Introduction

Traditional teaching in psychiatry has been that until proven otherwise the presence of olfactory, gustatory and visual hallucinations is a marker of underlying organic pathology. Visual hallucinations are seen in delirium or Lewy body disease, while olfactory and gustatory hallucinations raise the possibility of medial temporal pathology, especially epilepsy. The aims of this chapter are first to review the literature on olfactory hallucinations and second to discern any characteristics of olfactory hallucinations which allow those of organic aetiology to be distinguished from those of functional origin, such as those related to schizophrenia.

The subjective experience of olfactory phenomena in the absence of a stimulus has been generally referred to as an olfactory hallucination, though when experienced in the context of epilepsy the term olfactory aura has been preferred. In addition, other terms used to describe the same phenomenon have included olfactory reference syndrome, experiential hallucinations, and experiential responses. Each of these terms will be used in the course of this review and their origins explained. The available literature is replete with case series, case reports, highly selected populations and widely differing methods of assessment. While this limits the comparisons between studies, it is still possible to estimate the prevalence and nature of the olfactory hallucinations in many of the studies.

Epilepsy and olfactory auras

Numerous studies have examined groups of patients with regard to the presence and characteristics of olfactory hallucinations but there has been no attempt to standardise the descriptions across different studies, an understandably difficult task.