Automated otoacoustic emission (OAE) is a reliable well-recognized hearing screening tool in newborns if conducted after the first 24 h of delivery; performing the test in the first hours is found to be associated with false (Refer) results due to debris accumulation in the ear canal. The policy adopted in our hospital is to discharge mothers 6 h after normal delivery, so the well-baby nursery group consists mainly of newborns delivered by caesarian section or had their mothers admitted to the hospital due to various medical causes; on the other hand, NICU newborns are ill and are usually hospitalized for long durations, so they are more in number in our study than that is expected for the normal ratio of NICU to the well-baby group. On the other hand, NICU group neonates are more exposed to risk factors that are associated with hearing loss.
The newborn age at time of screening has wide range in the NICU group with maximum of 36 days compared to 19 days in the well-baby group, while the median age is almost the same in the two groups; this delay in testing is due to the inability to test ill neonates until their general condition had allowed this.
Regarding the gender distribution, we found that males are more dominant in the NICU group while females are dominant in the well-baby nursery group; when calculating the male to female ratio in the well-baby nursery group before excluding newborns discharged in less than 24 h of delivery, the ratio is almost equal to that seen in the NICU group. An important risk factor for hearing impairment is the genetic predisposition; this can be reflected by both consanguinity and family history of congenital hearing loss; in our study, consanguinity is present in 43% of NICU and 53% of well-baby nursery newborns; this reflects social and cultural issues; Dror et al. concluded in their study that the degree of parental consanguinity is significantly and directly associated with hearing loss . On the other hand, only 1.7–1.8% of the informers had admitted family history of hearing loss which is far less than the expected; this denial can be attributed to the social stigma of deafness in our society.
During prenatal period, there is marked exposure of mothers to ototoxic drugs and febrile infections; this indicates the poor antenatal care received by these mothers; the risk of neonatal hearing loss can be reduced if antenatal care is provided and encouraged; because congenital cytomegalovirus, toxoplasmosis and rubella are reportedly associated with late onset hearing impairment ( manifests during infancy or early childhood); hearing impairment due to these infections may not be detected during the neonatal period, so this necessitate the adoption of a follow-up hearing assessment program ; on the other hand, the recent introduction of rubella vaccination is expected to decrease the hearing affection related to rubella.
Additional risk factors peculiar to NICU group is prematurity (gestational age less than 37) which is a main indication for NICU admission and encountered in 20.7% of this group and low birth weight (≤ 1.5 kg) which was found in 1.9% of this group.
On the other hand, Rhesus incompatibility occurred in 8.8% of NICU neonates. Failure of neonates to cry immediately after birth is a known risk factor for hearing loss and was encountered in 15.2% of our NICU newborns. Febrile illness, rash, and convulsions were also observed and may potentiate the risk of hearing loss in our NICU series. Jaundice is evident in 15.6% of NICU neonates, but most of our cases were mild and did not necessitate exchange transfusion except in two cases (only 0.27%); hyperbilirubinemia requiring exchange blood transfusion is listed as a risk factor for hearing loss; this is consistent with reports from Nigeria, where the need for phototherapy exceeded exchange blood transfusion, but the Nigerian authors conclude that those who received phototherapy also being at significant risk for sensorineural hearing loss .
A serious risk factor is ototoxic drug exposure which is very prevalent in our NICU series accounting for 35% of neonates; it was considered in another study to be a major risk factor for hearing loss in newborns . Ototoxic drugs are administered with no available serum levels measurement in our hospital; so, serum drug level monitoring or using alternative drugs will be expected to dramatically reduce the risk of neonatal hearing impairment.
Dysmorphism was observed in our NICU series in 8.3% of neonates screened; the most frequent conditions known to be associated with congenital hearing loss observed in our neonates were hydrocephalus which was encountered in 3%, cleft palate in 0.65%, Down syndrome in 0.5%, and microphthalmia in 0.1%. Premature rupture of membrane (PROM) is a well-recognized risk factor associated with neonatal hearing loss; it was observed in 7.3% of our NICU series.
It is important to recognize that cumulative effect of risk factors on hearing function is expected and needs further studies with suitable statistical analysis; the more risk factors present will increase the chance for the newborn to develop significant hearing impairment ; another vital point to be stressed on is that some of these risk factors are associated with delayed-onset hearing impairment which may not manifest until late infancy, so follow-up programs beside risk factors elimination initiatives are to be considered without delay.
The estimated prevalence of hearing loss in the NICU group in our study is 10.8 per 1000 neonate; due to the high drop rate (50%), this figure of course is just an approximation of the true prevalence; the failure of compliance to our screening protocol highlights the unawareness of our societies and even health system towards early detection of hearing impairment in neonates. Failure to confirm the diagnosis is due to the unavailability of the ABR device in the hospital, the cost, and sometimes the unawareness or even the refusal of the parents. On the other hand, the estimated prevalence of the well-baby nursery group is 5.2 per 1000 which is very high compared to the international figures (1–2 per thousand) ; although not accurate due to the high drop rate, this must attract the attention regarding the real magnitude of congenital hearing loss in Sudan among the low risk (apparently healthy) group; genetic predisposition (in form of consanguinity and family history of hearing loss) is a major risk factor in this group and needs universal hearing screening program together with genetic analysis. According to the reviewed literature, the prevalence for NICU is ten times higher than that in the well-baby nursery ; while it is only doubled in our study, this may indicate a high prevalence of hearing loss in otherwise healthy (low risk) neonates in our community.