Familial amyloid polyneuropathy (FAP) is a heritable, progressive disease that causes the buildup of amyloids, or clumped proteins, in the nervous system and elsewhere. These amyloid deposits cause the death of nerve cells and interfere with the function of the tissue in which they accumulate.
Diagnosing FAP is challenging because the symptoms are similar to those of other neurodegenerative diseases. A thermoregulatory sweat test can help physicians determine a FAP diagnosis.
How is the thermoregulatory sweat test performed?
Sweat glands are controlled by nerves in the skin; autonomic nerve signals are sent from the brain to tell the glands to secrete sweat. In FAP patients, this nerve signal can be interrupted by the formation of amyloid deposits around the nerves.
The thermoregulatory sweat test is performed at a hospital or clinic. Patients have an area of skin (usually hands, feet, arms, or legs) dusted with a powder that responds to moisture by changing color. The patient sits or lies down in a warm, moist room to encourage sweating for a set period of time. The degree of color change at the end of the test is proportional to the amount of sweat produced, which is, in turn, is proportional to nerve activity.
What do results of the test indicate?
Results of the thermoregulatory sweat test indicate whether a patient has loss of nerve function, which may indicate a neurodegenerative disorder such as FAP.
On its own, the thermoregulatory sweat test does not mean that the patient has FAP specifically, but can indicate that a nerve defect is present.
The thermoregulatory sweat test can also be used to measure disease progression once a diagnosis has been reached. As the disease progresses, scores will change, reflecting the decrease in nerve function.
The thermoregulatory sweat test may be performed in combination with a quantitative sudomotor axon reflex test (QSART), another measure of nerve activity.
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