Objective diagnosis of small fiber impairment is difficult.Methods
We used the quantitative sudomotor axon reflex test (QSART) and axon-reflex-flare-test in the foot and thigh of 46 patients with peripheral neuropathy to assess C-fiber function in addition to conventional neurography and thermal threshold testing.Results
In all patients, small fiber impairment was suspected because of abnormal warmth detection thresholds (76% of all tested) and/or pain in the feet. A total of 83% had reduced axon-reflex flare areas and 17% lower QSART scores. Patients with pure small fiber neuropathy had higher rates of reduced flare areas (87.5%) and sweating rates (25.5%). There was no difference between patients with and without pain regarding thermotesting and axon-reflex testing.Conclusions
Both axon-reflex tests are helpful to identify objectively patients with small fiber impairment. Afferent and efferent C-fiber classes can be impaired differently. These tests detect small fiber impairment, but they cannot differentiate between painful and nonpainful neuropathy.