Loss of hair cells causes permanent deafness, because these cells can not regenerate. (science.sciencemag.org)
The sensory nerve cells of the cochlea subsequently die and permanent deafness results. (lsu.edu)
This form of deafness usually develops shortly after birth, where degeneration of blood vessels that supply blood to the cochlea (the auditory portion of the inner ear) results in permanent deafness. (petsmagazine.com.sg)