Standardization of anal sphincter electromyography: quantification of continuous activity during relaxation.
OBJECTIVE
Sphincter motor units (MUs) are continuously firing during relaxation. The quantification of this activity is a potentially useful electromyographic (EMG) parameter related to the number of MUs innervating the muscle and the level of motor neuron excitation. The aim of the present study was to develop a reliable, quantitative measure of continuous sphincter muscle activity during relaxation.


METHODS
EMG activity was analyzed during relaxation, 1 minute after insertion of the concentric needle electrode into four sites in the external anal sphincter (EAS) muscle. In 10 control subjects, 8 interference pattern samples were obtained by "turn/amplitude" analysis. In 35 control subjects, a multi-MUP count of continuously firing motor unit potentials (MUPs) was used, quadruplets with scores of 0-6 obtained, and the lower reference limits (95th percentile) calculated. This approach was then evaluated in 57 patients (182 muscles) with cauda equina or conus medullaris lesion (CECML) and 7 patients (13 muscles) with "idiopathic fecal incontinence."


RESULTS
The lower reference ("outlier") limits for MUP count were 0 0 6 6 and 0 1 2 2 for the subcutaneous, and 0 0 0 6 and 0 0 1 1 for the deeper EAS muscles. Both patient groups had a significantly diminished number of continuously firing MUPs, which was below the reference range in 43% of the EAS muscles from CECM patients and in 85% of muscles from patients with idiopathic fecal incontinence. In patients with CECML, the decrease in MUP count correlated with the severity of the lesion, as defined by the sensory deficit.


CONCLUSIONS
MUP count in sphincter muscles during relaxation is technically feasible, and it is a promising tool, particularly in patients with idiopathic fecal incontinence.
Podnar
Simon
S
Institute of Clinical Neurophysiology, Division of Neurology, University Medical Center Ljubljana, Slovenia. simon.podnar@kclj.si
Mrkaić
Mićo
M
Vodusek
David B
DB
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001003:Anal Canal / Q000502:physiology* / Q000503:physiopathology; D002420:Cauda Equina / Q000293:injuries; D004576:Electromyography / Q000379:methods* / Q000592:standards*; D005242:Fecal Incontinence / Q000503:physiopathology; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D009126:Muscle Relaxation / Q000502:physiology; D013118:Spinal Cord Diseases / Q000503:physiopathology