The "biro test" for autonomic dysfunction in carpal tunnel syndrome

The aim of this study was to assess whether autonomic dysfunction associated with carpal tunnel syndrome (CTS) can be assessed quantitatively with a (modified) biro test of finger sweating. Twenty-six hands of 16 patients with CTS were compared with 30 hands of 15 normal subjects. A device was constructed to measure the angle from the horizontal at which a biro slips from the finger (the critical angle). In the control subjects, no significant difference was found in the critical angle between the little and index fingers in either hand. By comparison, in subjects with CTS, the critical angle was significantly lower in the index finger than in the little finger, the mean difference being 8.65 degrees, indicating a quantifiable and significant difference of sweating of the median nerve-innervated index finger.