Definition and Overview
The autonomic testing protocol is a combination of standardized tests to evaluate the function of the sympathetic and parasympathetic nervous systems in an objective and reproducible manner. Since the autonomic nervous system controls a wide range of functions, including cardiovascular, sweating, digestion, and urogenital functions, it is difficult to determine the overall functional status through a single test. Therefore, a test battery that evaluates multiple effectors simultaneously is needed.
Preparation before inspection
Patient Preparation
For accurate test results, the following must be observed:
- Fasting for at least 3 hours before the test (water is allowed)
- Avoid consuming caffeine, nicotine, or alcohol on the day of the test
- Comfortable clothes, plenty of sleep
- Maintain laboratory temperature between 22 and 24°C
drug withdrawal
Discontinue medications that affect the autonomic nervous system before an appropriate period of time, in consultation with your doctor.
- Beta-blockers, alpha-blockers: more than 48 hours ago
- Anticholinergics, antihistamines: more than 48 hours before
- Tricyclic antidepressants, SSRIs: 5 to 7 days before (gradual reduction)
- midodrine, fludrocortisone: more than 24 hours ago
standard test battery
1. Cardiovascular parasympathetic function test
#### Heart Rate Variability (HRV)
Record the electrocardiogram at rest and analyze indicators in the time domain (RMSSD, pNN50) and frequency domain (LF, HF, LF/HF ratio). The HF (0.15~0.40 Hz) component reflects parasympathetic nerve activity, and the LF/HF ratio represents sympathetic-parasympathetic balance.
#### Deep breathing test
Heart rate fluctuations are measured while taking 6 deep breaths per minute. The expiratory-inspiratory heart rate difference (E:I ratio) is a sensitive indicator of parasympathetic function. There are normal standards for each age; more than 15 times/minute is considered normal, and less than 10 times/minute is considered abnormal.
#### Valsalva maneuver
While forced expiration is maintained for 15 seconds at a pressure of 40 mmHg, the four-phase response (phase I to IV) of heart rate and blood pressure is observed. Valsalva ratio (phase IV maximum heart rate/phase II minimum heart rate) reflects parasympathetic function, and a value of 1.21 or higher is normal.
2. Cardiovascular sympathetic function test
#### Active standing blood pressure test
Changes in blood pressure and heart rate are measured when standing from the supine position. Orthostatic hypotension is diagnosed when systolic blood pressure drops by more than 20 mmHg or diastolic blood pressure by more than 10 mmHg within 3 minutes of standing.
#### Head-up tilt test
Observe changes in blood pressure and heart rate for 20 to 45 minutes while passively maintaining a standing position of 60 to 70 degrees. It is essential in the differential diagnosis of vasovagal syncope, POTS, and orthostatic hypotension.
#### Valsalva maneuver blood pressure response
Analyze the four-stage response of blood pressure during the Valsalva maneuver. Failure of blood pressure recovery late in Phase II and loss of Phase IV overshoot suggest sympathetic dysfunction.
3. Sweating function test
#### Quantitative axon reflex sweating test (QSART)
Sweating is induced using acetylcholine iontophoresis, and the amount of sweating is quantitatively measured. It is performed in four areas: forearm, proximal lower extremity, distal lower extremity, and foot, and is useful for early diagnosis of small fiber neuropathy and autonomic neuropathy.
#### Thermoregulatory sweat test (TST)
After applying a sweating indicator (alizarin red powder) to the entire body, the room temperature is raised to increase body temperature by 1°C and the distribution of sweating is observed. Central-peripheral lesions are differentiated based on the pattern of anhidrosis area.
Interpretation of results
Composite Autonomic Severity Score (CASS)
The Composite Autonomic Severity Score (CASS), developed by the Mayo Clinic, quantifies the severity of autonomic dysfunction with a score of 0 to 10 by adding three subscores: sweating (0 to 3 points), cardiovascular adrenaline (0 to 4 points), and cardiovascular choline (0 to 3 points).
- 0 points: normal
- 1 to 3 points: mild dysfunction
- 4 to 6 points: moderate functional impairment
- 7-10 points: severe functional impairment
Clinical Interpretation Patterns
- Parasympathetic dominance disorder: decreased deep breathing heart rate variability + decreased Valsalva ratio, normal orthostatic blood pressure → early stage of diabetic autonomic neuropathy
- Widespread autonomic failure: decreased parasympathetic + sympathetic functions + orthostatic hypotension → multiple system atrophy, pure autonomic failure
- Selective sympathetic dysfunction: sweating abnormalities + orthostatic hypotension, relative preservation of parasympathetic function → dopamine β-hydroxylase deficiency
- POTS pattern: Increased heart rate by more than 30 bpm upon standing, no orthostatic hypotension, possibly accompanied by abnormal sweating.
Limitations of the test
Autonomic nerve test results are affected by age, gender, medication, water intake, laboratory environment, etc., so it should be performed under standardized conditions and normal values for each age should be applied. Additionally, abnormal test results do not necessarily indicate a specific disease and must be interpreted in conjunction with clinical findings.