Overview Of Risk Factors Of Congenital Hearing Loss 

Congenital hearing loss means hearing loss that is present in an infant at birth. There are case reports of 2 to 4 infants in a thousand are born with sensorineural hearing impairment, making congenital permanent hearing loss the most frequently occurring birth defect. Almost half of the congenital hearing loss cases are estimated to be caused by genetic factors, with the remainder of cases associated with environmental causes and prenatal factors. Congenital hearing loss in general can be caused by several environmental factors such as fetal ototoxic drug exposure, premature birth, congenital infections and the list goes on. The definition of hearing impairment is permanent, unilateral or bilateral, sensory or conductive loss, averaging 30 dB or more within the frequency range important for speech recognition. Hearing loss under these criteria has significant impacts on various aspects of a child’s development such as communication, cognition, behavior, socioemotional, and academic outcomes as well as subsequent vocational opportunities. Therefore, awareness regarding risk factors of congenital hearing loss should be created among the parents for them to prevent it. First and foremost, gestational age and low birth weight can play important roles in causing congenital hearing loss. Premature babies who have gestational age of fewer than 37 weeks and birth weight less than 1500g have a higher prevalence of hearing loss compared to full term babies.

A study has found that the prevalence of hearing loss in premature baby aged around 31 weeks is around 1. 2% up to 7. 5% while the prevalence of hearing loss of a premature baby with weight less than 1500 g is around 1. 4 % up to 4. 8%. Moreover, delayed maturation of the auditory system has been postulated as a concern in infants who are hospitalized in NICU. This is because premature babies who are sent to the neonatal care unit are more susceptible to necessary medical intervention such as venous access, assisted ventilation and ototoxic drugs which will then increase the likelihood of hearing loss. In addition, a previous study has shown that the maturation of the hearing system of the fetus accelerates between 32 and 34 weeks after conception. Thus, extra precautions must be taken by the parents of a premature baby if there is any suspected developmental delay, especially speech delay which may indicate hearing loss. On the other hand, ototoxic medication is one of the risk factors contributing to congenital hearing loss. The exposure could happen when the infant is admitted to neonatal intensive care for more than 5 days or from the mother during pregnancy. During the first trimester of pregnancy, the mother should avoid being exposed to ototoxic medication because that could affect the development of the offspring, especially in the sixth and seventh weeks which is the critical gestation period. Consequently, it can cause damage to auditory system in the fetus such as anomalies of the middle ear, absence of the eighth and ninth nerves and aplasia of the inner ear. For instance, exposure to ototoxic substances such as trimethadione which is used to treat women with seizures during pregnancy also gives rise to congenital hearing loss in the infant. Hence, women should be particularly cautious about the toxic effects of any prescribed or over-the-counter medication during pregnancy. They should always consult the doctors or pharmacists to discuss the risks of the medication which could affect the developing fetus. Besides, Cytomegalovirus (CMV) infection is also one of the risk factors that can lead to congenital hearing loss which a study by Medearis in 1964 showed that, over 40% of the surviving infants with cytomegalic inclusion disease(CID) were found to be deaf. CMV is known as human herpesvirus 5 and classified as a beta herpesvirus. Sexual activity and contact with young children are the most significant sources of primary CMV infection during pregnancy.

The transmission of CMV can be from mother-to-child in utero, intrapartum or during breastfeeding (Davis et al. 2017). A research has stated that children with sensorineural hearing loss (SNHL) in two groups of symptomatic and asymptomatic CMV had severe or profound hearing loss (>70 dB HL), with approximately 68% of asymptomatic children and 74% of the symptomatic children having severe or profound hearing loss (Fowler & Boppana 2006). These statistics have shown that CMV is a big contributor to congenital hearing loss and children with either symptomatic or asymptomatic CMV have higher chances to develop severe or profound hearing loss. Due to the absence of an effective vaccine, universal screening for CMV in pregnancy is recommended, so that primary infection could be diagnosed and potentially prevent the burden of disability due to congenital CMV. All in all, early detection is important for early intervention because the gap between the language comprehension development of hearing loss children without early intervention and normal hearing children grows over time. This eventually results in lacking in reading skills, cognition as well as socio-emotional development of the hearing loss children. One way to identify the hearing loss babies with high risks factors or without it at early stage is through universal newborn hearing screening. Newborn hearing screening program which is done within 2 weeks of birth has been running in many countries because it is thought that the children would become less developmentally disadvantaged if the permanent hearing impairment was diagnosed earlier. More than 50% of permanent hearing loss babies cases can be detected shortly after birth through this program.

In addition, this program is said to be successful because it is associated with improved development results at the age of 3 to 5 years in children with permanent hearing loss. Hence, early management planning and intervention to address hearing loss can be done with a multidisciplinary team approach which including the audiologist, pediatrician, speech therapist, or otolaryngologist before it significantly affects a child’s quality of life.

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