Sensorineural Hearing Loss

Anatomy and Pathophysiology:

Sensorineural hearing loss (SNHL) refers to hearing loss that is cochlear or retrocochlear in origin. For a review of ear anatomy, see Anatomy of the Ear. Briefly, sound waves traveling through air reach the inner ear when the stapes footplate contacts the oval window membrane. The sound waves then travel through the fluid-filled cochlea, causing mechanical deflection of the cochlear hair cells. This movement of the hair cells results in opening of voltage-gated ion channels and transduction of the stimulus into an electrical potential that is propagated through the auditory nerve to reach the midbrain. SNHL occurs when the cause of the hearing loss occurs at or after the sound waves have reached the inner ear. There are many etiologies of SNHL, including those that involve cochlear malformations, trauma to the cochlear hair cells or auditory nerve, and lesions of the central nervous system.

Congenital SNHL

Among children with SNHL, hereditary congenital disorders are common. Most of these are isolated disorders that exhibit an autosomal recessive inheritance pattern. Numerous genes have been identified that are thought to be associated with hereditary hearing impairment (HHI). One such gene, GJB2 (which codes for the protein connexin 26), is estimated to be mutated in 20% of children with congenital deafness. Because two specific mutations of this gene account for over half of the cases, connexin screening can be employed for families of children with HHI.

One-third of children with HHI have syndromic hearing loss, or SNHL that occurs in the context of a syndrome affecting multiple organs. The most common syndromes that involve hearing loss are summarized below.

Syndrome	Inheritance Pattern	Features
Usher syndrome	Autosomal recessive	SNHL, retinal anomalies. Accounts for 3-6% of congenital deafness and 50% of deaf-blindness. Hearing loss depends on clinical variant and may be moderate to profound, congenital or progressive. Some patients may have vestibular dysfunction as well.
Pendred syndrome	Autosomal recessive	SNHL, goiter, temporal bone anomalies. Accounts for 10% of hereditary deafness. Hearing loss is usually severe to profound. Patients may have vestibular aqueduct dilation and cochlear hypoplasia.
Waardenburg syndrome	Autosomal dominant	SNHL, white forelock of hair, and eye anomalies. Hearing loss is usually profound and bilateral.Temporal bone is usually normal on imaging.
Alport syndrome	X-linked	SNHL, nephritis (causing hematuria), eye anomalies. Type IV collagen disorder. Hearing loss is usually symmetric and progressive, involving high frequencies first.
Jervell-Lange-Nielsen syndrome	Autosomal recessive	Congenital deafness, prolonged QT interval, syncopal attacks. Hearing loss is usually bilateral, severe to profound.
Branchio-oto-renal syndrome	Autosomal dominant	External, middle, or inner ear abnormalities; branchial anomalies; and renal agenesis or dysplasia. Affects 2% of profoundly deaf children; hearing loss is mixed in 50%.

Epidemiology:

The incidence of congenital hearing impairment is 1 in every 1000 live births, making it the most common birth defect in the United States. Over half of these cases are due to a genetic cause. Up to half of all cases of autosomal recessive nonsyndromic hearing impairment are due to mutations in GJB2 (connexin 26).

Presbycusis affects an estimated 30% of individuals between the ages of 65-75, and up to half of people over age 75. There is no sex or race predilection.

Natural History:

The natural history of SNHL depends on the etiology. Certain causes of SNHL, such as noise-induced hearing loss and presbycusis, are progressive in nature and affect higher frequencies first before involving all frequencies. Congenital causes of SNHL are usually stable, although some may progress over time. Sudden sensorineural hearing loss is characterized by acute onset (over less than 3 days) and gradual recovery over days to weeks in some patients. Other patients may experience permanent, stable hearing loss.

Presentation:

Most congenital forms of SNHL present early in infancy or childhood; many may be detected on newborn hearing screening, which is standard in hospitals in the United States. Syndromic cases are typically associated with anomalies of other organ systems, which may be the presenting symptom. Occasionally, some genetic causes of SNHL may present with progressive hearing loss that is not diagnosed until later in childhood or adolescence. Symptoms of childhood hearing loss may include inattentiveness, failure to respond to conversation, or poor school performance for older children.

The presentation of presbycusis varies, but many patients report increased difficulty understanding speech, particularly in the presence of significant background noise. Onset is typically gradual. The presentation of noise-induced hearing loss is similar.

Patients with other causes of SNHL, such as infection, trauma, or Meniere’s disease typically have additional presenting symptoms, rather than isolated hearing loss.

Differential Diagnosis of Sensorineural Hearing Loss:

Evaluation:

History

The nature of symptom onset and progression is useful for identifying the etiology of the hearing loss. A sudden onset is more suggestive of an infectious or traumatic cause, whereas conditions like presbycusis and noise-induced hearing loss have a much more gradual onset. The patient history should also elucidate whether one or both ears are involved, and whether associated symptoms are present. Important associated symptoms include vestibular dysfunction (such as vertigo or imbalance), tinnitus, otorrhea, facial nerve dysfunction, and otalgia or headache. If the patient is a child, evidence of syndromic hearing loss may include history of cardiac, ophthalmologic, renal, thyroid, or craniofacial abnormalities. Family history of hearing loss or other otologic dysfunction should be elicited, particularly if there is suspicion for genetic SNHL. History of noise exposure, exposure to ototoxic substances, otologic or temporal bone trauma, and prior otologic surgery or disease should also be determined.

Physical Examination

The physical examination, including the otoscopic examination, may be normal in many patients with SNHL. A vestibular examination (detailed in Vestibular Disorders) should be performed, particularly if the patient has symptoms of vestibular dysfunction. A cranial nerve examination should also be done. In pediatric patients, the exam should include evaluation of other organ systems to assess for a syndromic cause.

The use of tuning fork testing can confirm a subjective complaint of hearing loss and determine whether the hearing loss is symmetric. The method for performing the Weber and Rinne tests is discussed in the previous chapter, Conductive Hearing Loss. Patients with a pure sensorineural hearing loss will have a normal (positive) Rinne test, with air conduction greater than bone conduction, while patients with a mixed hearing loss may have a negative Rinne test (bone conduction greater than air conduction). In unilateral or asymmetric sensorineural hearing loss, the Weber test lateralizes to the side of the unaffected or better hearing ear, respectively.

Audiologic Studies

Audiometric evaluation is essential for characterization of the degree and nature of hearing loss. A full assessment should include pure tone audiometry (air- and bone-conduction), speech audiometry measures, and tympanometry. Other tests, such as auditory brainstem response (ABR) testing and otoacoustic emissions (OAE), may be useful in certain cases for differentiating between cochlear and retrocochlear causes of hearing loss. Details regarding audiologic studies are given in Audiology.

Imaging Studies

In the adult patient, suspicion for a retrocochlear pathology (such as a vestibular schwannoma) is an indication for radiographic imaging. Magnetic resonance imaging (MRI) is the study of choice in these cases. A computed tomography (CT) scan of the temporal bone is generally ordered in the setting of acute trauma. In cases of congenital hearing impairment, imaging may be useful for identifying anatomic malformations of the cochlea, as well as the external and middle ear structures. Progressive hearing loss, suspicion for neurofibromatosis type 2, and evidence of retrocochlear pathology on audiometric testing are other indications for radiologic imaging in the pediatric patient.

Laboratory Studies

Patients with nonsyndromic congenital hearing impairment should be tested for GJB2 (connexin) mutation. Genetic tests for some other hereditary causes of SNHL are also available. Other tests for thyroid, cardiac, renal, and ocular function may be performed if there is suspicion for a syndromic cause.

Treatment

A key step in management of congenital hearing impairment is early diagnosis and intervention to facilitate language development. Syndromic conditions may require additional monitoring and treatment, such as beta blocker pharmacotherapy to address prolonged QT interval in Jervell-Lange-Nielsen syndrome. Genetic testing and counseling may be provided by a specialist if desired by the family.

Hearing aid amplification is routinely used for many patients with SNHL, particularly those with moderate or severe hearing loss . However, patients with profound hearing loss may not derive adequate benefit from hearing aids. In some of these cases, cochlear implantation may be a viable option. Details regarding cochlear implantation, including current indication guidelines, are given in Cochlear Implantation. Briefly, these devices consist of a microphone that receives auditory input, a processor that converts the mechanical sound waves into an electrical signal, and an electrode array that transmits the electrical signal directly to the auditory nerve. In patients without functioning auditory nerves (such as due to bilateral trauma or neoplasm), a brainstem auditory implant may be placed instead.

For patients with idiopathic sudden sensorineural hearing loss and no evidence of retrocochlear pathology on MRI, the American Academy of Otolaryngology-Head and Neck Surgery recommends oral or intratympanic corticosteroid therapy as an initial treatment. Additionally, hyperbaric oxygen therapy may also be considered as an option within 3 months of diagnosis. Although a viral etiology has been proposed as the cause of sudden sensorineural hearing loss, studies have not shown antiviral therapy to be effective in improving outcomes.

Complications, Prognosis and Follow-Up:

In cases of congenital hearing impairment, a schedule of frequent audiometric evaluation is recommended to monitor for disease progression and ensure that amplification is sufficient for language development. Similarly, patients at risk for (or with early signs of) noise-induced hearing loss should also undergo serial audiometric testing.

Most causes of sensorineural hearing loss result in irreversible damage; however, many patients can achieve satisfactory communicative function with the use of hearing aids. Patients who are deemed to be appropriate candidates for cochlear implantation typically have good outcomes. Up to 60% of adult cochlear implant recipients are able to converse using a normal telephone, and the vast majority report an overall improvement in quality of life. Children undergoing cochlear implantation have a more variable outcome, depending on the age at which the implant was placed. The best outcomes are achieved in children who are implanted before the age of 4.

Patients with sudden sensorineural hearing loss have an estimated 60% chance of recovering normal levels of hearing. The risk of permanent hearing loss increases in older patients and in those who have had symptoms lasting for greater than one month. A follow-up audiogram should be obtained approximately 6 months after diagnosis. In patients with persistent hearing loss after 6 months (with or without initial corticosteroid therapy), intratympanic steroids may be given as salvage treatment. Patients with temporary hearing loss resulting from noise exposure typically recover within 1-2 days.