Increased growth hormone response to apomorphine in Parkinson disease compared with multiple system atrophy

Background: Parkinson disease (PD) is often difficult to distinguish from p arkinsonian syndromes of other causes in early stages of the disease. In se arch of a suitable endocrinologic challenge test, we investigated dopaminer gic sensitivity in patients with de novo parkinsonian syndromes. Objective: We measured the growth hormone (GH) response to a subthreshold d ose of the dopamine l-dopamine 2 receptor agonist apomorphine hydrochloride to differentiate parkinsonian syndromes from PD. Patients and Methods: Seventeen patients with a clinical diagnosis of PD, 1 6 patients with a clinical diagnosis of multiple system atrophy, and 11 hea lthy controls. The GH response to a subthreshold dosage of apomorphine and to somatorelin (GH-releasing factor) was tested in a randomized order; on t he third day the protocol was repeated with a clinically effective dose of apomorphine. Results: The GH response to the low dose of apomorphine was significantly i ncreased in patients with PD when compared with patients with multiple syst em atrophy or the control subjects (multivariate analyses of covariance; un ivariate F test, all P < .05). In contrast, there were no significant group differences with use of the higher dose of apomorphine or in the somatorel in-induced GH release Conclusions: The GH response to a subthreshold dose of apomorphine appears to be a useful tool to identify patients with PD vs multiple system atrophy . The enhanced GH response to a subthreshold dopaminergic stimulus may refl ect a hypersensitivity of the extrastriatal dopamine receptors in PD.