Triggered electromyography-guided progressive fascicular subdivision to identify a functional cleavage plane in conjoined twin spinal cord separation: a dual-configuration case series

Technical Summary: Triggered Electromyography-Guided Progressive Fascicular Subdivision in Conjoined Twin Separation

Problem Statement
The surgical separation of conjoined twins with fused caudal spinal cords presents a critical challenge: identifying a safe neural cleavage plane when anatomical boundaries are indistinct. While intraoperative neurophysiological monitoring (IONM) has been utilized to determine nerve root ownership, previous literature lacks a systematic mapping sequence to formally define an electrophysiological endpoint. Specifically, prior reports have not described a structured process where progressive fascicular subdivision is followed by the repeated re-stimulation of each subdivided bundle to iteratively narrow the search for a separation point. Furthermore, existing IONM literature is largely limited to pygopagus configurations without significant intrinsic spinal cord pathology, leaving a gap in strategies for complex cases involving syringomyelia or myelomeningoceles.

Methodology
This study presents a dual-configuration case series involving two distinct pairs of conjoined twins: one pygopagus pair (Case 1) and one ischiopagus pair (Case 2) with severe intrinsic spinal pathology (continuous syrinx and anterior sacral myelomeningocele).

The core methodology employed a triggered electromyography (tEMG)-guided progressive fascicular subdivision approach:

1. Dual-Configuration Monitoring: A color-coded, dual-twin electrode setup was used to enable simultaneous, independent monitoring of both nervous systems. Red labels denoted Twin A, and blue labels denoted Twin B, with specific tracers for laterality and segmental levels.
2. Multimodal Surveillance: The protocol included transcranial motor evoked potentials (TcMEPs), somatosensory evoked potentials (SSEPs), free-running EMG, and bulbocavernosus reflex monitoring to ensure global neural integrity.
3. Iterative Mapping Strategy:
   • Initial tEMG stimulation of identifiable root bundles was performed to detect cross-innervation (responses in the co-twin's muscles).
   • Upon identifying overlapping innervation where no clear anatomical boundary existed, the fused neural tissue was progressively subdivided into smaller fascicular bundles under microscopic visualization.
   • Re-stimulation: Each isolated fascicle was re-stimulated using the same parameters (0.2–0.3 ms pulses, 0.1–5 mA) to reassess its functional contribution.
   • Endpoint Definition: This iterative cycle continued until a region was identified where stimulation produced no electromyographic responses in either twin across all monitored channels. This bilaterally electrically silent interval was defined as the functional cleavage plane.

Key Results

• Case 1 (Pygopagus): Preoperative MRI showed a fused conus medullaris at S2 with no distinct anatomical plane. Initial tEMG revealed bidirectional but asymmetric cross-innervation at S1–S2. Progressive subdivision successfully narrowed the overlap zone, identifying a bilaterally silent interval that guided microsurgical separation. Both twins were discharged without new neurological deficits.
• Case 2 (Ischiopagus): This case involved a 6-cm syrinx spanning both cords and an anterior sacral myelomeningocele. Initial mapping confirmed reciprocal sacral root overlap. Despite the complex pathology, the subdivision strategy identified distinct electrically silent intervals on both the Twin A and Twin B sides, guiding successful separation. Although Twin B experienced intraoperative desaturation leading to transient signal loss (systemic in nature, confirmed by Twin A's stable signals), both twins recovered without new deficits.

Key Contributions
The paper claims three primary contributions to the literature:

1. Formalization of a Systematic Endpoint: The study moves beyond the observation of bilateral electrical silence as a passive finding. It formalizes a stepwise operative sequence—progressive subdivision followed by iterative re-stimulation—to actively generate and confirm the functional cleavage plane as a defined surgical endpoint.
2. Applicability in Complex Pathology: The strategy was successfully applied in an ischiopagus configuration complicated by severe intrinsic spinal cord pathology (syrinx and myelomeningocele), demonstrating that electrophysiologically defined endpoints remain robust even when anatomical landmarks are obscured.
3. Configuration Independence: The consistent success across both pygopagus and ischiopagus configurations suggests that the functional cleavage plane concept is not specific to a single anatomical arrangement but is a transferable strategy for fused caudal neural elements.

Significance and Claims
The authors posit that this tEMG-guided progressive fascicular subdivision provides a systematic strategy for identifying a functional cleavage plane where anatomical boundaries are unreliable. By converting a pragmatic observation into a structured mapping protocol, the approach offers a defined electrophysiological endpoint for microsurgical separation. The study acknowledges its limitations, noting that the evidence is based on two cases and that long-term autonomic outcomes remain pending. However, it asserts that this framework provides a practical, reproducible methodology for neural separation in conjoined twins, particularly in scenarios where traditional anatomical dissection is insufficient.