Intraoperative identification of mixed activation profiles during hypoglossal nerve stimulation

Abstract

Study objectives: The effectiveness of hypoglossal nerve stimulation (HGNS) in the treatment of obstructive sleep apnea (OSA) depends on the selective stimulation of nerve fibers that innervate the tongue muscles that produce tongue protrusion (genioglossus) and stiffening (transverse/vertical) while avoiding fibers that innervate muscles that produce tongue retraction (styloglossus/hyoglossus). Postoperative treatment failures can be related to mixed activation of retractor and protrusor muscles, despite intraoperative efforts to identify and avoid nerve fibers that innervate the retractor muscles. This study describes a novel intraoperative protocol that more optimally identifies mixed activation by utilizing an expanded set of stimulation/recording parameters.

Methods: This study was a case series in a university hospital setting of patients undergoing unilateral hypoglossal nerve stimulation implantation for obstructive sleep apnea. Data included electromyographic responses in the genioglossus and styloglossus/hyoglossus to intraoperative stimulation with an implantable pulse generator using unipolar (- - -, o-o) and bipolar (+-+) settings.

Results: In a subset of patients (3/55), low-intensity unipolar implantable pulse generator stimulation revealed significant mixed activation of the styloglossus/hyoglossus and genioglossus muscles that was not evident under standard bipolar implantable pulse generator stimulation conditions. Additional surgical dissection and repositioning of the electrode stimulation cuff reduced mixed activation.

Conclusions: A novel intraoperative neurophysiological monitoring protocol was able to detect significant mixed activation during hypoglossal nerve stimulation that was otherwise absent using standard parameters. This enabled successful electrode cuff repositioning and a dramatic reduction of mixed activation.

Keywords: hypoglossal nerve stimulation; neurostimulation; obstructive sleep apnea; sleep surgery; upper airway stimulation.

Figure 1. Intraoperative neurophysiological monitoring protocol.

(1) The main trunk of the HGN is exposed, and a bipolar stimulation probe is used to stimulate four different HGN nerve sites A: Lateral (SG/HG), B: Superior medial (GG_O), C: Inferior medial (GG_H), D: C1 branches while recording EMG responses in GG and SG/HG muscles. Procedure as described previously.¹³ Additional stimulation at the breakpoint between SG/HG and GG_O is performed to identify late takeoff and hidden lateral nerve fibers. (2) Electrode cuff is targeted to include HGN branches that produced selective activation of GG, but not SG/HG. (3) Standard HGNS surgery is completed with placement of implanted pulse generator (IPG) in the chest and sensor lead in the intercostal space. (4) IPG is used to stimulate under three different polarities: Bipolar (+-+ ), unipolar (- - -), and unipolar (o-o) while measuring EMG activity and observing tongue motion. First, bipolar IPG stimulation is performed, progressing from high to low intensity. Next, unipolar IPG stimulation is performed, again progressing from high- to low-stimulation intensity. (5) If significant mixed activation is observed either by EMG (eg, SG/HG activity) or tongue motion, the electrode cuff is repositioned, and step 4 is repeated. Successful repositioning is confirmed by elimination of mixed activation measured by EMG and by direct intraoperative observation of unrestricted forward tongue protrusion in response to IPG stimulation. EMG = electromyography, C1 = first cervical nerve, GG = genioglossus, GG_H = horizontal compartment of the genioglossus, GG_O = oblique compartment of the genioglossus, HG = hypoglossus, HGN = hypoglossal nerve, IPG = implanted pulse generator, SG = styloglossus.

Figure 2. Patient 1.

(A) IPG stimulation under bipolar settings (0.3 V) did not clearly identify mixed activation. (B) IPG stimulation under unipolar settings (0.1 V) revealed mixed activation with significant SG/HG responses. (C) After further surgical dissection and cuff repositioning, mixed activation was significantly reduced under the same unipolar settings, as evidenced by a decrease in amplitude of SG/HG responses (post = 379 µV vs. pre = 520 µV) and an increase in amplitude of GG responses (post = 2,021 µV vs. pre = 173 µV). GG, genioglossus, HG = hypoglossus, IPG = implanted pulse generator, SG = styloglossus.

Figure 3. Patient 2.

(A) IPG stimulation under bipolar settings (0.3 V) did not identify mixed activation. (B) IPG stimulation under unipolar settings (0.3 V) revealed mixed activation with large SG/HG responses. (C) After further surgical dissection and cuff repositioning, mixed activation was significantly reduced under the same unipolar settings, as evidenced by a decrease in amplitude of SG/HG responses (post = 491 µV vs pre= 4,095 µV), which led to GG responses being higher in amplitude compared with SG/HG responses (post GG = 675 µV vs post SG/HG= 491 µV). GG, genioglossus, HG = hypoglossus, IPG = implanted pulse generator, SG = styloglossus.

Figure 4. Patient 3.

(A) IPG stimulation under bipolar settings (0.3V) did not clearly identify mixed activation. (B) IPG stimulation under unipolar settings (0.3 V) revealed mixed activation (C) After further surgical dissection and cuff repositioning, mixed activation was significantly reduced under the same unipolar settings, as evidenced by a decrease in amplitude of SG/HG responses (post = 617 µV vs pre = 753 µV) and an increase in the amplitude of GG responses (post = 1,258 µV vs pre = 966 µV). GG = genioglossus = HG = hypoglossus, IPG = implanted pulse generator, SG = styloglossus.