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NEW YORK (Reuters Health) - Hallucinated scents can, rarely, be a part of the "aura" that some people perceive before a migraine attack, a new study finds.
About 30 percent of people with recurrent migraines have sensory disturbances shortly before their headache hits.
Those disturbances, known as aura, are usually visual -- such as seeing flashes of light or blind spots. They can also include problems like tingling sensations or numbness, or difficulty speaking or understanding language.
But disturbances in the sense of smell -- so-called olfactory hallucinations -- have not been generally recognized as a part of migraine aura. They are not, for example, listed as an aura symptom in the international criteria doctors use to diagnose migraine.
"I think that's just because (olfactory hallucinations) have not been commonly reported," said Dr. Matthew S. Robbins, senior researcher on the new study.
But no one had done a systematic review of the medical literature on the subject until now, he told Reuters Health,
Robbins and his colleagues at the Montefiore Headache Center in New York reviewed 25 reported cases of patients with headaches (migraine in most cases) and olfactory hallucinations.
They also examined records from more than 2,100 patients seen at their center over 30 months. They found that 14 -- or just under 0.7 percent -- had described smelling scents in conjunction with their headaches.
"It's uncommon," Robbins said, "but it is distinctive."
Usually, the pre-migraine scents are not sweet. "The most common was of the burning or smoke variety," Robbins said.
Some headache sufferers described a general burning smell, while others said they smelled cigar smoke, wood smoke or burned popcorn.
After those burning scents, "decomposition" odors -- like garbage or sewage -- were the next most common.
A few people did describe pleasant odors, including the scent of oranges, coffee or, in one case, foie gras.
It's not clear why the hallucinated odors are most often unpleasant -- or why they are only rarely part of migraine aura.
But Robbins noted that, in general, aura symptoms are thought to involve a phenomenon called "cortical spreading depression" -- where a wave of increased electrical activity in nerve cells of the brain is followed by a wave of depressed activity.
That same phenomenon might underlie olfactory hallucinations, Robbins said. Since the brain's smell centers occupy much less real estate than its vision centers, that could, in theory, explain why phantom scents are so much less common than visual disturbances.
It's also possible that some people with migraines and olfactory hallucinations simply don't recognize the phenomenon, according to Robbins. You know something is wrong when you are seeing zigzag lines, for instance, whereas it's easy to assume that an odor is actually coming from somewhere.
A number of disorders, including Parkinson's disease, some epileptic seizures and brain tumors, can cause a person to smell scents that aren't there.
Knowing that migraines can be preceded by olfactory hallucinations might allow some headache sufferers to forgo "exhaustive medical workups" for other conditions, Robbins noted.
He stressed, however, that when those hallucinations arise without an accompanying headache, they should be thoroughly checked out.
It's estimated that about 11 percent of the world's population suffers from migraines. So even though olfactory hallucinations are an uncommon part of aura, there could still be a fairly large number of people who experience them, according to Robbins.
SOURCE: bit.ly/psoVEy Cephalalgia, online September 23, 2011.