Are there false-negative results of motor evoked potential monitoring in brain surgery?

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.