The autonomic nervous system (ANS) regulates physiologic processes, such as blood pressure, heart rate, body temperature, digestion, metabolism, fluid and electrolyte balance, sweating, urination, defecation, sexual response, and other processes. Regulation occurs without conscious control, i.e., autonomously. The ANS has two major divisions: the sympathetic and parasympathetic systems. ANS testing measures alterations in the R-R interval of the electrocardiogram (ECG) in response to parasympathetic and sympathetic system stimulation. The aim of such testing is to correlate signs and symptoms of possible autonomic dysfunction with objective measurement in a way that is clinically useful. Many organs are controlled primarily by either the sympathetic or parasympathetic system, although they may receive input from both; occasionally, functions are reciprocal (e.g., sympathetic input increases heart rate; parasympathetic decreases it).
The sympathetic nervous system is catabolic and activates fight-or-flight responses. Thus, sympathetic output increases heart rate and contractility, bronchodilation, hepatic glycogenolysis and glucose release, BMR (basal metabolism rate), and muscular strength; it also causes sweaty palms. Less immediately-life-preserving functions (e.g., digestion, renal filtration) are decreased.
The parasympathetic nervous system is anabolic; it conserves and restores. Gastrointestinal secretions and motility (including evacuation) are stimulated, heart rate is slowed, and blood pressure decreases.
Disorders of the ANS can affect any system of the body; they can originate in the peripheral or central nervous system and may be primary or secondary to other disorders. Symptoms suggesting autonomic dysfunction include orthostatic hypotension, heat intolerance, nausea, constipation, urinary retention or incontinence, nocturia, impotence, and dry mucous membranes. If a patient has symptoms suggesting autonomic dysfunction, cardiovagal, adrenergic, and sudomotor tests are usually done to help determine severity and distribution of the dysfunction.
ANS testing can be grouped into three general categories:
- Cardiovagal innervation - a test that provides a standardized quantitative evaluation of vagal innervation to parasympathetic function of the heart. Responses are based on the interpretation of changes in continuous heart recordings in response to standardized maneuvers and include heart rate response to deep breathing, Valsalva ratio, and 30:15 ratio heart rate responses to standing. A tilt table may be used, but is not required.
- Vasomotor adrenergic innervation - evaluates adrenergic innervation of the circulation and of the heart in autonomic failure. The following tests are included: beat-to-beat blood pressure and R-R interval response to Valsalva maneuver, sustained hand grip, and blood pressure and heart rate responses to tilt-up or active standing and must be performed with a tilt table.
- Sudomotor - function testing is used to evaluate and document neuropathic disturbances that may be associated with pain. The quantitative sudomotor axon reflex test (QSART), thermoregulatory sweat test (TST), sympathetic skin responses, and silastic sweat imprints are tests of sympathetic cholinergic sudomotor function.
Tests are useful in defining the presence of autonomic failure, their natural history, and response to treatment. They can also define patterns of dysautonomia that are useful in helping the clinician diagnose certain autonomic conditions. Selective autonomic failure (which only one system is affected) can be diagnosed by autonomic testing. An example is chronic idiopathic anhidrosis, where only sudomotor function is affected. Among the synucleinopathies, autonomic function tests can distinguish Parkinson’s disease (PD) from multiple system atrophy (MSA). There is a gradation of autonomic failure. PD is characterized by mild autonomic failure and a length-dependent pattern of sudomotor involvement. MSA and pure autonomic failure have severe generalized autonomic failure while Dementia with Lewy Bodies (DLB) is intermediate.
Syndromes of autonomic dysfunction which require formal autonomic function testing are relatively rare. Generally, only after excluding more common causes of autonomic signs or symptoms (e.g., hypotension, hyperhidrosis, and orthostatic tachycardia) may formal autonomic testing be indicated to exclude or confirm rarer autonomic disorders. Autonomic function testing is covered as reasonable and necessary when used as a diagnostic tool to evaluate symptoms indicative of vasomotor instability and the ANS testing is directed at establishing a more accurate or definitive diagnosis or contributing to clinically useful and relevant medical decision making for one of the following indications:
- To diagnose the presence of autonomic neuropathy in a patient with signs or symptoms suggesting a progressive autonomic neuropathy.
- To evaluate the severity and distribution of a diagnosed progressive autonomic neuropathy.
- To differentiate the diagnosis between certain complicated variants of syncope from other causes of loss of consciousness.
- To evaluate inadequate response to beta blockade in vasodepressor syncope.
- To evaluate distressing symptoms in a patient with a clinical picture suspicious for distal small fiber neuropathy in order to diagnose the condition.
- To differentiate the cause of postural tachycardia syndrome.
- To evaluate change in type, distribution or severity of autonomic deficits in patients with autonomic failure.
- To evaluate the response to treatment in patients with autonomic failure who demonstrate a change in clinical exam.
- To diagnose axonal neuropathy or suspected autonomic neuropathy in the symptomatic patient.
- To evaluate and treat patients with recurrent unexplained syncope or demonstrate autonomic failure, after more common causes have been excluded by other standard testing.
The following indications are considered not medically reasonable and necessary and will not be covered:
- Screening patients without signs or symptoms of autonomic dysfunction, including patients with diabetes, hepatic or renal disease.
- Testing for the sole purpose of monitoring disease intensity or treatment efficacy in diabetes, hepatic or renal disease.
- Testing results that are not used in clinical decision-making or patient management.
- Testing performed by physicians who do not have evidence of training, and expertise to perform and interpret these tests. Physicians must have knowledge, training, and expertise to perform and interpret these tests, and to assess and train personnel working with them. This training and expertise must have been acquired within the framework of an accredited residency and/or fellowship program or must reflect extensive continued medical education activities. If these skills have been acquired by way of continued medical education, the courses must be comprehensive, offered, sponsored or endorsed by an academic [institution] in the United States and/or by the applicable specialty/subspecialty society in the United States, and designated by the American Medical Association (AMA) as category I credit or the American Osteopathic Association (AOA).
- General professional standards with FDA clearance apply for all equipment used in ANS testing.
- Testing with ANSAR ANX 3.0 or a similar machine is considered investigational for screening and will not be covered.
Equipment for Autonomic Nervous System Studies:
Equipment with FDA clearance for heart rate variability measurements in response to paced respirations and exercises that tests only heart rate variability does not meet the full range of testing parameters required for testing of autonomic nervous system function; cardiovagal innervation and vasomotor adrenergic innervation, and does not ensure full test requirements, such as blood pressure monitoring and blood oxygen levels; nor do they incorporate proper testing conditions, such as the use of a tilt table. Providers may be asked to supply information on the equipment used to perform autonomic nervous system studies, to ensure that all studies performed meet the requirements of the procedure.