Introduction

It is estimated that 0.5 per 1000 neonates at birth have bilateral moderate to severe sensorineural hearing loss. Approximately 50% of this is estimated to be genetic in origin. This makes congenital deafness the commonest congenital disorder amenable to therapy (1).

For long the inadequacy of available technology for detection and hearing of children with hearing loss has been felt by clinicians. That early detection of deafness is the key to a successful rehabilitation program is well recognized. Till the 70’s behavioral testing was the only option available. Lack of a startle response to loud noise may be due to a hearing defect or failure to recognize the stimulus (1, 2), and this made the sensitivity of hearing detection by behavioural responses low. Even after the availability of audiometers and trained audiologists, the age of detection of congenital hearing loss often exceeded 2 years by which time hearing-impaired children were left far behind in speech and language skills. Deficits in hearing before acquiring speech and language skills also cause deficits in social and emotional development, behaviour, attention and academic achievement. It is thus essential that screening for hearing be instituted at an early age.

Advances

Three technical advances in the last two decades have made easy detection of hearing impairment an achievable goal. They are development of impedance audiometers, recognition of auditory evoked brain stem responses and the discovery of oto-acoustic emissions. Now objective measurement of hearing is possible at any age using this technology. The other factors that have made universal hearing screening a reality are introduction of digital hearing aids and the advent of Cochlear implantation technology.

MethodologyAcoustic Reflex Testing

Hensen observed the involuntary contraction of stapedius muscle in response to loud noise in 1878 (3). Early attempts to record reflexes in neonates were unsuccessful. This led to the false belief that acoustic reflexes were absent in new borns. Subsequent research has shown that these reflexes are present in nearly 100% of new borns, if an 800 HZ probe was used. The availability of newer technologies has relegated this modality to the background.

Auditory Brain Stem Response Testing

This technology has had the greatest influence in the field of paediatric audiology. The application of auditory brain stem responses (ABR) to neonates and premature infants was reported in 1975 (4). They also speculated on the role of ABR for mass neonatal hearing screening. By 1989, the American Speech Language-Hearing Association (ASHA) had endorsed ABR as a valid and reliable screening tool for infants at risk for hearing loss. The final technological leap forward was the invention of automated ABR screeners for evaluation of large populations by paraprofessionals.

However several important caveats must be kept in mind while considering ABR for mass screening for low-risk population.

ABR does not provide a direct measure of hearing.

Recording of a waveform does not document that an acoustic signal has been perceived by a child, it suggests that the neural pathway between the cochlea to the midbrain is intact. ABR thus provides an indirect estimation of hearing sensitivity.

Protocols for pass/refer separation are based on the computerized identification of wave V of the ABR. Wave form and latency depend on brain maturation.

Equipment and disposables are relatively expensive.

Data acquisition time is long compared with other available techniques. Inspite of the above, the available ABR screeners have the greatest potential among current technologies for achieving low false-positive rates in a mass screening project (5, 6).

Otoacoustic Emission (OAE) Testing

When 2 continuous pure tones of differing frequency (f1 and f2) were introduced simultaneously into a human ear, a distinct third tone (2f1 – f2) was measurable at low intensity in the external auditory canal. This is called distortion product otoacoustic emission (DPOAE) Kemp 1977 (6). If a series of click stimuli is given to the ear and background noise was removed, a low intensity echo of the click with a distinct wave configuration and latency of 6 msec was recorded. This transient evolved otoacoustic emission (TEOAE) became the basis for the most commonly used tool in infant hearing screening.

The advantages of TEOAE are:

OAE’s seem to be less sensitive to nervous system immaturity, as most of the inner ear development is complete early in the third trimester. TEOAE can be used in premature infants upto 32 weeks gestation.

TEOAE signal is strong and distinct so it can be recorded in real time (unlike ABR which is buried in background noise 50 times its intensity).

Data collection time may be as low as 4 seconds.

The disadvantages are that the external auditory canal has to be clean and the middle ear has to be pneumatized for accurate estimation of hearing level.

Screening – Current status

Several risk factors have been identified that predispose children to neonatal deafness. In 1972 screening of high-risk children based on ABCD’s of deafness was recommended:

Affected family – i.e. family history of deafness.

Bilirubin higher than 20 mg/dl

Congenital Rubella

Defects in ears, nose and throat (including absent and cleft palate)

Low birth weight (< 1500 gm).

According to JCIH (Joint committee on Infant Hearing 1990) guidelines: The goal of screening newborns for hearing impairment is to identify & initiate treatment by 6 months of age. Screening all newborn infants may not be cost effective. Screening only high risk neonates may appears more practical, however by this approach we will miss 50% of infants with severe to profound hearing loss. Hence universal hearing screening has been recommended and has been in place in some developed countries (1). Considering the Indian context, universal hearing screening may be a distant dream. We definitely need to develop clinical screening ability for babies at risk.

JCIH formulated a High Risk Register (HRR) to identify infants with higher risk of developing hearing impairment.

Family history of childhood hearing loss

Congenital infections: TORCH

Craniofacial anomalies

Birth Weight < 1500 gms

Hyperbilirubinemia requiring exchange transfusion

Bacterial meningitis and sepsis

Apgar score 0-3

Mechanical ventilation of 5 days or more

Ototoxic drugs

Encephalopathy – seizures

Abnormal neuroimaging

Symptomatic hypoglycemia

Stigmata of a syndrome known to include hearing loss

Ideally the methodology should detect all infants with significant bilateral hearing impairment (that is those with hearing loss more than 35dB in the better ear. False positive rate should not exceed 3%. The false negative rate (percentage of infants with significant hearing loss missed by the screening programme) should be 0% (3). Acceptable methodologies for physiologic screening include Otoacoustic emission (OAE) and ABR either alone or in combination.

In addition systems need to be in place to further evaluate neonates who fail the screen. In our institution we have used OAE for screening and ABR for those who failed the OAE screen. What is important is to set up a referral pattern to confirm the diagnosis and more important is to provide for rehabilitative measures. Data from USA suggests that 1 in 40 of referred infants will actually require rehabilitation (8, 9).

When to Screen

Currently the Mean age of detection is 14 months (range 2-3 years). The goal is to identify and initiate treatment by 6 months of age. All newborn infants should be screened and diagnosed within 3 months of birth. The high risk neonates should be screened before discharge. Ongoing surveillance during early childhood should continue. Failure to attain appropriate language milestones by 18 months of life and parental concern about hearing at any age should be sufficient reason to screen.

Conclusions

Universal newborn hearing screening is a much needed program. However in our country at risk screening probably needs to be initiated first. Newborn hearing screening needs specialized equipment that is costly. The delay in diagnosis of hearing impairment caused by not doing a screening of babies at risk may result in more expenditure and effort to rehabilitate these children. Before one dismisses the idea of neonatal hearing screening (at least for at risk babies) as a costly fad, one must remember that hearing loss at birth results in a double disability – deafness and mutism. Figure 1Protocol for Universal Screening

Neonatal Hearing Screening