Abstract
<title>Abstract</title> <p> <bold>Purpose:</bold> Surgical separation of conjoined twins with fused caudal spinal cords requires identification of a safe neural cleavage plane, yet anatomical boundaries are often indistinct. Although triggered electromyography (tEMG) has been used to determine nerve root ownership, a systematic mapping sequence to identify an electrophysiological boundary has not been formally described. This study describes a tEMG-guided progressive fascicular subdivision approach to identify a bilaterally electrically silent cleavage plane in two distinct conjoined twin configurations. <bold>Methods:</bold> Two cases (pygopagus and ischiopagus) were monitored using multimodal intraoperative neurophysiological monitoring, including tEMG, transcranial motor evoked potentials, somatosensory evoked potentials, and free-running electromyography. A dual-twin color-coded electrode configuration enabled simultaneous independent mapping of both nervous systems. Progressive fascicular subdivision with re-stimulation of each bundle was performed until a region electrically silent in both twins was identified. <bold>Results:</bold> Case 1 involved pygopagus twins with a fused conus medullaris at S2. Case 2 involved ischiopagus twins with a 6-cm syrinx spanning both spinal cords and an anterior sacral myelomeningocele. In both cases, progressive fascicular subdivision identified a functional cleavage plane, defined as a bilaterally electrically silent interval, which guided microsurgical separation. Both pairs of twins were discharged without new neurological deficits. <bold>Conclusion</bold> Triggered electromyography-guided progressive fascicular subdivision provides a systematic strategy for identifying the functional cleavage plane during separation of fused caudal neural elements. This mapping approach was applicable across two distinct conjoined-twin configurations. It remained robust in the presence of complex intrinsic spinal cord pathology, providing a defined electrophysiological endpoint where anatomical landmarks are unreliable. </p>