The six types of age-related hearing loss
Rex Banks, M.A. CCC-A Reg. CASLPO
Age-related hearing loss, known formally as presbycusis, is the cumulative effect of aging on hearing. It is progressive, occurs in both ears in equal amounts, and affects the high frequencies first. But what causes presbycusis? Let’s take a look at the six types of presbycusis.
Sensory presbycusis refers to atrophy (i.e. deterioration) with loss of the outer hair cells in the organ of Corti (structure in the cochlea that produces nerve impulses in response to sound vibrations). The base of the cochlea is affected first and the condition slowly progresses toward the inner tip of the cochlea. With sensory presbycusis there is a sharp drop in the ear’s ability to perceive high-frequency sounds (i.e. birds, alarms, etc.) Since this abrupt downward slope effects frequencies above speech, the ability to understand speech, known as “speech discrimination,” often remains relatively good. The process is slowly progressive and starts in mid-life.
Neural presbycusis refers to atrophy of nerve cells in the cochlea and auditory pathways to the brain. Researchers estimate that 2,100 neurons from a total of 35,500 are lost in each decade of life. The loss of 50% or more of the cochlear neurons is used as the criteria for neural presbycusis. This loss begins early in life and may be genetically predetermined. Effects are not noticeable until older age. People with neural presbycusis have a disproportionately severe decrease in speech discrimination ability compared to their level of hearing loss.
Metabolic or stria presbycusis results from atrophy of the stria vascularis. The stria vascularis provides the blood supply to the inner ear and maintains the chemical and bioelectric balance of the cochlea. Atrophy of the stria vascularis results in hearing loss represented by a flat hearing curve (meaning frequencies are affected in equal amounts) because the entire cochlea is affected. Speech discrimination is preserved. This process tends to occur in the last two to three decades of life, is slowly progressive and may be hereditary in nature.
Mechanical or cochlear conductive presbycusis is a degenerative thickening and stiffening of the basilar membrane of the cochlea. The basilar membrane supports the organ of Corti in the ear and aids in translating sound vibrations into electrical signals. Mechanical presbycusis correlates with a gradually sloping high-frequency sensorineural hearing loss that is slowly progressive. Speech discrimination is on par with the person’s level of hearing loss.
There are two other sub-categories of presbycusis. Mixed presbycusis refers to a combination of the above types of presbycusis. Finally, about 25% of all cases of presbycusis show none of the above characteristics and are classified as indeterminate presbycusis.
Although presbycusis can be put in various categories, distinguishing which one you have is no easy task. What we do know is that presbycusis is the most common cause of hearing loss in older adults and that various factors exacerbate it including heredity, medical disease and environmental factors. As a starting point, contact CHS for a hearing test and get the conversation going with one of our audiologists.