The arginine growth hormone stimulation test in bradykinetic-rigid parkinsonisms

The arginine growth hormone (GH) stimulation test differentiates the Parkinsonian variant of multiple system atrophy (MSA-P) from idiopathic Parkinson's disease (PD). Our aim was to evaluate the accuracy of the arginine GH stimulation test in distinguishing between PSP, MSA-P, and PD. We measured the GH response to arginine in serum samples of 26 MSA-P, 23 PSP, and 26 PD patients, and in 80 healthy controls. We used ANOVA followed by the Bonferroni test to compare GH values and peaks among groups. We used receiver operating characteristic curve analysis to establish the arginine cut-off level that best differentiated between MSA-P, PSP, and PD. The GH peak was significantly lower (P < 0.01) in MSA-P (1.46 +/- 0.29 microg/L) than in both PD (8.74 +/- 0.98 microg/L) and PSP (6.64 +/- 0.82 microg/L) patients, and controls (8.59 +/- 0.44 microg/L). Growth hormone peaked later in PSP patients than in PD patients and controls. At a cut-off level of 4 microg/L, arginine test distinguished MSA-P from PD with a sensitivity of 92% and a specificity of 96%, and MSA-P from PSP with a sensitivity of 78% and a specificity of 96%. The GH response to arginine differentiates MSA-P from PD and PSP with a good diagnostic accuracy. The neuroendocrine response to arginine of PSP patients differed from that of MSA-P patients, but was not identical to that of normal controls and PD patients. Our results suggest that the impairment of the central mechanisms modulating GH release differs between PSP and MSA-P.