Sudomotor Testing

For physicians

What is measured

Electrochemical skin conductance at the palms and soles. A low-voltage direct current is applied through stainless steel electrodes and the electrochemical reaction between the electrodes and chloride ions in eccrine sweat is quantified. The measurement reflects post-ganglionic sympathetic cholinergic fiber function — small unmyelinated C-fibers — and, indirectly, cutaneous microcirculation.

Evidence and positioning

A systematic review of electrochemical skin conductance identified 24 studies and appraised the method against established reference tests including the quantitative sudomotor axon reflex test, sympathetic skin responses, the thermoregulatory sweat test and skin biopsy, concluding that it expands the available arsenal for evaluating sudomotor dysfunction. It is a screening and characterization measurement, not a replacement for those reference tests where a definitive answer is required.

Diagnostic performance is population-dependent and should be quoted with its population. In a cohort of transthyretin familial amyloid polyneuropathy carriers compared with healthy controls, foot conductance showed 76% sensitivity and 85% specificity for detecting dysautonomia, and was the only measure that separated symptomatic patients with autonomic dysfunction from those without.

Why it belongs in a metabolic assessment

Nerve injury associated with impaired glucose tolerance and metabolic syndrome preferentially affects small fibers, and nerve conduction studies are relatively insensitive to it. The clinical corollary is the patient with burning feet and a normal conduction study, who is very often correctly symptomatic and incorrectly reassured. The American Diabetes Association’s position statement is explicit that screening should not await symptoms and that much distal symmetric polyneuropathy is painless.

Confounders to record

• Anticholinergic burden — antidepressants, bladder medications, antihistamines — reduces conductance directly.
• Skin preparation. Lotion, residue, moisture and callus all move the reading.
• Room temperature and recent activity.
• Dermatological disease at the contact sites.

Where it fits

Run with the vascular study when the question is a foot at risk, and with autonomic testing when the question is generalized dysautonomia. Interpreting the report.

Evidence and limitations

Written for a reader who wants the literature rather than the summary. Where the evidence is thin we say so and give both sides, and where a claim is mechanistic rather than demonstrated we label it as such.

Mechanism

Eccrine sweat glands are innervated by post-ganglionic sympathetic cholinergic C-fibers — thin, unmyelinated, long, and dependent on the cutaneous microcirculation they run alongside. That combination makes them metabolically expensive to maintain and vulnerable early. Sudomotor impairment and microcirculatory disturbance are described as among the earliest detectable stages of a distal peripheral neuropathy, and electrochemical skin conductance quantifies that function by reverse iontophoresis and chronoamperometry rather than by provoking and collecting sweat.

The mechanistic reason this matters for a metabolic clinic is that nerve injury associated with impaired glucose tolerance and the metabolic syndrome preferentially damages small fibers, while nerve conduction studies — which interrogate large myelinated fibers — are relatively insensitive to it. Skin biopsy with intraepidermal nerve fiber density has been the reference standard for confirming small-fiber involvement for exactly that reason.

What the evidence shows

A systematic review of electrochemical skin conductance identified 24 studies and classified them as preclinical, normative, comparative/diagnostic or interventional, appraising the method against established reference tests — the quantitative sudomotor axon reflex test, sympathetic skin responses, the thermoregulatory sweat test and skin biopsy. Its conclusion was measured: the method expands the available arsenal for evaluating sudomotor dysfunction, and its advantages and disadvantages relative to established tests should inform future guidelines.

Diagnostic performance figures are population-specific and should always be quoted with their population. In 221 Mexican patients with type 2 diabetes assessed against the Michigan Neuropathy Screening Instrument, abnormal hand or foot conductance had a sensitivity of 97% and a positive predictive value of 87% for detecting neuropathy in those with diabetes for five years or more, and 91% sensitivity with 88% positive predictive value in those with shorter duration. In a different population entirely — 133 transthyretin familial amyloid polyneuropathy carriers compared with 37 healthy controls — foot conductance showed 76% sensitivity and 85% specificity for detecting dysautonomia, and was the only measure separating symptomatic patients with autonomic dysfunction from those without.

A review of the technology argues that small nerve fiber dysfunction occurs early in metabolic syndrome and diabetes and may be the only neurological manifestation in small-fiber neuropathies, below the detection limits of traditional nerve function tests. Separately, in 211 patients with type 2 diabetes and overweight or obesity but no established atherosclerotic cardiovascular disease, sudomotor scores were significantly associated with calculated cardiovascular risk on multiple linear regression, and the prevalence of cardiac autonomic neuropathy in that cohort was 67.2%.

Limitations of that evidence

• Reference-standard disagreement. The comparators — the quantitative sudomotor axon reflex test, sympathetic skin responses, the thermoregulatory sweat test and skin biopsy — measure related but not identical things, so “agreement” means different things in different studies.
• Population dependence. Sensitivity and specificity are properties of a test in a population. A figure obtained in long-standing type 2 diabetes does not transfer to a screening population of asymptomatic adults, and a figure obtained in a hereditary amyloid cohort transfers to almost nobody else.
• Verification and spectrum bias. Several studies compare against a clinical screening instrument rather than a tissue standard, which inflates apparent agreement when both tools are influenced by the same clinical severity.
• Industry involvement. Some of the descriptive and normative literature has authors affiliated with the device manufacturer. That does not make the findings wrong; it does mean independent replication carries more weight.
• Confounding by medication and skin condition. Anticholinergic burden, skin preparation, callus, temperature and recent activity all move the reading and are unevenly controlled across studies.

What remains uncertain

The gap is outcome evidence. A registry search on 20 August 2026 for interventional and observational studies with an intervention matching Sudoscan, electrochemical skin conductance or sudomotor returned 41 registered studies — predominantly small, single-center, and designed around diagnostic agreement, prevalence or physiology across diabetes, Parkinson’s disease, fibromyalgia, Fabry disease, spinal cord injury and chemotherapy-induced neuropathy. What is not there is a large randomized trial testing whether screening asymptomatic adults with this measurement, and acting on the result, reduces ulceration, amputation or cardiovascular events.

That absence should be stated plainly rather than dressed up. It is also the normal state for a diagnostic measurement: the standard applied to a diagnostic is whether it measures what it claims to measure and whether the result changes management, and both of those are answerable from the literature above. It is worth adding that the trials which do exist in this space systematically exclude the patients a metabolic clinic actually sees — multiple comorbidities, polypharmacy, significant obesity, long symptom duration — so a null result in a selected population would not establish that the measurement is uninformative in a complex one.