Laryngeal advancement surgery improves swallowing function in a reversible equine dysphagia model J. E. VIRGIN, S. J. HOLCOMBE*, J. P. CARON, J. CHEETHAM † , K. A. KURTZ, H. A. ROESSNER, N. G. DUCHARME † , J. G. HAUPTMAN ‡ and N. C. NELSON ‡ Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, USA † Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA ‡ Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, USA. *Correspondence email: holcomb6@cvm.msu.edu; Received: 26.08.14; Accepted: 07.02.15 Summary Reasons for performing study: Pharyngeal dysphagia is a debilitating, sometimes fatal condition in horses, with multiple aetiologies. The pathophysiology is complex and not fully understood. Treatment is largely supportive. Laryngeal advancement surgery may diminish symptoms of dysphagia and improve swallowing in affected horses. Objectives: 1) to induce reversible moderate and marked pharyngeal dysphagia by regional anaesthesia of branches of the glossopharyngeal (IX), vagus (X) and hypoglossal (XII) nerves; 2) to characterise the dysphagia produced by each model; and 3) to determine whether laryngeal advancement surgery improves swallowing in these models. Study design: Experimental design using 6 adult horses. Methods: Two dysphagia models were produced by blocking IX, the pharyngeal branch of X and XII unilaterally (moderate model) and only the pharyngeal branch of X bilaterally (marked model) within the guttural pouches. Both models were performed on each horse before and after surgery in order to assess the effectiveness of the surgical procedure as a potential treatment for pharyngeal dysphagia. Dysphagia was scored by partly blinded observers on a scale of 0–12 based on observations of eating (nonblinded), endoscopic examinations and fluoroscopic swallowing (blinded), where 0 = normal swallow and 12 = severe dysphagia with tracheal aspiration. Data were analysed by 3-factor ANOVA, with significance taken as P<0.05. Results: Dysphagia models were reversible, and horses swallowed normally within 3 h of model induction. The marked dysphagia model impaired movement of feed from the base of the tongue to the oesophagus and caused severe airway contamination. The average dysphagia score (mean ± s.d.) for the marked dysphagia model was 10.6 ± 1.1 before surgery and 6.1 ± 4.3 after surgery (P = 0.007). Laryngeal advancement surgery did not significantly improve the dysphagia scores in the moderate model (P = 0.5). Conclusions: Laryngeal advancement surgery may improve swallowing and reduce aspiration in horses affected with diseases that cause pharyngeal dysphagia. Keywords: horse; laryngeal advancement surgery; tie-forward; dysphagia; pharynx Introduction Pharyngeal dysphagia describes defects in the effective transport of food, water or endogenous secretions from the oral cavity to the oesophagus [1,2]. This form of dysphagia may occur with central or peripheral neurological or muscular diseases and is reported to affect between 30 and 46% of horses with mycotic guttural pouch infections [3–5]. Clinical signs include nasal discharge of feed, coughing and dehydration. Tracheal aspiration and malnutrition may lead to pneumonia, cachexia and debilitation. Palliative therapy and alternative feeding methods can be attempted, but recovery from clinical signs takes days to months. Dysphagic horses are at times subjected to euthanasia owing to the uncertain prognosis for return of pharyngeal function and the duration, intensity and cost of supportive therapy [3,4]. An effective, practical and expedient treatment for pharyngeal dysphagia would reduce morbidity and mortality in affected horses. The pathophysiology of pharyngeal dysphagia is multifactorial and poorly understood. Neuromuscular deficits that diminish laryngeal elevation are implicated in the pathophysiology of human dysphagia [2,6]. In man, laryngeal elevation is a critical step in the pharyngeal phase of swallowing and is accomplished by contraction of the suprahyoid, thyrohyoid and pharyngeal muscles [2,6]. Contraction of the muscles responsible for laryngeal elevation opens the relaxed cranial oesophageal sphincter, formed in part by the cricopharyngeus muscle, to accept the food bolus. Laryngeal elevation also helps to protect the airway from aspiration by allowing passive retroversion or tilting of the epiglottis by the base of the tongue, shielding the adducted larynx as the bolus moves through the pharynx to the oesophagus. People with pharyngeal dysphagia receive medical and rehabilitation therapy before surgical intervention [7,8]. Laryngeal advancement surgery and partial upper oesophageal sphincter myotomy were curative in 9 of 17 people with severe dysphagia and life-threatening aspiration, and 3 of 17 were significantly improved [8]. Laryngeal elevation in man is the anatomical equivalent of dorsal laryngeal movement in horses. Deficits in dorsal laryngeal movement during swallowing may also produce pharyngeal dysphagia in horses. The pharyngeal phase of swallowing is controlled by branches of the glossopharyngeal (IX), vagus (X) and hypoglossal (XII) nerves [2,9]. These nerves lie within the medial compartment of the guttural pouch in horses. Most horses that develop dysphagia following guttural pouch infection or trauma have only one affected pouch [3,5]. A moderate dysphagia model was therefore developed in the present study by blocking IX, the pharyngeal branch of X (pX) and XII within the right guttural pouch. A marked dysphagia model was created based on evidence that bilateral blockade of pX produced severe pharyngeal dysphagia in research horses [10]. Laryngeal advancement surgery, or the ‘tie-forward’, was developed by Ducharme and colleagues to treat horses with exercise-induced intermittent dorsal displacement of the soft palate [11]. This procedure moves the larynx dorsal and rostral by advancing the thyroid cartilages towards the basihyoid bone [12], similar to laryngeal advancement in man [7,8]. The hypothesis of this study was that regional anaesthesia of IX, pX and XII would produce reversible models of pharyngeal dysphagia and that laryngeal advancement surgery would ameliorate clinical signs of dysphagia in these models. The aims of the present study were to develop 2 reversible models of pharyngeal dysphagia of different severity, to quantify the degree of dysphagia using a scoring system, and to demonstrate that laryngeal advancement surgery diminishes clinical signs of dysphagia in the moderate and marked models. 362 Equine Veterinary Journal 48 (2016) 362–367 © 2015 EVJ Ltd Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12430