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Just in time to protect patients from the dangers of holiday cheer, a new scholarly review from a British medical journal describes many harmful effects wrought by laughter.

Among the alarms it sounds: The force of laughing can dislocate jaws, prompt asthma attacks, cause headaches, make hernias protrude. It can provoke cardiac arrhythmia, syncope or even emphysema (this last, according to a clinical lecturer in 1892).

Laughter can trigger the rare but possibly grievous Pilgaard-Dahl and Boerhaave’s syndromes (see explanation below).

And ponder, briefly, the mortifying impact of sustained laughter on the urinary tract (detailed in a 1982 The Lancet paper entitled “Giggle Incontinence”).

At the very least, the new review could be considered an affirmation for the perpetually dour. If 2013 was the year of the worried well, the authors imply that 2014 is poised to be the year of the humorless healthy.

The analysis, “Laughter and MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful),” was drawn from about 5,000 studies. It appears in BMJ, formerly known as The British Medical Journal, which for more than 30 years has traditionally featured rigorously researched but lighthearted articles in its Christmas issue. A deputy editor, Dr. Tony Delamothe, said that the MIRTH study was indeed peer-reviewed — presumably by a doctor with a carefully managed sense of humor (or humour).

This year, companion studies in the issue include “Were James Bond’s drinks shaken because of alcohol induced tremor?” , “The survival time of chocolates on hospital wards: covert observational study,” and “Operating room safety: the 10 point plan to safe flinging” (among the cautions: “Before flinging, identify your target and the area beyond it” and “Never fling an instrument straight up into the air”).

Last spring the co-authors, Dr. Robin E. Ferner, an honorary professor of clinical pharmacology at the University of Birmingham, and Jeffrey K. Aronson, a fellow in clinical pharmacology at Oxford, who study the benefits and harms of medicines, discussed what benefit-harm they could explore in a gambit to win a coveted berth in BMJ’s Christmas issue. According to Dr. Delamothe, BMJ receives nearly 120 submissions and accepts about 30.

Dr. Ferner and Dr. Aronson considered holiday foods, for example, but their tastes were not in concert. “He likes sweet wines and I like dry wines,” Dr. Ferner explained. Then they found common cause: ”But we both like dry humor.”

They winnowed down the papers that mentioned laughter to 785, putting them into three categories: benefits (85), harms (114) and conditions causing pathological laughter (586).

The question was timely, they argue, because BMJ had not addressed laughter in a serious fashion in over a century. In 1898, it had published a case study of heart failure in a 13-year-old girl following prolonged laughter. The next year, the laughter problem was raised again, when an editorial writer, in response to an Italian doctor’s suggestion that telling jokes could treat bronchitis, dismissively proposed the term “gelototherapy” (Gelos was the Greek god of laughter; in Italian, gelato is ice cream.).

Laughter as therapy was slow to gain traction: in 1928, The Journal of the American Medical Association gave short shrift to Dr. James J. Walsh’s book, “Laughter and Health.”

The harms, however, have been scrutinized. A 1997 discussion of Boerhaave’s syndrome, a spontaneous perforation of the esophagus, a rare though potentially lethal event, mentioned that one unusual precipitating cause is laughter.

Then there is the mysterious Pilgaard-Dahl syndrome, identified in a 2010 article as a pneumothorax in middle-aged male smokers induced by laughter. It takes its name from Ulf Pilgaard and Lisbet Dahl, the Danish revue performers.

“I don’t think of the Danes as uproariously funny,” Dr. Ferner said, “and, having seen their show on YouTube, I still don’t, though, of course, I don’t speak Danish.”

There were other respiratory threats occasioned by laughter, he said. The popping of alveoli (the air sacs in the lungs, which together typically contain about 600 million): “If you’re going to make asthmatics laugh heartily,” Dr. Ferner said, “they might want to have an inhaler by their side.” (This, extrapolated from a 1936 experiment on the mechanism of laughter in asthmatics.)

There are choking hazards, such as ingesting food during belly laughs.

The MIRTH review did take an even-handed, cost-benefit approach to laughter, noting ample evidence of its salutary effects. It concluded that laughter’s benefits included reduced anger, anxiety and stress; reduced cardiovascular tension, blood glucose concentration and risk of myocardial infarction. “The benefit-harm balance,” the authors wrote, “is probably favourable.”

Studies in recent years concluded that laughter “reduces arterial wall stiffness” and “improves endothelial function.” And a 2008 study of patients with chronic obstructive pulmonary disease concluded that laughter inspired by Pello the clown improved lung function.

Indeed medical clowning has been observed and embraced. But one study left Dr. Ferner almost speechless. A 2011 fertility study reported that when a clown dressed as a chef de cuisine entertained would-be mothers for 12 to 15 minutes after in vitro fertilization and embryo transfers, the pregnancy rate was 36 percent, compared with 20 percent among a control group that was not entertained.

“Why a chef de cuisine?” Dr. Ferner asked plaintively.

Despite such a comprehensive look at the medical literature on laughter, Dr. Ferner felt there was still territory to be charted. “We don’t know how much laughter is safe,” he said. “There’s probably a U-shaped curve: laughter is good for you, but enormous amounts are bad, perhaps. It’s not a problem in England.”

Nonetheless, if you choose to peruse the MIRTH study or others in BMJ’s Christmas issue, the risk of outbursts of laughter, ranging from snickers to groans to guffaws, could be perilously high. You have been warned.