The Olfactory System and Its Disorders

Richard L. Doty


Semin Neurol. 2009;29(1):74-81. 

In This Article

Tests Of Olfactory Function

Accurate olfactory assessment is essential to (1) establish the validity of a patient's complaint; (2) characterize the specific nature of the problem; (3) reliably monitor changes in function over time, including those resulting from medical interventions or treatments; (4) detect malingering; and (5) establish compensation for permanent disability. Several patients who present with complaints of anosmia or hyposmia actually have normal function relative to their peers. Others are unaware of their dysfunction. In the case of PD, for example, ~90% of patients have a demonstrable olfactory loss, yet less than 15% are aware of the problem until being tested.

Electrophysiologic, psychophysical, and psychophysiologic tests are available for assessing smell function.[26,27] The most practical are psychophysical tests of odor identification and detection. The most widely used odor identification test, the University of Pennsylvania Smell Identification Test [UPSIT; known commercially as the Smell Identification Test™ (Sensonics, Inc., Haddon Heights, NJ)], was developed at our center and can be self-administered in 10 to 15 minutes by most patients in the waiting room and scored in less than a minute by nonmedical personnel.[28] This 40-item test, along with its briefer clones, is available in numerous languages and has been employed in hundreds of clinical and experimental studies. In this test, a patient is presented with 40 "scratch and sniff" odorant pads and is required to choose, from four response alternatives, an answer for each stimulus, even if none seems appropriate or no odor is perceived. This encourages careful sampling of the stimuli and provides a means for detecting malingering. Because chance performance is 10 out of 40, very low scores reflect avoidance, and hence recognition, of the correct answer. Norms based on responses from nearly 4000 people are provided, and an individual's percentile rank is established relative to persons of the same age and gender. Olfactory function can also be classified, on an absolute basis, into one of six categories: normosmia, mild microsmia, moderate microsmia, severe microsmia, anosmia, and probable malingering.

Threshold olfactory tests typically employ a dilution series of a stimulus in an odorless diluent, such as light mineral oil. In most clinical applications, the stimuli are presented via small sniff or squeeze bottles, or felt-tipped pen-like devices, using a series of ascending or descending concentration trials. As with odor identification tests, forced-choice response between odorant and blank trials is required. The reader is referred elsewhere for details on the various procedures used in assessing human olfactory thresholds.[29]

Despite the fact that bilateral testing detects most clinically meaningful cases of olfactory dysfunction, unilateral testing can detect deficits that are not ordinarily recognized. In general, bilateral tests measure the functioning of the better side of the nose.[30] To assess unilateral function, the naris contralateral to the tested side is occluded to prevent or minimize crossing of inhaled or exhaled air at the rear of the nasopharynx to the opposite side (so-called retronasal stimulation). In our clinic, we achieve this end by sealing the naris contralateral to testing using a piece of 3M Microfoam® tape (3M Corp, Minneapolis, MN) cut to fit its borders. The patient is instructed to sniff the stimulus normally and to exhale through the mouth.


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