Hearing Impairment 

Hearing loss is more prevalent than diabetes mellitus, myelomeningocele, all pediatric cancers, and numerous other medical conditions.^[1] However, medical professionals learn little about hearing impairment, about how to advise parents of children who are deaf or hard of hearing, or about the special considerations needed in the care of children with hearing loss.

In the past decade, recommendations for universal neonatal hearing screening resulted in numerous articles regarding the tests, the efficacy of testing, the role of the audiologist in amplification, and the importance of early intervention programs.^{[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]} The role of the primary care physician cannot be overemphasized. In many instances, the otolaryngologist develops a long-term relationship with patients and their families, caring for the patients through their spectrum of development, especially if the otolaryngologist is part of a cochlear implant program. In addition to the surgeon, most cochlear implant teams include audiologists, speech therapists, and, sometimes, social workers. These teams can be very helpful to deaf children, their families, and their primary care physicians.

Pediatricians play a crucial role in providing referrals to audiologists, otolaryngologists, and special programs. To do so, they must understand the nature of hearing loss and the equipment that can improve auditory reception, the linguistic and social development of children who have hearing impairment, and the educational and linguistic options available to children who are deaf or hard of hearing.

The goals must always be to integrate the child into the family and into society and to enable the growth and development of a healthy, confident child who is deaf or hard of hearing. To meet these goals, clinicians should use whichever communication strategy and equipment that is best suited for the individual child and his or her family.

Relevant anatomy and physiology

Sound waves arrive to the auricle and are channeled through the external auditory canal to the tympanic membrane. When they strike the tympanic membrane, the waves cause it to vibrate, setting off a chain of vibrations along the ossicles (malleus, incus, and stapes) to the membrane of the oval window at the entrance to the cochlea. This process amplifies environmental sound by approximately 20-fold.

The cochlea is the end organ of hearing and is shaped like a snail shell with 2.5 turns. Inside, 2 membranes longitudinally divide the cochlea into 3 sections: the scala tympani, the scala vestibuli, and the scala media. All 3 are filled with fluids of differing ion concentrations (similar to intracellular and extracellular constituents).

Along one of the membranes in the scala media, or cochlear duct, lie the internal and external hair cells. Movement of the stapes on the oval window creates a wave or vibration in the perilymph fluid of the cochlea. This fluid movement, which opens ion channels in the hair cells, displaces the hair cells, triggering an action potential and causing a nerve in the cochlea to fire to the brain.

Thousands of nerves representing more than 20,000 frequencies are located along the length of the cochlea; these nerves account for the hearing range. The microscopic nerves culminate in the cochlear portion of the eighth cranial nerve. The location of the vibration in the cochlea correlates with the frequency of the original pitch. Low-frequency sounds are near the apex, and high-frequency sounds are near the base.

Types of hearing loss

Conductive hearing loss (CHL) results from anything that decreases the transmission of sound from the outside world to the cochlea. Causes include abnormal formation of the auricle or helix, impaction of cerumen in the ear canal, effusions in the middle ear, or dysfunction or fixation of the ossicular chain. Otosclerosis is one of the most common examples.

An important cause of CHL is a cholesteatoma, a locally destructive but benign growth. Other neoplasms can affect the middle ear as well. Examples include glomus tympanicum or glomus jugulare, schwannomas of the facial nerve, and hemangiomas. Dehiscence of the roof of the middle ear (tegmen mastoideum), such as is caused by an encephalocele, can result in CHL. In CHL, sounds perceived by the brain are diminished but are generally not distorted.

Sensorineural hearing loss (SNHL) may result from disruptions in transmission after the cochlea. These disruptions may be a result of hair cell destruction in the cochlea or damage to the eighth cranial nerve. Sounds perceived by the brain are both diminished and distorted. The degree of distortion is independent of the degree of hearing loss (eg, mild hearing loss but very poor speech discrimination is possible).

Auditory dyssynchrony should be considered in the setting of no auditory brainstem response (ABR), no middle-ear muscle response, normal otoacoustic emissions, or normal cochlear microphonics.

Mixed hearing loss has components of both CHL and SNHL.

Categories of hearing loss

Regardless of the type, the American National Standards Institute defines hearing loss in terms of decibels (dB) lost, as follows:

• Slight hearing loss - 16-25 dB
• Mild hearing loss - 26-40 dB
• Moderate hearing loss - 41-55 dB
• Severe hearing loss - 71-90 dB
• Profound - More than 90 dB

Frequency

United States

Hearing loss occurs in approximately 5-10 per 1000 children in the United States. Roughly 1-3 in 1000 children is born with profound hearing loss, and 3-5 per 1000 are born with mild-to-moderate hearing loss that may affect language acquisition unless hearing, language, or both are aided.^[6] The prevalence of hearing loss requiring intervention among graduates from neonatal intensive care units (NICUs) is 1-4%. Acquired hearing loss in children may add another 10-20% to these numbers.^[14]

The prevalence of hearing loss in adolescents aged 12-19 years has increased in the United States compared with the previous decade.^[15] This increase was approximately one third greater from 2005-2006 than from 1988-1994. Interestingly, significant hearing loss (≥25 dB) was particularly increased, such that approximately 1 in 20 adolescents has this type of hearing loss. Noise-induced hearing loss contributes substantially to the increased incidence of hearing loss in adolescents.

Data from the United States Census show that almost 3% of the population in the workforce reports having some hearing loss, including CHL, SNHL, or mixed loss.

International

SNHL occurs in 9-27 per 1000 children worldwide.

Sex

No sex predilection is known. Some hereditary causes of deafness or acquired deafness may occur more frequently in one sex than the other. However, the overall prevalence of deafness is equal in male and female individuals.

Age

Most hearing loss in children is congenital or acquired perinatally.^[16] However, hearing loss may occur at any age. Approximately 10-20% of all cases of deafness are acquired postnatally, although some genetic causes of deafness result in hearing loss that begins during childhood or adolescence or is slowly progressive and therefore diagnosed in childhood or adolescence.