When Medical ‘Myths’ Outlast the Evidence

CHICAGO – Many physicians still hold beliefs despite the existence of clear evidence that they are incorrect, said a presenter at the annual meeting of the American College of Physicians.

Dr Douglas Paauw

These long-held pieces of dogma – or “medical myths” – were engraved during training or early in the careers of many physicians, and are difficult to overcome, noted Douglas Paauw, MD, professor of medicine at the University of Washington, Seattle.

“I think that myths persist because medical professionals get taught one way in training, given a ‘truth’ or ‘This is the way we do it,’ and then do not ever rethink, ‘Is it true?’ ” he said in an interview. “Studies pop up to question conventional wisdom, but unless the studies get highly publicized, they aren’t noticed.”

During his presentation, Paauw discussed three of what he considers to be some of the some of the medical myths that are in greatest need of being dispelled.

Shellfish Allergy and Radiocontrast

A myth persists that people with a shellfish allergy could have an allergic reaction when a contrast agent is used for a scan, he said.

This belief arose, because fish and shellfish contain iodine, and allergic reactions to seafood are fairly common, and contrast agents contain iodine, too, Paauw said.

The belief is widespread, with 65% of radiologists and 88.9% of interventional cardiologists saying they ask about seafood or shellfish allergies before administering contrast. And a third of radiologists and 50% of cardiologists said they would withhold contrast media or recommend a premedication for patients with such an allergy.

But the belief makes no sense, Pauuw said. Iodine is present in many other foods, including milk and bread, and allergies to shellfish are because of parvalbumin protein and tropomyosins, not iodine.

Colonoscopy Dogma

It’s been long believed that people need to be on a clear, liquid diet for 1 or 2 days and need to drink a bowel-prep liquid before a colonoscopy, noted Paauw.

But the evidence shows this isn’t necessary, he said.

A 2020 study found that a low-residual diet, allowing foods such as meat, eggs, dairy, and bread, were comparable to the clear liquid diet in terms of bowel prep and detection of polyps during the exam. The patients on the low-residual diet had less nausea, less vomiting, and less hunger, and expressed more willingness to have a repeat colonoscopy.

“Let them eat,” Paauw said in his presentation.

Metronidazole and Alcohol

There is a belief that patients shouldn’t drink alcohol if they are taking metronidazole, because of concerns about nausea, vomiting, flushing and other symptoms – also known as a disulfiramlike reaction, Paauw explained.

Case reports have been published, but the cases were presented as though a metronidazole-ethanol reaction was an established fact, and the authors didn’t provide evidence to justify this, Paauw said.

But it’s been shown in rat models that metronidazole can increase levels of acetaldehyde, the trigger of symptoms, in the colon, but not in the blood. And in a small placebo-controlled, randomized trial, six people were given metronidazole and ethanol and, after regular blood testing, no difference was seen in acetaldehyde blood levels, vital signs, or symptoms.

The Centers for Disease Control and Prevention has said that avoiding alcohol while taking metronidazole is unnecessary, said Paauw.

Sinus Headaches

Contrary to common belief, headaches thought to be “sinus headaches” are usually migraine headaches, Paauw said.

In one study, 2,991 patients with six headaches in the previous 6 months were self-diagnosed or were physician-diagnosed with sinus headaches. But 88% of these headaches met the International Headache Society criteria for migraine headache.

Paauw said he hopes that clinicians reconsider the evidence regularly when deciding how to treat their patients, and not rely on bits of dogma.

“They stay with us,” he said, “and sometimes there are other ways to do it.”

Shien Tze, MD, an internist in Fargo, N,D,, said that patients sometimes also hold misconceptions, based on outdated dogma, that he needs to dispel.

“I try to convince them that this is a myth that is not based on evidence, not based on science,” he said. “I think it depends on the way you say it. If you say it in a calm, firm, not wishy-washy way, the patients believe you.”

Paauw reported no relevant financial disclosures. He serves on the editorial advisory board of Internal Medicine News, and he contributes “Myth of the Month” and “Pearl of the Month” columns to this publication.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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