Sudoscan and Neuropad, tests that assess sweat gland function, are effective at diagnosing familial amyloid polyneuropathy (FAP) and assessing small nerve fiber damage, according to researchers.
Their study, “Assessment of autonomic innervation of the foot in familial amyloid polyneuropathy,” was published in the European Journal of Neurology.
Numerous studies have shown that mutations in the TTR gene, the underlying cause of FAP, lead to damages in small sensory and autonomic nerve fibers, those that control involuntary bodily functions such as blood pressure, temperature control, and digestion.
The damages are witnessed at the disease’s early stages where they appear as abnormal sensations in the legs and feet, such as numbness, tingling, or burning.
Several techniques have been used to diagnose small sensory fiber lesions, the neurophysiological techniques.
Now, researchers compared the effectiveness of these different methods to assess the autonomic nerve fibers function in the feet of FAP patients.
The study included seven carriers with no symptoms (group 1), seven patients with mild to moderate FAP (group 2), and seven patients with advanced to severe FAP (group 3).
The participants underwent four tests on their right foot to assess their autonomic nerve fibers function, specifically the EMLA test (EMLA is a numbing cream that is placed on the skin to provide pain relief) and the laser Doppler flowmetry (LDF), which measures skin blood flow.
Somatic nerve fibers — those we can voluntary control — were also evaluated by conducting nerve conduction studies, tests for sensations (vibration, cold, and warm), and laser evoked potentials, which can monitor pain and temperature signals.
The two sudomotor tests, Sudoscan and Neuropad, were able to distinguish between patients who are clinically asymptomatic carriers and patients with moderate or advanced FAP. This distinction failed with the EMLA test and the LDF tests.
Though both tests failed to assess vasomotor responses on the feet of FAP patients, this “does not mean that these types of small autonomic vasomotor fibers are spared in this clinical condition,” researchers wrote.
The sudomotor tests were more encouraging, and since the Neuropad test is simple and does not require a special setup, “it can be used in routine clinical setting,” they added. However, its results require confirmation with the Sudocan test.
“Our study shows that Neuropad may be valuable for detecting early TTR-FAP, but has much less interest in the follow-up of an overt neuropathy [nerve disease],” researchers wrote.
Detection thresholds for warm and laser-evoked potentials were also valuable at measuring voluntary small fibers function, whose performance outweighed the cold detection assays (the only assay test where results didn’t correlate with participants’ neuropathy impairment scores).
These results support the use of the Neuropad test to help diagnose FAP, although confirmation requires additional measurements with the Sudocan test. This specific test is a potential tool to follow the progression of damage to nerve fibers.
“Conversely, reliable tests for assessing vasomotor [constriction or dilatation of blood vessels] disturbances in limb extremities of TTR-FAP patients are still awaited,” researchers concluded.