HSC Section 3 - Trauma, Critical Care and Sleep Medicine

reduction in polysomnographic measures of disease sever- ity and significant improvement in patient-reported qual- ity of life measures that were maintained at 3 years follow-up, with overall low morbidity and good patient acceptance and adherence. 4–7 Further advancement of the therapy is ongoing to improve the technological com- ponents, to determine the most appropriate patient phe- notypes, to optimize stimulation parameters and titration protocols, and to refine the surgical implant technique. The focus of this article is on the latter: providing the lat- est knowledge on the neuroanatomy of the hypoglossal nerve, the step-by-step details and refinements of the cur- rent operative technique, and the knowledge pearls of perioperative and postoperative management. NEUROANATOMY OF THE HYPOGLOSSAL NERVE FOR UPPER AIRWAY STIMULATION The dilator muscle activity is crucial to maintaining upper airway patency, counteracting the negative intralu- minal pressure generated during inspiration. Activation of the genioglossus (GG), the main tongue protrusor, has been shown to be the principal mechanism for electrical stimulation of the hypoglossal nerve (CN XII) for treat- ment of OSA in early studies. 8,9 Heiser et al. confirm a direct association between postimplant tongue protrusion and reduction of OSA severity. 10 The placement of the stimulation electrode and the associated muscle recruit- ment play an important role in upper airway stimulation. The structural details of tongue muscles and CN XII innervation provide a basis to determine the site of stim- ulation for upper airway dilation. The CN XII innervates extrinsic and intrinsic muscles of the tongue, including protrusor GG; retrac- tors, styloglossus (SG) and hyoglossus (HG); and stiff- eners, transverse (T) and vertical (V). 11,12 As shown in Figure 1, from the main trunk of the CN XII that over- lies the HG muscle, the lateral branches supply HG, SG, and inferior longitudinal muscles (l-IL), whereas the medial branches innervate the GG, T/V, and inferior lon- gitudinal (m-IL) muscles. Overall, the CN XII gives off 50 to 60 branches along its entire length in the following order from the root to the apex of the tongue: geniohyoid (GH), superior longitudinal, SG, HG, I-IL, genioglossus (horizontal [GGh] and oblique [GGo] compartments), m- IL and T/V. The optimal site of stimulation, a functional break- point between retractors and protrusors along the course of the CN XII, is determined by excluding the most dis- tal HG branch (Fig. 1A,B), including the remaining dis- tal branches for stimulation. For exclusion of the HG from stimulation, there are average 3.3 6 1.5 nerve branches for HG muscles 13 stemming superiorly from the main trunk. The most distal HG branch typically innervates the anterior portion of the HG muscle, branching from the CN XII with an oblique angle (Fig. 1A). In some cases, the most distal HG fiber travels along the main trunk and requires fine dissection for separation from the medial branches (Fig. 1B). For inclusion of nerve fibers for stimulation in the order from superior to inferior branches as shown in Figure 1,

the I-IL serves the intrinsic tongue muscles that shorten and curl the tip inferiorly. The I-IL shortens the tongue and causes a retroflexion of the tongue base. The role of the I-IL for upper airway dilation is unknown. The T and V intrinsic muscles narrow and flatten the tongue, and their activation contributes to the contralateral extension of the tongue with unilateral CN XII stimula- tion. The GGo fibers innervate tongue body and generate midline depression of the tongue. The GGh fibers arise from the mental spine of the mandible and insert into the tongue base and hyoid. These fibers are the principal target for electrical stimulation to generate anterior dis- placement of the tongue base, enlarging the upper air- way. The GH muscle is supplied by one to three small fibers arising from the first cervical nerve (C1), which runs in the same general pathway as the CN XII nerve. The separation of the C1 from the CN XII varies from person to person, which determines the feasibility of including the C1 for stimulation. In most cases, the C1 separates from CN XII posterior to the hyoglossus Fig. 1. Schematic illustration of the tongue muscles and hypoglos- sal nerve (CN XII) branches. The CN XII branches innervate the following tongue muscles from proximal to the distal end: stylo- glossus (SG), hyoglossus (HG), inferior longitudinal (IL), genioglos- sus (horizontal and oblique compartments, GGh and GGo), transverse (T) and vertical (V) muscles. Comparing with part A, part B shows a variation of nerve branches, a more distal location for a lateral HG (lat. HG) branch and a C1 nerve branch that inner- vates the geniohyoid (GH).

Heiser et al.: Operative Techniques for UAS

Laryngoscope 126: September 2016

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