Levodopa in combination with carbidopa does not affect plasma arginine vasopressin levels in treatment-naïve older patients with Parkinson's disease: A before-after study

Several cases of syndrome of inappropriate antidiuresis (SIAD) induced by antiparkinson agents have been reported. Our previous study demonstrated that in some patients with Parkinson's disease (PD), pergolide and pramipexole stimulate elevations of plasma arginine vasopressin (AVP) levels even at doses that are lower than the ordinary maintenance dose. Although mean plasma AVP levels are significantly higher in treated PD patients than in treatment-naïve patients, neither disease severity nor levodopa/carbidopa dosage (range, 300/30-850/85 mg) correlates with plasma AVP levels. However, the effects of levodopa/carbidopa monotherapy on plasma AVP levels in older patients remain unknown. To address this issue, a 14-day before-after study was conducted. Subjects in this study were consecutive treatment-naïve patients with a diagnosis of possible PD who visited our clinic from November 2008 to September 2009. Patients had no conditions that could be associated with high plasma AVP levels. Twenty-five patients were treated with levodopa/carbidopa (100/10 mg) 3 times a day. A paired t-test was used to compare plasma AVP levels before and 14 days after initiation of treatment. Five patients dropped out of this study. In the remaining 20 patients (8 males and 12 females), no significant differences were observed between mean plasma AVP levels before and during treatment with levodopa/carbidopa. Monotherapy with levodopa/carbidopa (300/30 mg/day) does not affect plasma AVP levels in older PD patients and seems less likely to cause SIAD.