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Introduction
Obstructive sleep apnea (OSA) is a multi-factorial disorder where state-dependent loss of upper airway muscle tone and neuromuscular dysfunction result in cyclical upper airway obstruction during sleep. Hypoglossal neurostimulation is a new, functional approach to treating OSA that specifically addresses neuromuscular dysfunction, as opposed to conventional therapies such as positive airway pressure, oral appliance therapy, and reconstructive upper airway surgery, that act via direct, mechanical upper airway stabilization. Hypoglossal neurostimulation is an important new avenue for treatment, as conventional therapies are not successful in many patients due to insufficient effectiveness or efficacy, often related to low adherence.
The rationale for targeted hypoglossal neurostimulation (THN) is based on the lingual hydrostat model. The tongue is a muscular hydrostat, being a complex array of intertwined muscles that are relatively incompressible. To generate a myriad of shapes and various movements, tongue muscles attach to each other at various angles and support each other for various functions of the tongue in the hydrostat model. Complex co-activation is required for various physiologic activities and is supported as being needed for airway patency, as shown in animal studies where airway patency was greatest via truncal hypoglossal stimulation and lower with lesions that denervated the intrinsic tongue muscles. In human studies, when hypoglossal stimulation was performed via the nerve trunk or via surface stimulation of protrusors and retrusors, airflow was better or similar to just protrusor stimulation. The concept of hypoglossal nerve activation with co-activation of tongue muscles via nerve trunk stimulation was adopted by ImThera Medical, Inc., whose system ( Fig. 49.1 ) is the focus of this chapter (see Chapter 46 for Inspire Medical’s approach). ImThera Medical’s system at the time of publication is CE mark approved for use outside the United States but is still experimental in the United States and presently in Food and Drug Administration (FDA) pivotal trial testing.
The hypoglossal neuromuscular system has particular properties that are favorable for targeted neurostimulation. Tongue muscles—in particular, the posterior and tongue base muscles—have properties that make them more fatigue resistant and more favorable to exogenous stimulation, mitigating the risk of fatigue and dysfunction. In addition, the hypoglossal nerve trunk’s structure in the submandibular region facilitates targeted stimulation, as it is non-fasciculated , and thus surface stimulation at particular zones results in current spread to nearby neuronal bundles, which results in different tongue activities. Thus a system was designed where a six-contact stimulation cuff is placed on the trunk, and stimulation is near continuous, but cycled via multiple contacts to reduce the risk of muscle fatigue. The implantable pulse generator (IPG) has six independent constant-current channels that correspond to the six nerve-cuff contacts ( Fig. 49.2 ). Stimulation does not rely on timing via a sensing/triggering lead for respiratory synchronization, as used in another system, but rather via rotation of stimulation among the titrated contacts along the nerve periphery. The result of stimulation is reshaping and stiffening of the tongue, with mild protrusion, rather than mostly protrusion. With tongue activation, the pharyngeal walls become less compliant and dilate laterally (personal observations).
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