It is now recognized that orthostatic hypotension occurs as a complication of many
neurological diseases. An account is given of some of those in which it may develop.
The pathological lesions frequently allow studies to be made of physiological mechanisms
remaining in the isolated parts of the nervous system. For example transection of
the cervical spinal cord causes complete separation of peripheral sympathetic pathways
from the control of the brain. It was possible, therefore, to examine the control
of renin release and also cerebral blood flow regulation during change of blood pressure
in patients with some of these disorders. Evidence has been obtained that the renin-angiotensin
system is active in paraplegia and in many other patients with orthostatic hypotension.
In addition symptoms of cerebrovascular insufficiency are minimized by the retention
of autoregulation of cerebral blood flow (CBF) in patients with failure of sympathetic
function. Autoregulation maintains CBF within wide limits of blood pressure and symptoms
of orthostatic hypotension only develop if the blood pressure falls below the lower
limit, usually about 70 mm Hg.