In discussions of neurogenic hypertension, orthostatic disorders are rarely considered. Many orthostatic disorders are due to nonautonomic mechanisms, and there may sometimes be evidence of excessive sympathetic activation in some of the patients in response to the depressor pathophysiology, whatever it may be (46). However, in one syndrome—orthostatic intolerance or POTS (47), it is especially important to keep this potential relationship between an orthostatic abnormality and neurogenic hypertension firmly in mind. This syndrome is defined based on the evidence of sympathetic activation with upright posture and an absence of orthostatic hypotension. POTS has the dubious distinction of perhaps having the most names of any cardiovascular problem. Some of these names are listed in Table 2. Some of the most important and widely used ones include mitral valve prolapse syndrome, postural tachycardia syndrome, vaso-regulatory asthenia, neurasthenia, and idiopathic hypovolemia. A large number of names are deserved by this syndrome because it has a very large number of etiologies. Unfortunately, because of the difficulty in accurately diagnosing these many different pathophysiologies in the clinic, the names are not used in any clear pathophysiological way and therefore constitute more of a barrier than a door to improved understanding. The situation is complicated even further by the fact that many patients with chronic fatigue syndrome also meet the hemodynamic criteria for POTS. Indeed, sometimes the name of the condition depends more on the kind of a specialist that a patient visits. The endocrinologist may be most struck by the hypovolemic aspect of the illness and term the problem as idiopathic hypovolemia. The cardiologist may be struck by physical findings of mitral prolapse and diagnose as mitral valve prolapse syndrome. The rheumatologist may call the problem chronic fatigue syndrome, and the neurologist may call the problem as partial dysautonomia or hyperadrenergic orthostatic intolerance. When a patient is examined in a hypertension center, the hyperadrenergic features and the relative youth of a typical patient may more likely lead to the diagnosis of labile hypertension. Because there are probably significant numbers of patients in this last category among those we now call pre-hypertension, it is important to consider features that might lead to this diagnosis.
Patients with orthostatic intolerance or POTS have symptoms while standing that resemble those elicited by inadequate cerebral blood flow. On standing, they have increased heart rates of at least 30 bpm with dizziness, palpitations, poor exercise tolerance, and pre-syncopal symptoms, although syncope itself is very infrequent. Blood pressure is usually slightly higher than average in these patients and may be much higher on standing than when lying down. The form of this syndrome most likely to be encountered in a hypertension clinic is hyperadrenergic POTS.
Early investigators observing the tachycardia and hyperkinetic heart in these patients generally assumed that enhanced sympathetic activation or P adrenal receptor hypersensitivity was somehow involved. Friesinger et al. reported that these patients often had
Terms Used for Postural Tachycardia Syndrome
Postural tachycardia syndrome
Postural orthostatic tachycardia syndrome
Hyperadrenergic orthostatic intolerance
Mitral valve prolapse syndrome
Hyperkinetic heart syndrome
STT wave changes in the inferior electrocardiogram leads (48). These changes were more significant after upright posture and may in some cases have been heart rate related. Similar hyperadrenergic symptoms are sometimes seen in patients with baro-reflex failure and those having tumors involving the brainstem. Furthermore, destruction of the nuclei of the solitary tracts in rats yields a profoundly hyperadrenergic state that culminates in death within hours. This view was strengthened by the findings that plasma norepinephrine was often increased in POTS (49), and that a-2 agonists, P antagonists, and phenobarbital attenuated the tachycardia or at least relieved some of the symptoms.
One of the most important evidence in support of a central etiology for hyperadren-ergic POTS has emanated from studies in which both sympathetic and parasympathetic activities had been blocked by the NN nicotinic antagonist trimethaphan. With this agent, patients with POTS had greater decreases in sympathetic activity than control subjects. Systolic blood pressure decreased by 17 mmHg in patients with POTS but only 4 mmHg in control subjects under similar supine circumstances. Among the patients with POTS, the half of them having the greatest decrease (26 mmHg) after trimethaphan had greater pretrimethaphan supine systolic blood pressures, and greater supine and upright plasma norepinephrine levels than those who had a lesser response. However, the supine and upright heart rates were similar in both POTS subgroups. Analysis of simultaneous peroneal sympathetic nerve traffic and heart rate variability in patients with POTS suggests a greater increase in sympathetic tone to the heart than to the vasculature (50), a finding confirmed in studies of cardiac norepinephrine spillover, which is increased. The discordance seemed robust and may prove to be an important clue to the nature of the central pathophysiology of POTS. Currently, it is often impossible to identify patients with POTS likely to have enhanced central sympathetic outflow but some help is afforded by features noted in Tables 3 and 4. The use of such guidelines must however be approached cautiously because of our primitive understanding of the nature of POTS in most patients.
Recently, norepinephrine transporter dysfunction has been identified as a disorder presenting with tachycardia and mildly increased blood pressure in certain circumstances (51). Abnormalities in norepinephrine transporter function have been identified
Features of Hyperadrenergic and Neuropathic Postural Tachycardia Syndrome
Features suggestive of hyperadrenergic postural tachycardia syndrome Plasma norepinephrine levels > 1000 pg/mL Increased muscle sympathetic nerve activity
Increase in low-frequency/high-frequency ratio of heart rate variability Symptomatic benefit with low-dose clonidine Features suggestive of neuropathic postural tachycardia syndrome Plasma norepinephrine levels of high normal to 800 pg/mL
Absent galvanic skin response or abnormal quantitative sudomotor axon reflex test Other evidence of peripheral neuropathy Poor response to low-dose clonidine
Treatment of Hyperadrenergic Postural Tachycardia Syndrome
16 oz water; 2-3 times daily as needed (acts for ~1 h only) 10 g sodium diet Support garment
Propranolol: 10-20 mg; 2-4 times daily Clonidine: 0.05-0.10 mg; orally twice daily Methyldopa: 125-250 mg; half strength or twice daily Fludrocortisone: 0.05-0.30 mg; daily (attenuates tachycardia) Midodrine: 2.5-10 mg; three times daily (reflexly attenuates tachycardia) Phenobarbital: 30-100 mg; daily in some individuals with hypertension, though it remains uncertain if this is a primary or secondary event. Mice with norepinephrine transporter knockout have exhibited similar effects, with chronic stress-evoked increases in both heart rate and blood pressure (52). Furthermore, in response to the NN-nicotinic receptor antagonist, trimethaphan, which interrupts autonomic ganglionic transmission, blood pressure falls more in hypertensive subjects than in normal subjects, confirming the substantial role of sympathetic mechanisms in blood pressure maintenance.
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