Definition and Overview
The sudomotor function test is an autonomic test that evaluates the functional integrity of the sympathetic cholinergic C fibers that innervate the sweat glands. Sweating is one of the final effector responses of the autonomic nervous system and is one of the first functions to appear abnormal in small fiber neuropathy.
Existing nerve conduction tests can only evaluate thick myelinated nerves (Aβ, Aδ), so there are limitations in diagnosing small fiber (C fiber, Aδ fiber) lesions. The sweat function test is a key test that fills this diagnostic gap, and has a sensitivity of about 80% in diagnosing small fiber neuropathy.
Inspection type
Quantitative axon reflex sweating test (QSART)
QSART (quantitative sudomotor axon reflex test) is a standard sweat function test developed at Mayo Clinic.
The inspection principle is as follows. When acetylcholine is injected into the skin by iontophoresis, the sweating nerve endings are stimulated and adjacent sweat glands are activated through the axon reflex. The amount of sweating is quantitatively measured using a multicompartmental sweat cell.
The standard test areas are four: forearm, proximal lower extremity, distal lower extremity, and foot, and sweating (μL/cm²) is recorded for 5 minutes in each area. The decision is made by comparing it with the normal standard values for each age and gender.
Thermogenic sweating test (TST)
The thermoregulatory sweat test is a test that visually evaluates sweat distribution throughout the body. After applying a perspiration indicator (alizarin red or starch-iodine powder) to the entire body, the temperature of the sealed examination room is raised to increase core body temperature by approximately 1°C. The color of the indicator changes in the sweating area, allowing the distribution of sweating to be recorded through photographs.
The advantage of TST is that the distribution of sweating throughout the body can be identified at once, and central (preganglionic) and peripheral (postganglionic) lesions can be distinguished based on the anhidrosis pattern.
Sympathetic cutaneous response (SSR)
Sympathetic skin response is a simple test that measures electrical skin activity. Changes in skin potential caused by electrical stimulation, inspiratory stimulation, or psychological stimulation are recorded. The test is simple and can be performed with general electromyography equipment, but has limitations in that reproducibility is low and quantification is difficult due to habituation.
Quantitative Direct Indirect Perspiration Test (QDIRT)
QDIRT (quantitative direct and indirect test of sudomotor function) is a method to quantitatively evaluate direct and axonal reflex responses after acetylcholine iontophoresis at the level of individual sweat droplets. It has been reported to provide accuracy comparable to QSART while being relatively simple.
clinical application
small fiber neuropathy
In patients who do not have any abnormalities in conventional nerve conduction tests but have burning pain, numbness, or autonomic symptoms, abnormal sweat function tests are an important finding suggesting small fiber neuropathy. The combination of QSART abnormalities and skin biopsy (reduced intraepidermal nerve fiber density) is the gold standard for diagnosis.
Diabetic autonomic neuropathy
Abnormal sweating is one of the early signs of diabetic autonomic neuropathy. Abnormalities first appear in the distal part (foot) and spread to the proximal part as the disease progresses. Regular sweat function tests are useful in monitoring the progression of autonomic neuropathy.
CRPS
In complex regional pain syndrome (CRPS), sweating abnormalities (excessive or decreased) in the affected extremity are part of the diagnostic criteria. QSART allows objective comparison of the amount of sweating between the affected limb and the healthy limb.
POTS and more
Approximately 50% of patients with postural orthostatic tachycardia syndrome (POTS) are accompanied by decreased sweating in the distal region, suggesting peripheral sympathetic nerve dysfunction. Characteristic abnormal sweating patterns are also observed in multiple system atrophy, Parkinson's disease, and pure autonomic dysfunction.
Interpretation of results
Interpretation of QSART results
- Decreased sweating: suggests a peripheral lesion of the axonal reflex pathway (postganglionic sudomotor failure).
- Increased sweating: May occur during denervation supersensitivity or reinnervation.
- Distal selective reduction: This is the typical pattern of length-dependent small fiber neuropathy.
- Asymmetric findings: CRPS, suggestive of focal nerve lesion.
Interpretation of TST results
- distal anhidrosis: peripheral autonomic neuropathy
- Segmental anhidrosis: spinal cord lesion, sympathetic trunk lesion
- Widespread anhidrosis: Central causes such as multiple system atrophy, pure autonomic failure, etc.
Limitations of the test
Sweating function is affected by age, gender, body temperature, environmental temperature and humidity, medications, and skin condition. It is important to apply a standardized test environment and age- and gender-specific standards, and the diagnosis should be made in combination with clinical findings and other autonomic nerve test results rather than making a diagnosis based on a single test result alone.