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 different points along the median nerve when using the multiple point stimulation (MPS) technique and how this affects the accuracy of the motor unit number estimate (MUNE). Methods: Using the MPS technique, MUNE in the median innervated intrinsic hand muscles was done on two normal subjects, a patient with carpal tunnel syndrome and one with prior poliomyelitis. The collision technique was then used to confirm whether two motor unit action potentials (MUAPs) with similar configurations and sizes were generated by the same motor unit. A new MUNE 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 separate locations of the median nerve was negligible in normal subjects, this risk was much higher in patients with a depleted motor neuron pool. This resulted 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 recognized, can markedly affect the accuracy of the MUNE. The collision technique can be useful in minimizing this risk.