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 (De Leenheer et al. 2011). In general, genetic factors contribute to almost half of the congenital hearing loss cases while the rest are associated with environmental causes and prenatal factors. There are several environmental factors for congenital hearing loss such as fetal ototoxic drug exposure, premature birth, congenital infections, and the list goes on. The definition of hearing impairment is a permanent, unilateral, or bilateral, sensory or conductive loss, averaging 30 dB or more within the frequency range important for speech recognition (Robson 2006). It is doubtless that children’s development in different aspects such as reading, thinking, social skills, language, and academic performance in the future can be greatly influenced by hearing loss. Therefore, awareness regarding risk factors of congenital hearing loss should be created among the parents for them to take note of it.

First and foremost, gestational age and low birth weight can play important roles in causing congenital hearing loss. Premature babies who have a 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 (Kraft et al. 2014). A study has found that the prevalence of hearing loss in a 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% (Van Dommelen et al. 2015). Moreover, delayed maturation of the auditory system has been postulated as a concern in infants who are hospitalized in Neonatal Intensive Care Unit (NICU) (Univ Frei et al. 2015) because they are more susceptible to necessary medical intervention such as venous access, assisted ventilation and ototoxic drugs (Kraft et al. 2014). 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 a hearing loss.

On the other hand, ototoxic medication is one of the risk factors contributing to congenital hearing loss. The exposure can happen when the infant is admitted to NICU for more than 5 days (JCIH 2007) or from the mother during pregnancy (Roizen 2003). During the first trimester of pregnancy, the mother should avoid taking any ototoxic medication such as trimethadione which can give rise to congenital hearing loss in the fetus. 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 (Roizen 2003). 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 showed that, over 40% of the surviving infants with cytomegalic inclusion disease(CID) were found to be deaf (Fowler & Boppana 2006). CMV is known as human herpesvirus 5 and classified as a beta herpesvirus (Zaia 2016). 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 highly recommended, so that primary infection could be diagnosed and potentially prevent the burden of disability due to congenital CMV. (Walker et al. 2013)

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 cognition, communication, behavior as well as socio-emotional development of the hearing loss children. One way to identify the hearing loss babies with or without high-risk factors 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 (Korver et al. 2017). 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(Korver et al. 2010). Hence, early management planning and intervention to address hearing loss can be done with a multidisciplinary team approach which including the audiologist, speech therapist, or otolaryngologist before it significantly affects a child’s quality of life.