A predictive model for fatigue and its etiologic associations in primary biliary cirrhosis

Excessive day-time somnolence and autonomic dysfunction are biological processes prevalent in Primary Biliary Cirrhosis (PBC) that associate with fatigue. Here we explore how these biological associates inter-relate, and their cumulative impact upon typical clinical cohorts.

A predictive model for daytime hypersomnolence (Epworth Sleepiness Scale (ESS)) and autonomic dysfunction (Orthostatic Grading Scale (OGS)) was developed in a derivation cohort (n=124) and subsequently validated in a second cohort (n=114). Subjects also completed the disease specific quality of life tool, the PBC-40.

A composite predictive criterion (presence of either ESS > or =10 or OGS > or =4) for the presence of fatigue in PBC patients had a sensitivity of 0.71 (95% confidence intervals 0.59-0.81) and specificity 0.8 (0.67-0.9) (positive predictive value (PV); 0.84 (0.72-0.92), negative PV; 0.66 (0.53-0.78) for moderate or severe fatigue). Ninety-seven percent of severely fatigued patients (0% of non-fatigued) met the aetiology predictive criterion (chi(2) 49.6, P<.0001). Expression of both significant daytime somnolence and autonomic dysfunction was not associated with more severe fatigue, suggesting that there is a threshold effect for fatigue in PBC. When applied to a second independent cohort, the composite criterion retained strongly significant predictive value for fatigue.

A significant proportion of fatigue in PBC associates with one or both of autonomic dysfunction (OGS > or =4) and sleep disturbance (ESS > or =10). Those meeting both ESS and OGS criteria were not more severe fatigued than those meeting the diagnostic criterion for either OGS or ESS alone. A threshold effect for fatigue has implications for potential therapeutic interventions.