Colonic dilatation can occur in the babies due to lack of development of the small autonomic nerves in the large bowel (called Hirschsprung's Disease)
It would be very rare for this to present this late. Barium radiographs might alert a radiologist to this diagnosis. A rectal biopsy can show whether the nerves are present or not.
Otherwise, autonomic testing without other clinical autonomic abnormalities is likely not to yield anything, and your problem may be one of non-neurological origin.
Good luck
you could also have segmental hirschprungs where certain sections may not have innervation