Magnetic stimulation: C-8 root standardization based on arm length and arm position

Normal values for C-8 root distal latencies (DL) by magnetic stimulation (Magstim) have been reported; however, the methods of standardization have failed to consider the variables of arm length and arm position. Consequently, an artificial widening of the "normal" range occurs due to a false elevation of the standard deviation. In this study, Magstim of the C-8 nerve root was performed on 30 normal volunteers. Gender, hand temperature, age, arm length, and DL with the arm resting at the patient's side and fully abducted were recorded. DL was 13.3 +/- 1.1 ms with a side-to-side variation of 0.3 +/- 0.3 ms. The difference in DL between the arm at rest and in full abduction was 0.2 +/- 0.3 ms. Data analysis also demonstrated a direct correlation between arm length and DL. Utilizing this information a formula has been derived to more accurately describe the normal distribution of the C-8 root DL by Magstim.