Hearing loss may be either due to insufficient sound conduction from the outer to the inner ear ("conductive hearing loss"), or - much more frequently - to damage to the hair cells and neurons in the cochlea or to the auditory nerve ("sensorineural hearing loss").
Hearing loss may develop slowly over many years or set in acutely, e.g. following some noise or head trauma or due to some ototoxic medication (e.g. certain chemotherapy drugs or antibiotics). Sometimes the precise origin of acute hearing loss is unknown or may only be suspected such as case of idiopathic sudden deafness. Here, the source may be some infection, disruption in blood supply or autoimmune disorder.
While sensorineural hearing loss in the chronic stage is irreversible, all or part of it may recover in the acute stage thanks to cochlear repair mechanisms. The more severe the acute hearing loss is, the less likely spontaneous recovery becomes and the higher the risk for permanent damage and loss is. Usually, hearing recovery is most pronounced in the hours and days following the onset of acute hearing loss and tapering off over 4 to 5 weeks. In human beings, loss of cochlear hair cells or neurons is irreversible.
Chronic hearing loss may have serious impacts on professional and personal lives, e.g. through reduced job performance and earning power, impaired memory and ability to learn new tasks or reduced alertness and increased risk to personal safety (loss of monitoring of environmental warning sounds).