Clinical diagnosis occurs at symptom presentation

A correct diagnosis of PD allows appropriate forms of therapy and counselling to be made available to the patient.² As discussed previously, the progression of PD can be subtle at first, manifesting as relatively minor issues, such as experiencing difficulty rising from a chair.¹ Typically, younger individuals with PD show less rigidity and bradykinesia compared with older adults, which may delay the accurate diagnosis of their condition.^1,3

Diagnosis of PD most commonly occurs on presentation of one or more of the cardinal symptoms, such as bradykinesia, resting tremor, or rigidity.¹ In the early stages of the disease, the response to dopaminergic therapy is sustained.⁴ In the later stages of PD, the loss of neurons in the substantia nigra plus other (e.g., post-synaptic) factors mean that the beneficial effects of each dose of dopamine precursor (levodopa in its formulations) become progressively more short-lived.⁴

References:
1.DeMaagd G, Philip A. Parkinson’s disease and its management. Pharmacol Therapeut 2015; 40 (8): 504–510.

2.Berardelli A, Wenning GK, Antonini A, et al. EFNS/MDS-ES/ENS recommendations for the diagnosis of Parkinson’s disease. Eur J Neurol 2013; 20 (1): 16–34.

3.Suchowersky O, Reich S, Perlmutter J, et al.; Quality Standards Subcommittee of the American Academy of Neurology. Practice parameter: diagnosis and prognosis of new onset Parkinson disease (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 2006; 66 (7): 968–975.

4.Thanvi BR, Lo TCN. Long term motor complications of levodopa: clinical features, mechanisms, and management strategies. Postgrad Med J 2004; 80 (946): 452–458.