from Germany has shown an incidence as high as 160 cases per 100,000 per year.⁵ Some cases resolve spontaneously and medical attention is not pursued,⁶ therefore, the true incidence rate may be higher.

Despite a thorough search for an etiology, most cases remain idiopathic. Detailed investigation will show a specific cause in approximately 10% of patients. The term “idiopathic SSNHL” is often used to describe the remainder of patients. There has been, and continues to be, considerable debate regarding the pathogenesis of the disease in these patients. Three main theories exist to explain idiopathic SSNHL: viral infection, vascular compromise, and intracochlear membrane rupture. There is additional evidence to support a fourth explanation, immune inner ear disease. Each theory may explain a subset of SSNHL.⁷

The management of SSNHL should be focused on excluding known causes of the syndrome, especially conditions that require treatment.

Clinical Manifestations

Population studies of SSNHL show a wide age distribution, with an average of 50 to 60 years but there are cases reported in young (0 to 18 years) and elderly (> 60 years) population. Gender does not seem to be a risk factor. An equal distribution between left and right ears is expected. Bilateral involvement is rare (approximately 1 to 2% of cases), and simultaneous bilateral involvement is very rare.⁸

The most common presentation is a patient noticing a unilateral hearing loss on awakening from sleep. Others notice a sudden, stable hearing loss or a rapidly progressive loss. Occasionally, patients note a fluctuating hearing loss, but most patients have a stable loss.

A sensation of aural fullness in the affected ear is common and frequently is the only complaint. Tinnitus is present in the ear to a variable degree, (approximately 70%), and the hearing loss sometimes is preceded by the onset of tinnitus. It usually subsides within 1 month but may persist and even outlast the hearing loss. It usually has a roaring quality.

Vertigo or disequilibrium is present to a variable degree in approximately 40% of patients. Approximately 10% of patients have incapacitating vertigo that lasts for 4 to 7 days. Lesser degrees of vertigo may then persist for up to 6 to 7 weeks. Nausea and vomiting are usually associated with severe vertigo.

Headache is occasionally encountered, and symptoms of viral upper respiratory tract infections occur in as many as 25% of the patients in some series. Fever, usually of mild degree, may be present. Generally, however, the patient feels perfectly well except for the loss of hearing and tinnitus.

Usually the otoscopic examination is normal, but serous otitis media is occasionally observed and may add a conductive component to the loss of hearing.

Etiology

Sudden hearing loss can be subdivided into categories of defined causes and idiopathic SSNHL. Defined causes of sudden hearing loss are varied and uncommon (Table 28.1). Most cases of SSNHL are idiopathic.

Infectious Causes

Viral Infection

The precise cause of SSNHL has not been identified, but several pathophysiologic mechanisms have been proposed. Viral neuritis or cochleitis has long thought to be the most common cause of SSNHL, although much of the evidence for this is circumstantial. SNHL can complicate clinically evident infections with mumps, measles, herpes zoster, and infectious mononucleosis and with congenital rubella and cytomegalovirus. From studies of patients with SSNHL, this trail of evidence can be traced from reports of showing high prevalence of recent viral illness,^9,10 evidence of recent viral seroconversion, and temporal bone histopathology. The weakest of these links is the associated history of a recent viral illness. Of patients presenting with SSNHL, 17 to 33% report a viral-like upper respiratory infection within 1 month before the onset of their hearing loss.^9,11 With the possible exception of mumps parotitis and herpes zoster infections, however, the clinical diagnosis of viral infection is unreliable. Many viruses have been postulated as possible causes of SSNHL, but serological, epidemiological, and histopathological data are not conclusive.¹⁰ However, evidence for viral infections that affect the cochlea will be considered first.

Table 28.1 Differential Diagnosis in Sudden Hearing Loss

There are three routes of access of viral particles to the inner ear. Probably the most common is a viremia in the early phase of a viral illness in which virions are deposited within the membranous cochlea. In viral meningoencephalitis, for example, as a complication of measles, the virus may gain access to the inner ear through the perilymphatic space from the subarachnoid space via the cochlear aqueduct. A third possible route, proposed by Lindsay in 1959,12 is by direct extension from the middle ear to the inner ear in nonsuppurative otitis media during an upper respiratory infection. Once the virus particles have gained access to the membranous cochlea, replication of the virus leads to rapid pathophysiologic changes that are frequently reversible but that may be extremely destructive and result in permanent loss of hearing. From what is known of the development of viral lesions in other tissues, we may suppose that similar changes occur in the inner ear. Initially the vascular endothelium is invaded, and inclusion bodies may be seen in the endothelial cells in some viral diseases. The capillary endothelium is often swollen and may proliferate. These changes narrow the lumen of the capillaries and may reduce blood flow. Nevertheless, there is generally an overall hyperemia at the site of inflammation, and total effective blood flow may not be altered.¹³

Other Infections

Bacterial meningitis is a well-recognized and common cause of acquired severe to profound SNHL, affecting up to 35% of patients.¹⁴

It has been estimated that the incidence of syphilis in patients with SSNHL is 2% or less. Syphilitic hearing loss may manifest at any stage of the disease, and may be associated with other manifestations of syphilis, with vestibular symptoms, or alone. It may manifest with unilateral or bilateral SSNHL.

Lyme disease is a well-established etiology of acute facial paralysis, and it would not be unreasonable to assume that it could also cause SNHL. Hearing loss has not been associated with Lyme disease, however. The literature contains several descriptions of associations between positive Lyme titers and acute or chronic SNHL, but a causal relationship seems doubtful.

At autopsy, 88% of human immunodeficiency virus-positive patients have evidence of central nervous system involvement, and approximately 10% of patients with acquired immune deficiency syndrome (AIDS) present because of neurologic symptoms. SSNHL is not a common manifestation of AIDS, but it has been well documented in the literature.¹⁵

Vascular Compromise

The role of vascular occlusion versus viral infection in these idiopathic cases has been the subject of extensive debate over the years.

The cochlea derives its blood supply from the single terminal branch of the posterior cerebral circulation, labyrinthine artery, with no collateral vasculature. Cochlear function is exquisitely sensitive to change in blood supply. Thus, vascular compromise of the cochlea caused by thrombosis, embolus, reduced blood flow, or vasospasm has been thought by many authors to be an attractive hypothetical etiology for SSNHL.¹⁶ But similar to viral etiologies, a few cases of SNHL are clearly a result of vascular occlusion, but most cases remain idiopathic. Temporal bone studies have not found evidence of vascular occlusion in cases of idiopathic SSNHL¹³ although this line of investigation has the drawback that it typically occurs many years after onset of the hearing loss.

It has been believed that patients with diabetes have a higher incidence of idiopathic SSNHL than nondiabetics because of higher incidence of other acute cranial neuropathies and diffuse vascular abnormality in diabetes.¹⁷ Histologic studies of human temporal bones from patients with diabetes mellitus have not consistently found any abnormal alterations. And there was no significant difference in audiologic pattern between diabetic and nondiabetic patients with idiopathic SSNHL. In the absence of the ability to detect vascular occlusion acutely with high-resolution imaging at the time of hearing loss, a vascular etiology will continue to be difficult to establish conclusively for either diabetic or nondiabetic patients.

Round or oval window fistulas can occur congenitally, after stapedectomy, or after barotraumas. Some investigators18 theorize that these fistulas can occur after heavy lifting or straining or even spontaneously. Patients with such fistulas can have sudden or fluctuating SNHL and varying degrees of vestibular symptoms. There is no reliable test for the presence of such a fistula, and even surgical exploration is subject to error. Intracochlear membrane ruptures and fistulas have been documented pathologically in patients with endolymphatic hydrops.^11,19 Such breaks have been proposed to be an etiology of SNHL.

Acoustic Neuroma

The most common neoplastic disease associated with SSNHL is acoustic neuroma (vestibular schwannoma), which usually presents with unilateral tinnitus and progressive SNHL, although it can present with sudden loss in few cases. The SSNHL associated with acoustic neuroma can recover spontaneously.

The prevalence of acoustic neuroma among patients with SSNHL is less clear. Estimates range from 0.8 to 4%.²⁰ Several reports^11,21 have shown that approximately 10 to 15% of patients with acoustic neuromas will present with SSNHL as their initial symptom. There are no clear presenting symptoms that suggest that SSNHL may be a result of an acoustic neuroma.

Responsiveness of the hearing loss to treatment with steroids is an unreliable indicator that a retrocochlear lesion is not present. There have been many reported cases of steroid-responsive SNHL and SNHL with spontaneous recovery, which have been found to be caused by acoustic neuroma.²⁰

Autoimmunity

SNHL induced by an autoimmune process has gained greater acceptance since the concept was introduced in 1979 by McCabe.²² Subsequently, Harris in 1983 demonstrated experimentally that the inner ear is capable of mounting an immune response to antigenic challenge, and that the antibodies were indeed produced in perilymph rather than being transported from the middle ear or cerebrospinal fluid.²³

The finding that many patients with SNHL seem to benefit from glucocorticoid therapy,²⁴ and the finding of cross-reacting circulating antibodies in many patients with sudden and rapidly progressive SNHL suggest that at least a subset of SNHL cases are caused by inner ear autoimmunity.²⁵

The association of hearing loss in Cogan syndrome, systemic lupus erythematous, polyarteritisnodosa, and other autoimmune rheumatologic disorders has been well-documented.²⁶

Pharmacologic Toxicity

Many drugs have been associated with ototoxicity, but the hearing loss is usually slowly evolving. Nonetheless, there are case reports of SSNHL associated with penicillin, estrogens, phosphodiesterase-5 inhibitors, pegylated interferon and ribavirin, aminoglycosides, chemotherapeutic agents, and intravenous cocaine use.²⁷

Developmental Abnormalities

Large vestibular aqueduct syndrome²⁸ and enlarged internal auditory canal²⁹ are associated with SNHL, which frequently occurs in a stepwise fashion associated with minor head trauma. It seems plausible that other, as yet undefined, developmental abnormalities may predispose individuals to SSNHL, either spontaneously or after minor head trauma.

Endolymphatic Hydrops

Endolymphatic hydrops, an excess of endolymph in the scala media of the labyrinth, is a pathological finding that is commonly associated with Meniere disease-a clinical diagnosis, which classically presents with sudden unilateral hearing loss, tinnitus, and vertigo.³⁰ This probably constitutes only 5% of all patients with SSNHL. In a review of 1270 patients with Meniere disease, Merchant found that only 4.4% had initially been seen with SSNHL.³¹

Genetic Disorders

Several genetic diseases have been linked with SSNHL, including mutations of GJB2, the presence of a widened vestibular aqueduct in Pandred syndrome, renal tubular acidosis, brachio-oto-renal syndrome, and Fabry disease.³² Mutations in the mitochondrial 12S ribosomal RNA gene are associated with rapidly progressive SSNHL secondary to an increased susceptibility to aminoglycoside ototoxicity.³³ Capaccio et al in 2009 studied prothrombotic gene mutations in patients with SSNHL and cardiovascular thrombotic disease. The association between inherited and acquired prothrombotic factors in patients with SSNHL and thrombotic diseases in other sites suggests that multifactorial mechanism may underlie microvascular cochlear impairement.34

Miscellaneous Causes

SNHL of any degree can occur after closed or open head injury. The mechanism of injury in such patients has been shown pathologically to vary from mild loss to outer hair cells or cochlear membrane breaks to fracture across the labyrinth or intralabyrinthine hemorrhage.^1,35

Pseudohypacusis frequently manifests as a sudden loss. In most patients, malingering is readily apparent after initial audiologic studies. Especially pseudohypacusis seems to be the leading cause of sudden hearing loss in childhood and should be considered in every child with an abrupt onset of hearing loss.³⁶

In addition to noise-induced deafness, which can be divided into a slowly progressive deafness caused by prolonged intense noise exposure and acute trauma caused by a single exposure to very intense noise exposure and acute acoustic trauma caused by a single exposure to very intense sound.

Pathology

The pathology of sudden deafness presented by Schuknecht et al (1973) is similar to the findings in viral endolymphatic labyrinthitis and, of course, represents the final stage of these processes in patients in whom spontaneous recovery did not occur. Therefore, their findings can be considered the end result of the most virulent infections and do not represent the outcome in all cases.³⁵

The organ of Corti is often missing in the basal turns, and individual hair cells tend to be missing at higher turns. Ganglion cell populations are decreased at the basal turn are more normal toward the apex. The stria vascularis tends to be strophic. The tectorial membrane is often atrophic, rolled up, and ensheathed in a syncytium of cells on the limbus. Reissner membrane may be collapsed and adherent to the basilar membrane. The saccule is often involved, but the utricle and semicircular canals usually escape severe damage.^13,37

Clinical Assessment

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