Utility of laryngeal electromyography for establishing prognosis and individualized treatment after laryngeal neuropathies

Abstract

Laryngeal electromyography (LEMG) is a technique used to characterize neuropathic injuries to the recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN). The RLN and SLN innervate the laryngeal muscles to produce vocal fold (VF) motion and elongation, respectively. VF motion deficiencies can affect voice, swallowing, and breathing, which can greatly affect a patient's quality of life. Neuropathy-related VF motion deficiencies most often result from surgical interventions to the skull base, neck, or chest likely due to the circuitous route of the RLN. LEMG is ideally conducted by an electromyographer and an otolaryngologist using a team-approach. LEMG is a powerful diagnostic tool to better characterize the extent of neuropathic injury and thus clarify the prognosis for VF motion recovery. This updated review discusses current techniques to improve the positive and negative predictive values of LEMG using laryngeal synkinesis and quantitative LEMG. Synkinesis can be diagnosed by comparing motor unit potential amplitude during vocalization and sniff maneuvers when recording within adductor muscles. Quantitative turns analysis can measure motor unit recruitment to avoid subjective descriptions of reduced depolarization during vocalization, and normal values are >400 turns/s. By integrating qualitative, quantitative, and synkinetic data, a robust prognosis can help clinicians determine if VF weakness will recover. Based on LEMG interpretation, patient-centered treatment can be developed to include watchful waiting, temporary VF augmentation, or definitive medialization procedures and laryngeal reinnervation.

Keywords: nerve injury; peripheral nerve injury; recurrent laryngeal nerve; superior laryngeal nerve.

FIGURE 1

Cross sectional anatomy of muscles within the larynx. All intrinsic laryngeal muscles depicted in the figure are innervated by the recurrent laryngeal nerve. Muscles are typically named by the laryngeal cartilages that serve as the origin and insertion for each muscle. Only the cricothyroid muscle, which is extrinsic to the larynx, is innervated by the superior laryngeal nerve (not shown in the figure).

FIGURE 2

Laryngeal nerve anatomy and iatrogenic nerve injuries from surgery. The left vagus nerve with its two major branches that include the left recurrent laryngeal nerve and left superior laryngeal nerve are depicted in yellow. The colored regions represent the approximate location where the nerves can be injured during surgery: Carotid endarterectomy (purple), cardiac and pulmonary surgery (light red), esophageal and mediastinum surgery (green), anterior cervical discectomy and fusion (tan), thyroid and parathyroid surgery (blue).

FIGURE 3

Anterior view of the larynx (A) demonstrating placement of the needle electrode into the cricothyroid muscle on the surface of larynx. Lateral, cross section (B) showing how a superiorly and laterally angled needle electrode is placed through the cricothyroid ligament to record from the TA‐LCA muscle complex.

FIGURE 4

Turns and amplitude analysis in a patient with RLN injury. On the bottom right panel, note how the turns/second plots have shifted left below 400/s indicating neuropathy. Each blue square represents a brief 5 s analysis during vocalization with several points outside the normal cloud (red trace) indicating a component of neurogenic reinnervation. A sample of the MUPs used for an individual data point is shown to the left. The numbers on the top right are the actual data from each sample obtained during the analysis that are graphically depicted in the envelope below.

FIGURE 5

Two‐channel LEMG and voice recording in a patient with electrical synkinesis. Note that in the top trace, the LEMG signal from the TA‐LCA complex begins prior to vocalization (bottom trace) that is termed ‘setting’. A similar phenomenon is seen during a sniff. Electrical synkinesis is diagnosed because the voluntary MUP amplitude during a sniff is >65% of the amplitude during voicing.