Orthostatic hypotension is defined by a fall in systolic pressure > 20 mmHg or in diastolic pressure > 10  mmHg in response to assumption of the upright posture from a supine position within 3 min. There  may also be a lack of a compensatory tachycardia, an abnormal response that suggests autonomic  insufficiency, as may be seen in patients with diabetes or Parkinson's disease. Orthostatic hypotension  is a common cause of postural lightheadedness/syncope and should be assessed routinely in patients  for whom this diagnosis might pertain. Reference: : Harrison’s medicine page no  126