A sudden drop in hearing is a medical emergency, says NIH's National Institute on Deafness and Other Communication Disorders. It can come on all at once or over a few days and frequently affects only one ear, and the institute says to visit a doctor immediately.

Doctors call it sudden sensorineural hearing loss, or sudden deafness. Many people notice it when they wake up in the morning, the institute says. It may come with a feeling of fullness in the ear, dizziness or ringing in the ears.

People sometimes put off care because they blame allergies, a sinus infection or earwax plugging the ear canal, the institute reports. That delay matters. About half of people recover some or all of their hearing on their own, usually within one to two weeks, but the institute says waiting to get diagnosed and treated can make treatment less effective. Treatment delayed more than two to four weeks is less likely to help, according to the institute.

Experts estimate it strikes between one and six people per 5,000 every year, and the true number may be higher because it often goes undiagnosed, the institute says. It can happen at any age but most often affects adults in their late 40s and early 50s. Only about 10 percent of cases have an identifiable cause.

At the visit, the institute says a doctor should first rule out a blockage such as fluid or earwax. If no clear cause turns up, a hearing test called pure tone audiometry should be done within a few days of the first symptoms. Further tests, including blood work, an MRI or balance tests, may follow to look for an underlying cause.