Standardisation of anal sphincter EMG: high and low threshold motor units

Objective: The anal sphincter muscle has a proportion of low threshold moto r units (MUs) that are continuously active and other, recruitable high thre shold MUs. In standard EMG recordings, motor unit potentials (MUPs) of the later seem to be of higher amplitudes. A quantitative EMG study was perform ed to assess possible consequences of sampling MUPs at different levels of sphincter activation. Methods: Fifteen females without uroneurological disorders were studied. Af ter insertion, standard concentric EMG needle was left in the anal sphincte r muscle undisturbed for 1 min; then 30 s of the remaining continuous, and 1 min of voluntarily increased EMG activity were recorded on a DAT recorder . MUPs were collected and analysed by 'Multi-MUP' analysis. MUPs analysed d uring relaxation constituted the 'low threshold MUP pool'. MUPs sampled on activation were checked for those, already sampled during relaxation, (whic h were discarded), and the remaining MUPs constituted the 'high threshold M UP pool'. Parameters of both MUP pools were compared. Results: High threshold MUPs were found to be significantly larger than low threshold MUPs. Conclusions: EMG investigator should be aware of the differences of MUPs sa mpled at various anal sphincter activity levels. For the technique of 'Mult i-MUP' analysis sampling at an activity level which provides 3-5 MUPs per d etection site would seem practical, providing a standardised approach suita ble for comparing normative data with individual findings from most patient s. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.