Clinical meaning
Tinnitus is the perception of sound in one or both ears or in the head when no external sound source is present. It is often described as ringing, buzzing, hissing, whistling, clicking, roaring, or humming. Tinnitus is classified as either subjective (heard only by the patient, accounting for more than 95% of cases) or objective (heard by both the patient and the examiner, typically caused by vascular abnormalities, muscular contractions, or patulous Eustachian tube). Subjective tinnitus results from abnormal neural activity in the auditory pathway that the brain interprets as sound. The most common underlying mechanism involves damage to the outer hair cells of the cochlea, which are highly specialized sensory cells in the organ of Corti that amplify and fine-tune sound vibrations. When outer hair cells are damaged (most commonly by noise exposure, aging, or ototoxic medications), they can no longer properly modulate auditory nerve signals. The auditory cortex, deprived of normal input from the damaged frequency range, undergoes neuroplastic changes and begins generating spontaneous neural activity that is perceived as phantom sound. This is analogous to phantom limb pain, where the brain generates sensations from a missing body part. Noise-induced hearing loss (NIHL) is the most common preventable cause of tinnitus. Prolonged or repeated exposure to sounds above 85 decibels damages outer hair cells, particularly those tuned to high-frequency sounds (4,000-6,000 Hz), producing a characteristic high-pitched tinnitus and hearing loss at 4,000 Hz on audiometry (the noise notch). Presbycusis (age-related hearing loss) produces tinnitus through the same mechanism of outer hair cell degeneration but is bilateral and progressive. Meniere disease produces tinnitus along with episodic vertigo, fluctuating sensorineural hearing loss, and aural fullness due to endolymphatic hydrops (excess fluid in the endolymphatic space increases pressure on cochlear and vestibular structures). Otosclerosis produces tinnitus with conductive hearing loss from abnormal bone remodeling that fixes the stapes footplate, impeding sound transmission to the cochlea. Acoustic neuroma (vestibular schwannoma) is a benign tumor of the Schwann cells on the vestibular branch of cranial nerve VIII that produces unilateral tinnitus with progressive asymmetric sensorineural hearing loss -- this is the most important cause to rule out because it requires imaging and potential surgical intervention. Pulsatile tinnitus (synchronous with the heartbeat) is usually objective and suggests a vascular etiology such as carotid artery stenosis, arteriovenous malformation, glomus tumor, or intracranial hypertension. Ototoxic medications that can cause or worsen tinnitus include aminoglycoside antibiotics (damage cochlear hair cells), cisplatin (chemotherapy), loop diuretics (especially furosemide at high doses), high-dose aspirin (typically >4.5 g/day, usually reversible), and quinine.