Table 1. Types of presbycusis (based on Schuknecht [27, 28]).

Type	Description
Sensory	Caused by damage to outer hair cells at the basal turn of the cochlea (hearing loss in the high frequency range, 5% of cases)
Neural	Degeneration of the ganglion cells, shown by a moderate downward slope of the pure-tone threshold towards high frequency and a sharp drop in speech discrimination compared to the ‧pure-tone threshold (e55). Based on histological data, the loss of 50% or more of 35 500 cochlear neurons is used as the criterion of neural presbycusis. Otte et al. (e56) showed that about 2100 neurons are lost in every decade.
Metabolic	Caused by atrophy of the stria vascularis (slowly progressive hearing loss with flattening of the audiogram and good speech discrimination). The loss of strial tissue results in impaired K+ ‧recycling, leading in turn to a decrease in endolymphatic ‧potential (EP) (e57).
Cochlear conductive	This is described (hypothetically) as a degenerative change ‧resulting from stiffness of the basilar membrane (e55)
Mixed presbycusis	A combination of the types described above
Indeterminate presbycusis	In about 25% of cases of hearing loss, no association can be shown between the audiometric pattern and pathological changes in the cochlea (e55, e58)