Dysphagia is a swallowing difficulty that requires more time and effort to move food or liquid from the mouth to the stomach. Furthermore, dysphagia has been shown to be associated with malnutrition, dehydration, and increased length of hospital stay¹. The most common cause of dysphagia is stroke, and the prevalence of dysphagia that occurs after stroke is known to reach 19-81%^2,3. Vocal-fold paralysis is a common complication that can cause dysphagia. Leder et al. reported that vocal fold paralysis occurred in 56 (5.6%) of 1,452 patients with dysphagia⁴.

The mechanisms by which vocal fold paralysis causes dysphagia can be explained in various ways. Vocal fold paralysis is strongly associated with lary-ngeal movement. Since it is innervated by the vagus nerve, vocal fold paralysis is often accompanied by decreased laryngeal movement, and the larynx does not function properly during swallowing. Heitmiller et al.⁵ also suggested that dysphagia in unilateral vocal fold paralysis is caused by aspiration due to a lack of airway protection.

Various methods for the treatment of vocal fold paralysis have been proposed, and among them, vocal fold Teflon and Botox injection treatments have been tried^6,7. In addition to Botox vocal fold injection, it is reported that hyaluronic acid vocal fold injection is also effective^8-10.

In this case, we report a patient who developed dysphagia due to unilateral vocal fold paralysis after influenza infection, that was improved by vocal fold adduction through injection of hyaluronic acid (Res-tylane^Ⓡ, Q-Med, Uppsala, Sweden) into the paralyzed vocal folds. The present case was approved by the Institutional Review Board (IRB No. 2023-02-004). Written informed consent was obtained from each patient.

A 74-year-old female without any medical history was diagnosed with influenza infection on December 23, 2017, that improved after conservative treatment for 1 week. Dysphagia and voice changes occurred on January 2, 2018, and the patient underwent brain mag-netic resonance imaging (MRI) and computed tomo-graphy (CT) under the suspicion of stroke, but no abnormal findings were observed. Subsequently, the symptoms continued to worsen, and she lost about 7% of her weight in 3 weeks (reduced from 59 kg to 55 kg). On January 22, 2018, she was admitted to the emer-gency room of the Department of Rehabilitation Me-dicine. After admission, no abnormal findings were observed in the additional tests performed to rule out a stroke.

A Video Fluoroscopic Swallowing Study (VFSS) was performed on January 23, 2018, to evaluate dysphagia. There were no abnormalities in the oral stage, but 50% of the residue after swallowing was measured as semisolid, and aspiration was observed during swallo-wing in liquid. The Penetration Aspiration Scale (PAS) score was 6 points, and the Functional Dysphagia Scale (FDS) was 46 points.(Fig. 1)

Figure 1. VFSS results of pre, post in-tervention. (A) Before the hyaluronic acid injection, during swallo-wing aspiration and 90% of post swallowing residue is observed on the test of semisolid. (B) After the hyaluronic acid injection, aspiration is not observed and post swallowing residue is reduced to 10%. (C) Before the hyaluronic acid injec-tion, during swallowing aspiration is observed on the test of liquid. (D) After the hyaluronic acid injection, aspiration is not observed on the test of liquid, after the hyaluronic acid injection.

The following day, on January 24, 2018, we per-formed a close medical history-taking, neck CT, chest CT, and esophagogastroduodenoscopy to determine other neurological injuries that could cause dyspha-gia (e.g., blunt trauma and radiation therapy) and conditions such as lung cancer, thyroid cancer, lymph node metastasis, and esophageal cancer that can cause nerve compression due to mass. No specific findings or structural abnormalities were found. Ho-wever, laryngoscopy (MAJ-922^Ⓡ, Olympus, Japan) per-formed on the same day showed paralysis of the right vocal fold. When the test results were combined, it was thought that the patient’s dysphagia and voice changes were due to paralysis of the right vocal fold that occurred after influenza infection.(Fig. 2)

Figure 2. (A) Before the injection. (B) After the injection of Hyaluronic acid (Restylane^Ⓡ, Q-Med, Uppsala, Swe-den) in right vocal fold. The right vocal fold has been medialized after the injection of hyaluronic acid.

On January 26, 2018, vocal fold adduction was performed by injecting hyaluronic acid (Restylane^Ⓡ, Q-Med, Uppsala, Sweden). All procedures were per-formed under local anesthesia, and the patient in-haled 4% lidocaine nebulizer for 15 minutes. To anes-thetize the nasal cavity, 4% lidocaine was sprayed into the nasal passages. To anesthetize the larynx, 4% lidocaine was dripped onto the base of the tongue, epiglottis, arytenoid, and vocal fold while the patient phonated. Hyaluronic acid gel (Restylane^Ⓡ, Q-Med software, Uppsala, Sweden). A 25-G needle was passed through the cricothyroid membrane and the process was monitored using a transnasal flexible fiberscope (MAJ-922^Ⓡ, Olympus, Japan). The gel was slowly injected into the vocalis muscle anterior to the vocal process, and the injection continued until slight over-correction was evident. The mean volume of the inje-cted material was 0.5 ml¹⁰. After the injection, adduc-tion of the right vocal fold was confirmed.(Fig. 2)

To confirm the improvement of dysphagia, VFSS was performed 4 days after vocal fold injection. Con-sequently, during the swallowing of semisolids, no aspiration was observed and 10% of the residue was measured, that decreased compared to the initial test. In the liquid phase, no aspiration was observed during swallowing, and aspiration improved compared to the initial examination. The Penetration Aspiration Scale (PAS) score was 4, and the Functional Dysphagia Scale (FDS) score was 26.(Fig. 1)

Stroke is the most common cause of dysphagia, accounting for 19-81% of dysphagia cases, although it varies depending on the study^2,3. Leder and Ross⁴ explained that vocal fold paralysis can also be a cause of dysphagia, and approximately 5.6% of patients with dysphagia also have vocal fold paralysis. There-fore, in patients with dysphagia, stroke must be ruled out first, but if it is not, idiopathic vocal fold paralysis should also be considered as another cause of dysphagia such as lung and thyroid cancers. In this case, as no specific findings were observed in the stroke evaluation, we searched for other causes, such as unilateral vocal fold paralysis and influenza virus infection, confirmed a week prior. Therefore, it could be dysphagia due to idiopathic vocal fold paralysis caused by upper respiratory infection (URI). Bhatt et al. reported that idiopathic vocal fold paralysis occu-rred mostly from December to February, with the highest prevalence of URI. It was found that this patient also developed symptoms during this period¹¹.

The cause of idiopathic vocal fold paralysis after URI has not been clearly identified. However, post- viral vagal neuropathy is thought to be the cause¹². Amin and Koufman¹³ suggested two mechanisms for post-viral vagal neuropathy: (1) direct infection and inflammation of the vagus nerve and (2) secondary inflammatory response. The vagus nerve has both sensory and motor components and various symptoms appear. The vocal fold is directly innervated by the superior and inferior laryngeal nerves, that are bran-ches of the vagus nerve. Thus, vocal fold paralysis also occurs as one of several symptoms of post-viral vagal neuropathy¹².

Until now, studies on the rehabilitation of dys-phagia have been conducted mainly on stroke, and studies on other causes are scarce. Dysphagia is a serious disease that not only reduces the quality of life of patients, but can also lead to many compli-cations such as pneumonia and malnutrition. Recently, the treatment of dysphagia through intervention as well as conventional stroke rehabilitation is increasing. For example, the dysphagia team in the Department of Rehabilitation Medicine at our hospital in the past tried strengthening exercises for the upper sphincter of the pharynx and esophagus using a balloon and Botox injection of the upper esophageal sphincter in patients with cricopharyngeal dysfunction, and the therapeutic effect was confirmed¹⁴. However, it is not yet routinely implemented, and there are not many doctors who do so.

Since the 1980s, vocal cord injection has been atte-mpted as a treatment for voice changes in patients with idiopathic vocal fold paralysis in the otolary-ngology department, and various studies have been conducted. Several materials such as Teflon, Lipid and Botox have been tried previously. A recently tested material is hyaluronic acid, and its stability has been gradually proven^6,7,9,10. Hyaluronic acid (Resytlen^Ⓡ, Q-Med, Uppsala, Sweden), vivo-produced hyaluronic acid-based has high biocompatibility, that produces few foreign reactions and remains at the same vol-ume until fully absorbed¹⁰.

Various factors such as airway protection, laryn-geal elevation, apposition of the false cord, vocal fold, and effective cough are involved in swallowing. How-ever, the vocal fold plays the biggest role. A possible mechanism that influences the improvement of dys-phagia is the mechanical protection of the airway by medializing the vocal fold using hyaluronic acid. It may also reduce aspiration from the food residue in the piriform sinus because hypopharyngeal sump pressure was increased due to vocal fold closure compared to the paramedian fixation state¹⁵. Another possible mechanism is that the reduction of vallecular residue is due to the injection of hyaluronic acid into the vocal folds, creating a mass effect of 0.5 cc hyalu-ronic acid injection. This mass effect causes the inje-ction site and surrounding structures to be pulled, thus also pulling the epiglottis and reducing the valle-cular residue.

This case has some limitations. The patient was only followed up in the outpatient department for 4 months after discharge, during which the dys-phagia symptom improved; however, she did not return for further visits. Therefore, determining whe-ther the vocal cord palsy eventually recovered was difficult. Additionally, only vocal cord injection was performed without medication, such as oral steroids for vocal cord palsy. In the future, larger-scale, retro-spective, and long-term studies are necessary to investigate the effect of hyaluronic acid injection on dysphagia caused by vocal fold paralysis after influ-enza infection. Furthermore, comparative analysis and research on vocal cord injection and oral steroid use should be considered. In conclusion, patients com-plaining of dysphagia with unilateral vocal fold para-lysis due to viral infections other than stroke and hyaluronic acid injection into the vocal fold can be considered as a method to improve dysphagia. Since URI are increasing due to the recent COVID-19 pande-mic, this treatment is thought to have many effects in patients with dysphagia caused by vocal fold paralysis that occurs following URI. Further studies are needed to verify the efficacy and suitability of hyaluronic acid injection therapy.

This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

The authors have no financial benefits.