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.
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.
