The clinical significance of electrophysiological measures of olfactory function

To establish the detectability of olfactory event-related potentials (OERP) in relation to the results from psychophysical tests of olfactory function.

Fifty-nine men and 64 women (aged 19-89 years) participated all of whom presented themselves to a specialized "Smell and Taste Clinic" because of chemosensory complaints. Their olfactory function was assessed by means of psychophysical tests, e.g., assessment of odor threshold, odor discrimination, and odor identification, which were combined in a composite "Threshold Discrimination Identification" score ("TDI score"). OERP were obtained in response to the olfactory stimulant phenyl ethyl alcohol; their presence/absence was judged by a trained observer. Using logistic regression the TDI score was calculated in relation to the subjects' olfactory function at which the probability of the presence of OERP became greater than chance.

Based on psychophysical tests, 40 subjects were diagnosed with functional anosmia, 40 with hyposmia, and 43 subjects scored within the normal range. Causes of hyposmia and anosmia included congenital anosmia, Parkinson's disease, head trauma, infections of the upper respiratory tract, and sinunasal disease. A TDI score of 22.6 equivalent to "pronounced hyposmia" was identified as the turning point at which the probability of detection of OERP was higher than 50%. Its 95% confidence interval of 16.1-27.8 well reflected the range of hyposmia, i.e., was above significant loss of olfactory function (functional anosmia, TDI score<15.5) and below normal olfactory function (TDI score>30.5).

The present results indicate that the probability to detect an OERP becomes greater than 50% within a range of olfactory function that separates functional anosmia from normosmia. Presence of OERP clearly signifies presence of olfactory function while this is not always the other way around with absence of OERP.