Pitfalls in Trigeminal Nerve Mapping in Neurosurgery: The Mystery of the Unexpected Motor Responses.

OBJECTIVE

Intraoperative neurophysiological mapping can help identify the trigeminal nerve (TN) during surgeries where it is at risk. Low-stimulating currents may only recruit sensory branch stimulation and trigger reflex responses, potentially complicating interpretation during monitoring. This report aims to address this issue by examining cases to categorize TN responses in relation to varying stimulation intensities, identify stimulation patterns, and clarify common response characteristics in surgical contexts.

METHODS

Seven cerebellopontine (CP) angle surgery cases with TN exposure were studied. Concentric bipolar stimulation was applied over the TN from 0.05 to 4 mA, and responses were recorded from various muscles innervated by the cranial nerves under monitoring (V, VII, IX, X, XI, XII).

RESULTS

Low stimulation currents (particularly below 0.5 mA) may elicit reflex responses over hypoglossal and facial innervated muscles, more often than motor responses, when the TN is stimulated.

CONCLUSION

A stimulation protocol starting at 0.05 mA and increasing to at least 1 mA or until compound muscle action potentials are observed in trigeminally innervated muscles is recommended for accurate TN mapping in CP angle surgery. These results may be helpful to avoid misinterpreting reflex responses in non-TN-innervated muscles.