Making the diagnosis of Parkinson’s disease (PD) is a common clinical situation faced by neurologists, geriatricians, and general physicians. In the UK, about 30 to 40 patients are diagnosed with PD every day.¹ However, the diagnosis is not always easy. The diagnostic reference standard for idiopathic PD is still histopathology, based on the loss of dopaminergic neurons in the substantia nigra, with Lewy bodies in the surviving neurons (fig 1).² But clearly any reference standard that requires postmortem examination is not very helpful to a clinician faced with a living patient. Unfortunately, there is no antemortem equivalent of the Lewy body.

The diagnosis, therefore, remains a clinical one in which laboratory and radiological investigations play only a small part. Clinicians must rely on interpreting a combination of clinical features, their onset, symmetry, progression, and response to treatment. This diagnostic process is subjective and so, unsurprisingly, is liable to error; two hospital based studies in the early 1990s found that only about three quarters of patients with a final clinical diagnosis of PD had it confirmed at postmortem (fewer when the clinical diagnosis at presentation was considered).^3,4 Although more recent studies suggest that the accuracy of diagnosis has improved, to perhaps 90% or more,^5,6 these came from highly specialised units, which may not be representative of most departments that see parkinsonian patients.

Several attempts have been made to improve diagnostic accuracy by developing better clinical criteria for parkinsonism and PD. Two of the most commonly used are the UK Parkinson’s Disease Society (PDS) Brain Bank criteria,² and the Gelb criteria⁷ (see boxes 1 and 2). These may improve the accuracy of the diagnosis of PD, …