Orthostasis = standing upright. Orthostatic intolerance can then be defined as "the development of symptoms during upright standing relieved by recumbency". Often, illnesses producing orthostatic intolerance include disorders of blood flow, heart rate and blood pressure regulation that, while most easily demonstrable during orthostatic stress, are present in all positions. The term "dysautonomia", signifying autonomic dysfunction, has been frequently applied to this list of disorders, but recent data suggest that autonomic function may be normal in many variants of these conditions but have to cope with unusual circulatory demands. A simple and common example of this is the finding of postural hypotension and postural tachycardia when dehydrated. Therefore it is probably best to denote the type of symptomatic illness by the more vague term "orthostatic intolerance" rather than the more specific term"dysautonomia" which may not be correct. Acute orthostatic intolerance usually manifests as syncope (fainting). Many syncopal patients have no intercurrent illness; between faints they are well.
intolerance implies day-to-day symptoms including dizziness in all patients,
and often altered vision (blurred, ‘white outs’, ‘black-outs’), fatigue, nausea, neurocognitive deficits, disordered thermoregulation,
palpitations, headache, tremulousness, hyperpnea, difficulty breathing,
sweating, and pallor.
These produce day-to-day disability; estimates suggest that over a million
Americans are affected, mostly young women, who may be prevented from gainful
employ or school attendance.
Evidence suggests that the changes in heart rate, blood pressure, and cerebral blood flow that produce orthostatic intolerance may be related to abnormalities in the interplay between blood volume control, the cardiovascular system, the autonomic nervous system and local circulatory mechanisms that regulate these basic physiological functions. Therefore, the control of the circulatory system should be considered neurovascular, resulting from the interactions between neurological and cardiovascular systems. These interactions, may alter blood flow through direct effects on blood pressure, by redistribution of blood flow resulting in altered blood return to the heart and drastic reductions in the amount of blood that the heart can pump (cardiac output), and by local flow regulatory mechanisms that impair cerebral blood flow . This results in decreases in blood flow to the skin and muscles producing pallor, and decreases in blood flow to the brain producing light-headedness or loss of consciousness although respiratory changes may produce hypocapnea and cerebral vasoconstriction that precedes changes in blood pressure. This rather striking hyperpnea and associated dyspnea may even be produced in healthy volunteer subjects and is often observed prior to overt circulatory failure in fainting and chronic orthostatic intolerance. Separate evidence also indicates the potential for abnormal central nervous system autoregulation which normally spares the brain from the consequences of low flow states. While there is often evidence for impaired neurovascular interactions while supine, orthostasis (standing up) profoundly stresses the control mechanisms. Thus symptoms of neurovascular regulatory failure are often first appreciated during orthostatic challenge analogous to the use of exercise as a challenge to aerobic conditioning and during heart failure.
Therefore, while neurovascular abnormalities can be assessed supine and often noninvasively, orthostatic stress testing (analogous to exercise stress testing) is frequently employed. While standing may be the most physiological orthostatic stressor, head-up tilt table testing (HUT), has become the standard stress test for orthostatic integrity and thus of neurovascular regulatory competence. In addition a research procedure called lower body negative pressure may provide physiological challenges which in many respects emulate and complement orthostatic stress. However, most laboratories use upright tilt to diagnose syncope and other orthostatic disability. The test is simple - using a motorized table with a foot support to raise the patient from supine to approximately 60-70 degrees upright without the use of the patient's own muscles. At the Center for Pediatric Hypotension we perform research using supine and upright measurements of neurovascular integrity. Often these combine Orthostatic stress testing and instrumentation designed to measure circulatory and orthostatic compensatory activity. These include determinations of , heart rate and blood pressure variability, baroreflex assessment, autonomic assessment, peripheral and central measures of blood flow, vascular capacitance, and permeability properties to obtain information concerning the physiology and thus relative risk for fainting and OI in vulnerable patients and in healthy volunteer control subjects. Patterns of blood pressure and heart rate during HUT take diverse forms which relate to the underlying mechanism of disease and which may help individualize specific therapy.
Patients with orthostatic intolerance diagnosed by these techniques can often be helped by medical therapy or other forms of intervention. Many of such patients have received little effective therapy for their problems and some have received treatment for diseases (such as seizure disorders) which they do not have. In both instances appropriate use of tilt-testing, autonomic assessment, and vascular integrity may both specify and simplify their therapy.