Introduction
 Multiple system atrophy (MSA) is characterized clinically by the combination of autonomic dysfunction, extrapyramidal dysfunction, and cerebellar dysfunction. The lower urinary tract dysfunction (LUTD) is prevalent and severe in MSA. In particular, voiding symptoms is common and many MSA patients usually show large post-void residual (PVR). 
 Although, large PVR in MSA is caused by impaired bladder contraction attributable to the neurodegeneration of parasympathetic nucleus in sacral cord innervating bladder, the definitive criteria for detrusor underactivity in urodynamic findings in MSA are not well established.
 There are several methods evaluating detrusor contraction. Although, Schafer’s nomogram is popular in men with benign prostatic enlargement, this nomogram is not validated for women. Watts factor is available for both genders, however complex calculation is needed which limit its use in clinical practice. Bladder contractility index (BCI) based on the projected isovolumetric pressure (PIP) described as PIP5=Pdet@Qmax+5Qmax is frequently used for both genders. However, some studies recommend to use PIP1=Pdet@Qmax+Qmax for old women instead of using PIP5 which might overestimate bladder contractility in women. In general, PIP can be described as 
PIP k=Pdet@Qmax+kQmax. We aimed to clarify which “k” value is suitable for representing bladder contractility in terms of correlations to PVR during pressure flow study.

We retrospectively reviewed 133 patients with MSA (male n=74, female n=59, mean disease duration 3.2 years) who had several lower urinary tract symptoms and underwent urodynamic study. All patients in this study were diagnosed as probable or possible MSA according to Gilman’s second consensus criteria. We excluded MSA patients who could not void during pressure flow study and MSA patients having comorbid urological conditions such as benign prostatic hyperplasia. We calculated various PIP values using formula PIP k=Pdet@Qmax+kQmax by increasing “k” value from 0.1 to 10 in increments of 0.1. We calculated the correlations between each PIP k (k=0.1 to 10.0) and PVR during pressure flow study in both genders.

In male MSA patients, mean Pdet@Qmax was 38.9±2.0 cmH2O, mean Qmax during pressure flow study was 7.65±1.0 ml/s, mean PVR during pressure flow study was 245.3±14.6ml. In female patients, mean Pdet@Qmax was 26.0±1.7 cmH2O, mean Qmax during pressure flow study was 9.19±1.0 ml/s, mean PVR during pressure flow study was 184.3±17.4ml. Mean Pdet@Qmax and Q max during pressure flow study was significantly smaller and PVR during pressure flow study was significantly larger in male patients compared to female patients. The correlational coefficients between PIP k and PVR during pressure flow study showed significant negative correlation when “k” value is larger than 1.2 in female patients and correlational coefficients reached plateau level when “k” value is larger than 6.0 (Figure 1b). In male patients, the correlational coefficients between PIP k and PVR during pressure flow study showed significant correlations in all “k” value larger than 0.1 and showed largest negative correlational coefficients when “k” value is 2.0 (Figure 1a).

The present study suggested that appropriate “k” value for PIP might differ depending on pathological condition and gender with respect to correlation to PVR. PIP2 (Pdet@Qmax+2Qmax) might be appropriate for evaluating bladder contractility in male MSA patients and “k” value larger than 6 might be appropriate in female MSA patients. The relationships between “k” value and the corresponding correlational coefficients between PIP k and PVR are quite different between male and female MSA patients. Because MSA usually do not show bladder outlet obstruction (BOO) and neurodegenerative changes in autonomic nervous system including sacral parasympathetic nucleus innervating bladder is pathological hallmark, PVR is largely influenced by impaired bladder contractility rather than BOO in MSA. Although, “k” value of 5 is commonly used in both gender and “k” value of 1 is occasionally used in female patients, appropriate “k” value should be examined depending on pathological conditions and gender.

The appropriate PIP k formula might differ depending on pathological conditions and gender in terms of correlations to PVR during pressure flow study.