Olfactory hallucinations in primary headache disorders: case series and literature review.

Background: Olfactory hallucinations (phantosmias) have rarely been reported in migraine patients. Unlike visual, sensory, language, brainstem, and motor symptoms, they are not recognized as a form of aura by the International Classification of Headache Disorders.

Methods: We examined the clinical features of 39 patients (14 new cases and 25 from the literature) with olfactory hallucinations in conjunction with their primary headache disorders.

Results: In a 30-month period, the prevalence of phantosmias among all patients seen at our headache center was 0.66%. Phantosmias occurred most commonly in women with migraine, although they were also seen in several patients with other primary headache diagnoses. The typical hallucination lasted 5-60 minutes, occurred shortly before or simultaneous with the onset of head pain, and was of a highly specific and unpleasant odor, most commonly a burning smell. In the majority of patients, phantosmias diminished or disappeared with initiation of prophylactic therapy for headaches.

Conclusions: We propose that olfactory hallucinations are probably an uncommon but distinctive form of migraine aura, based on their semiology, timing and response to headache prophylaxis.