Audiology and hearing health professionals in developed and developing countries describes a field of study and practice that examines the global disparities in the availability, accessibility, and standards of audiological care and hearing health. The central professional in this field is the audiologist, a practitioner uniquely qualified through academic and clinical training to provide services related to the prevention, identification, assessment, diagnosis, and treatment of hearing loss and vestibular disorders, as defined by the American Academy of Audiology. The core theme of the field is the critical analysis of how the role, density, and recognition of this profession differ drastically between well-resourced health systems in developed nations and the challenges faced in developing countries, which include a critical shortage of trained audiologists, a lack of educational programs, and inequitable access to services.
According to the World Health Organization (WHO), approximately 250 million people worldwide have a disabling hearing impairment (i.e., moderate or worse hearing loss in the better ear). Of these 250 million people, two-thirds live in developing countries. Therefore, it is not surprising that adult-onset hearing loss ranks 15th amongst the leading causes of the Global Burden of Disease (GBD).
Prevention According to the World Health Organization, "half of all cases of hearing loss can be prevented through primary prevention. Some simple strategies for prevention include: immunizing children against childhood diseases, including measles, meningitis, rubella and mumps; immunizing adolescent girls and women of reproductive age against rubella before pregnancy; screening for and treating syphilis and other infections in pregnant women; improving antenatal and perinatal care, including promotion of safe childbirth; avoiding the use of ototoxic drugs, unless prescribed and monitored by a qualified physician; referring babies with high risk factors (such as those with a family history of deafness, those born with low birth weight, birth asphyxia, jaundice or meningitis) for early assessment of hearing, prompt diagnosis and appropriate management, as required; and reducing exposure (both occupational and recreational) to loud noises by creating awareness, using personal protective devices, and developing and implementing suitable legislation.
Hearing loss due to otitis media can be prevented by healthy ear and hearing care practices. It can be suitably dealt with through early detection, followed by appropriate medical or surgical interventions."
Screening for hearing loss Due to the lack of readably available hearing screenings in developing countries, hearing loss is often left undetected in all ages. Reasons for the lack of hearing screenings is in part due to limited personnel and financial resources, according to McPherson and Olusanya (2008).McPherson & Olusanya (2008). In: Audiology in Developing Countries. Nova Science Publishers, Inc, pp. 74-97. Hearing screenings are considered a high priority component of a hearing healthcare program in large due to the fact that early detection of hearing loss in infants can allow for prompt assessment, detection, and intervention of congenital and early onset hearing loss. Furthermore, research has shown the importance of early intervention during the critical period of speech and language development (Yoshinaga-Itano et al., 1998).Yoshinaga-Itano, C., Sedey, A. L., Coulter, D. K., & Mehl, A. L. (1998). Language of early-and later-identified children with hearing loss. Pediatrics, 102(5), 1161-1171. Lastly, hearing screenings may be able to detect transient hearing losses that may dissipate with appropriate medical intervention.
Currently, actions have been taken in developing countries to prevent hearing loss. These actions include: promotion of immunization against known causes of hearing loss (e.g., measles, mumps, and rubella); improved care of mothers before and during child delivery; and education on the use or misuse of ototoxic drugs. Unfortunately, the healthcare systems in the majority of developing countries are unable to uphold these standards of care.
Definition and principles According to the World Health Organization (1971),World Health Organization. (1971). Mass health examinations. Public Health Papers NO 45. Geneva: World Health Organization. screening is a "medical investigation that does not arise from a patient’s request for advice for specific complains. The term covers all types of examination and does not refer to their speed or accuracy." Similarly Harford et al. (1978)Harford, E. R. (1978). Impedance Screening for Middle Ear Disease in Children: Proceedings of a Symposium Held in Nashville, Tennessee, June 20–22, 1977. Grune & Stratton. states, "screening is a process by which individuals are identified who may have disease or disorders that are otherwise undetected" and which many have "findings of asymptomatic cases" (Haggard & Hughes, 1991).Haggard, M. P., & Hughes, E. (1991). Screening children's hearing: a review of the literature and the implications of otitis media. HM Stationery Office. When considering an appropriate screening program for a developing, or developed, country, Wilson and Jungner (1968)Wilson, J. M. G., & Jungner, G. (1968). Principles and practice of screening for disease. World Health Organization. Public Health Paper, (34). recommend the following ten basic principles that screening program should observe: "The condition to be screened for should be an important health problem There should be an accepted treatment for cases identified Facilities for diagnosis and treatment should be available There should be a recognizable latent (early, asymptomatic) stage in the condition There should be a suitable test to employ in screening The test should be acceptable to the population The natural history of the condition should be understood There should be an agreed policy on whom to treat as patients The cost of case-finding (including diagnosis and treatment of those diagnosed) should be non-wastefully balanced in relation to expenditure on medical care as a whole Case-finding should be an ongoing process and not a ‘one-off’ project."
Tools In developing countries, the predominant method of detecting children with hearing loss is through parental suspicion due to the child’s inappropriate response, or lack of response, to sound and occurs at a mean age of 22 months (Gopal et al., 2001;Gopal, R., Hugo, S. R., & Louw, B. (2001). Identification and follow-up of children with hearing loss in Mauritius. International Journal of Pediatric Otorhinolaryngology, 57(2), 99-113. Mukari et al, 1999;Mukari, S. Z., Vandort, S., Ahmad, K., Saim, L., & Mohamed, A. S. (1999). Parents' Awareness and Knowledge of the Special Needs of Their Hearing-Impaired Child. Med J Malaysia, 54(1). Olusanya et al., 2005Olusanya, B. O., Luxon, L. M., & Wirz, S. L. (2005). Childhood deafness poses problems in developing countries. BMJ: British Medical Journal, 330(7489), 480.). Early detection of hearing loss requires reliable, valid, easy to apply, and safe hearing screening battery in order to be used in developing countries. A hearing screening is considered valid, according to McPherson and Olusanya (2008), "if it detects the majority of subjects with the target disorder (high sensitivity) and excludes most subjects without the disorder (high specificity) and if a positive test indicates the presence of the disorder (high positive predictive value)." Two objective screening tests available for use in infants are otoacoustic emissions (OAEs) and auditory brainstem response (ABR).
An OAE is an electrophysiologic measure of the integrity of the outer hair cells in the cochlea. Two types of OAEs are transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs). This test is relatively quick, non-invasive, and does not require the sleep or sedation. The OAE screener results in either a pass or fail result, making it easy to read without requiring the screener to have audiological expertise. Furthermore, OAEs have a high sensitivity (>90%) and specificity (>96%) based on a two-stage screening (Davis et al., 1997;Davis, A., Bamford, J., Wilson, I., Ramkalawan, T., Forshaw, M., & Wright, S. (1996). A critical review of the role of neonatal hearing screening in the detection of congenital hearing impairment. Health technology assessment (Winchester, England), 1(10), i-iv. JCIH, 2000Joint Committee on Infant Hearing (JCIH) (2000). Joint Committee of Infant Hearing 2000 Position Statement: Principles and guidelines for early hearing detection and intervention programs. Pediatrics, 106, 798-817.). Disadvantages of OAEs include its sensitivity to conductive hearing loss, which may occur within the first few days of life due to a vernix plug in an infant's ear canal, does not detect auditory neuropathy, and DPOAEs may miss mild hearing loss.
ABR is an electrophysiologic measure of the function of cranial nerve eight and the auditory pathway in the brainstem. Three electrodes are placed on the scalp in order to record electrical responses from auditory stimuli. Recordings may be measured when the baby is sleeping, sedated, or in a quiet state. ABR recordings are correlated with the degree of hearing loss, for click ABR this range is from 1k to 4k Hz within 10 to 15 dB HL. The screening version of ABR, or automated ABR (AABR), is designed to produce a simple pass or fail result. AABR has high sensitivity (>90%), high specificity (>96%) and low positive predictive value (19%) (Vohr et al., 2001;Vohr, B. R., Oh, W., Stewart, E. J., Bentkover, J. D., Gabbard, S., Lemons, J., ... & Pye, R. (2001). Comparison of costs and referral rates of 3 universal newborn hearing screening protocols. The Journal of Pediatrics, 139(2), 238-244. Watkin, 2001Watkin, P. M. (2001, December). Neonatal screening for hearing impairment. In Seminars in Neonatology, 6(6), pp. 501-509.). One advantage of AABR is that it is able to identify auditory neuropathy; however, the disadvantage is that AABR may miss mild sensorineural or exclusively low frequency hearing loss. Therefore, it is recommended that hearing screening programs utilize a two-stage screening protocol that is made up of TEOAEs and AABR. Combined, these two tests have the most favorable combination of specificity, sensitivity, acceptability and high coverage in hospitals with a wide range of birth rates (Kennedy et al., 2000;Kennedy, C., Kimm, L., Thornton, R., & Davis, A. (2000). False positives in universal neonatal screening for permanent childhood hearing impairment. The Lancet, 356(9245), 1903-1904. Vohr et al., 2001). McPherson and Olusanya (2008) write, "evidence from ongoing infant hearing screening programs has shown that these tests are acceptable to parents because they are not invasive, painless and quick to administer. They are currently employed in developed countries and in a growing number of developing countries." Lastly, it is recommended that the two-stage screening protocol be performed on newborns prior to hospital discharge in order to decrease the number of infants lost to follow-up care.
Hospital-based hearing screening Screening newborns in the hospital prior to discharge eliminates the need to ask parents to return specifically for their child to have a hearing test and allows healthcare professionals to satisfy an ethical obligation to ensure infants have been assessed for hidden abnormality prior to discharge. Parents may be less likely to return to the hospital for an invisible and non-life-threatening disorder in their apparently normal newborn babies.