Role of autonomic evaluation in the diagnosis and management of syncope

Syncope: there is more than haemodynamic failure

Convulsive syncopes may be particularly difficult to differentiate from epileptic seizures. Recurrent syncopes are caused by autonomic or non-autonomic failure. In this report, we present a 22-year-old woman who was misdiagnosed as epileptic and whose first symptoms during neurocardiogenic syncope occurred prior to asystole. The patient sensed her aura and reached for the alarm button 2 s before a cardiac asystole was documented in the ECG. We conclude that so far unspecified autonomic mechanisms play a significant role in our patient leading to the symptoms of dizziness and nausea heralding her attacks prior to the occurrence of asystole.

Autonomic testing: common techniques and clinical applications

Laboratories able to test autonomic function are increasingly available and rely on batteries of well-accepted, noninvasive tests. Tests of parasympathetic cardiovagal, sympathetic vasoconstriction, and sudomotor (sweating) function are most commonly employed. Common examples include heart rate variability to various challenges, Valsalva maneuver, standing and tilt-table studies, and various sudomotor methods. New techniques and technical refinements continue to be described. Most studies rely on perturbations of complex systems and not direct assessment. Testing has helped to improve disease recognition and prompted advances in classification, pathophysiology, and treatment. Major areas impacted include hereditary and immune-mediated autonomic neuropathy, diabetic autonomic neuropathy, distal symmetric polyneuropathy, Parkinson disease and other autonomic failure syndromes, orthostatic intolerance, and unexplained syncope.

Methods of evaluation of autonomic nervous system function

Disturbances of the autonomic nervous system play a crucial role in the pathogenesis and clinical course of many diseases. Recently, rapid development has occurred in the clinical assessment of autonomic function. Various procedures have been described as diagnostic tools to monitor autonomic dysfunction. Some of them are mostly used for research purposes. Many, however, have found their place in routine clinical evaluation. Our paper presents selected methods of assessment of the autonomic nervous system with particular emphasis on those that are useful in diagnosis and treatment of diseases of the cardiovascular system. We discuss multiple tests based on cardiovascular reflexes, methods of studying heart rate variability as well as direct catecholamine measurements. Moreover, we outline tests of sudomotor function and microneurography.

beta-adrenergic receptor blockers and the treatment of vasovagal syncope: more nails in the coffin!

beta-Adrenergic receptor blockers are one of a number of therapeutic agents promoted as having beneficial effects in vasovagal syncope. In this issue of Clinical Science, Eldadah and co-workers have investigated the effect of the beta-adrenergic receptor blocker propranolol in preventing syncope in a double-blind cross-over trial in eight subjects with a diagnosis based on tilt table testing and elevated plasma adrenaline levels during syncope. Of these, seven did not respond and the authors therefore suggest that this drug has no role in the management of vasovagal syncope. Their laboratory-based study, however, raises a number of issues regarding terminology, choice of subjects, the value and role of investigations directed towards diagnosis and in understanding pathophysiological mechanisms, and the relevance of such trials to individual subjects with vasovagal syncope.

The ACCF/AHA Scientific Statement on Syncope

The American College of Cardiology Foundation (ACCF) and the American Heart Association (AHA) have recently published, in both the Journal of the American College of Cardiology (JACC) and Circulation, a Scientific Statement on the Evaluation of Syncope ('Statement'). This Scientific Statement was commissioned to provide guidance for clinicians regarding the evaluation of patients who present with 'syncope'. The Statement was not intended to be a formal set of practice guidelines. However, in the absence of generally accepted practice guidelines in North America, the Statement's potential impact on clinical care may be more far-reaching than expected; it may erroneously be considered to be the authoritative 'de-facto' guideline document. This commentary, submitted by a multidisciplinary consortium of more than 60 physicians with expertise in the management of transient loss of consciousness (TLOC), points out that in many respects the ACCF/AHA Syncope Statement fails to address long-standing clinical errors associated with the evaluation of episodes of apparent TLOC, including syncope. If not appropriately revised, the current Statement may lead to both inadequate patient care as well as a potentially damaging legal environment for physicians undertaking evaluation of patients who present with transient loss of consciousness.

Quantitative studies of autonomic function

Dysfunction of the peripheral and central autonomic nervous system is common in many neurological and general medical diseases. The quantitative assessment of sympathetic and parasympathetic function is essential to confirm the diagnosis of autonomic failure, to provide the basis for follow-up examinations, and potentially to monitor successful treatment. Various procedures have been described as useful tools to quantify autonomic dysfunction. The most important tests evaluate cardiovascular and sudomotor autonomic function. In this review, we therefore focus on standard tests of cardiovascular and sudomotor function such as heart-rate variability at rest and during deep breathing, active standing, and the Valsalva maneuver, and on the sympathetic skin response. These tests are widely used for routine clinical evaluation in patients with peripheral neuropathies. Refined methods of studying heart-rate variability, baroreflex testing, and detailed measures of sweat output are mostly used for research purposes. In this context, we describe the spectral analysis of slow modulation of heart rate or blood pressure, reflecting sympathetic and parasympathetic influences, and consider various approaches to baroreflex testing, the thermoregulatory sweat test, and the quantitative sudomotor axon reflex test. Finally, we discuss microneurography as a technique of direct recording of muscle sympathetic nerve activity.