Lower cranial nerve responses after laryngeal stimulation in posterior fossa surgery: a study of medullary complex motor functions.

OBJECTIVE

Complex motor responses (CMRs) can be recorded from lower cranial nerves after laryngeal stimulation. Long latency responses (LLRs) are observed in the soft palate and tongue. This study aimed to describe these responses, evaluate their intraoperative reproducibility, and assess correlations between signal loss and postoperative outcomes.

METHODS

The laryngeal adductor reflex (LAR) was elicited while LLRs were recorded bilaterally from the soft palate and tongue. Permanent signal loss was defined as an amplitude reduction greater than 50% affecting two LLRs. Postoperative outcomes were then analysed.

RESULTS

LLRs were recorded in 50% of tongue recordings and 47.8% of soft palate recordings, compared with a 92% detection rate for LAR. Significant associations were found between postoperative swallowing deficits and true positive permanent LLR signal changes. The odds ratio was 22.2 (95% CI: 3.5-141). Positive and negative likelihood ratios for swallowing outcomes were 6.30 and 0.28, respectively.

CONCLUSION

LLRs provide a novel method to monitor cranial nerves IX, X and XII within an integrated medullary network, acting as coordinated bulbar motor sequences. While they may offer a practical surrogate marker for complex motor functions like swallowing, their preservation primarily suggests the structural integrity of these shared brainstem pathways.

SIGNIFICANCE: Our study demonstrates the reproducibility of CMRs after laryngeal stimulation in asleep patients. These results may be useful for monitoring medullary complex motor functions of the brainstem.