This calls attention to the fact that taste depends largely on the volatile particles in foods and beverages, which reach the olfactory receptors through the nasopharynx, and that the perception of flavor is a combination of smell, taste, and tactile sensation. This can be proved by demonstrating that patients with anosmia but without a complaint of ageusia are able to distinguish the elementary taste sensations on the tongue (sweet, sour, bitter, and salty). The olfactory defect can be verified readily enough by presenting a series of nonirritating olfactory stimuli (vanilla, peanut butter, coffee, tobacco) and asking the patient to sniff once and identify them. If the odors can be detected and described, even if they cannot be named, it may be assumed that the olfactory nerves are relatively intact (humans can distinguish many more odors than they can identify by name). If they cannot be detected, there is an olfactory defect. Ammonia and similar pungent substances are unsuitable stimuli because they do not test the sense of smell but have a primary irritating effect on the mucosal-free nerve endings of the trigeminal nerves.
The value of testing smell in one nostril at a time has been questioned, for example by Welge-Luessen and colleagues, who studied olfactory groove meningiomas. They found, contrary to expectations, that this test was not sensitive to the presence of a unilateral lesion, ostensibly because of mixing of air in the nasopharynx as well as crossing of medial olfactory stria fibers as described earlier. Nonetheless, other experience suggests that rapidly sniffing through one nostril does briefly allow segregation of each side of the nasal cavities and can detect unilateral lesions.
A more elaborate scratch-and-sniff test has been developed and standardized by Doty and colleagues (University of Pennsylvania Smell Identification Test). In this test the patient attempts to identify 40 microencapsulated odorants, and olfactory performance is compared with that of ageand sex-matched normal individuals. Unique features of this test are a means for detecting malingering and amenability to self-administration. Air-dilution olfactory detection is an even more refined way of determining thresholds of sensation and of demonstrating normal olfactory perception in the absence of odor identification. The use of olfactory evoked potentials is being investigated in some electrophysiology laboratories, but their reliability is uncertain. These last two refined techniques are essentially research tools and are not used in neurologic practice.
The loss of smell usually falls into one of three categories: nasal (in which odorants do not reach the olfactory receptors), olfactory neuroepithelial (caused by destruction of receptors or their axon filaments), and central (olfactory pathway lesions).
Viewed from another perspective, in an analysis of 4,000 cases of anosmia from specialized clinics, Hendriks found that the three most common diagnoses were viral infection of the upper respiratory tracts (the largest group), nasal or paranasal sinus disease, and head injury.