Intraoperative Monitoring of Cranial Nerves III, IV, and VI: A Scoping Review

Background: In recent decades, the use of intraoperative neuromonitoring (IONM) of cranial nerves (CN) during cranial base surgeries has increased dramatically. Despite this increase in utilization, relevant literature investigating the use of IONM to monitor CN III, IV, and VI remains scarce, and there remains no consensus on the optimal strategy for doing so. With this scoping review, we aimed to summarize the current state of the literature on this topic.

Methods: We performed a systematic review of the literature to summarize the use of IONM for CN III, IV, and VI in accordance with the PRISMA-ScR guidelines. Two databases (PubMed and Scopus) were searched from the date of their inception through April 2024. Search terms were (((“cranial nerve” AND (“monitoring” OR “neuromonitoring”)) AND (“III” OR “IV” OR “VI”)) AND (“intraoperative”).

Results: Nineteen studies met the inclusion criteria. Of those, 17 (89.5%) reported monitoring of cranial nerve III, 11 (57.9%) reported monitoring of cranial nerve IV, and 16 (84.2%) reported monitoring of cranial nerve VI. Fourteen (73.7%) studies used variations of electromyography (EMG) monitoring, four (21.1%) used electrooculography (EOG), and one study (5.3%) used a novel needle electrode (NNE) for IONM. The three most common specified conditions necessitating intraoperative monitoring of CN III, IV, and/or VI were pituitary adenoma (34.7%), chordoma (8.6%), and angiofibroma (5.7%). Similarly, the three most commonly specified surgical approaches were transellar (39.8%), tranclival (27.0%), and transpterygoidal (5.7%). Immediately following surgery, the mean reported percentage of patients with any EOM deficits was 12.3%. However, the mean reported incidence of EOM deficits at 6 months of follow-up or longer was 5.2%. It was also found that rates of postoperative CN deficits may be lower in cases of EOG use compared with EMG use (p = 0.01).

Conclusion: IONM of CN III, IV, and VI remains an under-investigated topic. Our review suggests that the use of EMG for IONM is associated with a statistically significant increased risk of post-operative CN deficit when compared with the use of EOG. Similarly, There were no reported cases of CN deficits at 6 months follow-up in cases of EOG use. These results indicate that the use of EOG may be associated with more favorable outcomes in post-operative functioning of the CN III, IV, and VI. Of the 19 studies included in our review, only one study reported any post-operative complications attributed to the IONM technique. This suggests that the techniques used in IONM confer a low risk for iatrogenic intraoperative injury. Regardless, further investigation with larger cohorts of patients using control groups is necessary to further our understanding of how to best implement each technique in clinical practice.

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Publication History

Article published online:
07 February 2025