2) Dysautonomic Orthostatic Intolerance

Included in this group are patients with true "orthostatic hypotension" defined by the American Autonomic Society to be a persistent fall in systolic blood pressure of >25 mmHg within 3 minutes of assuming the upright position (31). This group harbors patients with autonomic failure. Autonomic failure includes primary forms such as primary autonomic failure and multiple system atrophy, and more common secondary forms occurring with Parkinsonís disease and diabetes. Dysautonomia may also be drug induced. Pediatric causes are rare and include familial dysautonomia as the only "relatively" common variants as well as subtle early changes in diabetes and other disorders. Acute forms may occur during infectious and inflammatory diseases or be related to peripheral neuropathies, e.g. Guillian Barre syndrome. Standard tests of circulatory autonomic function such as timed breathing and the quantitative Valsalva maneuver demonstrate signs of circulatory autonomic dysfunction. Other manifestations of dysautonomia may be present including pupillary, gastrointestinal, and sweating abnormalities. Neurological damage such as occurs in cerebral palsy, trauma, etc may result in some autonomic dysfunction in addition to other neurologic disability. Responses to orthostasis in such patients differ from those in truly dysautonomic patients in that compensatory mechanisms may adapt the patient to orthostasis (e.g. increased blood volume) which seldom occurs in the dysautonomic.

Dysautonomic orthostatic intolerance is depicted in the figure. Blood pressure falls while there is no significant change in heart rate throughout the course of the tilt. The appropriate response of the baroreflex to hypotension is tachycardia, which fails to occur in these illnesses. Patients may be so brittle that they are hypertensive supine, hypotensive upright, and lose consciousness due to overzealous splanchnic vasodilation (vasoactive intestinal polypeptide?) after every heavy meal.

Treatment modalities favor volume loading and midodrine which, as noted, often results in recumbent hypertension. Specific therapy for chronic disease is largely experimental and acute therapy for acute illness remains specific for the specific disease.

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Normal
Postural Tachycardia Syndrome
Vasovagal Syncope
Dysautonomic Orthostatic Intolerance