Tests of Olfaction: Use, Indications, and Interpretation

Approximately 5% of the general population suffers from anosmia, unrelated to chronic nasal problems. Olfactory dysfunction includes: (1) quantitative disorders—anosmia (inability to detect odors) and hyposmia (decreased olfactory function)—and (2) qualitative disorders—parosmia (altered odor perception) and phantosmia (smell of nonexistent odors). Olfactory dysfunction often presents as a loss of taste, reflecting the contribution of the retronasal route to the sense of taste.

The three most common causes of olfactory disorders are: (1) sinonasal disease (30%), (2) postviral olfactory loss (20%), and (3) head trauma (15%). Olfactory loss is also common in patients with idiopathic Parkinson's disease and Alzheimer's disease.

There are a variety of commercially available olfactory tests. The most widely used in clinical practice are the University of Pennsylvania Smell Identification Test (UPSIT) and the “Sniffin' Sticks” test battery. The UPSIT involves 40 microencapsulated odors in a scratch-and-sniff format; it has only one use, assessing odor identification. The Sniffin' Sticks test is based on pen-like, odor-dispensing devices and has multiple uses: tests for odor threshold, discrimination, and identification.

Whatever comprises a clinical olfactory test, it should reliably distinguish between anosmic, hyposomic, and normosomic subjects wherever it is used. Thus, the test should be used after validation with cross-cultural adaptation.

In addition, as psychophysical tests depend on patients' responses, they are not reliable in cases of malingering. Objective methods of olfactory assessment are needed for medicolegal cases such as fMRI, event-related potentials, and brain SPECT.

A short course of oral steroids is the most useful tool in the management of anosmia due to sinonasal disease. In postviral and traumatic cases, there is no proven therapy. Counseling and olfactory retraining can help these patients. Cessation of smoking may help to restore olfactory function.