FUNCTION -- Olfaction

TEST
Tests of cranial nerve I, the olfactory nerve, begin with an assessment of the patency of the nasal breathing passages. The patient should occlude a nostril and demonstrate that air passes freely through the open nostril with both inspiration and expiration. Then, the demonstration should be repeated for the other nostril. The test itself involves occlusion of a single nostril while the eyes of the patient are closed. The patient should inhale gently through the open nostril in close proximity to a common odorant (e.g., vanilla, ground coffee, fresh orange, etc.). The patient should be asked first whether the odorant can be detected. If detectable, the odorant should be identified by name. If the patient fails at the task, the test should be repeated with at least one distinct odorant. The test should be repeated for the opposite nostril using entirely new odorants.

INTERPRETATION
The failure to detect and identify odorants (anosmia) unilaterally often indicates damage to the central nervous system, commonly involving cranial nerve I itself, the thalamus, the frontal lobe or connections among these structures. Etiology may involve traumatic injuries, aneurysms, hemorrhages, or tumors. Bilateral failures to identify odorants may also reflect central pathology, but peripheral causes must be excluded. Common peripheral sources of anosmia are the smoking of tobacco, colds, flues, allergies, and the nasal consumption of cocaine. Importantly, it is not unusual to see changes in personality in persons exhibiting anosmias of central origin (particularly social inappropriateness and emotional lability). Also evident may be perseveration, short-term mnemonic deficits, decreased motivation, mutism, and decreased mobility. Always note, however, that it is not uncommon for neurologically intact individuals to exhibit generally poor olfactory senses bilaterally. Further, some patients may exhibit the otherwise benign failure to detect and/or identify a specific odorant (i.e., odorant-specific anosmia), while the general sense of smell seems to be preserved.

PITFALLS AND SAFEGUARDS
Longer or repeated exposures to a given odorant do not usually help the patient with identification. Do not use irritating substances in tests of cranial nerve I (e.g., ammonia, hot pepper). Take care to avoid passage of solid material into the nostrils. Keep the odorants concealed from the view of the patient before and during the test. Prepare to use more than one odorant (for each nostril), particularly when testing persons familiar with the neurological examination (e.g., repeat patients; professional or trainee clinicians) – knowledgeable patients may be able to make educated guesses as to the identity of the odorant, thus defeating the test.