Sniffing out Parkinson disease: Can olfactory testing differentiate parkinsonian disorders?

BACKGROUND:: Akinetic-rigid syndromes are a heterogeneous group of diseases with overlapping clinical manifestations. A recent American Academy of Neurology practice parameter indicated that olfactory testing was "probably useful" for differentiating idiopathic Parkinson disease (IPD) from other diseases with features of parkinsonism. OBJECTIVE:: To determine the diagnostic accuracy of olfactory testing for differentiating IPD from other parkinsonian disorders. METHODS:: The objective was addressed using a structured, evidence-based, critically appraised topic format. This format includes development of a clinical scenario, focused and answerable clinical question, search strategy, evidence appraisal, reporting and interpretation of results, commentary, and bottom line conclusions. Participants included neurology consultants and residents, clinical epidemiologists, a medical librarian, and neurologists with expertise in movement disorders. RESULTS:: Two studies evaluated the diagnostic accuracy of olfactory testing within a broad spectrum of parkinsonian syndromes. Each study examined different test methods [University of Pennsylvania Smell Identification Test; "Sniffin' Sticks"] and both were compromised by several potential biases in sampling and outcome evaluation. The University of Pennsylvania Smell Identification Test is moderately sensitive (77%; likelihood ratio 0.27) and specific (85%; likelihood ratio 4.9) for differentiation of IPD from non-IPD syndromes, but less specific (62%; likelihood ratio 2.0) for distinguishing IPD from multiple system atrophy. CONCLUSION:: The diagnostic accuracy of olfactory testing for differentiating IPD from other disorders is insufficient to justify its routine clinical use but available evidence is derived from small samples and studies of questionable validity. Recommendations for future research of olfactory testing for diagnosis and disease predication are discussed.