Laryngeal and swallow dysregulation following acute cervical spinal cord injury

Abstract

Laryngeal function is vital to airway protection. While swallow is mediated by the brainstem, mechanisms underlying increased risk of dysphagia after cervical spinal cord injury (SCI) are unknown. We hypothesized that loss of descending phrenic drive affects swallow and breathing differently, and loss of ascending spinal afferent information alters swallow regulation. We recorded electromyograms from upper airway and chest wall muscles in freely breathing pentobarbital-anesthetized cats and rats. Inspiratory laryngeal activity increased ~two-fold following C2 lateral-hemisection. Ipsilateral to the injury, crural diaphragm EMG amplitude was reduced during breathing (62 ± 25% change post-injury), but no animal had complete termination of activity; 75% of animals increased contralateral diaphragm recruitment, but this did not reach significance. During swallow, laryngeal adductor and pharyngeal constrictor muscles increased activity, and diaphragm activity was bilaterally suppressed. This was unexpected because of the ipsilateral-specific response during breathing. Swallow-breathing coordination was also disrupted and more swallows occurred during early expiration. Finally, to determine if the chest wall is a major source of feedback for laryngeal regulation, we performed T1 total transections in rats. As in the C2 lateral-hemisection, inspiratory laryngeal recruitment was the first feature noted. In contrast to the C2 lateral-hemisection, diaphragmatic drive increased after T1 transection. Overall, we found that SCI alters laryngeal drive during swallow and breathing, and reduced swallow-related diaphragm activity. Our results show behavior-specific effects, suggesting SCI affects swallow more than breathing, and emphasizes the need for additional studies on the effects of ascending afferents from the spinal cord on laryngeal function.

Voluntary Cough Testing as a Clinical Indicator of Airway Protection in Cervical Spinal Cord Injury

“…The diaphragm, the primary inspiratory muscle, may be active in precise coordination with laryngeal muscles during swallowing to (1) draw the bolus inferiorly or (2) assist in sustaining brief apnea during functional swallowing. This coupling between airway protection during swallowing with inspiratory and compression phases of cough further supports the theory that the larynx serves as a dual valve during swallowing, a valve which is susceptible to disruption following injury to the cervical spine 5 …”

“…A critical component of both swallowing and cough generation is timely and complete laryngeal closure 3 . Injury to the cervical spinal cord may impair respiratory sufficiency, respiratory–swallow coordination, and/or laryngeal and respiratory motor drive, all of which can undermine laryngeal closure as well as exacerbate dysphagia and dystussia 5,22–24 . Recent retrospective data on cervical spinal cord injury suggest peak expiratory flow during voluntary cough testing (VCT) correlates with airway protection during swallowing, however, the study did not find sufficient sensitivity and specificity, explore composite measures of cough, nor investigate laryngeal kinematics during swallowing 12 …”

“…This prevention‐reaction continuum against airway invasion is bookended by two mechanisms: (1) a functional swallow to prevent material from entering the lower airway (i.e., aspiration), and (2) an adequate cough response to eject invasion 3 . Although each are on separate ends of the airway protection continuum, a strong relationship between swallowing and voluntary cough abilities has been demonstrated through in vivo animal models 4,5 and across several neurogenic populations 6–11 . Similarly, survivors of cervical spinal cord injury often present with disruption to both swallowing (i.e., dysphagia) and cough mechanisms (i.e., dystussia) 1,12–21 .…”

“…3 Injury to the cervical spinal cord may impair respiratory sufficiency, respiratory-swallow coordination, and/or laryngeal and respiratory motor drive, all of which can undermine laryngeal closure as well as exacerbate dysphagia and dystussia. 5,[22][23][24] Recent retrospective data on cervical spinal cord injury suggest peak expiratory flow during voluntary cough testing (VCT) correlates with airway protection during swallowing, however, the study did not find sufficient sensitivity and specificity, explore composite measures of cough, nor investigate laryngeal kinematics during swallowing. 12 Given that dysphagia following traumatic cervical spinal cord injury (tCSCI) is characterized by impaired laryngeal vestibule closure and high percentages of "silent" aspiration (i.e., aspiration without overt clinical signs such as coughing), 2,25 we hypothesized a functional relationship exists between voluntary cough abilities and swallowing safety for survivors of tCSCI.…”

Oropharyngeal Dysphagia in Acute Cervical Spinal Cord Injury: A Literature Review

“…During swallowing, this contributes to a favorable pressure gradient (i.e., a suction effect) for bolus flow across the upper esophageal sphincter [51]. Recent studies in animal models of cSCI demonstrate that the absence of diaphragmatic activation during swallowing disrupts negative pressure generation in the esophagus and results in disordered laryngeal muscle activity, contributing to penetration and aspiration [52]. These preliminary findings suggest that damage to the phrenic nerve and diaphragm innervation following cSCI may also directly contribute to swallowing dysfunction.…”