Orthostatic Hypotension Syndromes: Autonomic

Orthostatic hypotension (OH) results from failure of the autonomic cardiovascular reflexes that normally maintain cerebral perfusion during upright posture. When a person stands, gravity shifts approximately 500-1000 mL of blood from the thorax to the lower extremities and splanchnic vasculature, reducing venous return and cardiac output. In healthy individuals, arterial baroreceptors in the carotid sinus and aortic arch detect the pressure drop within seconds and trigger a compensatory sympathetic response: increased heart rate, arteriolar vasoconstriction, and venous return augmentation via venoconstriction — restoring blood pressure within 30-60 seconds. In neurogenic orthostatic hypotension, this baroreflex arc is disrupted at any point: afferent baroreceptor pathways, central autonomic processing centers (nucleus tractus solitarius, rostral ventrolateral medulla), or efferent sympathetic fibers. Multiple system atrophy (formerly Shy-Drager syndrome) causes progressive degeneration of central autonomic nuclei with accumulation of alpha-synuclein inclusions in glial cells, producing severe neurogenic OH combined with cerebellar ataxia, parkinsonism, and urogenital dysfunction. Pure autonomic failure involves peripheral postganglionic sympathetic neuron degeneration. Parkinson disease and diabetic autonomic neuropathy also impair sympathetic efferents. The hallmark differentiating neurogenic from non-neurogenic OH is the heart rate...