In this study it appears that autonomic disfunction as in orthostatic parasympathetic withdrawal is common in AS:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1752931/More overt autonomic disfunction of either sympathetic or parasympathetic nervous system is found in Chronic fatigue syndrome and in conditions of postural hypotension or orthostatic intolerance such as POTS or OI. All of these conditions have the cardinal hallmark of fatigue as the main symptom.
Secondly, autonomic disfunction is suggested as the possible cause of fatigue in other systemic autoimmune illnesses such as Sjorgen SYndrome and RA and is commonly found in more severe forms in MS.
