Hearing impairment is the essential sensory pathway for development of speech & language and presence of even mild hearing loss in early childhood may affect speech, language, learning and psychosocial development.
Incidence: Moderate to severe hearing loss is estimated to be present in 0.1-0.5% children, apart from a large number of undetected cases with mild hearing impairment.
Etiology: Common causes of Hearing impairment 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 the treatment.
b) Sensory-neural hearing toss (SNHL) due to damage to the cochlea or the auditory nerve. These Hearing impairment are usually irreversible.
c) Mixed Hearing loss, where early treatment of CHL is necessary to minimize the disability.
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 impairment as well as whether it is unilateral or bilateral. Unilateral hearing defects are associated with problem in localizing the source of the sound.
Severity of hearing loss is categorized on the basis of unperceived sound decibels (db), as mild (16-30 db), Moderate (31-50 db), severe (5 0-70 db) and profound (> 70 db). Normal conversational speech produces — 55 db of sound. Only 5-10% of Hearing impairment children have severe or profound deafness.
Diagnosis of Hearing impairment depends on routine screening of high-risk cases or on clinical suspicion of hearing impairment, with following age- appropriate tests —
a) Crib-O-Gram in newborns.
b) Audiometry to differentiate the high-tone deafness (SNHL) from low-tone deafness (CDL). Various types of audiometric evaluations, suitable in different age groups includes Behavioral response audiometry (<6 months), Visual reinforcement audiometry (6 months-3 years), Play audiometry (3-5 years) and Pure-tone audiometry for children >5 years.
Brainstem evoked response audiometry (BERA) is simplest and most reliable test for early diagnosis of peripheral hearing loss even in newborns. Ideally all high-risk newborns should be subjected to BERA at 3, 6 and 12 months.
Common Causes of Hearing impairment
A) Conductive hearing impairment: Congenital anomalies of middle ear and Acquired defects (Ear canal: otitis media, foreign body, cerumen — Tympanic membrane: perforation, sclerosis — Ossicles: cholestatoma, otoscierosis, tumors)
B) Sensorineural hearing impairment: 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 & lesions of VIII cranial nerve – Drugs: Aminogycosides, Quinine – Toxins: Lead poisoning)
C) Central Auditory problems: Cerebral palsy and Mental retardation
Indications for hearing impairment Assessment
A) Presence of high-risk factors: Family history of hearing impairment, High-risk neonates (Intrauterine infections or drug exposure – Preterms & very Low birth weight – Perinatal asphyxia – Neonatal sepsis, hyperbilirubinemia), Craniofacial anomalies, Cerebral palsy or mental retardation, CNS infections: Meningitis, encephalitis and Chronic otitis media
B) Clinical suspicion of hearing impairment: 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 and Learning disabilities/poor school performance.
c) Impedance audiometry helps to check patency of eustachian tube, tympanic membrane and ossicles by typanometry, 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 ‘5 and Weber’s tests) helps to distinguish CHL from SNFTL. In CHL, bone conduction is better than air conduction, while both are affected in SNIFIL.
Management: Early diagnosis and appropriate management of hearing impairment is essential for normal speech development. Children with hearing aids given before 6 months of age develop better speech than the untreated children. 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.