Use of anal sphincter electromyography during operations on the conus medullaris and sacral nerve roots.
The mechanical activity of the anal sphincter can be translated into electrical activity and recorded on graph paper or an oscilloscope. The activity of the anal sphincter may be extrapolated to activity of the external urethral sphincter because both are striated muscles innervated by the pudendal nerve that arises from S-2, S-3, and S-4. Stimulation of these nerves causes contraction of the sphincter muscles, and a deflection of the recording device occurs. This technique was employed intraoperatively in monitoring operations on the conus medullaris and sacral nerve roots in 10 patients with spinal dysraphism (age range, 3 weeks to 15 years). Their diagnoses were tethered conus, 4; lipomeningocele, 3; spinal hamartoma, 1; syringocele, 1; and sacral arachnoiditis, 1. With general anesthesia, and the patient in the prone position, an electrode-containing anal plug was inserted or two needle electrodes were inserted into the anal sphincter muscle. The electrodes were connected to the electromyography recording stylus of the urodynamic bladder diagnostic unit. During the spinal operation, whenever a structure could not be identified clearly, it was stimulated with the disposable electrical stimulator and, if oscillations of the stylus occurred (indicating contraction of the anal sphincter), the structure was preserved. This technique permitted spinal operations in these 10 patients without changes in neurological or urological function.