Hearing loss can be due to many different causes. One way of classifying hearing loss is whether it is due to loss in transmission or conduction of sound through the structures of the ear (i.e conductive hearing loss) – CHL, or if it is due to a problem in detection and conversion of sound into neural impulses at the level of the inner ear (i.e sensorineural hearing loss) – SNHL.
Sensorineural Hearing Loss (SNHL)
Probably the most common cause of SNHL is age-related hearing loss (presbyacusis); this is due to degeneration of the inner hair cells of the cochlea (a part of the inner ear). Decrease in function may also occur due to prolonged exposure to loud noise (noise-induced hearing loss) NIHL. Rare causes of acquired SNHL include autoimmune ear disease (AIED), some forms of otosclerosis, Meniere’s disease, vestibular schwannoma.
Sudden (sensorineural) hearing loss (SSNHL) is an important cause of SNHL that needs to be diagnosed and treated early to have the best chance of recovery.
Conductive Hearing Loss (CHL)
Typical causes for CHL in the external ear include obstruction of the ear canal with earwax or debris from infection. In the case of middle ear effusion (“water in the ear”) or infections, the presence of fluid (instead of air) in the middle ear impairs the transmission of sound.
The eardrum transmits sounds to the inner ear via the ossicles (a chain of 3 tiny bones in the middle ear). If there is any disruption of this system, patients will experience CHL. The most common cause is a hole in the eardrum (tympanic membrane perforation). Other causes include cholesteatoma, ossicular dislocation or erosion of the ossicles.
Conversely, a stiffening of the ossicular chain can also impair sound transmission. This may occur in tympanoslerosis or otosclerosis.\
Mixed Hearing Loss
Mixed hearing loss occurs when there is both sensorineural as well as conductive hearing loss. Causes of SNHL and CHL (e.g. presbyacusis and tympanic membrane perforation) may co-exist. Alternatively, a condition may cause both SNHL and CHL (e.g. otosclerosis).
Congenital and Inherited Hearing Loss
Some patients may be born with impaired hearing, either as an inherited genetic trait or as result of problems in the development of the ear. Some of these may be associated with other abnormalities (i.e. part of a syndrome). With the introduction of universal newborn hearing screening (UNHS) in Singapore almost 20 years ago, hearing loss is diagnosed at the earliest age so that timely intervention can be implemented.