Autonomic Testing

Louis H. Weimer

INTRODUCTION

Recognition of disorders producing autonomic dysfunction continues to increase the demand for noninvasive clinical testing in order to validate clinical diagnoses. Because autonomic systems affect virtually all organ systems, symptoms produced are multiple and varied, often affecting functions not considered by neurologists. Because many autonomic complaints can appear to be nonspecific, objective assessment is desirable in many instances. Laboratories testing autonomic function are increasingly available in part due to reliable noninvasive techniques. However, formal training opportunities in this neurologic specialty remain limited.

TABLE 27.1 Selected Tests of Autonomic Function

Cardiovagal
Well established
	HR variability to cyclic deep breathing
	HR response to the VM (VR)
	HR response to standing (30:15 ratio)
Other
	Diving reflex/cold face test
	HR variability at rest
	HR response to cough
	Spectral analysis of HR signals (frequency domain)
	Transfer function analysis (nonlinear dynamics)
Adrenergic
Well established
	BP response to the VM (phase IV and late phase II)
	BP response to orthostatic stress
		– Head-up tilt
		– Standing
Other
	Sustained handgrip test
	Squat test
	BP response to alternate stressors
		– Lower body negative pressure
		– Neck suction
		– Lying down
		– Liquid meal
	Plasma catecholamine levels (supine/standing)
	Microneurography
	Mental stress tests
	Cold pressor test
	Spectral and transfer function BP analysis
Sudomotor
Well established
	Sympathetic skin response (SSR)
	Quantitative sudomotor axon reflex test (QSART)
	Thermoregulatory sweat test (TST)
	Silastic sweat imprint testing
	Skin biopsy for sweat gland innervation
Additional or Investigational Methods
	Pharmacologic challenges
	Vasomotor testing
	Pupillary testing (pharmacologic)
	Pupillometry, pupillography
	Urodynamics/cystometrogram with bethanechol
	GI motility studies
	GI manometry
	Salivary testing/Schirmer test
	Penile plethysmography, papaverine injection
	Neuroendocrine tests
	Neurogenic flare test
	Quantitative direct and indirect test of sudomotor function (Q-DIRT)
	Cardiac PET scanning
HR, heart rate; VM, Valsalva maneuver; VR, Valsalva ratio; BP, blood pressure; GI, gastrointestinal; PET, positron emission tomography.

Unlike other systems, autonomic function is not directly assessed. Instead, responses of complex overlapping reflex loops are measured after controlled perturbations, most commonly heart rate (HR), blood pressure (BP), and sweating. Techniques to evaluate autonomic function are numerous and continue to be devised, but only a limited number are considered to be suitable for routine clinical application (Table 27.1). Tests of cardiovagal, adrenergic, and sudomotor function are most commonly performed and are recognized, standard clinical measures. Consensus recommends use of a standardized testing battery in a controlled setting. Bedside screening tests complement a clinical evaluation; some techniques can be performed with limited equipment such as an electrocardiography (ECG) or electromyography (EMG) machine. The primary testing goal is to identify autonomic failure and to assess and quantify disease severity. In some cases, determining the systems involved, such as parasympathetic, sympathetic, or panautonomic, can refine diagnostic possibilities. In most instances, localization to central, preganglionic, or postganglionic dysfunction is not possible. The effects of medications, environmental conditions, dehydration, and acute illness should be minimized during testing. Selected commonly performed tests are briefly discussed in this chapter.

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