Sudden hearing loss is defined as a rapid onset, occurring over a 72-hour period, of a subjective sensation of hearing impairment in one or both ears.

Sudden sensorineural hearing loss is a subset of Sudden hearing loss that is sensorineural in nature such as abnormality of the cochlea, auditory nerve, or higher aspects of central auditory perception or  processing, and meets specific audiometric criteria: a decrease in hearing of ≥30 decibels (dB), affecting at least 3 consecutive frequencies (AC+BC).

Idiopathic sudden sensorineural hearing loss is defined as sudden sensorineural hearing loss with no identifiable cause despite adequate investigation. the most pressing of these are vestibular schwannoma (acoustic neuroma), stroke and malignancy.

A maximum of 32% to 65% of cases of SSNHL may recover spontaneously. Prognosis for recovery is dependent on a number of factors, including patient age, presence of vertigo at onset, degree of hearing loss, audiometric configuration, and time between onset of hearing loss and treatment.