Hallucinated scents such as a burning or rotten smell, or even the scent of foie gras, can be a part of the "aura" that some people perceive before a migraine attack, although it is rare, according to a U.S. study.
About 30 percent of people with recurrent migraines have sensory disturbances shortly before their headache hits, known as aura, but these are usually visual, such as flashes of light or blind spots. Tingling sensations or numbness, or difficulty speaking or understanding language, may also appear.
But the study, conducted by Matthew Robbins and colleagues at the Montefiore Headache Center in New York, found that a small number of people described smelling scents in conjunction with their headaches.
"It's uncommon, but distinctive," said Robbins, noting that disturbances in the sense of smell, known as olfactory hallucinations, have not been covered in a systematic review of medical literature before.
Researchers reviewed 25 reported cases of patients with headaches, migraines in most cases, and olfactory hallucinations. They also examined records from more than 2,100 patients seen over 30 months. Fourteen people, or just under 0.7 percent, had described smelling scents ahead of their headaches.
"The most common was of the burning or smoke variety," Robbins said.
Some sufferers described a general burning smell. others said they smelled cigar smoke, wood smoke or burned popcorn.
"Decomposition" odors, such as garbage or sewage, were the next most common smell reported. A few people described pleasant odours, including the scent of oranges, coffee or, in one case, foie gras.
About 11 percent of the world's population suffers from migraines, so even though olfactory hallucinations are an unusual part of aura, there could still be a fairly large number of people who experience them, Robbins said.
It's not clear why the hallucinated odors are most often unpleasant, or why they are only rarely part of migraine aura.
But 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, Robbins said.
That same phenomenon might underlie olfactory hallucinations -- and because the brain's smell centers occupy much less space than its sight centers, that could, in theory, explain why phantom scents are so much less common, he added.
It is also possible that some people with migraines and olfactory hallucinations do not recognize the phenomenon, he added. People know something is wrong when they see zigzag lines, but it is easy to assume a small is actually coming from somewhere.
Since some disorders, such as Parkinson's disease, can cause a person to smell scents that are not present, any such hallucinations without an accompanying headache should be checked out, he warned.