Comparison of multiple point stimulation and incremental stimulation in patients with amyotrophic lateral sclerosis

Abstract

To compare two common techniques for motor unit number estimation (MUNE), multiple point stimulation and incremental stimulation to determine which is preferable in patients with amyotrophic lateral sclerosis (ALS). Surface recorded motor unit action potentials of median nerve or thenar muscle were measured in 60 ALS patients and 60 controls. The maximal baseline to negative peak compound muscle action potential (CMAP) amplitude was recorded. For multiple point stimulation, the stimuli sites included the skin of wrist, 6 cm above the wrist, elbow and 6 cm above the elbow. Individual motor unit responses were obtained by moving the stimulating electrode and isolating threshold responses with distinct morphologies. Then, with finely graded stimulus intensity at one point, 3 steps in a CMAP were investigated. 10 - 12 different single motor unit action potentials (SMUPs) were recorded. For incremental stimulation, stimulus intensity was slowly increased from subthreshold levels until a small all-or-none response was evoked. The intensity was slowly increased until the response increased in a quantal fashion. This process was repeated for a total of 10 increments. Individual motor unit amplitudes were obtained by subtracting amplitudes of each response from that of prior response. Both techniques were performed twice, electrodes changed and results averaged. For controls, MUNE was 228 +/- 30 for multiple point stimulation and 198 +/- 26 for incremental stimulation. Test-retest correlation coefficients and coefficients of variation for mean of two MUNE were 0.88 - 0.91 and 13.20% - 15.24% for multiple point stimulation, 0.86 and 13.30% - 15.65% for incremental stimulation. For ALS patients, MUNE was 64 +/- 6 and 59 +/- 7 respectively. Both MUNE methods are similarly reproducible and are equally effective at documenting progression of a lower motor neuron disorder in ALS patients.