Neurodegenerative Disease Parkinson’s Disease Olfactory Function in Parkinson’s Disease Antje Haehner,1 Thomas Hummel2 and Heinz Reichmann3


1. Assistant Professor, Department of Otorhinolaryngology and Department of Neurology; 2. Professor, Department of Otorhinolaryngology; 3. Professor of Neurology, Department of Neurology, University of Dresden Medical School


Abstract


Impairment of olfaction is a characteristic and early feature of Parkinson’s disease (PD). Recent data indicate that >95% of patients with PD present with significant olfactory loss. Deficits in sense of smell may precede clinical motor symptoms by years and can be used to assess risk of developing PD in otherwise asymptomatic individuals. This article summarises the available literature on olfactory function in PD, which indicates that olfactory tests improve the diagnostic process in these patients; however, it is still unclear which lesion causes olfactory loss in PD.


Keywords Parkinson’s disease, parkinsonism, smell, olfaction, diagnosis


Disclosure: The authors have no conflicts of interest to declare. Received: 5 April 2010 Accepted: 25 May 2010 Citation: European Neurological Review, 2010;5(1):26–9 Correspondence: Antje Haehner, Department of Otorhinolaryngology, University of Dresden Medical School, Fetscherstr. 74, 01307 Dresden, Germany. E: antje.haehner@uniklinikum-dresden.de


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 to 74% in the work of Hawkes et al.,4 published by Doty et al.5


and Ward,3 In our recent multicentre study6 respectively, up


or as high as 90% in a study 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 disease4,5,7


and the clinical severity of PD as measured 26


by means of the Hoehn and Yahr scale and the Unified Parkinson’s Disease Rating Scale (UPDRS),8


or results from an odour-discrimination task.10


although some studies found a


correlation between the severity of PD and certain measures of olfactory function, namely latencies of olfactory event-related potentials9


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 reports by Stern and colleagues,7


the current findings are in contrast to who reported significantly better


odour identification scores in patients with tremor-predominant PD (n=40) than in cases with postural instability gait disorder- predominant 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.


Olfactory Dysfunction as an Early Symptom of Parkinson’s Disease


on questioning prior to olfactory testing, nine of 37 patients (24%) indicated an awareness of a decrease of olfactory function that


PD patients frequently report reduction in their sense of smell that occurs a few years prior to the onset of motor symptoms. However, the unawareness of patients of smell deficits may account for the inconsistent results described in retrospective surveys. In a small study13


© TOUCH BRIEFINGS 2010


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