The explanation of the finding is postulated to be the release of a vasodilator from activated mast cells, supported by a raised urinary methylhistamine.
4 These patients, as might be expected, may be worsened by beta-blockers but may respond to H1 and H2 histamine blockers with or without alpha-methyldopa. This third sub-type of POTS may present raised blood pressure at times in association with flushing and offer a differential diagnosis from phaeochromocytoma. This sub-type may only be a small percentage of those with POTS but, for them, there may be a specific therapy, anti-histamines not beta-blockade.