The rate of difficulty with oral intake in this study was 30.4%. Older age, longer length of hospitalization, longer duration of mechanical ventilation, tracheostomy, diabetes, and higher levels of CRP and PCT were associated with difficulty in oral intake in patients with severe COVID-19.
The rate of dysphagia in patients with severe COVID-19 has been reported to be approximately 30–90%5,6,7,8,9,10,11. The prevalence of difficulty with oral intake in this study was not as high as reported previously. Differences between our results and those of previous reports may arise from differences in inclusion criteria, definition of feeding outcome, or dysphagia. Since the number of patients with severe COVID-19 was high during this period, we were unable to assess swallowing function in all patients. Nine patients in relatively good general condition were transferred to other hospitals before swallowing function tests or oral intake. Therefore, the actual rate of dysphagia may have been low.
In this study, older age, longer length of hospitalization, longer duration of mechanical ventilation, tracheostomy, and diabetes were associated with the prevalence of difficulty in oral intake. These results are consistent with previous reports5,6,7,8,9,10,11. Patients with severe COVID-19 are prone to dysphagia due to muscle weakness caused by prolonged intubation9, cerebrovascular events, encephalomyelitis, encephalopathy, peripheral neuropathy, and myositis14. The mechanisms of postintubation dysphagia in COVID-19 patients include impaired swallowing dysfunction resulting from intensive care, endotracheal intubation, or tracheostomy, as well as swallowing dysfunction due to the pulmonary dysfunction characteristic of COVID-19. Macht et al. identified the potential mechanisms of dysphagia in the ICU, including oropharyngeal and laryngeal trauma, neuromuscular weakness, reduced laryngeal sensitivity, altered senses, gastroesophageal reflux, and impaired synchronization of breathing and swallowing.15 The decrease in swallowing function due to the tracheotomy has been indicated as a decrease in sensory input due to disruption of the upper airway, decreased subglottic air pressure, and disuse atrophy of the laryngeal structures16,17,18.
A characteristic complication of COVID-19 is reduced lung function due to shortened and weakened breathing; some patients may develop pulmonary fibrosis. In particular, the course has been observed to be more severe in elderly men with complications19,20. Breathing and swallowing are highly emphasized movements21. Respiratory pauses during swallowing prevent postswallowing bolus aspiration22, and disruption of respiratory rhythm due to tachypnea is associated with dysphagia22.
Swallowing function generally deteriorates with age due to age-related changes23 or underlying diseases. Therefore, clinicians should be aware of the underlying dysphagia, especially in older patients with severe COVID-19. Longer durations of mechanical ventilation and tracheostomy might have a negative impact on swallowing function.
The use of ECMO and serum levels of LDH and CK were not associated with the prevalence of difficulty in oral intake in this study. ECMO is used when ARDS is difficult to treat with invasive mechanical ventilation. Higher levels of LDH have been associated with a higher risk of poor outcomes in COVID-19 patients, reflecting a severe form of interstitial pneumonia (IP)24. COVID-19 has also been reported to be associated with viral myositis attributed to direct myocyte invasion or the induction of autoimmunity, and enzyme markers, such as CK, are elevated25. Therefore, the use of ECMO and higher levels of LDH and CK are associated with the severity of COVID-19, reflecting ARDS, IP, and myositis. The results of this study suggest that the severity of ARDS, IP, and myositis is not associated with the prevalence of dysphagia. Several previous reports have shown that the severity of the disease is associated with dysphagia9,26; however, our results were inconsistent with these previous reports. This may be related to differences in the definition of dysphagia. However, more studies are required to confirm these findings.
Higher levels of CRP and PCT at the time of intubation were significantly associated with difficulty in oral intake, suggesting that patients with bacterial infections or aspiration pneumonia before intubation tend to have a longer duration of mechanical ventilation and dysphagia. Dysphagia has also been reported to be prevalent even in non-intubated patients with COVID-1927,28. Together with our results, these findings suggest that patients with higher levels of CRP and PCT may have dysphagia and aspiration pneumonia.
In this study, older age, longer length of hospitalization, longer duration of mechanical ventilation, tracheostomy, diabetes, and higher levels of CRP and PCT at intubation were associated with dysphagia in patients with severe COVID-19. Postoperative oral intake should be performed with caution in patients with these risks. It is difficult to perform swallowing function tests, including FEES, in all cases of severe COVID-19 due to the risk of infection for healthcare workers. Swallowing function tests are recommended for patients with these risk factors.
This study has several limitations. First, this study was retrospective. Because the number of patients was small, we were unable to perform other statistical tests, such as multivariate regression. However, a large-scale prospective study is required to confirm these findings. Second, we performed only the MWST or FEES and did not perform other tests, such as VFSS or high-resolution manometry. Therefore, the detailed mechanisms underlying dysphagia remain unknown. More studies are required, including other swallowing function tests, to confirm the mechanism of dysphagia caused by severe COVID-19. Third, controls other than those with COVID-19 were excluded from this study. We were unable to assess the influence of laryngeal inflammation caused by SARS-CoV-2 infection. More studies that include a control group, are required. Fourth, there was a risk of bias, as we could not assess swallowing function in all patients with severe COVID-19. Due to the high number of patients with severe COVID-19 during this period, patients in relatively good general condition were transferred to other hospitals before swallowing function test or oral intake. Therefore, the incidence of dysphagia may have been low.
