15 - Olfaction in Parkinsonian Syndromes

Summary

Introduction

Interest in the smell dysfunction of patients with extrapyramidal disorder has increased in recent years with the recognition that most patients with Idiopathic Parkinson's Disease (IPD) are hyposmic and the possibility that olfactory disorder might be an initial event preceding the classical signs of the disease. This has been aided by ¹⁸F-dopa positron emission spectroscopy (PET) scan and latterly by the less expensive dopamine transporter scan (DATScan) technique, both of which image cerebral dopamine distribution and raise the possibility of presymptomatic diagnosis. Thus, olfactory disorder can act as a biomarker of a pending disease and may afford the possibility of neuroprotective therapy.

Olfactory testing

Most clinicians do not enquire about olfaction let alone perform any tests of it. At least one-third of the subjects with hyposmia are unaware of their defect (Hawkes et al., 1997) and others complain of loss of taste instead: thus, it is insufficient simply to ask a patient about their sense of smell. Local nasal disease has to be excluded by clinical examination, endoscopy and ideally computed tomography/magnetic resonance imaging (CT/MRI), but a useful clue is that when anosmia is intermittent, the problem is probably conductive i.e. air cannot reach the olfactory neurons in the nose. Conversely, continual anosmia is characteristic of sensorineural loss.

Age has a profound effect on smell function and our recent analysis of smell identification score (Hawkes et al., 2005) showed that ageing effects start at 36 years (Figure 15.1).