Perception › Hallucination

Olfactory Hallucination

An olfactory hallucination is smelling something that is not actually present, with no external source for the odour. The smell is often, though not always, unpleasant, and it can occur in brief episodes or persist over longer periods.

Because the sense of smell is closely tied to certain structures deep in the brain, persistent olfactory hallucination has traditionally prompted particular attention to possible neurological causes alongside psychiatric ones.

Also known as

Phantosmia

The general medical term for a smell hallucination, used in both neurology and psychiatry.

Olfactory reference syndrome

A related but distinct pattern in which the person holds a fixed, false belief that they themselves emit a foul odour, which is a belief rather than a perceptual experience.

Recognition features

• A smell is perceived with no external source that could explain it
• Content is most often unpleasant (burning, decay, sewage) though pleasant odours are also described
• May be brief and episodic, or more sustained
• Occurs independently of what the person is actually smelling in their environment

Eliciting it at interview

• Ask directly, since smell is easy to overlook in a standard review of hallucinations: "Do you ever notice smells that don't seem to have a source, or that other people say they can't smell?"
• Ask whether the smell is pleasant or unpleasant, and whether it is linked to any particular time or situation
• Distinguish this from a fixed belief that one's own body smells bad, which is a different, delusional phenomenon

Distinguishing features

• Differs from olfactory reference syndrome, which is a delusional belief about self-odour rather than a perceptual experience
• Differs from a genuine, verifiable smell that others can also detect, however unpleasant
• Differs from a vivid olfactory memory, which the person recognises as recollection rather than a present-moment perception

Common misconceptions

• That an unpleasant smell hallucination is always linked to guilt or self-disgust; it is a perceptual phenomenon and does not necessarily carry symbolic meaning
• That olfactory hallucination is exclusively psychiatric; it is also a recognised feature of certain neurological conditions, including some forms of epilepsy

Related phenomena

Hallucination · Gustatory Hallucination

Practice this

A patient repeatedly reports smelling burning rubber, on and off over several days, though no one else in the room notices anything and no source can be found on careful checking of the environment. What is this? · A patient reports smelling her late mother's distinctive perfume, briefly and vividly, while looking through an old photo album, and immediately recognises this as a memory triggered by the photographs rather than a present, real smell. Before revealing the answer: is this an olfactory hallucination? · A patient reports both an unusual taste and an unusual smell with nothing present to account for either. Before revealing the answer, decide what distinguishes a gustatory hallucination from an olfactory hallucination. · A patient mentions smelling something odd occasionally. Which single follow-up question would most usefully help clarify whether this is an olfactory hallucination? · A patient describes smelling burning rubber that no one else can detect, and a thorough check of the environment finds no source. Why does the involvement of other people in checking matter here?

Illustration

A 45-year-old woman reports smelling burning toast for a few seconds at a time, several times a week, always followed by a strange, heavy feeling. There is never any toast in the house when it happens, and she has started to wonder whether something is wrong with her sense of smell rather than assuming, at first, that it might be psychiatric in nature.

References

• Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 3, 'Disorders of Perception'.