Continuous Vagal Neuromonitoring Using the Laryngeal Adductor Reflex: Can Preincision Dyssynchrony Predict Intraoperative Nerve Behavior?

Understanding the variability of the electrophysiologic laryngeal adductor reflex

OBJECTIVE

The laryngeal adductor reflex (LAR) is vital for airway protection and can be electrophysiologically obtained under intravenous general anesthesia (IGA). This makes the electrophysiologic LAR (eLAR) an important tool for monitoring of the vagus nerves and relevant brainstem circuitry during high-risk surgeries. We investigated the intra-class variability of normal and expected abnormal eLAR.

METHODS

Repeated measures of contralateral R1 (cR1) were performed under IGA in 58 patients. Data on presence/absence of cR2 and potential confounders were also collected. Review of neuroimaging, pathology and clinical exam, allowed classification into normal and expected abnormal eLAR groups. Using univariate and multivariate analysis we studied the variability of cR1 parameters and their differences between the two groups.

RESULTS

In both groups, cR1 latencies had coefficients of variation of <2%. In the abnormal group, cR1 had longer latencies, required higher activation currents and was more frequently desynchronized and unsustained; cR2 was more frequently absent.

CONCLUSIONS

cR1 latencies show high analytical precision for measurements. Delayed onset, difficult to elicit, desynchronized and unsustained cR1, and absence of cR2 signal an abnormal eLAR.

SIGNIFICANCE

Understanding the variability and behavior of normal and abnormal eLAR under IGA can aid in the interpretation of its changes during monitoring.

Recent Developments of Intraoperative Neuromonitoring in Thyroidectomy

At present, intraoperative neuromonitorization (IONM) with surface electrode-based endotracheal tube (ETT) is a standard method in thyroidectomy and can be performed either intermittently IONM (I-IONM) or continuously IONM (C-IONM). Despite the valuable contribution of I-IONM to the thyroidectomy, it still has limitations regarding the recording electrodes and stimulation probe. New approaches for overcoming the limitations of I-IONM and developing the method are taking attention. Most of the technical issues of IONM with surface electrode-based ETT are related with inadequate contact of electrodes to the vocal cords. Nowadays, efficiency of various recording electrodes is under investigation. Recording electrodes such as needle electrodes applied to thyroarytenoid or posterior cricoarytenoid muscle (PCA), surface electrodes applied to the PCA, and needle or adhesive electrodes applied to the tracheal cartilage or skin, can make safe recordings similar to the ETT electrodes. Despite their invasiveness, needle electrodes record higher electromyography (EMG) amplitudes than tube electrodes do. Adhesive surface electrodes make safe EMG recordings, although amplitudes of these electrodes are usually lower than those of the tube electrodes. These different types of electrodes are less affected by tracheal manipulations and amplitude changes are lower compared to the tube electrodes.

During C-IONM, an additional stimulation probe is applied to the vagus nerve after dissecting the nerve circumferentially. Recently, without applying a probe, a new continuous monitorization method called laryngeal adductor reflex CIONM (LAR-CIONM) using sensorial, central, and motor components of LAR arch which is an automatic, primitive brainstem reflex protecting the tracheoesophageal tree from foreign body aspiration, has been implemented. Afferent track of LAR communicates laryngeal mucosa to the brainstem by internal branch of the superior laryngeal nerve and efferent track reaches larynx through recurrent laryngeal nerve. Total outcome of LAR activation is the closure of laryngeal entry by bilateral vocal cord adduction. In LAR-CIONM, a stimulus is given by an electrode from one side of surface electrode-based ETT and amplitude response of the LAR at the vocal cord is followed on the operation side. Recently, it has been reported that real-time EMG response can be obtained with stimulation probe cables applied to dissectors or energy devices during the dissection through I-IONM.

Continuous Laryngeal Adductor Reflex Versus Intermittent Nerve Monitoring in Neck Endocrine Surgery

OBJECTIVE

Intraoperative neuromonitoring (IONM) techniques aim to identify and potentially prevent nerve injury during surgeries. Prior studies into the efficacy of recurrent laryngeal nerve (RLN) IONM convey mixed results, with some claiming equivalence between IONM and no monitoring at all. The goal of the current study was to compare continuous RLN monitoring using the laryngeal adductor reflex (LAR) to intermittent RLN monitoring (intermittent IONM) to determine whether continuous monitoring reduces the incidence of intraoperative RLN injury during neck endocrine surgeries.

METHODS

In this observational, historical case-control study, a historical cohort of patients monitored with intermittent-IONM (group 1, n = 130) were compared to prospectively collected data from consecutive nerves-at-risk monitored continuously with the LAR (LAR-CIONM, group 2, n = 205), at a single center by a single surgeon. The test benefit ratio and relative risk reduction (RRR) for LAR-CIONM over intermittent IONM were calculated.

RESULTS

For group 1, nine nerves at risk exhibited intraoperative LOS with transient postoperative vocal fold (VF) hypomobility (n = 2) or immobility (VFI, n = 7). For group 2, two nerves at risk (0.98%) had sudden intraoperative LAR LOS following bipolar cautery, resulting in postoperative transient VFI (P = .004). In each group, there was one case of permanent postoperative VFI. The test benefit rate ratio for LAR-CIONM demonstrated a dramatic effect at 5.23, with an RRR of 81.0%.

CONCLUSION

LAR-CIONM significantly decreased rates of postoperative transient VF paralysis and paresis over intermittent IONM alone (P = .004). Surgeons should be aware of the benefits and limitations of intermittent IONM versus CIONM. Intermittent IONM, although useful in nerve mapping and intraoperative decision making, has minimal benefit for the prevention of nerve injury, whereas CIONM can potentially reduce nerve injury rates and improve patient outcomes.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:230-236, 2021.