Hallucinations related to smells (phantosmia) can occur in a number of neurologic or psychological conditions.
Features of phantosmia:
(1) perception of an odor in the absence of an odorant stimulus
(2) exclusion of other explanations such as hyperosmia
(3) a negative impact on the patient (frequent, unpleasant, disruptive, etc)
Categories:
(1) type of smell: An unpleasant phantosmia is called a cacosmia.
(2) whether one (unirhinal) or both (birhinal) nares are affected
(3) transient vs prolonged/recurrent
(4) peripheral (involves olfactory receptor neurons) vs central (involves cortical neurons)
Conditions associated with transient olfactory hallucination:
(1) drug or alcohol withdrawal
(2) chemical exposure
(3) acute paranasal inflammation
Conditions associated with prolonged or recurrent phantosmia:
(1) migraine
(2) Parkinson's Disease
(3) thrombosis of the internal carotid artery or stroke
(4) intracanial aneurysm
(5) schizophrenia
(6) depression
(7) associated with radiation therapy to the brain
(8) trauma to or surgery through the cribriform plate
(9) seizure disorder
(10) chronic paranasal inflammation
(11) brain tumor
(12) pregnancy
In some patients a precise cause is not identified.
