Definition and Overview
The autonomic function test battery is a diagnostic protocol that systematically evaluates the function of the sympathetic and parasympathetic nerves using several standardized tests. Because it is difficult to sufficiently evaluate the complex functions of the autonomic nervous system with a single test, a combination of complementary tests is performed.
The autonomic reflex screen (ARS) developed by Low et al. at the Mayo Clinic is the most widely used standard protocol. This protocol includes cardiorespiratory testing using heart rate variability, Valsalva maneuver, standing tilt test (or standing test), and QSART.
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Heart rate variability test (parasympathetic nerve evaluation)
The deep breathing test measures heart rate fluctuations (E:I ratio) during six deep breaths per minute. The difference between maximum and minimum heart rate during breathing is a normal response, and a decrease in this difference suggests parasympathetic (vagus) dysfunction.
Valsalva ratio is the maximum heart rate during the Valsalva maneuver divided by the minimum heart rate after the maneuver and is an indicator of parasympathetic nerve function. If it is below the age-adjusted lower limit of normal, it is judged abnormal.
The 30:15 ratio is the R-R interval ratio of the 15th and 30th heartbeats after standing. The reflex response of heart rate increase (15th beat) immediately after standing up and subsequent decrease (30th beat) is evaluated and reflects parasympathetic function.
Blood pressure response test (evaluation of sympathetic adrenergic activity)
Blood pressure response during the Valsalva maneuver assesses sympathoadrenergic function. Late phase II (IIb) blood pressure recovery and phase IV blood pressure overshoot are normal responses, and loss of this pattern indicates abnormal sympathetic nerve function.
In the tilt table test or active standing test, blood pressure changes are measured for 3 minutes after standing. Orthostatic hypotension is diagnosed when systolic blood pressure drops by more than 20 mmHg or diastolic blood pressure drops by more than 10 mmHg when standing.
QSART (Sympathocholinergic/Sweat Function Assessment)
The quantitative sudomotor axon reflex test (QSART) quantifies sweat secretion by stimulating the axon reflex of eccrine sweat glands with acetylcholine iontophoresis. Measures four areas: the proximal forearm, proximal lower leg, ankle, and top of the foot, and evaluates the distal-proximal functional distribution of sympathetic cholinergic fibers.
CASS (Comprehensive Autonomic Score)
The composite autonomic scoring scale (CASS) is a scale that quantifies the battery test results from 0 to 10 points and indicates the severity of autonomic dysfunction as a single score.
CASS consists of three subscores. Parasympathetic score (0 to 3 points): based on cardiorespiratory testing, Valsalva ratio, and 30:15 ratio. Adrenergic score (0 to 4 points): based on orthostatic blood pressure response and Valsalva blood pressure response. Sympathetic cholinergic score (0 to 3 points): based on QSART.
A CASS score of 0 is normal, 1 to 3 points are mild, 4 to 6 points are moderate, and 7 to 10 points are severe autonomic dysfunction. CASS is useful for follow-up evaluation of autonomic abnormalities and monitoring treatment response.
Preparation before inspection
Standardized preparation is necessary for accurate test results. Avoid consuming caffeinated beverages (coffee, tea, energy drinks) and alcohol starting the day before the test. Fasting for at least 3 to 4 hours on the day of the test is recommended. Discontinuation of medications that affect the cardiovascular system (beta-blockers, alpha-blockers, anticholinergics, tricyclic antidepressants, etc.) should be decided after consulting with the attending physician. Smoking is prohibited on the day of the test. Rest in the supine position for at least 30 minutes before the test.
clinical application
Diabetic autonomic neuropathy: Used to diagnose and evaluate the severity of cardiac autonomic neuropathy, including the 5 tests recommended by the ADA. POTS and orthostatic hypotension: Orthostatic blood pressure and heart rate responses are used for accurate subtype classification and mechanism evaluation. Distinguishing between multiple system atrophy and Parkinson's disease: It helps in differentiating the two diseases based on the extent and pattern of autonomic nerve damage. Small fiber neuropathy: Detecting early damage with QSART. It is also used to evaluate autoimmune autonomic ganglionopathy.