Defining
Characteristics
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.
| Symptoms
of Orthostatic Intolerance |
|
Lightheadeness |
|
Headache
|
| Fatigue
|
| Neurocognitive/Sleep
Disorders |
| Visual
Disturbances
(black/white/spots)
|
| Exercise
intolerance
|
| Weakness
|
| Hyperpnea/Dyspnea
|
| Tremulousness
|
| Sweating
|
| Anxiety/Palpitations
|
| |
Chronic orthostatic
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.
Chronic orthostatic intolerance is associated with
postural tachycardia syndrome (POTS). When symptoms of chronic orthostatic intolerance are present, signs of
POTS are almost always found. Most forms of orthostatic intolerance are
not typically life threatening although circumstances can conspire to create
considerable risk.
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.
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Orthostatic Intolerance