Reinnervation of anal sphincter muscle by low and high-threshold motor neurones

The external anal sphincter (EAS) has continuously active low-threshold and recruitable high-threshold motor units (MUs), the latter being 'larger'. O n performing concentric needle electromyography (EMG) of the EAS, the high- threshold MUs seemed to reveal more neuropathic changes than the low-thresh old MUs. To verify this hypothesis, low- and high-threshold motor unit pote ntials (MUPs) were compared in patients with neuropathic EAS and controls. Fifteen subjects without pelvic disorders and 29 patients with sequela afte r cauda equina lesions were studied. In patients, only muscles ipsilateral to severe perianal sensory loss were Included. MUPs were sampled using mult i-MUP analysis during relaxation ('low-threshold'), and on activation ('hig h-threshold' MUs). MUP parameters of low- and high-threshold MUs from contr ols and patients were compared, as was the sensitivity and specificity with which MUPs were classified as normal or pathological (using discriminant a nalysis). MUP changes due to reinnervaton, and the sensitivity and specific ity in classifying MUPs as normal or pathological were not significantly di fferent between the low- and high-threshold MUPs. Stronger activation of EA S does not improve discrimination between neuropathic and normal MUPs. New EMG techniques for sampling sphincter MUPs at higher activation levels woul d seem not to yield additional information. (C) 2001 Editions scientifiques et medicales Elsevier SAS.