This kind of vertigo occurs more frequently in older adults, apparently because the protein coating that holds the crystals in place weakens with age. “Like an old Post-it note, after 60 or 70 years the stickiness wears off,” said Dr. Carol Foster, an otolaryngologist who directs the Balance Laboratory at the University of Colorado Hospital.

Mercifully, the dizziness often resolves on its own within a few days or a couple of weeks. That also makes it difficult to calculate how many people suffer from vertigo, since some never seek treatment or get a correct diagnosis if they do.

When Dr. Oghalai and his colleagues evaluated 100 patients who came to a geriatrics clinic at Baylor College of Medicine over two weeks, none of whom had previously reported dizziness, they found that 9 percent had undiagnosed B.P.P.V. Balance experts think it is even more widespread.

Though the vertigo itself isn’t dangerous, the Baylor study found that patients who had it were more likely to fall, which can cause serious injuries, and were less able to handle daily activities like bathing and dressing. They were also more likely to have received a depression diagnosis.

If the vertigo persists, “it can be very debilitating,” said Dr. Susan Herdman, professor emerita of rehabilitation medicine at Emory University. Some patients come in feeling so unsteady that they are holding onto walls and furniture, or even using a wheelchair.

On average, patients with this disorder have already visited five other physicians before they arrive at Emory’s Dizziness and Balance Center, Dr. Herdman said.

Along the way, they may have undergone lots of expensive, unnecessary tests. They may also have been taking prescription anti-nausea drugs for months, or even years, which may slow the perceived spiraling but don’t stop the vertigo.