Hearing loss is not uncommon. At least 1 neonate per 1,000 births has this and the incidence increases with age as many genetic causes have delayed onset and damage to hearing accumulates.
We have not been very effective in the timely recognition of hearing loss. For example, Copian reported that 4.6% of children referred to him for assessment of developmental delay had permanent hearing loss. He was often first to make this diagnosis, the average age of recognition was 24 months, and the usual delay between parents' first suspicion and physicians' referral was approximately 1 year.1 A National Institutes of Health Consensus Statement indicated the average age for first diagnosis of congenital hearing loss is approximately 3 years.2 Based on mis and evidence that treatment before age 6 months can virtually eliminate speech, communication, and cognitive delays,1"3 this Consensus Statement recommended screening for all infants before 3 months of age and before discharge from the nursery when possible. A 1994 American Academy of Pediatrics (AAP) recommendation agrees.4
However, the problem of timely recognition of deafness is not yet solved. As presented in this issue, the equipment and personnel needed to test hearing in a neonate are specialized. So many hospitals and regions, especially in rural and innercity areas, cannot yet offer routine hearing screens at birth. Add to this that 20% to 30% of impairment is acquired during early chñdhood2 and you can see the challenges we face in finding and treating all these children.
So, much of this responsibility will continue to fall on our alertness to the possibility of hearing loss during each wellchild visit. In recognition of this, the AAP and others have published risk factors for hearing loss and recommend screening when these are found. This list of indications includes diverse historical and physical examination clues at various ages. It is thus somewhat complex (see Tables 1 and 2 of the Applebaum article) and difficult to memorize. And it is hard to have such indications pop into mind spontaneously during a well-child visit. Nevertheless, these risk factors remain the way to detect hearing loss when neonatal screening was normal, not done, or abnormal but lost to follow-up. This editorial is based on the concept that it is easier to remember risk factors if you understand and organize the pathogenesis of the disorders into a system.
TRACKING SOUND THROUGH THE HEARING MECHANISM5,6
Sound has two qualities, intensity or loudness, measured in decibels (dB), and frequency or pitch, measured in cycles per second (Hz). Hearing can be screened in a cooperative 6 year old by an inexpensive and simple office audiometer that delivers pure tones through earphones. As few as four audiometer controls are needed: one for loudness (from approximately 10 to 90 dB); a second for frequency in octaves at, say, 250, 500, 1,000, 2,000, 4,000, and 8,000 Hz; a third for directing sound to the right or the left ear (or to mastoids for bone conduction in upgraded models); and a fourth for turning the signal on and off. The child lifts his hand on the same side when he hears the sound. Normal hearing is defined as perceiving these signals at 15 dB (or less), at each frequency. Twenty decibels is often used for screening and results are plotted on an "audiogram." If the child fails the screen, loudness can be increased and plotted as the "threshold" or lowest intensity perceived by the child at each octave. Intensity is based on a logarithmic scale, so an increase of 3 dB represents a doubling of sound energy. A child who has a threshold of 45 dB is said to have a "45 dB hearing loss." Mild hearing loss usually represents thresholds of approximately 25 to 40, moderate is 40 to 60, severe is 60 to 90, and profound is more than 90. Authors vary these definitions slightly. Further information about these techniques, codes for billing, and referral sources is at <www.betterhearing.org / > by clicking "A Physician's Guide."
For hearing to occur, sound first enters through the external ear, consisting of the auricle (pinna), the meatus, the external auditory canal, and the lateral tympanic membrane. The external canal collects and focuses sound for the drum. Next, sound is transmitted to the middle ear by the medial side of the drum and mechanically passes through the tympanic cavity by movements of the malleus, incus, and stapes. Besides these ossicles with their ligaments, the middle ear is characterized by openings to the mastoid cavity and eustachian tube, plus the oval and round windows. The middle ear is an "acoustic transformer" that matches the lower acoustic impedance of the external ear to the higher impedance of fluids in the inner ear. Sound travels better in water than in air but loses 30 dB when transferred between surfaces. The middle ear eliminates this loss. The stapes ends in a footplate that seals loosely but precisely into the oval window, like a piston. Sound is transmitted as mechanical movement (vibrations) through the oval window to the fluid in the inner ear.
The inner ear contains the main sense organs for hearing (the cochlea) and balance (the semicircular canals). The cochlea is shaped like a snail's shell with three tubes inside that spiral toward the apex in parallel. The outer two are connected at the apex. The central tube is a blind pouch and contains "hair cells" that change sound energy to electrochemical stimuli for the eighth nerve. Sound enters at the base (large end) of the cochlea from the oval window and spirals down the cochlea through one tube, passes into the other at the apex, and spirals back. This second tube ends at a membrane covering the round window. As sound makes this trip, mechanical vibrations are transmitted to the central tube and move the "hairs" (or cilia) that protrude from the hair cells in lines along this tube. Each hair cell is connected to an afferent fiber of the eighth nerve. The fibers leave the cochlea in a line that follows the spiral of the cochlea and are wound into the auditory nerve. The cochlea is "tuned" so high frequencies are transmitted to hair cells near the base while low frequencies can proceed to the hair cells at the apex. This means that each nerve fiber will carry a specific frequency and that the outer fibers in the eighth nerve carry higher frequencies while inner ones bring lower frequencies. This allows humans to recognize tones as the eighth nerve enters the auditory region of the cortex by way of the lateral brain stem and the pons.
HEARING LOSS ALONG THIS ROUTE8,8
The types of hearing loss plus the causes and risk factors for each can be expressed relative to the steps of hearing. The failure of any step can result in hearing loss. First, deafness can be central or peripheral. Central causes are anything that disrupts the processing and interpretation of auditory impulses in the central nervous system. So cerebral palsy or organic brain damage by asphyxia; head trauma; perinatal events such as prematurity, intracranial hemorrhage, severe hyperbilirubinemia, or persistent fetal circulation; or mental retardation can be a risk factor for central hearing loss.
Peripheral hearing loss can be divided into conductive, sensorineural, or mixed (both). The outer and the middle ears conduct sound to the inner ear, so anything that physically impedes this produces a conductive loss. Knowing the path of sound makes the causes or risk factors for conductive loss obvious. Working inward, this includes stenosis or atresia of the meatus or canal; obstruction of the canal by wax or foreign body; a perforated drum; abnormal pressure or fluid in the middle ear; cholesteatoma; and fracture, otosclerosis, or absence of the ossicles.
Sensorineural hearing loss involves defects in the inner ear and its eighth nerve attachments. Causes include agenesis, congenital infections, meningitis, head trauma, and fistulae that allow cochlear fluid to leak from the oval or round windows. The most sensitive link in the chain for sensorineural hearing is the hair cells. They can be damaged by high levels of noise, or by toxic agents such as aspirin, aminoglycoside antibiotics, furosemide, chemotherapy, or x-ray.
Noise can damage hearing in two ways. "Burst" -like explosions may stretch and tear movable inner ear parts. Alternatively, prolonged exposure to sounds above 100 dB with no break (even brief interruptions of noise decrease damage) can injure hair cells. Noise at 140 dB and higher is dangerous to hearing for even brief periods. Damage tends to involve hair cells nearer the base first. So an early audiogram will reveal a spike of hearing loss at 4,000 Hz. As damage continues, the defect widens over high frequencies. A selective high-frequency loss is therefore typical of the sensorineural type.
Noise-induced hearing loss is becoming a major concern because home and recreational noises may be increasing and fetal and neonatal developmental periods seem to have increased susceptibility. The AAP has recommended that the Occupational Safety and Health Administration consider pregnancy in setting occupational noise standards to prevent fetal hearing loss and voiced concern about the noise inherent in neonatal intensive care.7 Note again the need for a routine hearing screen in the latter group. Teens are at risk now from rock concerts, "megabase" music systems, and motorized equipment. There is also growing concern that a substantial part of the hearing loss that accompanies aging is due to repeated noise damage accumulating over life. Parents and patients can find more information at the web address listed earlier by clicking "Information on Hearing Conservation." They should also know that initial loss is usually temporary so hearing recovers within 24 hours, and that temporary hearing loss and tinnitus after loud noise are warning signs for permanent hearing loss.
In summary, conductive hearing loss is usually less than 40 dB, and is rarely more than 60 dB in children. It is usually treatable by removing the obstruction or amplification. Conductive loss can be distinguished by thresholds that are higher for air than bone conduction on audiogram. Air and bone thresholds are equal in normal hearing and usually equally reduced in sensorineural loss. Losses also tend to be more even or random over tympanogram frequencies.
Pediatricians can remember anomalies that suggest referral for a hearing test based on embryology. The outer and middle ears are endodermal in origin and develop from the first and second branchial arches (the former also gives rise to the mandible). So other anomalies from these arches increase chances that hearing is impaired as well. These include pits or ear tags near the auricle; branchial cleft cysts or sinuses from the arches; micrognathia; a dysplastic auricle, meatus, or canal; and other craniofacial anomalies. The neural elements of the inner ear (including the cochlear hair cells) are ectodermal in origin. This means that other ectodermal and neural crest anomalies are risk factors for hearing loss. So almost any abnormality of the eyes, including hypertelorism, or pigmentary defects of hair and facial skin, and neural tumors suggest hearing screen.
Finally, it is always prudent to refer a child for a hearing screen if the family history is positive for deafness or if a parent has concerns about the hearing of the child.
1. Copian J. Deafness: ever heard of it? Delayed recognition of permanent hearing loss. Pediatrics. 1987;79:206-213.
2. NIH Consensus Statement. Early Identification of Hearing Impairment in Infants and Young Children. 1993;11:1-24.
3. Yoshinaga-Itano C, Sedey AL, Coulter DK, Mehl AL. Language of early- and later-identified children with hearing loss. Pediatrics. 1998;102:1161-1171.
4. American Academy of Pediatrics Joint Committee on Hearing. 1994 position statement. Pediatrics. 1995;95:152-156.
5. Rolland PS, Marple BF, Meyerhoff WL. Hearing Loss. New York: Thieme; 1997: 1-256.
6. Norther JL. Hearing Disorders. Boston: Little Brown; 1984:253-265.
7. American Academy of Pediatrics Committee on Environmental Health. Noise: a hazard for USe fetus and newbom. Pediatrics. 1997;100:724-727.