Essentials of Diagnosis
- May affect patients of all ages
- For patients who have unilateral hearing loss
- Weber tuning fork test lateralizes to the unaffected side
- Rinne tuning fork test demonstrates air conduction greater than bone conduction
- Pure-tone thresholds result in equally diminished air and bone conduction
- Speech discrimination testing less than 90% correct.
General Considerations
Hearing loss is extremely common and has a wide spectrum ranging from a nearly undetectable degree of disability to a profound loss of ability to function in society. Nearly 10% of the adult population has some hearing loss. Often, this impairment presents early in life. One to three of every 1000 newborn in the United States is completely deaf, and more than 3 million children have hearing loss. However, hearing loss can present at any age. Between 30% and 35% of individuals over the age of 65 have a hearing loss sufficient to require a hearing aid. Forty percent of people over the age of 75 have hearing loss.
Hearing loss can result from disorders of the auricle, external auditory canal, middle ear, inner ear, or central auditory pathways. In general, lesions in the auricle, external auditory canal, or middle ear cause conductive hearing loss. The focus of this chapter is sensorineural hearing loss that tends to result from lesions in the inner ear or eighth nerve. See Table 52–1 for a list of the common causes of hearing loss.
| Category | Example |
|---|---|
| Developmental and hereditary | |
| Syndromic | Alport syndrome, Usher syndrome |
| Nonsyndromic | Large vestibular aqueduct syndrome |
| Infectious | Otitis media, CMV, syphilis, labyrinthitis |
| Pharmacologic toxicity | Aminoglycosides, loop diuretics, antimalarials, salicylates |
| Trauma | Head injury, noise-induced, barotraumas, irradiation |
| Neurologic disorders | Multiple sclerosis |
| Vascular and hematologic disorders | Migraine, cryoglobinemia, sickle cell, blood dyscrasia |
| Immune disorders | Polyarteritis nodosa, HIV |
| Bone disorders | Otosclerosis, Paget disease |
| Neoplasms | Vestibular Schwannoma, meningioma |
| Unknown etiology | Presbycusis, Meniere disease |
Classification
Sensorineural hearing loss may result from damage to the hair cells caused by intense noise, viral infections, fractures of the temporal bone, meningitis, cochlear otosclerosis, Meniere disease, and aging. The following drugs can also produce sensorineural hearing loss: ototoxic drugs (eg, salicylates, quinine, and the synthetic analogs of quinine), aminoglycoside antibiotics, loop diuretics (eg, furosemide and ethacrynic acid), and cancer chemotherapeutic agents (eg, cisplatin).
Presbycusis, age-associated hearing loss, is the most common cause of hearing loss in adults. Initially, it is characterized by symmetric, high-frequency hearing loss that eventually progresses to involve all frequencies. More important, the hearing loss is associated with a significant loss in clarity.
Congenital malformations of the inner ear cause hearing loss in some adults. Genetic predisposition alone or in concert with environmental influences may also be responsible.
Neural hearing loss is due mainly to cerebellopontine angle tumors such as vestibular schwannomas (acoustic neuromas) or meningiomas; it may also result from any neoplastic, vascular, demyelinating (eg, multiple sclerosis), infectious, or degenerative disease, or trauma affecting the central auditory pathways.
Human immunodeficiency virus (HIV) infection leads to both peripheral and central auditory system pathology and is associated with sensorineural hearing impairment.
A person can have both conductive and sensory hearing loss, which is termed mixed hearing loss. Mixed hearing losses are due to pathology that can affect the middle and inner ear simultaneously; causes include otosclerosis involving the ossicles and the cochlea, transverse and longitudinal temporal bone fractures, head trauma, chronic otitis media, cholesteatoma, and middle ear tumors. Some inner ear malformations can also be associated with mixed hearing loss. These include large vestibular aqueduct, lateral semicircular canal dysplasia, superior canal dehiscence, and a bulbous lateral end of the internal auditory canal (IAC); the latter is associated with the absence of the bony partition between the basal turn of the cochlea and the IAC (as seen in the stapes gusher syndrome).
Etiology
The predominant etiology of hearing impairment in children has evolved with advances in medical knowledge and therapeutics. Historically, infectious disorders such as otitis media, maternal rubella infections, cytomegalovirus (CMV), and bacterial meningitis as well as environmental factors such as intrauterine teratogenic exposure or ototoxic insult were the dominant causes of congenital and acquired hearing losses. The introduction of antibiotics and vaccines, along with improved knowledge and enhanced awareness about teratogens, has led to a decline in hearing loss resulting from infections and environmental agents.
Currently, more than half of childhood hearing impairment is thought to be hereditary; hereditary hearing impairment (HHI) can also manifest later in life. HHI may be classified as either nonsyndromic hearing loss, in which hearing loss is the only clinical abnormality, or syndromic hearing loss, in which hearing loss is associated with anomalies in other organ systems.
Pathogenesis
Hearing occurs by air conduction and bone conduction. In air conduction, sound waves reach the ear by propagating in the air, entering the external auditory canal, and setting the tympanic membrane in motion; the movement of the tympanic membrane, in turn, moves the malleus, incus, and stapes of the middle ear. The structures of the middle ear serve as an impedance-matching mechanism, improving the efficiency of energy transfer from the air to the fluid-filled inner ear. Hearing by bone conduction occurs when the sound source, in contact with the head, vibrates the bones of the skull; this vibration produces a traveling wave in the basilar membrane of the cochlea.
Cochlear neurons send fibers bilaterally to a network of auditory nuclei in the midbrain, and impulses are transmitted through the medial geniculate thalamic nuclei to the auditory cortex in the superior temporal gyri. At low frequencies, individual auditory nerve fibers can respond more or less synchronously with the stimulating tone. At higher frequencies, phase locking occurs so that neurons alternate in response to particular phases of the sound wave cycle. Three things encode the intensity of sound: (1) the amount of neural activity in individual neurons, (2) the number of neurons that are active, and (3) the specific neurons that are activated.
Nearly two thirds of hereditary hearing impairments are nonsyndromic and the remaining one third is syndromic. Between 70% and 80% of nonsyndromic HHI is inherited in an autosomal recessive manner; another 15–20% is autosomal dominant. Less than 5% is X-linked or maternally inherited via the mitochondria.
Extensive progress has been made in the identification of genes responsible for syndromic and nonsyndromic HHI. Over 110 loci harboring genes for nonsyndromic HHI have been mapped with an equal number of dominant and recessive modes of inheritance; of these, 51 different genes have been cloned. In general, the hearing loss associated with dominant genes has its onset in adolescence or adulthood and varies in severity (mimicking presbyacusis), whereas the hearing loss associated with recessive inheritance is congenital and profound. Among the genes associated with human deafness, GJB2 encoding for connexin 26 is significant for being associated with nearly 20% childhood deafness. Furthermore, two frame-shift mutations, 35delG and 167delT, account for more than 50% of the cases making population screening feasible. The 167delT mutation is primarily prevalent in the Ashkenazi Jews where it is predicted that 1:1765 individuals will be homozygous and affected. The hearing loss associated with GJB2 mutations can be variable but is generally severe to profound at birth. In addition, the hearing loss can be variable among the members of the same family, suggesting that other genes likely influence the auditory phenotype.
