The study of electrochemical skin conductance enabled by the SUDOSCAN is a non-invasive method used to assess the function of small nerve fibres, particularly those involved in sweating. It falls within the field of peripheral neurology and the autonomic nervous system, notably for the diagnosis and monitoring of so-called "small-fibre" neuropathies and dysautonomic disorders.
Physiological Principle
The skin, particularly on the palms of the hands and the soles of the feet, contains numerous eccrine sweat glands, innervated by small-calibre autonomic nerve fibres (unmyelinated C-fibres). These fibres are part of the sympathetic nervous system and control sweating.
Electrochemical skin conductance is based on the concept that:
a low electrical stimulation applied to the skin triggers an electrochemical reaction,
this reaction involves the chloride ions present in sweat,
the skin's ability to conduct the current therefore depends on the proper functioning of the sweat glands and their innervation.
Consequently, impairment of the small nerve fibres leads to decreased sweat function, resulting in a measurable decline in conductance.
Measurement Methodology
The examination is generally performed using dedicated devices (such as the Sudoscan, for example), according to a standardised protocol:
The patient places the palms of their hands and the soles of their feet on electrodes.
A low-intensity electrical current is applied.
Chloride ions migrate under the influence of the electrical field.
The device measures the electrochemical conductance (in microsiemens, µS).
Values are obtained separately for the hands and the feet, the latter being often affected earlier in neuropathies.
Interpretation of the Examination
Results are expressed in µS, with thresholds depending on age and the device used.
In general:
high values → normal sweat function,
decreased values → small-fibre dysfunction.
Electrochemical skin conductance primarily reflects:
the integrity of sudomotor autonomic fibres,
the density and functionality of the sweat glands,
indirectly, the state of the peripheral small nerve fibres.
This is not a direct anatomical measurement (like a skin biopsy), but rather a functional test.
Value in Small-Fibre Neuropathies
Small-fibre neuropathies (SFN) selectively affect:
C-fibres (pain, temperature, autonomic functions),
and sometimes Aδ-fibres (fast pain).
These neuropathies can be linked to various causes:
diabetes,
prediabetes,
autoimmune diseases,
toxins,
idiopathic causes.
Benefits of Skin Conductance:
Early detection: it can reveal impairment before abnormalities appear on conventional tests (EMG is often normal).
Functional assessment: it measures a direct consequence of sweat denervation.
Fast and non-invasive test: highly useful in consultations or for screening.
A decrease in conductance (especially in the feet) is highly suggestive of small-fibre involvement.
Value in Dysautonomia
Dysautonomic neuropathies affect the autonomic nervous system (sympathetic and parasympathetic), with manifestations such as:
orthostatic hypotension,
digestive disorders,
sweating abnormalities,
cardiac symptoms.
In this context, skin conductance makes it possible to:
objectively confirm sudomotor impairment,
complement other autonomic tests (tilt-table test, heart rate variability),
monitor the progression of the autonomic dysfunction.
Advantages and Limitations
Advantages:
non-invasive,
fast (a few minutes),
reproducible,
requires no special preparation.
Limitations:
does not replace a skin biopsy (the gold standard for fibre density),
can be influenced by:
skin hydration,
ambient temperature,
certain dermatological conditions,
does not always differentiate the underlying causes of neuropathy.
Role in the Diagnostic Strategy
The study of skin conductance is part of a multimodal approach:
clinical evaluation (neuropathic pain, dysautonomia),
neurophysiological tests (EMG),
autonomic tests,
skin biopsy (intraepidermal nerve fibre density).
It is particularly useful:
for screening (e.g. early diabetic neuropathy),
for longitudinal follow-up,
as a complementary functional investigation.
Conclusion
Electrochemical skin conductance is a modern, simple, and valuable tool for investigating small nerve fibre function, particularly in neuropathies and dysautonomia. Although it does not replace reference diagnostic tests, it serves as an excellent first-line and follow-up screening tool, allowing for a rapid and objective assessment of sudomotor function.
YF
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