Also started taking nortriptyline at the recommendation of my gastroenterologist for severe IBS. It's only been a few days and already feeling much better. Two years ago results of nerve conduction tests showed neuropathy in both arms from shoulder on down. That got better with resumption of humira after trying some other biologics. They also told me to stop mowing and line trimming our 5 acres of lawn because of the vibrations. A benefit?
Regarding the nortriptyline the idea is to "remap" the nervous system. It could be this is also a factor in how much and what kind of pain one feels from A.S. and related diseases. I've had success using the Quell device which essentially cuts my pain from 75 - 90% depending on environmental factors. So it won't surprise me if nortriptyline also helps. As a side note, I've sometimes felt well enough with these various treatments that I wonder if I have a disease at all. So I stop using the quell and sure enough with a day a lot of the pain returns.
Excerpt from NIH article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2669938/
"Visceral hypersensitivity and dysregulation of central pain perception in the brain-gut axis is considered to play a pivotal role in the pathophysiology of IBS. IBS patients have a lower sensory threshold to colonic and rectal balloon distention and electrical stimulation; therefore, beneficial effects of antidepressants can be explained by partial increment in central pain threshold. Other mechanisms by which antidepressants might express their effect include anticholinergic effects, regulation of GI transit and peripheral antineuropathic effects[24,25]. The results from the current meta-analysis show that TCAs induce clinical response and reduce abdominal pain score in patients with IBS."