Use of the collision technique to improve the accuracy of motor unit number estimation

Objective: To evaluate the risk of stimulating the same motor axon at diffe rent points along the median nerve when using the multiple point stimulatio n (MPS) technique and how this affects the accuracy of the motor unit numbe r estimate (MUNE). Methods: Using the MPS technique, MUNE in the median innervated intrinsic h and muscles was done on two normal subjects, a patient with carpal tunnel s yndrome and one with prior poliomyelitis. The collision technique was then used to confirm whether two motor unit action potentials (MUAPs) with simil ar configurations and sizes were generated by the same motor unit. A new MU NE was recalculated after the repeated inclusion of the same motor unit had been excluded. Results: While the risk of stimulating the same motor axon at widely separa te locations of the median nerve was negligible in normal subjects, this ri sk was much higher in patients with a depleted motor neuron pool. This resu lted in marked distortion of the calculated size of the average single MUAP and, consequently, inaccuracy of the MUNE. Conclusions: The inadvertent inclusion of the same motor units, if not reco gnized, can markedly affect the accuracy of the MUNE. The collision techniq ue can be useful in minimizing this risk. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.