A partial or total inability to hear is a common sensory condition affecting people of all ages. Hearing loss ranges from mild, where faint sounds are missed, to profound, often termed deafness. Determining when the loss first occurred is a fundamental distinction. This timing helps medical professionals identify the underlying causes and determine the most appropriate course of action.
Congenital vs. Acquired Hearing Loss
Hearing loss is primarily categorized based on when it first manifested. Congenital hearing loss is present at the time of birth or develops very shortly thereafter. This type results from factors that occurred before, during, or just after the birthing process.
Acquired hearing loss develops later in life, after a person was born with typical hearing. This distinction shifts the focus of investigation away from prenatal factors toward external or later-occurring medical events. Both congenital and acquired forms vary in severity and may affect one or both ears.
Primary Causes of Deafness Present at Birth
Congenital deafness is complex, with causes divided into two main categories: genetic and non-genetic factors. Genetic factors account for more than half of all congenital hearing loss cases in children. This occurs due to variations in genes that affect the structure or function of the auditory system.
Genetic causes are classified as either syndromic or non-syndromic. Syndromic hearing loss is one feature of a larger syndrome that involves other medical issues, such as Usher syndrome or Down syndrome. Non-syndromic hearing loss is more common, accounting for about 70 percent of genetic cases, where hearing loss is the sole symptom. Most genetic hearing loss is inherited in an autosomal recessive pattern, requiring a child to receive the altered gene from both parents.
Non-genetic causes often stem from events during pregnancy or the perinatal period. Prenatal infections are a leading cause, with Cytomegalovirus (CMV) being the most common infectious agent affecting a developing fetus. Other infections like rubella and toxoplasmosis can also impact auditory development. Prematurity and low birth weight are associated with a higher risk. Severe jaundice (hyperbilirubinemia) and a lack of oxygen at birth (birth asphyxia) can also damage the structures involved in hearing.
Newborn Screening and Early Detection
Identifying congenital hearing loss is standardized through universal newborn hearing screening programs. These programs ensure that nearly all infants are screened before leaving the hospital, typically using objective, physiological measures. The goal is to detect the presence of hearing loss, not to identify its cause.
Two primary screening methods are used: Otoacoustic Emissions (OAEs) and Auditory Brainstem Response (ABR) testing. The OAE test measures a faint sound echo produced by the outer hair cells of the cochlea in response to a stimulus. The ABR test uses electrodes placed on the baby’s head to measure electrical activity in the auditory nerve and brainstem as sounds are played.
Global guidelines, known as the 1-3-6 guidelines, mandate a rapid timeline for follow-up testing and intervention.
1-3-6 Guidelines
- All infants should be screened by one month of age.
- Any confirmed diagnosis should be established through comprehensive audiological evaluation by three months.
- If hearing loss is confirmed, appropriate early intervention services should begin by six months of age.
This early timeline maximizes a child’s speech, language, and cognitive development.
Causes of Hearing Loss That Develop Later in Life
Acquired hearing loss is often the result of environmental factors, diseases, or the aging process. Age-related hearing loss, known as presbycusis, is the most common form worldwide. It affects high-frequency sounds first and progresses gradually in both ears. Presbycusis results from natural wear on the inner ear structures, particularly the sensory hair cells in the cochlea, diminishing their ability to transmit sound signals to the brain.
Noise exposure is another major contributor to acquired hearing loss. Both chronic exposure to moderate noise levels and acute exposure to loud sounds can permanently damage the outer hair cells in the inner ear. This damage is cumulative, meaning a lifetime of exposure increases the risk of developing hearing loss.
Certain medications are ototoxic, meaning they can cause temporary or permanent damage to the ear. Examples include some chemotherapy drugs and specific classes of antibiotics, such as aminoglycosides. Medical professionals closely monitor the risk of hearing damage when these drugs are necessary for severe infections or illnesses.
Infections that occur after birth can also lead to acquired hearing loss. Bacterial meningitis, which causes inflammation of the membranes surrounding the brain and spinal cord, is a severe cause of sensorineural hearing loss. Other childhood infections like measles and mumps can also damage the auditory system. Finally, head trauma or a perforated eardrum can result in sudden or temporary conductive hearing loss.