Neurocardiogenic Syncope and Related Disorders of Orthostatic Intolerance

Abstract

HomeCirculationVol. 111, No. 22Neurocardiogenic Syncope and Related Disorders of Orthostatic Intolerance Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBNeurocardiogenic Syncope and Related Disorders of Orthostatic Intolerance Blair P. Grubb, MD Blair P. GrubbBlair P. Grubb From the Cardiology Department of Medicine, the Medical University of Ohio, Toledo, Ohio. Search for more papers by this author Originally published7 Jun 2005https://doi.org/10.1161/CIRCULATIONAHA.104.482018Circulation. 2005;111:2997–3006“We shall not cease from exploration and the end of all our exploring will be to arrive where we started and know the place for the first time”— —T.S. Eliot, Four QuartetsSyncope, defined as transient loss of consciousness and postural tone with spontaneous recovery, has both challenged and perplexed physicians since the dawn of recorded time. The earliest written accounts come from Hippocrates, and the word syncope itself is derived from an old Greek term meaning “to cut short” or “interrupt.” Recurrent episodes of syncope may result from a large number of different disorders, all of which cause a transitory reduction in cerebral blood flow sufficient to disturb the normal functions of the brain. Over the last 2 decades, considerable attention has been given to types of syncope that occur due to a centrally mediated (or “reflex”) fall in systemic blood pressure, a condition that has been referred to as vasovagal (and later neurocardiogenic) syncope. However, research into the nature of this disorder revealed that it is but one aspect of a broad and varied group of disturbances in the normal functioning of the autonomic nervous system (ANS), each of which may result in orthostatic intolerance, hypotension, and ultimately syncope. Continued investigations into the nature of these similar yet different disorders has led to the development of a system of classification that attempts to more accurately reflect our understanding of these conditions and their interrelationships.1The present system of classification has proven both functional and clinically relevant and includes a group of disorders that most investigators have thought to be principally autonomic in nature. Because both the cardiologist and the cardiac electrophysiologist frequently are expected to both diagnose and treat these conditions, the following review is designed to provide a basic framework for understanding their causes, clinical presentations, diagnosis, and management.Autonomic Nervous SystemTo survive in the world, all animals must possess the ability to make moment by moment alterations that permit their internal environment to remain stable despite dramatic changes in their external environment. This includes not only changes in ambient temperature, humidity, and barometric pressure but also the ability to react quickly to the presence of perceived danger. The principal neural mechanisms by which this “homeostasis” is maintained and regulated are governed by the hypothalamus via its 2 effector systems, which include the ANS and the endocrine system.2Although the adoption of upright posture represents one of the defining moments in human development, it nonetheless provided a unique challenge to a blood pressure control system that had principally evolved to meet the needs of animals who spent the majority of their time in a dorsal position.1–4 The ANS provides the principal means for both the short- and long-term responses to changes in position (the renin-angiotensin-aldosterone system also plays a role but over a longer time frame).5 In the normal person, approximately 25% to 30% of blood volume is located in the thorax when they are supine. On standing, there is a gravity-mediated downward displacement of roughly 300 to 800 mL of blood to the vasculature of the abdomen and lower extremities.6 This represents a volume drop of between 25% and 30%, half of which occurs within the first few minutes of standing. This sudden redistribution of blood results in a fall in venous return to the heart. Because the heart can only pump the blood that it receives, this causes a fall in stroke volume of ≈40% and a decline in arterial pressure. The reference point around which these changes occur is called the venous hydrostatic indifference point (HIP) and is defined as the site in the vascular system where pressure is independent of posture (the venous HIP is around the diaphragm, the arterial HIP is near the level of the left ventricle).5 Compared with the arterial HIP, the venous HIP is dynamic in nature and is influenced by factors such as the degree of vascular compliance, intravascular volume, and muscular activity. While standing, contractions of the leg muscles (in conjunction with the venous valve system) actively pump blood back to the heart and move the venous HIP closer to the level of the right atrium.In addition, standing produces a substantial increase in the transmural capillary pressure present in the dependent areas of the body, which causes a rise in fluid filtration into tissue spaces. This process reaches a steady state after ≈30 minutes of upright posture and can produce a decline in plasma volume of up to 10%.Successful maintenance of upright posture (and cerebral perfusion) requires the interplay of several cardiovascular regulatory systems. Orthostatic stabilization occurs within 1 minute. The exact response to postural change differs with standing (an active process) compared with responses seen during head-up tilt (a more passive process). Wieling and van Lieshout7 have described 3 phases of orthostatic response. These consist of (1) the initial response (during the first 30 seconds), (2) the early steady state alteration (at 1 to 2 minutes), and finally (3) the prolonged orthostatic period (after at least 5 minutes upright).7Immediately after head-up tilt, cardiac stroke volume remains normal despite the decline in venous return (believed to occur due to blood left in the pulmonary circulation). This is followed by a gradual fall in both arterial pressure and cardiac filling. This causes activation of 2 different groups of pressure receptors consisting of the high-pressure receptors of the carotid sinus and aortic arch and the low-pressure receptors of the heart and lungs.5 Within the heart, there are mechanoreceptors linked by unmyelinated vagal afferents in all 4 cardiac chambers. These mechanoreceptors produce a tonic inhibitory effect on the cardiovascular control centers of the medulla, in particular on the nucleus tractus solitarii. The baroreceptive neurons of the nucleus tractus solitarii directly activate cardiovagal neurons of the nucleus ambiguous and dorsal vagal nucleus while inhibiting the sympathoexcitatory neurons of the rostral ventrolateral medulla.3The reduced venous return and fall in filling pressure that occur during upright posture reduce the stretch on these receptors. As their firing rates decrease, there is a change in medullary input, which triggers an increase in sympathetic outflow. This causes a constriction not only of the systemic resistance vessels but of the splanchnic capacitance vessels as well. In addition, there is a focal axon reflex (the venoarteriolar axon reflex) that can constrict flow to the skin, muscle, and adipose tissue. This may contribute up to 50% of the increase in limb vascular resistance seen during upright posture.7During head-up tilt, there is also activation of the high-pressure receptors in the carotid sinus. The carotid sinus contains a group of baroreceptors and nerve endings located in the enlarged area of the internal carotid artery, just after its origin from the common carotid artery. Here, the mechanoreceptors are located in the adventitia of the arterial wall.8 The afferent impulses generated by stretch on the arterial wall are then transmitted via the sensory fibers of the carotid sinus nerve that travels with the glossopharyngeal nerve. These afferent pathways terminate in the nucleus tractus solitarii in the medulla, near the dorsal and ambiguous nuclei.4 The initial increase in heart rate seen during tilt is thought to be modulated by a decline in carotid artery pressure. The slow rise in diastolic pressure seen during upright tilt is believed to be more closely related to a progressive increase in peripheral vascular resistance.The circulatory changes seen during standing are somewhat different from those seen during tilt. Standing is a much more active process that is accompanied by contractions of muscles of both the leg and abdomen, which produces a compression of both capacitance and resistance vessels and results in an elevation in peripheral vascular resistance. This increase is sufficient to cause a transient increase in both right atrial pressure and cardiac output, which in turn causes an activation of the low-pressure receptors of the heart. This provokes an increase in neural traffic to the brain, with a subsequent decrease in peripheral vascular resistance, which can fall as much as 40%.6 This can allow a fall in mean arterial pressure of up to 20 mm Hg that can last for up to 6 to 8 seconds. The decline in pressure is then compensated for by the same mechanisms as during head-up tilt.The early steady state adjustments to upright posture consist of an increase in heart ate of ≈10 to 15 bpm, an increase of ≈10 mm Hg, and little or no change in systolic blood pressure.7 At this point, compared with supine posture, the blood volume of the thorax has fallen by 30%, cardiac output has increased by 30%, and heart rate is 10 to 15 bpm higher.5Continued upright posture also activates a series of neurohormonal changes, the exact extent of which depend on the patient’s volume status. The greater the volume depletion, the greater the degree of activation of the renin-angiotensin-aldosterone system (and vasopressin). However, one of the most important aspects of the body’s ability to compensate for continued orthostatic stress is the influence of arterial baroreceptors (particularly the carotid sinus) on peripheral vascular resistance. At any given moment, ≈5% of the body’s blood is in the capillaries, 8% is in the heart, 12% is in the pulmonary vasculature, 15% is in the arterial system, and 60% is in the venous system.6 The inability of any one of these mechanisms to operate adequately (or in a coordinated manner) may result in a failure of the body to compensate to either an initial or prolonged orthostatic challenge. This, in turn, would result in systemic hypotension, which, if sufficiently profound, could lead to cerebral hypoperfusion and subsequent loss of consciousness.Conditions That Occur as a Result of Disturbances in Orthostatic ControlA growing number of autonomic disturbances of orthostatic regulation have been identified. Although in many ways similar, each has features that make it unique. In attempting to classify them into a useful system, one should remember that when we look at nature, in many ways we see what we wish to see that fits what we know about it at that moment. Conditions such as supraventricular tachycardia and long-QT syndrome were each first thought to be a single entity but with time were found to be composed of a variety of subgroups. To attempt to make sense of the apparent chaos of nature, we try to organize and classify it into a coherent framework and system that fits with both our knowledge and our expectations.9 Thus, any system of classification is in many ways arbitrary, subject to debate, and in a constant process of revision and refinement. The system presented in the Figure follows that developed by the American Autonomic Society and attempts to represent our current understanding of these disorders in a clinically useful framework.10 In some ways, all autonomic disturbances can be thought of as being either primary or secondary in origin. The primary forms tend to be idiopathic, unassociated with other diseases, and are either acute or chronic in nature. Conversely, the secondary forms are seen in conjunction with another disease process (such as diabetes and amyloidosis) or alternatively occur owing to a known biochemical or structural deficiency or after exposure to known toxic agents (such as alcohol and heavy metals). A brief description of each subtype follows, along with references for the reader who desires a more in-depth discussion. Download figureDownload PowerPointDisorders of ANS associated with orthostatic intolerance.Reflex SyncopesThe reflex syncopes are a group of disorders that occur because of a sudden failure of the ANS to maintain adequate vascular tone during orthostatic stress, resulting in hypotension (frequently associated with bradycardia) that results in cerebral hypoperfusion and loss of consciousness. The 2 most frequent types of reflex syncope are neurocardiogenic (vasovagal) syncope and carotid sinus syndrome. The other types of reflex syncopes are often referred to as situational because they are often associated with specific activities or conditions. Neurocardiogenic syncope (NCS) can be quite varied in presentation.11 It tends to occur in younger patients and tends to exhibit 3 distinct phases that consist of a distinct prodrome (usually lightheadedness, nausea, diaphoresis, or visual changes) followed by a sudden loss of consciousness. Recovery is usually quite rapid, and postictal states are rare. However, nearly one third of patients (most commonly older adults) will experience few, if any, prodromal symptoms, and loss of consciousness occurs suddenly, with little warning. The cause of neurocardiogenic syncope is unclear. It can be brought on by pain, emotional distress, or by prolonged standing (especially in very warm environments), yet many episodes occur without any specific provocation. Although still the subject of considerable debate, it is presently believed that the pathophysiology of NCS often is related to prolonged orthostatic stress.12 This results in an increased amount of peripheral venous pooling to a point great enough that the venous return to the heart falls so precipitously that a significant increase in ventricular inotropy occurs. This hypercontractile state is believed to activate mechanoreceptors that would normally fire only during stretch.13 The sudden increase in neural traffic to the medulla is thought to mimic the conditions seen in hypertension, thus causing an apparent “paradoxic” decline in sympathetic activity that results in hypotension, bradycardia, and ultimately syncope.14 It should be remembered, however, that other stimuli (for example fear, strong emotion, or epileptic discharges) can elicit nearly identical responses, which raises the possibility that these patients may have an inherent susceptibility to these events. During head-upright tilt-table testing, these individuals will demonstrate a relatively sudden fall in blood pressure that is often (but not always) followed by a decline in heart rate (sometimes to the point of asystole).Sutton and Peterson15 have noted that there is a striking similarity between the hemodynamic collapse profile seen during NCS and carotid sinus syndrome, which suggests that they may be different aspects of the same underlying condition. Indeed, it has been postulated that all the reflex syncopes occur in susceptible individuals when sudden activation of mechanoreceptors from any site (rectum, esophagus, bladder, or cough) produces a similar response. Recent studies on defecation syncope have supported this concept. What distinguishes the reflex syncopes from the other conditions discussed herein is that between episodes of syncope, these patients complain of few (if any) autonomic symptoms. Thus, in this group, the autonomic system appears to function in a relatively normal manner, despite being somewhat “hypersensitive,” as opposed to other conditions in which the autonomic system appears to “fail,” functioning at a level inadequate for the body’s needs, which results in varying degrees of orthostatic intolerance.11Primary Autonomic Failure: Chronic FormsThe first of the autonomic failure syndromes described was by Bradbury and Eggleston in 1925.16 They used the phrase “idiopathic orthostatic hypotension” to describe the condition because of its apparent lack of other features; however, this term does not adequately describe the generalized state of autonomic failure that occurs in these patients, characterized by disturbed bowel, bladder, sudomotor, thermoregulatory, and sexual function (without somatic involvement). At present, the condition is referred to as pure autonomic failure (PAF).17 PAF usually begins between ages 50 and 70 years, and it displays a 2:1 male-to-female ratio.18 Symptoms are slow and insidious in onset, usually beginning with vague complaints of orthostatic weakness, dizziness, and lightheadedness that are often dismissed by physicians as insignificant.19 As their orthostatic hypotension worsens, patients experience near syncope or syncope, which often prompts the patient to seek initial medical attention. In men, the earliest symptoms are impotence and loss of libido, whereas in women, urinary retention and incontinence occur first. This condition is characterized by orthostatic hypotension, syncope or near syncope, constipation, urinary retention, inability to sweat, and impotence.20 Although PAF may lead to severe functional impairment, only rarely is it fatal.A second, much more severe form of autonomic failure (associated with somatic defects) was subsequently described by Shy and Drager in 1960.21 As opposed to the situation in PAF, these patents not only have severe orthostatic hypotension but also have iris atrophy, urinary and rectal incontinence, loss of sweating, external ocular palsy, rigidity, tremor, and impotence. The condition is now referred to as multiple system atrophy (MSA) and usually begins in the fifth to sixth decade of life (although the disorder may occasionally begin in the late 30s or early 40s), with men affected twice as often as women.18 As in PAF, syncope and near syncope are the symptoms that most frequently prompt the patient to seek medical attention. Although MSA patients may start with symptoms that suggest PAF, they later begin to develop symptoms of somatic nervous system involvement, in addition to central nervous system involvement.22 Indeed, MSA is divided into 3 different subtypes depending on the somatic system involved.18,19 The first group displays a motor tremor similar to that seen in Parkinson’s disease (some authors refer to this group as having striatonigral degeneration on the basis of autopsy findings).23In contrast to true Parkinson’s disease, there tends to be more rigidity than tremor, yet the rigidity lacks the typical “cogwheel” or “lead pipe” rigidity seen in Parkinson’s disease.24,25 Patients with parkinsonian-type MSA frequently display a loss of facial expression and limb akinesia. The second subtype of MSA is characterized by prominent cerebellar and/or pyramidal features (also called olivopontocerebellar atrophy/degenerative form on the basis of autopsy findings).26 These patients have a significant gait disturbance and truncal ataxia that may impede the person from standing without support. Slurring of speech and loss of diction are common, as is a mild intention tremor of the extremities. The third group of MSA patients displays a combination of parkinsonian and cerebellar features and are referred to as having “mixed” MSA.22 A recent autopsy study reported that between 7% and 22% of patients diagnosed with Parkinson’s disease during life were found to have neuropathologic changes diagnostic for MSA after death.24 The natural history of MSA is one of relentless progression after onset, with most patients dying within 5 to 8 years after onset (although rarely, individuals have survived up to 20 years). Apnea, aspiration, and respiratory failure are the most frequent causes of death.19Over the last decade, considerable attention has been given to a relatively new subgroup of disorders currently referred to as the postural tachycardia syndrome (POTS). This appears to be a milder, less severe form of autonomic insufficiency that is characterized by excessive increases in heart rate while in the upright position.27 There are 2 primary forms of the disorder that have been identified. The more common variety is referred to as the peripheral (or partial) dysautonomic form.28 The hallmark of this group is a persistent tachycardia while upright, which can achieve rates of 160 bpm or higher, that typically is associated with complaints of severe fatigue, exercise intolerance, palpitations, near syncope, lightheadedness, and dizziness. Many patients also complain of cognitive impairment and visual disturbances and of intolerance to the heat while at the same time always feeling cold. During tilt-table testing, they will display a significant increase in heart rate of >30 bpm in the first 10 minutes or will achieve a maximum heart rate of >120 bpm over the same time frame, often associated with only a modest decline in blood pressure.29 The pathophysiology appears to be a failure of peripheral vascular resistance to increase adequately in the face of orthostatic stress. This results in excessive blood pooling in the dependent areas of the body, which is then compensated for by an increase in heart rate and inotropy. Investigators have reported that in some patients, POTS may be the earliest sign of autonomic dysfunction and that occasional patients (≤10%) may later progress to PAF. Onset may occur after a viral infection, trauma, or surgery or may be associated with the joint hypermobility syndrome.30The second type of POTS is referred to as the “β-hypersensitivity” or “central” form. In this form, there is believed to be an inadequate feedback process that arises from above the level of the baroreflex.30 Although the initial heart rate responses to upright posture are adequate, the brain does not know when to stop the response, and heart rate continues to elevate. Some patients who have this form of POTS will demonstrate orthostatic hypertension and postural tachycardia. Although the clinical presentations of both groups are similar, the hyperadrenergic form is often associated with migraines, excess sweating, and tremor. Serum catecholamine levels are usually quite high (serum norepinephrine is often >600 ng/dL), and patients exhibit an excessive response to isoproterenol infusion (>30 bpm increase in response to 1 μg/min).27Recent investigations have suggested a genetic basis for this group of disorders. A landmark study by Shannon et al31 has identified the exact genes responsible for this subgroup in 1 family with a number of severely affected members. The defect was in the genetic code for a protein responsible for recycling norepinephrine in the intrasynaptic cleft, which allowed for excessively high levels of serum norepinephrine. There may be multiple genetic forms of this disorder, and studies are under way to determine this.Acute Autonomic DysfunctionAlthough uncommon, the acute autonomic neuropathies are dramatic in presentation.32 Onset is quite sudden and is heralded by severe diffuse failure of both the parasympathetic and sympathetic systems, whereas the somatic system remains unaffected.33 Orthostatic hypotension is often so severe that sitting up in bed results in syncope. Complete loss of sweating and severe bowel and bladder dysfunction are common, causing nausea, abdominal pain, bloating, and vomiting.34 Heart rates drop to 45 to 55 bpm, with complete chronotropic incompetence. Recent studies completed by Verino et al35 have demonstrated that many of these patients have high levels of antibodies to acetylcholine receptors in the autonomic ganglia, which suggests that the disorder is autoimmune in nature.Secondary Causes of Autonomic DysfunctionThere are a number of different disorders that may disturb normal autonomic function, and the interested reader is directed elsewhere.9 It is important to keep in mind that any autonomic disturbance may be part of a larger disease process (for example, malignancy). A number of different drugs may either cause or contribute to orthostatic hypotension (Table 1). Systemic illness that affects multiple organs (eg, cancer, diabetes mellitus, amyloidosis, and renal failure) may disturb autonomic function to a degree that orthostatic hypotension and syncope occur. Recent studies have demonstrated a link between orthostatic hypotension and Alzheimer’s disease.36 Orthostatic hypotension may also occur owing to single enzyme abnormalities. Examples include isolated dopamine B-hydroxylase deficiency and nerve growth factor deficiency.37TABLE 1. Pharmacological Agents That May Cause or Worsen Orthostatic IntoleranceACE inhibitorsα-Receptor blockersCalcium channel blockersβ-BlockersPhenothiazinesTricyclic antidepressantsBromocriptineEthanolOpiatesDiureticsHydralazineGanglionic blocking agentsNitratesSildenafil citrateMonoamine oxidase inhibitorsClinical FeaturesThe main feature that is shared by each of these disorders is a disruption in cardiovascular regulation to such an extent that postural hypotension and/or tachycardia or bradycardia occurs. Historically, orthostatic hypotension has been described as a >20 mm Hg fall in systolic blood pressure or a >10 mm Hg fall in diastolic blood pressure over a 2-minute period after standing. Yet a less dramatic decline in blood pressure associated with symptoms may be equally important. Often, patients will display a gradual progressive decline in blood pressure over a more prolonged period of time (≈10 to 15 minutes) that will also result in symptoms. Whether symptoms occur will depend on the rate at which blood pressure falls and the total degree of change. The reflex syncopes tend to exhibit abrupt falls in blood pressure that are often associated with a definitive prodrome. In contrast, the loss of consciousness in the orthostatic (or “dysautonomic”) syncopes tends to be slow and gradual.37 This more gradual fall in blood pressure is often not perceived by a number of older patients, who often will report these episodes as “drop attacks” that occur with little warning. Those who do perceive the fall in blood pressure will relate feeling lightheaded, having blurred or tunnel vision, or having visual disturbances. One distinguishing factor between reflex and dysautonomic syncope is that the latter usually is not associated with diaphoresis and bradycardia.19 Dysautonomic syncopes tend to be more common in the morning after arising from sleep and are worsened by any condition that enhances peripheral venous pooling (eg, heat, alcohol, or fatigue). Patients with PAF and MSA may develop severe chronotropic incompetence with a relatively fixed heart rate (50 to 70 bpm).20 Some patients with significant autonomic dysfunction (particularly diabetes) may display a combination of upright hypotension and supine hypertension, which is believed to be due to an inability to adequately vasoconstrict when upright or vasodilate when prone.19 At times, it is difficult to fully distinguish between the various disorders, because a considerable degree of overlap may occur (not dissimilar to the situation seen in chronic obstructive pulmonary disease).DiagnosisThe most important aspect of evaluation is a comprehensive history and physical examination. When did symptoms first appear? Is the main problem syncope, lightheadedness, or fatigue? When do symptoms tend to occur, and what factors make them worse or better? Are there other organ symptoms that appear to be involved? A detailed, careful history and physical examination will have a much greater yield than the indiscriminate ordering of multiple laboratory examinations. Indeed, testing should proceed in a prudent and directed fashion to evaluate the diagnosis suggested by the history and physical examination.It is beyond the scope of this review to provide a detailed discussion of every autonomic disorder and the various laboratory tests used in their evaluation. The interested reader is referred to several excellent texts on the subject.2–4 Special attention should be given to identify any prescription or over-the-counter medications that could cause or exacerbate hypotension, as well as to recognize the possibility of illicit drug or alcohol abuse.Because the autonomic centers of the brain are not readily accessible to direct measurement, autonomic function principally is evaluated by observing the responses of different functions or organ systems to a variety of pharmacological or physiological challenges.38 The simplest and most important of these is the recording of heart rate and blood pressure in the supine, sitting, and immediate upright positions, as well as at 3 and 5 minutes after being upright. Blood pressure measurements should be made with the arm extended horizontally (to minimize any hydrostatic effect produced by having the arm in a dependent position).18 Because the responses noted during standing differ from those seen during passive tilt, we often perform tilt-table testing on these patients. There is an extensive body of literature on tilt-table testing that is reviewed in detail elsewhere.39 There are other tests of autonomic function available that provide useful information in selected patients, the details of which are available elsewhere.11,38Briefly, however, some aspects of tilt-table testing will be outlined. Tilt-table testing is based on the concept that an orthostatic stress, such as prolonged upright posture, could be used to cause venous pooling and thereby provoke the previously discussed responses in predisposed individuals.39 In contrast to standing, the patient is strapped