Multimodal Intraoperative Neuromonitoring in Intradural Spinal Tumors: A Detailed Case Series Highlighting the Role of D‑Wave Monitoring

Intradural spinal tumors, including intramedullary and extra‑ medullary lesions, pose significant surgical challenges due to the narrow spinal canal and dense arrangement of ascending and de‑ scending neural tracts. Although gross total resection remains the primary surgical goal, preservation of neurological function is par amount. Multimodal intraoperative neuromonitoring integrates MEP, SSEP, and D‑wave monitoring to provide real‑time functional assessment of corticospinal and sensory pathways. Among these modalities, D‑wave monitoring has been shown to be the most re‑ liable predictor of long‑term motor outcomes because it directly reflects corticospinal tract conduction.

Study Design: Retrospective case series.

All patients underwent tumor resection under multimodal IONM including: - Transcranial motor evoked potentials (MEP) Somatosensory evoked potentials (SSEP) of median and posterior tibial nerves - Epidural D‑wave recording

Alert Criteria: -MEP amplitude reduction >50% - D‑wave am‑ plitude reduction >50% - Significant SSEP latency prolongation or amplitude reduction

Anesthesia Protocol:Total intravenous anesthesia with avoid‑ ance of long‑acting neuromuscular blockade and maintenance of stable mean arterial pressure to optimize monitoring reliability

Case 1

A 26‑year‑old male presented with a D10–D12 intramedul‑ lary tumor. Intraoperatively, left lower limb MEP signals were lost during tumor resection while D‑wave amplitude remained pre‑ served. Immediate postoperative weakness (1/5) improved to 4/5 by postoperative day 5 with restoration of ambulation (Figure 1-3).

Case 2

A 27‑year‑old male with a cervical intradural tumor demon‑ strated intraoperative reduction of left upper limb MEP amplitude to 50% with subsequent recovery to baseline. D‑wave signals re‑ mained stable. No postoperative motor deficit was observed (Fig‑ ure 4-7).

Case 3

A 38‑year‑old female with thoracic intramedullary schwanno‑ ma developed bilateral lower limb MEP reductions intraoperatively while D‑wave signals were preserved. Postoperative transient sen‑ sory symptoms resolved within 25 days (Figure 8-11).

Case 4

A 9-year-old boy with a thoracic low-grade glial tumor devel‑ oped bilateral reduction in motor evoked potentials (MEPs) along with a decrease in D-wave amplitude to approximately 50% during tumor debulking. With intraoperative adjustments, the D-wave am‑plitude gradually recovered to nearly 80% of the baseline by the time of wound closure. Postoperatively, the patient exhibited motor weakness that improved significantly by the time of discharge (Fig‑ ure 12-16).

Preservation or recovery of D-wave amplitude across cases demonstrated a strong correlation with favorable postoperative motor outcomes. In instances where transient loss of motor evoked potentials (MEPs) occurred without a sustained decline in D-wave amplitude, neurological deficits were typically temporary and showed subsequent recovery. These findings highlight the com‑ plementary role of D-wave monitoring in distinguishing reversible intraoperative corticospinal tract disturbances from permanent injury.

Intradural spinal tumor surgery carries significant risk because of manipulation of delicate neural structures, particularly the cor‑ ticospinal tracts. Multimodal intraoperative neurophysiological monitoring (IONM) has therefore become a key strategy for reduc‑ ing postoperative neurological morbidity and improving surgical safety [1].

Motor evoked potential (MEP) monitoring provides real-time assessment of corticospinal tract integrity and allows early de‑ tection of motor pathway compromise during tumor resection. However, MEP signals are highly sensitive to anesthetic depth, hy‑ potension, temperature fluctuations, and transient surgical manip‑ ulation. Consequently, isolated MEP loss may not necessarily indi‑ cate irreversible neurological injury [2-4].

D-wave monitoring records direct corticospinal tract volleys through epidural electrodes and is relatively resistant to anesthetic influences and neuromuscular blockade, making it a more stable in‑ dicator of corticospinal tract integrity [5]. Preservation of ≥50% of baseline D-wave amplitude has consistently been associated with favorable long-term motor outcomes, even in situations where mus‑ cle MEPs transiently disappear [3]. Recent studies further demon‑ strate that D-wave changes are strong independent predictors of long-term postoperative motor dysfunction during intramedullary spinal cord tumor surgery [6].

The addition of somatosensory evoked potential (SSEP) moni‑ toring complements motor pathway monitoring by evaluating dor‑ sal column function. Multimodal interpretation of MEPs, SSEPs, and D-waves enables simultaneous assessment of sensory and motor tracts, improving intraoperative decision-making and reducing the risk of permanent neurological deficit [7].

Recent evidence suggests that integration of D-wave monitor‑ ing within multimodal IONM improves the ability to continue tu‑ mor resection safely after transient MEP loss, facilitating maximal safe tumor removal while maintaining functional outcomes [8].

This case series highlights several important clinical implica‑ tions: 1. Transient MEP loss alone should not mandate termina‑ tion of tumor resection if D-wave signals remain preserved [6]. 2. D-wave monitoring allows surgeons to proceed with cau‑ tious tumor debulking while maintaining corticospinal tract integrity [3]. 3. Interpretation of multimodal neuromonitoring signals rather than reliance on a single modality is essential for optimal intraoperative decision-making [1].

The main limitations of this study include the small sample size and retrospective design. Larger prospective multicenter studies are needed to establish standardized alarm criteria and predictive models for neurological outcomes following intradural spinal tu‑ mor surgery

Microcirculation - provides blood to the brain substance and thus forms the local cerebral circulation;

Macrocirculation forms a pool of general blood supply on the surface of the brain [8].

In the microcirculation subsystem, a morphofunctional unit is distinguished in the form of a set of microvessels that allow blood to be supplied to individual specialized cell populations. This is the so-called vascular module. This scheme is most pronounced at the level of the cerebral cortex in its projection areas [9]. The selectivity of the transport of components from the blood to the brain tissue is provided by the blood-brain barrier. This barrier is formed due to the spatial and functional structure of filling the intercellular space with astrocytes [10]. Vascular epithelial cells are layered on top of each other in such a way that the vessels become almost impermeable [11].

Due to this circulatory system, the internal chemical environment of the brain is an isolated system consisting of many isolated subsystems. Only in some areas of the brain, such as the suture nucleus, hypothalamus, and others, is the permeability to a number of chemicals, such as acids, increased. This is necessary for the needs of the chemoreceptors grouped there [10]. At the same time, the penetration of water, carbon dioxide and oxygen through the capillary wall occurs almost unhindered. Multiple contacts between tightly fitting endotheliocytes form the intima layer of the cerebral arterioles. In addition, the subendothelial zone contains so-called myoendothelial junctions [12]. In this way, due to the ability to secrete vasoactive substances – factors of contraction and relaxation of smooth muscle muscles, endotheliocytes independently control the contractile activity of arterioles. The nexuses and plasma outgrowths of the muscle layer, due to their numerous contacts, are the structure of electrotonic conduction. This allows you to quickly excite the entire smooth muscle layer.

Multimodal intraoperative neuromonitoring plays a crucial role in enhancing surgical safety during intradural spinal tumor resec‑ tion by providing real-time assessment of neural pathway integrity. Among the available modalities, preservation of D-wave amplitude serves as one of the most reliable predictors of postoperative mo‑ tor function. Incorporation of D-wave monitoring alongside motor evoked potentials and somatosensory evoked potentials can aid in distinguishing reversible intraoperative changes from true cor‑ ticospinal tract injury. Therefore, whenever technically feasible, D-wave monitoring should be routinely utilized during intramedul‑ lary spinal cord tumor surgery to optimize neurological outcomes.

This study was conducted in accordance with the ethical stan‑ dards of the institutional and national research committee and with the principles of the Declaration of Helsinki. Formal ethics commit‑ tee approval was not required for this work as it represents a ret‑ rospective clinical observation/case-based report without patient identifiers.

Written informed consent for publication of clinical details and images was obtained from the patient/guardian.

The authors declare that they have no conflict of interest.

The authors received no financial support for the research, au‑ thorship, and/or publication of this article.

All authors contributed to the conception, data collection, anal‑ ysis, manuscript drafting, and final approval of the manuscript.

Data supporting the findings of this study are available from the corresponding author upon reasonable request.

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