Functional variability of sacral roots in bladder control.

Object. Sacral roots are involved in sensory, autonomic, and motor innervation of the lower limbs and perineum. Theoretically, it can be assumed that the S-3 root level innervates the bladder; however, clinical practice shows that this distribution can vary. Few researchers have studied this variability. Methods. The authors conducted a retrospective study involving 40 patients who underwent surgery requiring an electrophysiological exploration of the sacral roots. They performed stimulations for the monitoring of muscular (3 Hz, 1 V) and bladder responses under cystomanometry (30 Hz, 10 V). Results. Although the S-3 roots were involved in bladder innervation in all cases, they were exclusively involved (i.e., the only nerve roots involved) in only 8 of 40 cases. In the remaining 32 cases, other sacral nerve roots were involved. The most common association was S-3+S-4 (12 cases), followed by S-2+S-3 (6 cases), S-2+S-3+S-4 (5 cases), and S-3+S-4+S-5 (2 cases). Stimulation of S-2 could sometimes induce bladder contraction (15 cases, 40%); however, the amplitude was often low. S-4 nerve roots were involved in 24 of 40 cases (60%) in the bladder motor function, whereas S-5 roots were only involved 7 times (17%). Occasionally, we noticed a horizontal asymmetry in the response, with a predominant response from the right side in 6 of 7 cases, always with a major S-3 response. Conclusions. This is the first study showing a significant horizontal and vertical variability in the functional distribution of sacral roots in bladder innervation. These results show the variability of cauda equina syndromes and their forensic implications. These data should help with the monitoring of sacral roots and the performance of several tasks during surgery, including neurostimulation and neuromodulation.