The Physiologic Impact of Unilateral Recurrent Laryngeal Nerve (RLN) Lesion on Infant Oropharyngeal and Esophageal Performance

Abstract

Recurrent laryngeal nerve (RLN) injury in neonates, a complication of patent ductus arteriosus corrective surgery, leads to aspiration and swallowing complications. Severity of symptoms and prognosis for recovery are variable. We transected the RLN unilaterally in an infant mammalian animal model to characterize the degree and variability of dysphagia in a controlled experimental setting. We tested the hypotheses that 1) both airway protection and esophageal function would be compromised by lesion, 2) given our design, variability between multiple post-lesion trials would be minimal, 3) variability among individuals would be minimal. Individuals' swallowing performance was assessed pre- and post-lesion using high speed VFSS. Aspiration was assessed using the Infant Mammalian Penetration Aspiration Scale (IMPAS). Esophageal function was assessed using two measures devised for this study. Our results indicate that RLN lesion leads to increased frequency of aspiration, and increased esophageal dysfunction, with significant variation in these basic patterns at all levels. On average, aspiration worsened with time post-lesion. Within a single feeding sequence, the distribution of unsafe swallows varied. Individuals changed post-lesion either by increasing average IMPAS score, or by increasing variation in IMPAS score. Unilateral RLN transection resulted in dysphagia with both compromised airway protection and esophageal function. Despite consistent, experimentally controlled injury, significant variation in response to lesion remained. Aspiration following RLN lesion was due to more than unilateral vocal fold paralysis. We suggest that neurological variation underlies this pattern.

Pre-pharyngeal Swallow Effects of Recurrent Laryngeal Nerve Lesion on Bolus Shape and Airway Protection in an Infant Pig Model

“…Thus, swallows in which aspiration occurs already have different properties from safe swallows before the swallow reflex proper has been triggered. As aspiration is much rarer in animals without lesion, the effect is less strong ([12] and table 2). Although the bolus area measured here is a two dimensional proxy for volume, previous work [25] has shown that the correlation between bolus area and bolus volume is strong.…”

“…The clear effect of recurrent laryngeal nerve lesion on bolus area, bolus shape and their relationship to airway protection outcomes adds to the body of knowledge indicating that RLN lesion affects the swallow in a more fundamental way than simply by preventing vocal fold closure [12]. The apparent reduction in the area of boluses associated with safe swallow outcomes in infants with lesion is particularly intriguing in this regard, as it suggests that RLN lesion may impact on the oro-pharyngeal system’s ability to handle large boluses safely.…”

“…Not all IMPAS scores are present in all individuals owing to natural variation in the severity of response to lesion; in particular one individual did not aspirate post lesion. Furthermore, silent aspiration is considerably rarer in unlesioned pigs than in lesioned pigs, although it does occur [12]. As a result the design could not be perfectly balanced.…”

“…We use our validated infant pig model [8,11,12] in order to do controlled nerve injuries and collect amounts of data impossible in vulnerable infant clinical populations. Infant pigs are a good model for infant humans through similar size [13], similar anatomy (a fleshy, muscular tongue, muscular cheeks and an intranarial larynx) and a similar mode of drinking (tongue based suction rather than lapping) [14].…”

Animal Models for Dysphagia Studies: What Have We Learnt So Far

“…Surgical or pharmacological models of stroke in rodents have been common, but are now being developed and validated for examining dysphagia [19, 44]. Injuries that are associated with dysphagia, such as damage to branches of the vagus nerve, recurrent or superior laryngeal nerve [11, 12, 16, 38], or other sensory deficits [18], can be relatively easily duplicated in animal models.…”