Recent estimates suggest that screening for a relatively small number of genetic and environmental causes for hearing loss in newborns will identify the etiology for as many as 70% of those who either have congenital hearing loss or are at risk for late-onset pre-lingual hearing loss. Screening tests for the relevant causes of deafness are already commercially available. All newborn infants would require screening to detect pre-symptomatic infants at risk for delayed-onset pre-linguistic hearing loss, but the tests can also be used in infants with identified hearing losses. As is true of most genetic tests, a positive test result would provide a reliable indication of the cause of hearing loss in an infant who is deaf, but a negative test result would not exclude other mutations or other genetic or environmental causes.
Molecular screening tests require a small sample of the child's blood, which is mailed to any one of several laboratories that offer tests for one or more of the following conditions: Connexin 26, Connexin 30, Pendred syndrome, the mitochondrial A1555T mutation, and cytomegalovirus (CMV). Screening results typically are reported within 48 hours of specimen receipt. To exclude congenital CMV infection as the cause for hearing loss, however, the blood sample must be obtained within the first few weeks of life.
Screening for these specific causes of deafness provide an affordable method to detect the most common causes of congenital and delayed-onset pre-lingual deafness. Families in which the probable cause for the hearing loss is identified should seek professional advice from primary care physicians, geneticists, otolaryngologists, or experts in infectious disease, if appropriate, for a full evaluation and counseling about the significance of test results. In cases of negative test results, parents interested in a more thorough assessment of possible genetic mutations associated with congenital hearing loss should see a geneticist for a diagnostic assessment.
One may ask why it is important for a clinician to know the etiology of a pediatric patient's hearing loss. The answer is clear—knowing the cause of a child's hearing loss may improve clinicians' abilities to provide gold-standard service.
Parents Want to Know
A recent survey by Roush and Harrison (2002) asked parents what information they would like to receive pertaining to their child's hearing loss. Parents reported their most pressing concern at the time of initial diagnosis was the cause of their child's hearing loss. Based upon personal experience, some parents noted that uncertainty regarding the cause of hearing loss at the time of diagnosis made it more difficult to address intervention issues. Other families noted that they worried about the cause of hearing loss for years after the initial diagnosis and often erroneously associated the hearing loss with something innocuous during pregnancy, such as drinking too many caffeinated beverages. One mother expressed relief when genetic screening revealed her 3-year-old son's hearing loss was attributed to a Connexin 26 mutation. She had never before revealed her fear that the loss was due to an over-the-counter decongestant taken during pregnancy.
Parents often feel guilty about their child's hearing loss when they learn of the cause. As part of genetic counseling, a geneticist can explain that everyone carries abnormal genes. In the case of recessive traits, which include most—but not all—causes of deafness, it is only when both parents carry the same "weak" gene that a child risks inheriting a "double dose" of the gene and becoming deaf. A genetic diagnosis includes important implications concerning the hearing status of future children. In whatever way the parents decide to use this information, they will not again face the totally unexpected birth of a child who is deaf, and most families are relieved to know the cause and its implications.
In any pediatric patient with pre-lingual hearing loss of unknown cause, audiologists should routinely document that they have urged the parents to seek genetic counseling. For other genetic diseases, physicians have sometimes been held liable for assuring their patients that "lightning never strikes twice."
Audiological and Rehabilitative Intervention
Although some studies have indicated that the etiology of hearing loss does not necessarily limit receiving a benefit from such hearing technology as a cochlear implant, some particular diagnoses—such as Connexin 26 mutations and Pendred syndrome—typically are associated with good outcomes for cochlear implantation. Thus, a family may feel more comfortable about moving forward with cochlear implantation knowing that it has provided favorable outcomes for others with the same type of hearing loss as their child.
Additionally, cochlear implants are now advocated for children as young as 6 months of age. Assessment of minimal response levels to acoustic stimuli is challenging when working with children with hearing loss during their first few months of life. The recommendation of a cochlear implant for children less than 1 year old may be based at least partially upon electrophysiological assessment. Inclusion of genetic screening into the assessment battery may provide knowledge of the etiology of the hearing loss, which—along with electrophysiological assessment and the subjective report of the child's family and speech-language pathologist—allows cochlear implant teams to make better-informed recommendations of cochlear implantation for children under 1 year old.
For example, electrophysiological results consistent with profound hearing loss, along with a finding of a Connexin 26 mutation, would support the recommendation of cochlear implantation at an early age. Moreover, anecdotal experience has suggested that some pediatricians are not convinced by results of electrophysiologic assessments that indicate permanent congenital hearing loss. The diagnosis of such a hearing loss is met with much less skepticism when it is accompanied by unequivocal genetic evidence.
Knowledge of certain etiologies also may change the management of services provided for a child with hearing loss. For example, Pendred syndrome is typically associated with an enlarged vestibular aqueduct, an anatomical abnormality that often results in fluctuating or progressive hearing loss. Consequently, frequent audiologic assessments should be conducted to characterize the degree and configuration of the child's hearing loss and to ensure hearing aids are fitted appropriately.
The increased likelihood of a fluctuating/progressive hearing loss may influence the choice of amplification provided for a child given the need for flexible electroacoustic characteristics to account for possible changes in hearing sensitivity. In addition, downward progression of hearing sensitivity associated with enlarged vestibular aqueduct is frequently coincident with physical trauma and/or barotrauma, so families can be counseled to avoid activities that may place the child at risk (i.e. contact sports or scuba diving).
CMV is one of the leading causes of acquired congenital hearing loss in the United States. Most people are infected with CMV after the neonatal period and have no signs or symptoms. However, when CMV is contracted in utero, serious neurological and sensory deficits, including hearing loss, may occur. The only way to determine if a child's hearing loss is attributable to CMV is to screen for the virus in the neonatal period, specifically within the first two to three weeks of life.
The neurological deficits associated with congenital CMV often result in serious cognitive, speech, and language delays. When congenital CMV is confirmed as the probable etiology for congenital hearing loss, hearing health care professionals are better-equipped to discuss a prognosis for development of speech, language, and auditory skills. In this country, unilateral deafness—which is most commonly caused by CMV—is considered to be a clinically significant abnormality because of substantial evidence that as a group, children with unilateral deafness exhibit poor school performance. Additional intervention may be proactively identified and provided to address possible cognitive delay.
Enhancing Medical Intervention
Pendred syndrome also may be associated with goiter, which usually forms during adolescence or early adulthood. An awareness of the potential for goiter allows for the development of a routine schedule for evaluation with possible referral to an endocrinologist. Early detection of goiter typically improves the prognosis for successful treatment through hormonal therapy and other pharmacological interventions.
Screening for CMV—which is responsible for up to 25% of congenital hearing loss—must take place within the first few weeks of life to determine whether the infection is congenital or acquired. Infections acquired postnatally are typically asymptomatic, while congenital CMV may cause serious disabilities, such as hearing loss, visual impairment, and neurologic abnormalities, but in other affected infants, hearing loss is the only recognized abnormality. When a physician knows an infant has congenital CMV, various assessments may be ordered in a timely fashion and early intervention may be provided.
Hearing health care providers should expanding the traditional battery of assessments to include the provision of molecular screening for children with hearing loss. Knowledge of the etiology of the hearing loss is appreciated by families and improves intervention services.