HEARING DISORDERS

Hearing is the essential sensory pathway for development of speech and presence of even mild hearing loss in early childhood may affect speech, learning and psychosocial development.

Severity of hearing loss is categorized on the basis of unperceived sound decibels (db), as mild (16-30 db), moderate (31-50 db), severe (50-70 db) and profound (>70 db), i.e. a child with profound hearing loss cannot hear a sound even as loud as 70 db and can hear sounds only louder than 70 db normal conversational speech produces ~55 db of sound.

Incidence: Moderate to severe hearing loss is estimated to be present in ~0.1-0.5% children, apart from a larger number of undetected cases with mild hearing impairment. Less than 10% of hearing impaired children have severe or profound deafness.

Etiology: Common causes of hearing impairment (Table 3.13) are broadly classified into 4 groups:

a. Conductive hearing loss (CHL) due to interference in the mechanical transmission of sounds to the inner ear. These disorders respond well to treatment.

b. Sensorineural hearing loss (SNHL) due to damage to the cochlea or the auditory nerve. These disorders are usually irreversible. Most cases of congenital hearing loss are sensorineural hearing loss in nature and genetic in origin. Mutations of a single gene GJB2 may be responsible for over 50% cases of congenital SNHL.

c. Mixed hearing loss, where early treatment of CHL is necessary to minimize the disability.

TABLE 3.13: Common causes of hearing loss

A. Conductive hearing loss

• Congenital anomalies of middle ear

• Acquired defects:

- Ear canal: Otitis media, foreign body, cerumen

- Tympanic membrane: Perforation, sclerosis

- Ossicles: Cholesteatoma, otosclerosis, tumors

B. Sensorineural hearing loss

• Pre-natal causes:

- Familial

- Genetic: Pendred syndrome, Alport syndrome

- Congenital cochlear anomalies

- Intrauterine (TORCHS) infections

• Post-natal causes:

- CNS infections: Meningitis, encephalitis

- Trauma: Head injury, noise trauma, surgery

- Tumors and lesions of VIII cranial nerve

- Drugs: Aminogycosides, quinine

- Toxins: Lead poisoning

C.

• Cerebral palsy

• Mental retardation

*TORCHS: Toxoplasmosis, rubella, CMV, herpes, syphilis

d. Central auditory problems: These children have normal propagation of auditory stimuli to central auditory areas, but further processing, i.e. comprehension of spoken words, is impaired. Audiograms are typically normal in these cases.

Clinical presentation and age of diagnosis depends on the severity and type of hearing loss as well as whether it is unilateral or bilateral. Lack of startle response in the newborn to a sound is often the earliest indicator of hearing impairment, apart from others listed in Table 3.14. Unilateral hearing defects lead to problems in localizing the source of the sound.

Diagnosis of hearing problems depends on routine screening of high-risk cases or on clinical suspicion of hearing impairment, with following age-appropriate tests:

a. Oto-acoustic emissions (OAE) test is a simple bed-side screening test to assess neonatal screening, though with poor sensitivity. It is performed by a special machine, placing the small probe in the newborn's ear canal. When the machine triggers a click sound, sound waves travel from the external ear to inner ear and cause outer hair cells of the cochlea to move, producing sound detected by the probe. It indicates normal cochlear function.

b. Audiometry brain stem response (ABR) is the more reliable screening test to assess the hearing function in infants and to differentiate the high-tone deafness (SNHL) from low-tone deafness (CDL). Various types of audiometric evaluations, suitable for different age groups include-behavioral response audiometry ( 5 years.

Brainstem evoked response audiometry (BERA) is the most reliable test for early diagnosis of hearing loss even in newborns.

TABLE 3.14: Indications for hearing assessment

A. Presence of high-risk factors

• Family history of hearing impairment

• High-risk neonates: Preterm, asphyxia, hyperbili­rubinemia

• Craniofacial anomalies

• Cerebral palsy or mental retardation

• CNS infections: Meningitis, encephalitis

• Chronic otitis media

B.

• No startle response at birth

• No turning of head towards the sound by 3 months

• No response to name-call by 10 months

• No response to gesture-free command by 18 months

• Delayed language development

• Learning disabilities/poor school performance.

c. Impedance audiometry helps to check patency of eustachian tube, tympanic membrane and ossicles by tympanometry, acoustic impedance testing and measurement of acoustic reflex thresholds.

d. Local examination of ear canal and testing of air vs. bone conduction by tuning-fork tests (Rinne's and Weber's tests) helps to distinguish CHL from SNHL. In CHL, bone conduction is better than air conduction, while both are affected in SNHL.

Management: Early diagnosis and appropriate manage­ment of hearing impairment is essential for normal speech development. Children given hearing aids before 6 months of age develop better speech than untreated children or those given hearing aids later. Principles of management include:

• Correction of CHL etiology, e.g. otitis media;

• Correction of SNHL by hearing aid or cochlear implants for sound amplification;

• Communication training with use of non-verbal methods, e.g. sign-language, lip-reading, etc.;

• Modified schooling, e.g. seating them on front benches or in special schools;

• Psychological and behavioral counseling for child, parents and teachers.

Prevention: Prevention of congenital deafness has been included as a disability for early identification and remedial action under Rashtriya Bal Swasthya Karyakram (RBSK) by screening of infants and children under age 18 years by a mobile team and provision of appropriate treatment at District Early Intervention Centres.

Indian Academy of Pediatrics recommends universal screening of newborns for hearing impairment by a two-step process, as follows:

• All newborns should be screened before the discharge from hospital with OAE and if it 'fails', should be repeated after 4-6 weeks during first immunization visit. If OAE fails again, then, these infants should be subjected to behavioral response audiometry (BERA).

• All babies admitted to intensive care unit should be directly screened via ABR.

• All babies with abnormal ABR should undergo detailed evaluation, hearing aid fitting and auditory rehabilitation, before six months of age.

The goal is to screen all newborn babies before one month, diagnose hearing loss before three months and start intervention before six months of age.

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