It is usually difficult to differentiate multiple system atrophy (MSA) from Parkinson’s disease (PD) in the early stage because parkinsonism is present in PD and MSA (especially the parkinsonian type of MSA). Although levodopa responsiveness is useful in differentiating MSA from PD, some MSA patients show levodopa responsiveness in the early stage. Lower urinary tract dysfunction (LUTD) is prevalent and severe in PD and MSA. We previously reported that post-void residual (PVR) is significantly larger in MSA patients compared to PD patients and PVR is useful in differentiating MSA from PD [1,2].
 However, PVR depends on maximum bladder capacity, and patients with smaller bladder capacity might have smaller PVR, suggesting that smaller PVR does not necessarily represent mild voiding dysfunction.
 Voiding efficiency (VE) might probably represent voiding dysfunction in patients with small bladder capacity and therefore might be appropriate in differentiating MSA from PD. 
 We, therefore, aimed to examine whether voiding efficiency is useful in differentiating MSA from PD as compared to PVR.

We retrospectively reviewed PD patients (n=85, mean age 66.9 years, mean duration 3.2 years) and MSA patients (n=237, mean age 63.3 years, mean duration 3.2 years) who underwent a urodynamic study. We examined whether PVR and VE during the pressure-flow study (PFS) are useful in differentiating MSA from PD by performing receiver operating characteristics (ROC) analysis.

PVR was significantly larger in MSA patients (136.8±7.6ml) compared to PD patients (49.7±8.4ml) (p<0.01) and VE was significantly smaller in MSA patients (33.9±2.0%) compared to PD patients (79.5±3.0%) (p<0.01). The area under the curve (AUC) used for differentiating MSA from PD was 0.86 for VE and 0.81 for PVR with a statistically significant difference (p<0.01) (Figure).

Our result suggested that VE might be more appropriate than PVR in differentiating MSA from PD. Because some MSA patients show reduced bladder capacity, indicating that PVR does not necessarily represent the degree of voiding dysfunction, the present result might be helpful for clinical practice. We evaluated the PVR during PFS rather than during the free-flow study because we previously reported that PVR during PFS had a larger AUC compared to PVR during the free-flow study [2]. 
 Because MSA patients usually showed both storage and voiding dysfunction attributable to the degeneration in spinal autonomic neurons, the precise examination of LUTD is usually difficult in MSA patients. The present result might suggest that VE is useful for differentiating MSA from PD.

VE might be helpful in differentiating MSA from PD compared to PVR.

Yamamoto T, Sakakibara R, Uchiyama T, Yamaguchi C, Nomura F, Ito T, Yanagisawa M, Yano M, Awa Y, Yamanishi T, Hattori T, Kuwabara S. Receiver operating characteristic analysis of sphincter electromyography for parkinsonian syndrome. Neurourol Urodyn. 2012 Sep;31(7):1128-34. doi: 10.1002/nau.22208. Epub 2012 Mar 30. PMID: 22473520.
Yamamoto T, Asahina M, Yamanaka Y, Uchiyama T, Hirano S, Fuse M, Koga Y, Sakakibara R, Kuwabara S. The Utility of Post-Void Residual Volume versus Sphincter Electromyography to Distinguish between Multiple System Atrophy and Parkinson's Disease. PLoS One. 2017 Jan 6;12(1):e0169405. doi: 10.1371/journal.pone.0169405. PMID: 28060892; PMCID: PMC5217958.