There is convincing evidence from numerous studies using both psychophysical and electrophysiological approaches that olfaction is markedly reduced in Parkinson’s disease (PD). In light of the current data on smell loss in PD patients, olfactory dysfunction has to be seen as a cardinal symptom of the disease that is even more consistent than the classic symptom of tremor.1 Consequently, structured and validated tests of olfactory function should be a mandatory part of the early and differential diagnosis of PD. Olfactory tests are quick and easy to use and testing can be performed in a reliable fashion even in non-specialised centres.
Olfactory Loss in Parkinson’s Disease
The association between olfactory dysfunction and PD was noted more than 30 years ago.2 Virtually all studies performed since then have shown olfactory disturbances in PD patients. However, data on the prevalence of olfactory dysfunction in PD range from 45 and 49% in the pioneering studies of Ansari and Johnson2 and Ward,3 respectively, up to 74% in the work of Hawkes et al.,4 or as high as 90% in a study published by Doty et al.5 In our recent multicentre study6 using a comprehensive testing method in a large sample of PD patients (n=400) from three independent populations, the prevalence of olfactory dysfunction in people with PD was greater than previously reported in terms of norms obtained in healthy young subjects. More than 96% of PD patients were found to present with olfactory dysfunction (see Figure 1). When using age-dependent normative criteria, 74.5% of this study population was diagnosed with olfactory loss (see Figure 2). Furthermore, >80% of PD patients with smell loss were functionally anosmic or severely hyposmic regardless of the olfactory test being used for diagnosis.