In roughly 85 to 90 percent of cases, sudden sensorineural hearing loss (SSHL) has no identifiable cause. The remaining 10 to 15 percent trace back to a specific trigger: a viral infection, a blood flow problem, an autoimmune reaction, a tumor, or damage from medications. Clinically, SSHL is defined as a hearing drop of 30 decibels or more across at least three connected sound frequencies, all within a 72-hour window. That’s enough to make normal conversation sound like a whisper.
Despite the high rate of “unknown cause” diagnoses, researchers have identified several well-supported mechanisms that explain how the inner ear can lose function so quickly. Understanding these can help you recognize the urgency of the situation and know what to expect if it happens to you or someone close to you.
Viral Infections and Inner Ear Inflammation
Viral infection is one of the most studied explanations for sudden hearing loss. Several viruses can reach the cochlea, the snail-shaped structure in your inner ear responsible for converting sound into nerve signals, and cause direct damage or trigger an inflammatory response that destroys delicate hearing structures.
Cytomegalovirus (CMV) is among the most clearly linked. Studies of inner ear tissue show that CMV causes swelling and inflammation in the cochlea, and viral proteins have been found in the organ of Corti, which houses the hair cells that detect sound. The damage appears to come both from the virus itself and from the immune system’s aggressive reaction to viral proteins in the inner ear.
Herpes simplex virus (types 1 and 2) can also cause hearing loss and dizziness. Animal studies have found scarring inside the cochlea, loss of outer hair cells, and viral particles inside cochlear nerve fibers after infection. Varicella zoster virus, which causes chickenpox and shingles, has been linked to bleeding in the cochlear nerve and destruction of parts of the organ of Corti. Measles causes degeneration of cochlear neurons, particularly in the base of the cochlea, which handles high-frequency sounds. Mumps damages hair cells and the protective coating around the hearing nerve. HIV has been detected directly in auditory hair cells and along key structures of the inner ear.
Rubella causes direct cell death in the organ of Corti and the stria vascularis, a tissue layer that maintains the chemical environment the cochlea needs to function. In many of these viral cases, the damage isn’t just from the virus killing cells. The immune system’s attempt to fight the infection inside such a small, sensitive structure can be equally destructive.
Blood Supply Disruption
The inner ear depends on a single small artery, the labyrinthine artery, for its entire blood supply. There is almost no backup circulation. When blood flow through this artery drops significantly, the cochlea can be starved of oxygen within minutes, leading to rapid hearing loss.
Blockages in the vertebrobasilar system, the network of arteries running through the back of the neck and base of the brain, are one documented cause. Thrombosis (a blood clot) in the basilar artery or a lack of flow in the vertebral artery can reduce circulation to the labyrinthine artery enough to damage the cochlea. In documented cases, imaging has confirmed complete loss of blood flow in the vertebral artery, and hearing improved only after the blockage was treated with a stent.
Carotid artery disease can also play a role, especially when combined with vertebrobasilar insufficiency. A reduction of more than 60 percent in carotid blood flow, measured by ultrasound, can impair the microcirculation feeding the inner ear. These vascular causes are particularly important to identify because they can signal a broader risk of stroke, not just hearing loss.
Autoimmune Reactions
The immune system can sometimes turn against the inner ear’s own tissues. In autoimmune inner ear disease, the body produces antibodies that target specific proteins in the cochlea. The most studied of these is a 68-kilodalton protein, later identified as heat shock protein 70 (HSP-70). Antibodies against this protein have been found in the blood of patients with sudden hearing loss, and hearing has recovered in some of these patients after treatment with immune-suppressing medications, which strongly supports the autoimmune explanation.
Other inner ear proteins that the immune system may mistakenly attack include cochlin, collagen, and a protein found in the protective sheath around auditory nerve fibers. The immune response in the inner ear relies on inflammatory signaling molecules and immune cells including macrophages and T lymphocytes, which can cause collateral damage to the very structures they’re meant to protect.
SSHL has also appeared as a symptom of broader autoimmune conditions: lupus, rheumatoid arthritis, Crohn’s disease, Cogan’s syndrome, multiple sclerosis, and autoimmune hepatitis, among others. In these cases, the hearing loss is one manifestation of a system-wide immune dysfunction rather than a standalone event.
Neurological Conditions
Multiple sclerosis (MS) deserves its own mention because it can cause sudden hearing loss through a distinct mechanism: demyelination. MS strips away the protective myelin coating on nerve fibers, and when a demyelinating lesion forms along the vestibulocochlear nerve or in the brainstem where that nerve connects, the result can be rapid hearing loss. MRI scans of MS patients who develop sudden hearing loss have shown new active lesions at the point where the hearing nerve enters the brainstem. In MS patients, sudden hearing loss can essentially function as a relapse of the disease.
Tumors on the Hearing Nerve
A vestibular schwannoma, sometimes called an acoustic neuroma, is a benign tumor that grows on the nerve connecting the inner ear to the brain. Between 1 and 5 percent of patients who present with sudden hearing loss are eventually found to have one of these tumors. A study of 499 patients treated for SSHL found tumors at the junction of the cerebellum and brainstem on the affected side in 3 percent of cases.
These tumors grow slowly and usually cause gradual hearing decline, but they can occasionally compress the blood supply or nerve fibers suddenly enough to produce rapid hearing loss. This is one of the key reasons imaging is recommended after a sudden hearing loss diagnosis.
Medications and Toxic Exposures
More than 200 medications are considered potentially ototoxic, meaning they can damage the sensory cells inside the inner ear that handle hearing and balance. The two most commonly recognized categories are aminoglycoside antibiotics (used for serious bacterial infections) and platinum-based chemotherapy drugs like cisplatin and carboplatin. These can cause irreversible damage to the inner ear.
Loop diuretics, prescribed to help the kidneys remove excess fluid, are another known category. Quinine, used for malaria, and high doses of aspirin can also affect hearing, though aspirin-related hearing changes are more often temporary. Environmental toxins including mercury, lead, tin, and carbon monoxide round out the list of potential ototoxic exposures.
Why Most Cases Go Unexplained
The fact that 85 to 90 percent of SSHL cases never receive a definitive cause can be frustrating. Part of the problem is that the inner ear is tiny, deeply embedded in bone, and nearly impossible to biopsy in a living person. Many of the suspected mechanisms, like a brief viral infection that resolves before testing or a transient vascular spasm, leave no trace by the time a patient reaches a clinic. The diagnosis of “idiopathic” SSHL doesn’t mean nothing happened. It means the event couldn’t be caught or confirmed with current tools.
Why Timing Matters for Treatment
Regardless of cause, the single most important factor in treatment is speed. High-dose oral steroids are the standard first-line treatment, and they work best when started within the first two weeks of symptom onset. Treatment can be attempted up to six weeks after the hearing loss begins, but the chances of meaningful recovery drop significantly after that window closes.
Without any treatment at all, somewhere between 32 and 65 percent of people experience some degree of spontaneous recovery. That’s a wide range, and it reflects the reality that outcomes vary enormously depending on the severity of the initial loss, the underlying cause, and individual factors. The uncertainty in that number is exactly why early treatment matters: steroids can improve the odds for people who wouldn’t have recovered on their own, and there’s no reliable way to predict which group you’d fall into.