Are there False-negative Results of Motor Evoked Potential Monitoring in Brain Surgery?

Abstract

Object: The present study explores the causes of occasional postoperative pareses despite unchanged or fully recovered intraoperative motor evoked potentials (MEPs) in supratentorial brain surgeries.

Methods: In a prospective, observational design, MEP monitoring results, motor outcome, and perioperative imaging were correlated in 200 procedures for brain tumours and cortical dysplasias critically related to motor areas and pathways.

Results: Persisting pareses after unchanged or recovered MEPs occurred in four cases due to delayed ischemia, or venous congestive oedema. Transient new deficit in four cases after stable MEP monitoring occurred due to inadvertently strong stimulation bypassing the target lesion, due to marked postresectional oedema, and after cortical transsections for alleviation of epilepsy.

Discussion and Conclusions: With technically adequate MEP monitoring, truly false-negative results missing manifest corticospinal impairment do not occur. However, sustained vascular dynamics (vasospasm, congestive oedema) may cause delayed pareses which are missed, or hardly reflected by intraoperative MEP changes. Even minor MEP changes must therefore be observed to prevent impending motor deficit.

Zusammenfassung

Ziel: Die Studie untersucht die Gründe für das gelegentliche Auftreten neuer postoperativer Paresen trotz intraoperativ unveränderter oder vollständig erholter motorisch evozierter Potenziale (MEPs).

Methoden: Bei 200 Operationen von Hirntumoren und kortikalen Dysplasien in kritischer Lagebeziehung zu motorischen Arealen und Faserbahnen wurden das motorische Outcome sowie die Ergebnisse des MEP – Monitorings und der perioperativen Bildgebung prospektiv korreliert.

Ergebnisse: Bleibende Paresen trotz erhaltener oder erholter MEPs traten in vier Fällen auf, verursacht durch verzögerte Ischämien bzw. venöse Stauungsödeme. Vorübergehende neue Paresen trotz stabilen intraoperativen MEPs in weiteren vier Fällen waren durch unbeabsichtigte Stimulation kaudal der Zielläsion erklärbar, durch postresektionelles Ödem, und durch rein kortikale Affektion nach epilepsiechirurgischen multiplen subpialen Transsektionen.

Diskussion und Schlussfolgerungen: Bei technisch korrekter Durchführung des MEP – Monitorings kommen falsch-negative Ergebnisse – im strengen Sinne von nicht erfassten manifesten Schäden der kortikospinalen Bahn – nicht vor. Allerdings können fortdauernde vaskuläre Prozesse (Vasospasmus, verzögertes Stauungsödem) verspätete Paresen verursachen, die von den intraoperativen MEP – Messungen nicht ausreichend erfasst werden können. Auch geringe MEP – Veränderungen müssen beachtet werden, um drohende motorische Defizite abzuwenden.

Key words

motor evoked potentials - intraoperative monitoring - brain tumor

Schlüsselwörter

motorisch evozierte Potenziale - intraoperatives Monitoring - Hirntumor

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Correspondence