Bilateral vs. unilateral electromyographic examination of the external anal sphincter muscle.
OBJECTIVE
In patients with cauda equina or conus medullaris lesions, bilateral electromyographic (EMG) examination of the subcutaneous external anal sphincter (EAS) muscles has been suggested. In spite of its circular shape, EAS should be considered as two separate semicircular muscles. The aim of the present study was to test the hypothesis that information obtained by bilateral examination of the EAS muscle outweighs inconvenience due to additional needle insertions.


METHODS
A group of 67 patients with clinical and radiological data supportive of cauda equina or conus medullaris lesion was studied. From the subcutaneous EAS muscles motor unit potentials (MUPs) were sampled by the standard concentric EMG needle electrode, and an advanced EMG system with template operated multi-MUP analysis. Severity of EMG abnormalities was determined, and compared in 48 pairs of the left/right subcutaneous EAS muscles.


RESULTS
In 18 patients, bilateral EMG abnormalities were found, in five of them asymmetry of involvement was demonstrated. Unilateral MUP abnormalities were found in 22 patients. The sensitivity of unilateral EMG analysis was 57% and of bilateral examination, 83%.


CONCLUSIONS
In patients with suspected cauda equina lesions, as a rule, bilateral EMG of the subcutaneous EAS muscle is recommended.
Podnar
Simon
S
Division of Neurology, Institute of Clinical Neurophysiology, University Medical Center Ljubljana, 1525 Ljubljana, Slovenia. simon.podnar@kclj.si <simon.podnar@kclj.si>
D000328:Adult; D000368:Aged; D001003:Anal Canal / Q000502:physiology* / Q000503:physiopathology; D004566:Electrodes; D004576:Electromyography; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009460:Neurologic Examination; D011128:Polyradiculopathy / Q000503:physiopathology; D012677:Sensation / Q000502:physiology; D013117:Spinal Cord Compression / Q000503:physiopathology