Free-hand electrode placement for intraoperative monitoring of extraocular cranial nerves in skull base surgery: preliminary experience and feasibility assessment

Postoperative dysfunction of the oculomotor (CN III) and abducens (CN VI) nerves remains a major determinant of disability after skull base surgery for tumors. This study assessed the feasibility, safety, and diagnostic performance of a surgeon-controlled, free-hand extraocular muscle electrode placement for corticobulbar motor evoked potentials (cb-MEPs) and direct nerve stimulation (DNS). This monocentric, observational, retrospective study enrolled 40 patients scheduled for skull base tumor surgery, with planned intraoperative monitoring of CN III and/or VI. Curved needle electrodes were placed free-hand by the neurosurgeon at the scleral–muscular junction of the medial and/or lateral rectus, and cb-MEPs and DNS were recorded; evaluability required reproducible baselines. Primary endpoint was 3-month cranial nerve palsy. Diagnostic accuracy was calculated, and Spearman correlations tested the relationship between intraoperative percentage amplitude change and postoperative deficit severity. Placement succeeded in all cases (mean 10 min) with one transient conjunctivitis (2.5%). Stable baseline cb-MEPs occurred in 20/37 (CN III) and 18/31 (CN VI). For CN III, cb-MEPs showed sensitivity 66.7% and specificity 100%; amplitude reduction correlated with postoperative severity (ρ = 0.94, p < 0.001). DNS was evaluable in 22/26, with sensitivity 83.3% and specificity 100%. For CN VI, cb-MEPs showed sensitivity 75.0% and specificity 96.8%, with correlation to severity (ρ = 0.88, p < 0.001). DNS elicited responses in 15/22, with sensitivity 75.0% and specificity 100%. This neurosurgeon-performed, free-hand technique enabled rapid, safe, and reproducible electrode placement for extraocular cranial nerve monitoring during skull base surgery. When baselines are obtainable, cb-MEPs and DNS provide highly specific, actionable feedback aligned with postoperative outcomes. These findings support pragmatic adoption and prospective multicenter validation.