Definition: sudden, unexplained sensorineural hearing loss of at least 20 dBHL in at least two adjacent frequencies within 72 hours. Typology: low frequency decline, high frequency decline, flat decline, total deafness. Epidemiology: no difference between men and women, slightly more on the left than on the right. Etiology: possible causes vascular diseases, viral infections, autoimmune diseases, infectious diseases, tumors, etc. Causes: mental tension, stress, mood swings, irregularities in life, sleep disorders, etc. Pathogenesis: low-frequency descending type mostly due to membrane vagus fluid accumulation, high-frequency descending type mostly due to hair cell loss, flat descending type mostly due to vascular pattern dysfunction or inner ear vasospasm, total deafness mostly due to inner ear vascular embolism or thrombosis. Clinical manifestations: sudden onset of hearing loss; tinnitus; ear stuffiness; vertigo or dizziness; auditory hypersensitivity or hard of hearing; abnormal peri-aural sensation; mental or psychological symptoms. Examination: 1. Mandatory items: Otological examination, tuning fork examination, pure tone audiometry, acoustic conductance and stapedius reflex, nystagmus examination with vertigo. 2. Spare items: OAE, ABR, cochlea electrogram, speech audiometry (speech recognition threshold and speech recognition rate), MRI including internal auditory tract (temporal bone CT if appropriate), laboratory tests (blood routine, blood biochemistry, coagulation function, CRP), pathogenic tests (mycoplasma, syphilis, herpes virus, varicella virus, HIV, etc.), vestibular and balance function tests. Note: MRI and ABR are generally not recommended within one week of onset unless stroke must be ruled out. Treatment: glucocorticoids + improvement of blood rheology + nutritional neurology + antioxidants, etc. (Hyperbaric oxygen is not recommended as first choice). Prognosis: 78.16% recovery rate for low-frequency descending type, only 14.29% for total deafness (data from the Chinese Multicenter Institute of Sudden Deafness).