The day she fell flat on her face in her own kitchen, Regina Grossman was doing nothing weird.

She was sitting in her Silver Spring home when the phone rang and she stood up to answer it.

Not that she tripped or slipped or stumbled or swooned. She was sober and perfectly alert. Not a banana peel in sight. But all of a sudden the world went whirlybirds.

"I felt like I was standing on my head with the room going all around," she recalled. "A very uncomfortable feeling."

It lasted about a minute. She never blacked out. Then

the kitchen stopped whirling and Grossman got her bearings back, lucky to get away with only a bruise on her

arm.

That was last July. Several times in the previous few weeks, she had felt the same helpless spinning sensation. The spells came without warning and, seemingly, without cause.

She'd be lying in bed and turn her head to the right,

and suddenly the whole room would start orbiting around

her.

"It was like the old song," Grossman recalled: "Stop the world! I want to get off."

The word Grossman used when she called the doctor was "dizziness." For patients and doctors alike, that word is shorthand for a disconcerting set of symptoms that can include lightheadedness, loss of balance, disorientation, nausea, fear of fainting, fear of falling down.

It's not unusual. More than 90 million American adults -- many more than voted in last Tuesday's elections -- have experienced a dizziness or balance problem, according to a National Institutes of Health task force. Most people beyond age 70 report at least occasional dizziness and imbalance, and more than half of the accidental deaths in elderly Americans result from people losing their balance and falling down.

"Dizziness is a very common complaint," said David S. Zee, professor of neurology and otolaryngology at Johns Hopkins Medical Center in Baltimore. "It's ubiquitous in the elderly. If you look at complaints of why people over 70 go to doctors, dizziness is probably No. 1."

Complaints of dizziness or imbalance send more people to a doctor's office in this country than any other symptom except lower back pain. More than 5 million Americans went to the doctor in 1985 because of vertigo -- a spinning sensation -- about 10 percent of them severely disabled.

Dizziness is not a disease, but a symptom or cluster of symptoms. It can be a warning sign of a serious medical problem, such as a lack of blood flow to the brain. Sometimes, it is a side effect of taking medication. In very rare cases -- far less than 1 percent -- it indicates a brain tumor.

But most often, dizzy spells are a sign of something out of whack in the body's delicate sense of balance, which is concentrated in the inner ear. Problems with the balance mechanism of the inner ear can have many causes, including head injury, infection or nerve degeneration.

Vertigo, another term for dizziness, is not, despite common misunderstanding, a fear of heights (which is normal) but a disorienting sense of spinning. (The root of "vertigo" means "to turn" in Latin.)

The symptoms are almost infinitely varied. They can be severe or mild, long-lasting or brief, complex or simple, frequent or rare. The good news for patients is that in most cases, dizzy spells can be prevented or controlled with medication, exercise or "the tincture of time." Sudden Vertigo

The kind of dizziness Regina Grossman felt -- known as benign positional vertigo (BPV) -- is especially disconcerting because it comes on so suddenly and with such routine activity. Any sudden tilt of the head -- looking up at the top kitchen shelf or washing one's hair in the shower -- can bring on a topsy-turvy sensation.

"You suddenly feel as if you're not quite sure where your feet are or where the ground is," said Preston C. Calvert, a Washington neurologist who specializes in treatment of dizziness and balance problems.

BPV, the most common form of dizziness Calvert sees in his private practice at the Neurology Center, is usually treatable with medication or physical therapy.

"This can be a terrifically powerful condition -- to turn over in bed and lose all orientation and maybe start vomiting," neurologist Zee said. "But sometimes, the worse the symptoms are, the more benign the condition."

Inside each inner ear are three semicircular canals that sense rotation of the head and two tiny sacs that sense vertical and horizontal motion. Each sac contains calcium granules -- like microscopic grains of sand -- floating in a gel. When the granules, called otoliths, are pulled by gravity (as in an elevator) or acceleration (as in a subway car) against sensitive nerve fibers, they remind the brain which way it is moving.

But if the inner ear is disturbed by injury, infection or loss of blood supply, the otoliths sometimes float free of their sac and land on the sensor in one of the semicircular canals that normally senses rotation.

The brain then thinks the head is rotating when it isn't, or when it is merely tilting to one side or moving forward. That's what happened when Regina Grossman jumped up to answer the phone.

Grossman, 66, learned to sit or lie still whenever dizziness came over her and wait for it to pass. Closing her eyes only made it worse. She was doubly scared because she had had back surgery to repair a slipped disc a few weeks before, and a fall could redamage the disc. In fact, the unnatural posture of her neck during the surgery may have aggravated a problem in her inner ear that led to the dizziness, she said.

For a time, her dizziness got worse and more frequent. Motion sickness medicine didn't seem to help. Well-meaning friends tried to tell Grossman things like, "You have to remember you're not as young as you used to be."

"Just what people like to hear at a time like this!" Grossman said. "You don't want sympathy, believe me, just understanding."

Finally, her doctors prescribed a set of exercises deliberately designed to trigger an attack of dizziness -- so that her brain and inner ear could begin to "unjam" the confusing signals and overcome them. She can laugh about them now, but at the time she "didn't know which was worse -- the exercise or the dizziness."

She would sit on the bed with her hands over her ears, then lean down sharply to the right, pause and sit up quickly. She did that three times a day for about two weeks. Each time it brought on an attack of dizziness, but gradually they subsided.

Today, she is back at work at the Jewish Council on Aging and several volunteer jobs, free of symptoms and "going all the time." Keeping Balance

Most people don't think much about their sense of balance -- until something goes wrong with it or they've had too much to drink. But keeping the body from continually falling over even while it runs, turns corners, climbs steps, jaywalks and bends over to pick up paper clips is no easy challenge.

The body maintains its balance through a complicated system of signals to the brain, which coordinates movements of muscles to keep the head and body steady. The signals travel so fast and the muscles react so automatically -- as when a person steps off a curb -- that they are called reflex actions. The slowest reflex takes about a tenth of a second.

Without those signals and the reflexes they trigger, a person literally would have no sense of which way is up.

To keep the body oriented, the brain relies on signals from the eyes, ears, muscles, joints, skin and the vestibular system -- an intricate network of canals, chambers, hair cells and nerve endings in the inner ear. The entire vestibular system is about the size of an olive.

The balance system is well protected inside the ear. But the very quality that makes it so effective -- acute sensitivity -- also makes it vulnerable to being thrown out of kilter by a blow to the head, medication, infection or nerve degeneration.

"Many people have no idea that the inner ear is involved in balance problems," said Jerry Underwood, managing director of the Vestibular Disorders Association, an information and support group based in Portland, Ore. "They think: Ears? Hearing.

"So they haven't a clue when they start getting these weird symptoms." Making the Diagnosis

There's no single diagnosis, and no single treatment. One reason patients resort to the catch-all term "dizziness" is that its myriad specific symptoms, including various forms of disorientation, are hard to describe in words.

"Dizziness is a very vague term," said Marianne Schuelein, assistant professor of neurology and pediatrics at Georgetown University Medical Center. "The first thing the doctor has to do is find out what the patient means by 'dizzy' "

"These patients are among the most difficult to diagnose and manage in medicine," said F. Owen Black, a neuro-otologist and director of neuro-otologic research at the Dow Neurological Sciences Institute at Good Samaritan Hospital and Medical Center in Portland, Ore.

Diagnosis usually begins with lots of questions for the patient and may proceed through a battery of tests -- of vision, hearing, reflexes, nerves, head movement. If there is any reason to suspect a possible tumor, a brain scan is in order. Mostly, the doctor wants to hear exactly when the dizzy spells occur and what they feel like.

That can be difficult. It's easier to describe a sound or a sight than a feeling of motion or disequilibrium, Black notes. He sometimes asks patients to draw a picture of what they feel is happening to them during a dizzy spell.

"We really don't have a language for it," said Ralph E. Naunton, an otolaryngologist and director of the division of communication sciences and disorders at the National Institute on Deafness and Communicative Disorders.

What's more, because dizzy spells come and go, patients usually have to describe their symptoms to a doctor while looking and feeling quite normal.

The task of pinning down a diagnosis is doubly difficult because symptoms of dizziness or disorientation often mimic other conditions, including multiple sclerosis and clinical depression.

"Many physicians kind of turn their mind off when they hear the word 'dizziness' or don't try to distinguish between the different kinds," said Calvert. "Very often, if it were recognized it would be easy to treat."

Some patients find themselves dismissed as "crazy" or "hypochondriac." Since an occasional dizzy spell doesn't change their appearance, they have a hard time persuading friends that there's anything wrong. Yet they may find themselves afraid to drive, unable to go to work, wary of venturing outside home.

"It's a very scary, very debilitating feeling," said Gaye Cronin, an occupational therapist in Atlanta who suffered vertigo for two years after a freak accident. While adjusting a patient's wheelchair, she leaned forward to hear the patient's question when one of the wheelchair tires blew out, six inches from her right ear. The explosion of air blew a tiny hole in the wall between her middle and inner ear, causing a fluid leak that discombobulated her balance system.

After surgery to repair the leak -- with a skin graft from the back of her outer ear -- Cronin has overcome almost all her symptoms of vertigo. She has only a slight hearing loss and works at a clinic specializing in balance disorders. In three years, she has seen about 400 patients experiencing balance problems from a head injury, a damaged nerve, infection, drug reaction or normal aging.

Not all of them can overcome their dizziness, but Cronin has an advantage in counseling and treating such patients: She has been there. She knows they're not making symptoms up. "I can definitely tell patients: 'I know how you feel,' " she said. 'Normal' Dizziness

The dizziness a child might feel after turning somersaults or when stepping off a merry-go-round is normal. The dizziness Regina Grossman felt while reaching for the phone in her kitchen isn't.

"Normal" dizziness comes when the body's balance system is working properly but is momentarily overloaded or confused. The child stepping off the merry-go-round has briefly gotten used to whirling and must suddenly readjust to standing on solid ground. The adjustment takes a few seconds, during which the child may stumble or fall.

Similarly, nearly every astronaut gets some degree of motion sickness while learning to float "weightless" in space without the pull of gravity that humans take for granted to help them tell up from down. "It was a closely guarded secret in the early days of the space program -- it didn't fit their macho image," said Naunton, who worked with NASA.

A prime cause of motion sickness in space is "sensory conflict." The sensory messages the astronauts get from their eyes, muscles and inner ears are clashing. Two examples: an astronaut sitting in a spacecraft directly in front of another astronaut who is upside down, or looking outside the window and seeing earth above and sky below. "Makes me nauseated just to think about it," Naunton said. Common Fear: Brain Tumor

It is a rare patient who suddenly starts having dizzy spells and doesn't think: brain tumor. A key part of diagnosing dizzy spells is the effort to rule out or pin down such serious or life-threatening conditions.

"A lot of the time I spend with patients is trying to find out what they are most afraid of and then reassuring them that that isn't what they have," neurologist Calvert said. Usually, what they are most frightened by is a stroke or a brain tumor.

Two kinds of tumors can cause dizziness (though dizziness alone, without hearing loss in one ear, is rarely a sign of a turmor). The most common are benign tumors of the eighth cranial nerve, which carries both hearing and balance information to the brain stem. Known as acoustic neuromas, they are slow-growing and usually operable. They don't spread, as cancerous tumors do, but can inflict nerve damage and affect hearing if they're not detected early and removed by surgery.

Cancerous brain tumors are much rarer -- but life-threatening. This year there will be an estimated 15,600 new cases in the United States and 11,000 deaths,

Sophisticated new computerized X-rays and other imaging techniques, such as CT scans and MRIs, have revolutionized the field in the past 10 years. Doctors can now routinely detect brain tumors as small as a pea -- when they can be surgically removed without loss of hearing. Treatment and Recovery

Neurologists and other doctors who see lots of patients with dizziness express wonder at the resilience and adaptability of the human brain and balance system. That is why many -- though not all -- forms of dizziness are less alarming, and more treatable, than they seem at first.

"The brain is amazingly tolerant," said neurologist Calvert. "It's designed to adjust and adapt to changes in the body's sensory input."

In some cases, "treatment" involves no treatment at all besides careful monitoring of symptoms until they moderate or go away. In other cases, it's a matter of adjusting the dose or type of medication a patient is taking for another illness.

Dizziness sometimes responds to medication, such as motion sickness drugs or decongestants to relieve pressure in the ear.

Surgery can prevent or relieve some kinds of dizziness by repairing leaks in the membranes of the inner ear (using skin grafts from the back of the outer ear). Brain tumors, particularly benign tumors, are removed by surgery if detected early enough.

But often, therapy is designed mainly to help the brain "unjam" the disturbances in the signals it receives. That's the purpose of the leaning-and-tilting exercises that Regina Grossman went through.

No matter what the treatment, it is likely to rely at least in part on the brain's ability to adjust. The human brain is "remarkably good" at recovering its balance," said Naunton.

Loss of one inner ear, for example, produces a devastating effect for a while -- dizziness, nausea, vomiting. But very often, he said, the brain and the sense of balance in the other ear adapt to the loss and overcome those symptoms. And not just in the young.

The dramatic recuperative powers of the brain were once thought "to be less so in older people," Naunton said. "But there's a good deal of skepticism about that now." More Information

National Institute on Deafness and Other Communication Disorders, Bethesda, Md. 20892. Tel: (301) 496-7243. TDD: (301) 402-0252.

National Institute of Neurological Disorders and Stroke, Bethesda, Md. 20892. Tel: (301) 496-5751.

Vestibular Disorders Association (formerly the Dizziness and Balance Disorders Association of America), 1015 N.W. 22nd Ave., Portland, Ore. 97210. Tel: (503) 229-7705.

Acoustic Neuroma Association (a support group for people with benign tumors of the brain), P.O. Box 398, Carlisle, Pa. 17013. Tel: (717) 249-4783.