Association of Serum Indirect Bilirubin Concentrations with Motor Subtypes of Parkinson's Disease

Introduction: We aimed to investigate the change of serum indirect bilirubin (IBIL) concentrations in patients with Parkinson's disease (PD) and whether IBIL concentrations were associated with the motor subtypes of PD. Methods: A case-control study was performed to evaluate differences in bilirubin concentrations between 78 PD subjects and 78 controls. Venous blood samples were collected, and total bilirubin (TBIL), direct bilirubin (DBIL), and IBIL concentrations were analyzed between PD subjects and controls. PD patients were classified into three motor subtypes: tremor-dominant (TD), intermediate (I), and postural instability and gait disorder (PIGD). It was evaluated whether there were differences in IBIL concentrations between the different motor subtypes and between motor subtypes and controls. Results: PD patients had lower IBIL concentrations compared to controls (6.51 +/- 4.03 vs. 10.82 +/- 4.61, p< 0.001). There was no significant difference in IBIL concentrations between PD males and PD females (6.66 +/- 3.64 vs. 6.22 +/- 4.79, p =0.655). IBIL concentrations had negative relationships with levodopa-equivalent daily dose (LEDD) (R = -0.452, p < 0.001) and positive relationships with tremor score (R = 0.360, p = 0.001). IBIL concentrations were significantly lower for PIGD than for TD subtype (4.88 +/- 4.03 vs. 9.00 +/- 4.15, p< 0.001). The lower IBIL concentrations in PD compared to controls were mainly driven by the PIGD patients. Conclusions: PD subjects showed lower levels of IBIL compared to controls. Higher IBIL levels were associated with TD motor subtype in PD, which could be related to the antioxidative properties of IBIL.