When singer Neil Sedaka stays here, he turns off the air conditioning in his hotel suite, no matter how hot it is, and sets up a half-dozen humidifiers. He avoids iced drinks, stays out of the sun, has a small device attached to the microphone to spray a fine mist in his face while performing - and gets his big toes massaged twice a day by a therapist.

Sedaka is neither a hypochondriac nor a fruitcake. He is only trying to ward off the most dreaded disorder of The Strip, "Vegas throat." Brought on by the extreme dryness of the desert air, showrooms filled with billows of smoke and the strenuous show schedules in this entertainment capital, Vegas throat threatens the talented tonsils of almost every entertainer who comes here. The malady, marked by dryness soreness, costa hotel-casinos millions in lost revenue yearly by silencing performers for varying periods of time.

Not every star goes to such extremes. But most of those who perform here take some extra precautions to ensure that they aren't sidelined by Vegas throat. Frank Sinatra, Paul Anka, Diana Ross, Dionne Warwick and Neil Diamond all become throat patients automatically when they have an engagement in Las Vegas, their doctors say.

According to the physicians who treat it, true Vegas throat is a specific ailment suffered by performers whose vocal cords are affected by the sudden drop in humidity when they arrive here. The vocal cords themselves haven't any mucus-secreting glands, and when the surrounding tissues get dried out, the throat can become severely irritated under the strain of belting out show tunes in smoky rooms. To the performers themselves, Vegas throat is any disorder that affects their singing.

The most popular prescription is for medicine to increase mucus secretion in the throat, helping to lubricate the vocal cords. Most treatment, however, is up to the patient. Smoking is out. Humidifiers are in. No alcohol, because it has a drying effect, but plenty of other fluids, preferably uniced. Smile on stage but try not to laugh too hard; guffaws irritate the throat. Many performers also sniff a weal saline solution to keep nasal and throat moist.

They put up with it all for a very good reason. Las Vegas is by far the highest-paying spot on the nightclub circuit. A big name can make $25,000 a night here, and only the most serious throat problem will keep a performer off the stage at those rates. "Your throat feels like cardboard," says Pia Zadora, who lost her voice in her first engagement here last year. "I even had a nosebleed, and my eyes were so dry they wouldn't open." But, aided by steam inhalation and salt-water gargles, she didn't miss a show.

Dionne Warwick says, "The only way I can keep going in Las Vegas is to shut up." When offstage, she speaks as seldom as possible, and her social life is minimal. The climate and working conditions make Las Vegas her most taxing place to perform, but she never misses a chance to come here. "It's every performer's goal, the entertainment mecca - and really lucrative," she adds.

No one can say accurately how many performances are canceled due to Vegas throat or similar ailments. Few performers will admit that they are subject to it, out of worry that this might affect their bookings with hotel-casinos. One big-name female star swears that she has never been affected, thanks to preventative measures, but the Vegas producer who books her act says that after two weeks here, "She's a lead-pipe cinch for throat trouble."

Though physicians generally won't say which of their patients were knocked off the stage for what reason, they make it clear that Vegas throat causes more than a few cancellations - and that the performers don't want it known. "Some of my patients would rather enter my office through the back door," say Dr. Edward Kantor, a Beverly Hills throat specialist who is sometimes flown to Las Vegas to minister to performers.

Cancellations mean big revenue losses to the 40 hotel-casinos here that count on lounge and showroom acts to draw crowds. About 10 million people went to these shows last year. If a big-name performer conks out, the casinos still must pay salaries to stagehands and others without taking in the $25 to $30 a head that patrons would have paid to see the act. More important, the gaming tables fail to get money from people who would have come to see the show and then stayed to wager.

"Conservatively speaking, we could have a $250,000 cut in revenue if a performer misses one night," says a spokesman for the Riviera Hotel-Casino. (Neil Sedaka played to packed houses in the 1,000 seat Versailles Room at the Riviera during a recent two-week engagement and didn't miss a performance.)

If a headliner falls prey to Vegas throat and is out just a night or two, no replacement is usually hired, and the house remains dark. "You don't replace a Sinatra for one show," says Bill Weinberger Jr., vice president of casino marketing for Caesars Palace, where the singer often appears. Any substitute would be a disappointment for most customers, he says, "and besides, it costs us $500 just to change the marquee." Because Sinatra's voice is more delicate now than when he was in his prime, Caesars counts on losing at least one night when he's in town.

The economic importance of Vegas throat makes local specialists in throat disorders a powerful group indeed. They give entertainers high priority, often making "house calls" and watching from the wings as a client performs. They may recommend that a particularly taxing high note be eliminated from a singer's repertoire, or that the whole show be shortened to avoid vocal strain.

Or they can recommend that their patients not sing at all. A leading throat specialist here, Dr. Sidney Boyers, says that the recommended 12 to 15 cancellations last year to protect his patient's health, and that several of these involved advanced cases of true Vegas throat. All told, the cancellations no doubt cost the hotel-casinos millions of dollars.

At least one performer, the iron-larnyxed Wayne Newton, doesn't worry about Vegas throat. A local resident, he sings at Summa Corp. casinos 40 weeks a year and says he has never had the malady, though he had to cut short his act last Tuesday at the Desert Inn Hotel because of an apparent case of stomach flu and high fever. He's also insympathetic to some performers who complain of it, saying that they don't take care of themselves. "They boogie in the discos until all hours; Vegas is a big party for them," he declares. "That's the real trouble. The throat is a muscle, and if you take care of it properly, it gets stronger with work." Of course, Newton, who remains here most of the time, doesn't have to cope with the changing humidity that doctors blame Vegas throat on.

Doctors who treat the problem, however, say that rest is sometimes imperative. Hotel-casinos generally respect a physician's recommended cancellation of a show when a performer's health is at stake, but there are exceptions. "You've got a few where the dollar is almighty," says Boyers, recalling the time he told a show manager that his female lead in a musical production would risk a burst blood vessel if she performed. "The casino insisted she go on, the vessle broke, and that was it, says Boyers. She was sidelined for six months.