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Sudomotor Function Test

Sudomotor Function Test

Sudomotor function testing: QSART and other methods for evaluating sympathetic cholinergic sudomotor pathways, clinical applications in autonomic neuropathy diagnosis.

2026-03-29

At a Glance

Sudomotor function testing: QSART and other methods for evaluating sympathetic cholinergic sudomotor pathways, clinical applications in autonomic neuropathy diagnosis.

Definition and Overview

The sudomotor function test is an autonomic nervous system assessment that evaluates the functional integrity of sympathetic cholinergic C fibers innervating the sweat glands. Sweating is one of the final effector responses of the autonomic nervous system and is among the first functions to show abnormalities in small fiber neuropathy.

Conventional nerve conduction studies can only evaluate large myelinated fibers (A-beta, A-delta), limiting their ability to diagnose small fiber (C fiber, A-delta fiber) lesions. Sudomotor function testing is a key examination that fills this diagnostic gap, demonstrating approximately 80% sensitivity in the diagnosis of small fiber neuropathy [1].

Types of Tests

Quantitative Sudomotor Axon Reflex Test (QSART)

QSART (quantitative sudomotor axon reflex test) is the standard sudomotor function test developed at the Mayo Clinic [1].

The test principle is as follows: acetylcholine is delivered to the skin via iontophoresis, stimulating sudomotor nerve terminals. Through the axon reflex, adjacent sweat glands are activated. Sweat output is quantitatively measured using a multicompartmental sweat cell.

The standard test sites are 4 locations: the forearm, proximal leg, distal leg, and foot. Sweat output (microL/cm2) is recorded over 5 minutes at each site and compared against age- and sex-specific normative values for interpretation.

Thermoregulatory Sweat Test (TST)

The thermoregulatory sweat test (TST) is an examination that visually assesses whole-body sweat distribution. After applying an indicator powder (alizarin red or starch-iodine powder) to the entire body surface, the temperature in a sealed testing room is raised to increase core body temperature by approximately 1 degrees C. Areas of sweating are revealed by color changes in the indicator, allowing photographic documentation of sweat distribution [2].

The advantage of TST is its ability to assess whole-body sweat distribution in a single examination, and anhidrosis patterns can help differentiate central (preganglionic) from peripheral (postganglionic) lesions.

Sympathetic Skin Response (SSR)

The sympathetic skin response (SSR) is a straightforward test that measures cutaneous electrical activity. It records skin potential changes in response to electrical stimulation, inspiratory stimulation, or psychological stimuli. While the test is simple and can be performed with standard electromyography equipment, it has limitations including poor reproducibility due to habituation and difficulty with quantification.

Quantitative Direct and Indirect Test of Sudomotor Function (QDIRT)

QDIRT (quantitative direct and indirect test of sudomotor function) is a method that quantitatively evaluates both direct and axon reflex responses at the individual sweat droplet level following acetylcholine iontophoresis [3]. It has been reported to provide accuracy comparable to QSART while being relatively simpler to perform.

Clinical Applications

Small Fiber Neuropathy

In patients with burning pain, tingling, and autonomic symptoms despite normal nerve conduction studies, abnormal sudomotor function testing is an important finding suggestive of small fiber neuropathy. The combination of QSART abnormalities and skin biopsy (reduced intraepidermal nerve fiber density) represents the diagnostic gold standard [5].

Diabetic Autonomic Neuropathy

Sudomotor dysfunction is one of the early signs of diabetic autonomic neuropathy. Abnormalities first appear distally (in the feet) and progress proximally with disease advancement. Regular sudomotor function testing is useful for monitoring the progression of autonomic neuropathy [1].

CRPS

In complex regional pain syndrome (CRPS), sweat abnormalities (either excessive or diminished) in the affected limb are part of the diagnostic criteria. QSART enables objective comparison of sweat output between the affected and unaffected limbs.

POTS and Other Conditions

Approximately 50% of patients with postural orthostatic tachycardia syndrome (POTS) demonstrate distal sudomotor reduction, suggesting peripheral sympathetic nerve dysfunction. Characteristic sudomotor abnormality patterns are also observed in multiple system atrophy, Parkinson's disease, and pure autonomic failure [4].

Interpretation of Results

QSART Result Interpretation

  • Reduced sweat output: Suggests a peripheral lesion in the axon reflex pathway (postganglionic sudomotor failure).
  • Increased sweat output: May indicate denervation supersensitivity or reinnervation processes.
  • Selective distal reduction: A typical pattern of length-dependent small fiber neuropathy.
  • Asymmetric findings: Suggestive of CRPS or focal nerve lesions.

TST Result Interpretation

  • Distal anhidrosis: Peripheral autonomic neuropathy
  • Segmental anhidrosis: Spinal cord lesion, sympathetic chain lesion
  • Widespread anhidrosis: Central causes such as multiple system atrophy, pure autonomic failure

Limitations of the Test

Sudomotor function is influenced by age, sex, body temperature, ambient temperature and humidity, medications, and skin condition. Standardized testing environments and application of age- and sex-specific normative values are essential. Diagnosis should not be based on a single test result alone but rather on a comprehensive assessment integrating clinical findings and other autonomic function test results [2].

Frequently Asked Questions

It is performed when there is abnormal sweating of the hands and feet (hyperhidrosis or anhidrosis), suspected small fiber neuropathy, screening for autonomic neuropathy in diabetes, objective assessment of CRPS, or evaluation of unexplained autonomic dysfunction. It is particularly useful when small fiber lesions are suspected despite normal nerve conduction studies [1].

QSART uses acetylcholine iontophoresis and only a mild tingling sensation may be felt from the weak electrical current applied to the skin. Since it is a non-invasive procedure, pain is virtually absent. The sympathetic skin response (SSR) test utilizes startle responses from electrical or inspiratory stimuli, minimizing discomfort.

Distal sweat reduction suggests length-dependent small fiber neuropathy or diabetic autonomic neuropathy. Unilateral sweat abnormalities are suggestive of CRPS, and widespread anhidrosis may indicate autonomic failure or multiple system atrophy. Excessive sweating has been reported in conditions such as POTS [4].

Medications that affect sweating, such as anticholinergics, antihistamines, and tricyclic antidepressants, should be discontinued at least 48 hours prior. On the day of testing, moisturizing creams and lotions should not be applied to the test sites, and caffeine and nicotine should be avoided. Testing room temperature (22-24 degrees C) and humidity must be maintained at constant levels.

QSART quantitatively measures sweat output, providing high reproducibility and excellent sensitivity. SSR is simpler but has low reproducibility due to habituation and normal/abnormal determination can be subjective. QSART is suitable for precise diagnosis, while SSR is appropriate as a screening test [2].

References

  1. [1] Low PA, Caskey PE, Tuck RR, Fealey RD, Dyck PJ (1983). "Quantitative sudomotor axon reflex test in normal and neuropathic subjects." Annals of Neurology, 14: 573-580. DOI PubMed
  2. [2] Illigens BMW, Gibbons CH (2009). "Sweat testing to evaluate autonomic function." Clinical Autonomic Research, 19: 79-87. DOI PubMed
  3. [3] Gibbons CH, Illigens BMW, Centi J, Freeman R (2008). "QDIRT: quantitative direct and indirect test of sudomotor function." Neurology, 70: 2299-2304. DOI PubMed
  4. [4] Cheshire WP, Freeman R (2003). "Disorders of sweating." Seminars in Neurology, 23: 347-358. DOI PubMed
  5. [5] Novak V, Freimer ML, Kissel JT, Sahenk Z, Periquet IM, Nash SM, Collins MP, Mendell JR (2001). "Autonomic impairment in painful neuropathy." Neurology, 56: 861-868. DOI PubMed

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