Is clonidine growth hormone stimulation a good test to differentiate multiple system atrophy from idiopathic Parkinson's disease?

Clonidine, a centrally active alpha (2)-adrenoreceptor agonist used to lowe r blood pressure, has been proposed to differentiate central from periphera l autonomic deficits and multiple system atrophy (MSA) from untreated idiop athic Parkinson's disease (IPD). A lack of growth hormone (GH) increase aft er clonidine infusion is found in patients with MSA, but not in those with IPD or with pure autonomic failure. We studied 19 IPD and 7 MSA patients to assess whether this test could be used in clinical practice to distinguish MSA from IPD, whatever the stage of the disease. Serum GH levels were meas ured 15, 30, 45 and 60 min after a 10-min infusion of 2 mug/kg clonidine. G H levels remained stable after clonidine infusion in all 7 MSA patients but increased in only 12 of the 19 IPD patients, while remaining stable in the other 7. No correlation was found with the presence of orthostatic hypoten sion. We conclude that the GH response to clonidine infusion has a very hig h sensitivity (100% in our series and in previous studies) for the diagnosi s of MSA. However, this response cannot be used as a diagnostic test becaus e of its poor specificity.