Most common cause of ohostatic hypotension -

Ohostatic hypotension: defined as a reduction in systolic blood pressure of at least 20 mmHg or diastolic blood pressure of at least 10 mmHg within 3 minutes of standing or head-up tilt on a tilt table, is a manifestation of sympathetic vasoconstrictor (autonomic) failure. Causes of neurogenic ohostatic hypotension include central and peripheral autonomic nervous system dysfunction Autonomic dysfunction of other organ systems (including the bladder, bowels, sexual organs, and sudomotor system) of varying severity frequently accompanies ohostatic hypotension in these disorders. ref : Harrison&;s principle of internal medicine,20th edition,pg no. 175