Phantosmia

Excerpt

Phantosmia is a qualitative olfactory disorder wherein an odorant is perceived in the absence of an identifiable stimulus. This is distinct from the closely-related qualitative olfactory disorder, parosmia, wherein an erroneous odorant is perceived in response to an identifiable stimulus.

Phantosmia has other names, including "olfactory hallucinations" or "phantom odors." Olfactory disorders are often classified as being either conductive or sensorineural. Conductive olfactory dysfunction involves obstruction of an odorant from interacting with the olfactory mucosa. Sensorineural dysfunction is a result of impaired transmission or processing of olfactory stimuli.

Other classifications, including central and peripheral, have also been used to describe phantosmia. Peripheral causes of phantosmia are thought to arise from disruption and alteration of the olfactory mucosa and sensory receptors, whereas central phantosmia is believed to originate from aberrant central integration and interpretation.

Phantosmia can have detrimental effects on a patient's quality of life. The sense of smell is a critical component of the enjoyment of life. It is intimately involved with taste, social life, and the ability to perceive potential environmental dangers.

Anatomy and Physiology

The olfactory epithelium is a pseudostratified columnar neuroepithelium at the superior nasal vault, between the septum and middle turbinate. The basal cell layer of the olfactory epithelium contains stem cells, serving as the basis for its regenerative nature. Superficial to the basal layer lie olfactory receptor neurons, which transmit odorant signals through the olfactory nerve with or without inputs from the trigeminal nerve to the central nervous system, lending to odor perception.

Each olfactory neuron expresses a specific G-protein coupled receptor, which, when bound by odorants, leads to signal transmission through the cribriform plate to the olfactory bulb. Within the olfactory bulb, olfactory nerves of similar converge onto specific glomeruli to synapse with second-order neurons.

Second-order neurons carry olfactory signals within the olfactory tract to primary olfactory cortex structures such as the piriform cortex, rostral entorhinal cortex, and the periamygdaloid cortex. Further processing and integration occur in the secondary and tertiary olfactory networks, comprised of areas including the thalamus, hypothalamus, and dorsolateral frontal cortex.