1) Classic simple faint (Vasovagal Syncope)

This is depicted in the figure. Typically patients easily tolerate the early parts of tilt with little change in blood pressure or sensorium. Following a variable period of time – on the order of 7 to 20 minutes, patients develop orthostatic symptoms of nausea, dizziness, heat, heavy breathing, and sweatiness associated most commonly with a small initial slow fall in blood pressure (which can be seen if the figure is inspected closely). In short order there is an abrupt drop in blood pressure and heart rate. The early fall in blood pressure is coincident with a decrease in vasoconstriction (the peripheral arteries vasodilate) which normally occurs as part of the neurovascular compensation required to maintain blood pressure with orthostasis. Blood pressure and heart rate may plummet precipitously, and asystole may occur. When this happens there is a rapid loss of central nervous system activity and often a dysinhibition of peripheral neurologic responses resulting in muscular movements mimicking a tonic-clonic seizure. This is denoted "convulsive syncope". That no true seizure activity is present has been confirmed as early as the 1950’s by Gastaut and associates and later reconfirmed using HUT methods by Grubb and coworkers in the 1990’s.  Such episodes, while relatively uncommon, are quite dramatic and such phenomena are periodically "rediscovered" by beginning practitioners of the HUT art. There are several sidebar observations on simple faint that stem from the convulsive variant:

a) If the episode occurs rapidly the patients can be injured. It is estimated that approximately 15% of patients with simple faint are injured overall during a faint. This can take relatively innocuous forms as a superficial cut while falling or can be more pernicious when a car is overturned. In the latter case it is evident that treatment for fainting is necessary.

b) Fainting usually is short lived; upon assuming recumbence the patient usually awakes after a few seconds. But some patients do not awake immediately and prolonged sleep-like states have occurred. These may mimic post-ictal states.

c) A patient persistently maintained upright in a severe simple faint (for example proceeding to asystole) can potentially die. Although there is no reported pediatric death during HUT, the same cannot be said for adult testing. The figure illustrates one possible scenario for death which is the rapid onset of asystole. This is often designated "malignant syncope" or "convulsive syncope" because of the frequent association with seizure like movements (mostly tonic posturing without clonus). Gastaut in the 1950's showed that these 'seizures' were not be related to brain seizure activity but rather to cessation of CNS activity (flat-line) and peripheral nerve escape. Pacemaker insertion has been advocated in some adults with convulsive/asystolic syncope but remains controversial. There is no literature on the use of pacemakers in the pediatric age range. Maintaining such a patient upright would be fatal.

A corollary is that upright tilt testing is potentially lethal and should be only performed by experienced personnel. Similar events may also occur in the real world, so called "telephone booth syncope’. This has decreased since telephone booths are largely anachronistic. However, recently one of my patients with known vasovagal syncope managed to prop herself up with impending faint and nearly required a full resuscitation. Patients should be placed supine or allowed to fall to a recumbent position. Recumbence invariably resolves all symptoms and signs.

 

 


 

Until recently most research concerning orthostatic intolerance was primarily concerned with syncope. Even here there is no consensus about mechanism. The most popular proposed mechanism holds that fainting results from a stretch reflex from the left ventricle. The reflex is presumably activated by an underfilled (due to reduced venous return), overly contractile (due to sympathetic activation), left ventricle. This results in a “paradoxical reflex” mediated by unmyelinated C-fibers coursing from the ventricle to the CNS and causing vagally mediated bradycardia as well as vasodilation Treatment which increases blood volume would be expected to help relieve underfilling while negative inotropic agents should help to reduce cardiac contractility. However, just because a reflex is possible, does not make it probable. Both hypercontractility and decreased ventricular stretch have been called into question by recent research. Also, patients receiving cardiac transplants retain the ability to faint , which implies that the ventricular receptor theory cannot explain all simple faints. Most significantly, animal research performed by Hainsworth and associates have convincingly demonstrated that once coronary baroreflex function is separated from ventricular receptor action, little in the way of Bezold-Jarisch like reflex activity remains under physiologically achievable conditions. Other theories of fainting include epinephrine or renin surges and vasopressin decreases. This would rationalize the common use of isoproterenol as adjunctive provocation . Such surges have been shown to occur in those who faint and often take many minutes to develop. However, it remains unclear whether these humoral changes are the cause of the hemodynamic abnormalities or the result of attempted compensation for decreased blood pressure and peripheral resistance during incipient fainting. A decrease in cerebral blood flow has also been shown to occur in syncopal patients and may precede a large fall in blood pressure .  Cerebral syncope is thought to exist independent of blood pressure changes. Also, blood flow is similarly impaired in chronic orthostatic intolerance in which hypotension does not usually occur . Other proposed mechanisms include changes in CNS neurotransmitters such as serotonin, norepinephrine, neuropeptide Y and substance P . In our laboratory, we have recently demonstrated a relation between postural fainting and excess splanchnic pooling which in turn produces an unusual degree of thoracic hypovolemia. Causation has not been established. In summary it is fair to say that we still have no precise understanding of the mechanism or mechanisms of simple faint. 


Treatment
Without a clear mechanism there is no clear treatment. Moreover, many patients with infrequent simple faints, who do not injure themselves and who do not have convulsive syncope, may require no specific therapy above training in aversive maneuvers. The simplest of these maneuvers is lying down although leg crossing, bending at the waist, squatting and other maneuvers may also be effective. Increased fluid and salt intake is always helpful in ameliorating the initial thoracic hypovolemia of orthostasis. Lower body exercise, particularly isometric exercise, can be a genuine help by enhancing the muscle pump and by increasing venous tone in the lower extremities. Elastic support hose can be useful at times but are often unacceptable to children. Other investigators have advocated a regimen of progressively longer quiet standing as a form of "orthostatic training". In terms of medication, often beta-1 blockade works well. This may have its basis in reducing putative hypercontractility but other possible roles for beta-1 blockade include blunting the release of epinephrine or renin, which are modulated by beta-1 receptors and central effects. Other possible medications include fludrocortisone (florinef), which retains sodium and water at the expense of small potassium wasting and has modest if any corticosteroid side effects. In addition to its other actions, florinef may aid in sensitizing alpha-receptors and blocking vasodilation. A new, direct acting alpha-1 agonist, midodrine (proamatine) has been used to good effect in many patients with assorted forms of orthostatic intolerance. Other agents have included alpha-2 adrenergic agents (both clonidine and its obverse yohimbine) which have been used in select patients. Disopyramide has been used occasionally but controlled studies do not support its efficacy. Recently, selective serotonin reuptake inhibitors have been used to good effect in a variety of orthostatic disabilities. These seem to interfere with hypotensive responses at a central level. Grubb and associates have demonstrated efficacy of sertraline and fluoxetine in a series of controlled studies. The studies were performed after careful psychiatric screening had ruled out significant depression. Personal experience bears this out and the SSRI’s remain a useful medication for many forms of orthostatic intolerance.

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Postural Tachycardia Syndrome
Vasovagal Syncope
Dysautonomic Orthostatic Intolerance