For patients

Sudomotor dysfunction: when sweating goes wrong

Sweat glands are controlled by some of the smallest and most fragile nerves in the body. When those nerves degrade, sweating changes — and that change is measurable.

The word is sudomotor, from the Latin for sweat. It is a nerve function, and it has nothing to do with any infection.

What sudomotor function is

Sweat glands are driven by post-ganglionic sympathetic cholinergic fibers — small, unmyelinated C-fibers that run alongside the tiny blood vessels of the skin. Because those fibers are long, thin and unmyelinated, they are metabolically expensive to maintain and among the first to be damaged by high glucose, inflammation and poor microcirculation.

That makes sweat gland function a useful early signal. Sudomotor impairment and microcirculatory disturbance can be among the earliest detectable stages of a distal peripheral neuropathy, before the larger nerves show anything.

What it feels like

• Feet that no longer sweat at all — dry, cracked heels, skin that splits.
• Sweating that has moved: less in the feet and legs, more in the head, face and trunk.
• Night sweats or drenching sweats after eating, in someone with long-standing diabetes.
• Heat intolerance, because sweating is the body’s main way of shedding heat.
• Skin that feels different — smoother, shinier or thinner than it used to.

A surprising number of people never mention any of this, because dry feet do not feel like a medical problem. They are often the earliest sign in the chart.

How it is measured

Sudomotor testing uses electrochemical skin conductance. You place your palms and the soles of your feet on stainless steel plates, a very low voltage is applied, and the device measures the electrochemical reaction between the electrodes and the chloride ions in sweat. It takes about three minutes and you feel nothing.

Electrochemical skin conductance has been reviewed systematically as a simple, non-invasive and fast method for detecting sudomotor dysfunction, and compared against established reference tests including the quantitative sudomotor axon reflex test, sympathetic skin responses, the thermoregulatory sweat test and skin biopsy. In one study of a hereditary amyloid polyneuropathy population, foot conductance detected autonomic dysfunction with 76% sensitivity and 85% specificity — a figure that belongs to that population and that condition, not to everyone.

Why the feet matter most

Loss of sweating in the feet is not only a nerve finding. Dry, cracked skin is how foot ulcers begin, and a foot that has also lost protective sensation will not report the injury. That combination is the reason sudomotor testing sits next to the vascular study in this suite rather than in a separate appointment.

If you are a clinician: the same subject written for the person ordering the test is at the full test description.

Common questions

Is sudomotor testing the same as a sweat test for cystic fibrosis?

No. That test measures chloride concentration in sweat to diagnose cystic fibrosis. This one measures how well the nerves driving the sweat glands are working. What each assessment measures.

My feet sweat too much, not too little. Does that count?

Yes. Both excessive and absent sweating point at autonomic dysfunction; the pattern and distribution are what distinguish them. Burning feet at night.

Does it hurt?

No. The current is very low and most people report no sensation. What the appointment feels like.

What if it is abnormal?

An abnormal result is a reason to look for a cause and to protect the feet, not a diagnosis by itself. Small-fiber versus large-fiber neuropathy.

References

• Novak P. Electrochemical skin conductance: a systematic review. Clinical Autonomic Research. 2019;29(1):17–29. doi:10.1007/s10286-017-0467-x
• Castro J, Miranda B, Castro I, de Carvalho M, Conceição I. The diagnostic accuracy of Sudoscan in transthyretin familial amyloid polyneuropathy. Clinical Neurophysiology. 2016;127(5):2222–2227. doi:10.1016/j.clinph.2016.02.013

Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, medical director of Measura. Last reviewed August 20, 2026.