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Neurology Now:
doi: 10.1097/01.NNN.0000412281.39813.37
Departments: Ask the Experts

I have “olfactory hallucinations” in which I smell smoke. What could be causing this?

DeVere, Ronald M.D.

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Ronald DeVere, M.D., Fellow of the American Academy of Neurology (AAN), is the director of the Taste and Smell Disorder Clinic in Austin, TX. He has been in private practice for more than 30 years and has 15 years of experience in evaluating patients with taste and smell disorders. Dr. DeVere is also the coauthor, with Marjorie Calvert, of the AAN's patient book, Navigating Smell and Taste Disorders (Demos Health, 2010).

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OLFACTORY HALLUCINATIONS

Q I have “olfactory hallucinations” in which I smell smoke. What could be causing this?

Dr. RONALD DEVERE RESPONDS:

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A Olfactory hallucinations are perceived abnormal smells—usually unpleasant—that are not actually present in the physical environment. They can come from a number of different areas of the smell system. The length of time these smells last depends on the cause. If the smell of smoke occurs suddenly and continues for less than a few minutes, the site of origin is likely the smell region of the inner temporal lobe of the brain, called the uncus. The source could be an abnormal electrical discharge or “firing” in the brain (a seizure). Potential causes of this abnormality could be a brain tumor, inflammation, stroke, or an injury following head trauma. Confirming the cause requires an imaging study of the brain (MRI) and a brain-wave test (EEG). Usually, results of smell testing will be normal to minimally abnormal in a person who is experiencing this type of seizure. If a seizure disorder is suspected, antiseizure medications may be used to prevent a seizure and thus eliminate the smell.

Olfactory hallucinations lasting more than a few minutes to several hours are usually due to a disturbance of the smell system in the nose (olfactory organ or olfactory nerves) or in the olfactory bulb, which sits just inside the skull above the upper nose level. The term for this type of olfactory hallucination is dysosmia. Common causes of dysosmia are head and nose injury, viral damage to the smell system after a bad cold, chronic recurrent sinus infections and allergy, and nasal polyps and tumors. The brain is usually not the source. In these instances, sense of smell for other odors is often impaired as well, and the results of smell testing typically are abnormal.

Dysosmia usually disappears with time (three months to two years) without treatment. A thorough evaluation for the mentioned causes may include an MRI of the olfactory system and a nasal endoscopy, in which an ear, nose, and throat (ENT) physician looks inside nasal and sinus passages with a magnified scope. Dysosmia can be treated with normal saline nose drops administered with the head lowered (the top of the head should be pointing to the floor). It may also improve with some medications, such as gabapentin—a medication normally used for seizure disorders but that has also been shown to prevent unpleasant odors arising from injured smell receptors or their nerve branches. The use of gabapentin in this instance is considered off label, which means it is not approved by the FDA for this indication. This doesn't mean the medication is not effective and safe, but rather that the drug has not been officially studied and evaluated by the FDA for this condition.

The source of olfact...
The source of olfact...
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©2012 American Academy of Neurology

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