The stimulation electrode, which is designed in the form of a cuff to envelope the anterior (protrusor) branches of the hypoglossal nerve receives electrical stimulation from the implanted pulse generator, implanted above the pectoralis muscle of the chest wall. This design allows for collaborative awake and overnight titration of the device as directed by a sleep medicine physician. Attention is paid not only to the voltage “strength” administered with each pulse but also the degree of synchronization between respiration and stimulation, as well as pattern of pulse administration. Our experience remains that true success and adaptation to therapy requires not just meticulous surgical technique but a diligent approach to postoperative therapeutic titration to achieve a comfortable, yet effective, voltage for maintaining airway patency. Thus, akin to initiation of CPAP, UAS requires regular follow-up and device fine-tuning with patient comfort taken into consideration to achieve optimal results, and patient expectation should be aligned with this process.
Success in UAS relies heavily on appropriate presurgical evaluation and clinical phenotyping. The following surgical indications have been demonstrated in the Stimulation Therapy for Apnea Reduction (STAR) trial and subsequent 3-year clinical follow-up: AHI between 15 and 80 events/hour (with ≤ 25% central apneas) and a BMI ≤ 32.8
As OSA often results from multi-level airway collapse, UAS targets an increase not only in the diameter of the retropalatal/oropharyngeal airway space but also the antero-posterior hypopharyngeal airway. Original criteria for implantation excluded patients with a pattern of complete circumferential collapse (CCC) noted on dynamic airway evaluation during pre-implant drug-induced sleep endoscopy (DISE). DISE aims to precisely target dynamic airway collapse patterns during simulated (propofol or midazolom induced) sleep.
The effects of UAS are dependent on upper-airway cross-sectional area, particularly diameter. In patients who demonstrate CCC, the anteroposterior direction of activation derived from the UAS stimulus is unable to overcome CCC. In a recent prospective study, our group demonstrated that CCC can be converted to an airway collapse pattern compatible with UAS implantation, using a modified palatopharyngoplasty prior to UAS implantation. By stabilizing the lateral walls of the oropharyngeal airway with pre-implant palatal surgery, UAS is able to successfully direct widening of the airway cross-sectional area in an antero-posterior fashion. This exciting finding potentially allows for expansion of current indications, thus opening treatment to a wider patient population.9 Still, UAS remains highly studied in a relatively uniform patient population with data in more diverse subsets requiring further directed attention to expand and better define optimal patient populations for treatment.
From the perspective of improving patient adaptation and tolerance in UAS, a well-established concept in the CPAP literature can be applied, as explained by the Starling resistor model. The starling resistor is comprised of two rigid tubes connected by a collapsible segment in between. In parallel, the pharynx is a collapsible muscular tube connected on either end by the nose/nasal cavity and the trachea – both of which are bony/cartilaginous, noncollapsible structures. As has been shown in the use of CPAP, the same pressure required to maintain stability of the collapsible muscular pharynx via nasal breathing may lead to pharyngeal collapse when applied orally.10 This concept has also been directed towards UAS with our clinical experience demonstrating that oro or oronasal breathers tend to require a higher amplitude to maintain airway patency versus nasal breathers. This is an important area for future-directed study as medically/surgically improving nasal breathing in UAS subjects may subsequently lower amplitude requirements and improve patient tolerance.
Future direction to allow for improvement in the technology for application in a broader populational segment, external or alternative device powering mechanisms, along with MRI Compatibility and reducing the number of required external incisions will continue to broaden the patient selection criteria. As we move from a “stimulation” to a precision-tailored “stimulation and titration” approach, the mid to long term data supporting UAS remains very promising with 5-year follow up demonstrating sustained polysomnographic and subjective reported outcomes in well selected patients.
Dr. Awad is Assistant Professor – Department of Otolaryngology/Head & Neck Surgery, and Chief – Division of Sleep Surgery; Northwestern University, Chicago, Illinois. Dr. Capasso is Associate Professor – Department of Otolaryngology/Head & Neck Surgery, and Chief – Division of Sleep Surgery; Stanford Hospital and Clinics, Stanford, California.
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