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. These data also confirmed numerous previous studies in terms of the missing correlation between olfactory loss and both duration of disease^4,5,7 and the clinical severity of PD as measured by means of the Hoehn and Yahr scale and the Unified Parkinson’s Disease Rating Scale (UPDRS),⁸ although some studies found a correlation between the severity of PD and certain measures of olfactory function, namely latencies of olfactory event-related potentials⁹ or results from an odour-discrimination task.¹⁰ A follow-up study in 27 PD patients¹¹ revealed that psychophysically tested olfactory function changes in an unpredictable manner. Thus, the olfactory deficit appears to be variable over time such that some patients develop anosmia while other patients convert from anosmia to hyposmia.

In terms of olfactory function, we did not find major differences between subtypes of PD, namely tremor-dominant PD, akinetic–rigid PD and mixed-type PD. While this confirms previous observations in a small sample of 37 patients,¹² the current findings are in contrast to reports by Stern and colleagues,⁷ who reported significantly better odour identification scores in patients with tremor-predominant PD (n=40) than in cases with postural instability gait disorderpredominant PD (n=23). Although differences between studies may be due to the type of olfactory test used, sample size, normative data and age distribution (which varied between these investigations), available data allow the conclusion that olfactory dysfunction is a highly reliable symptom of the disease.

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