Recurrent Laryngeal Nerve Injury

Etiology

Given the coursing nature of the recurrent laryngeal nerve, there are many injuries that can cause pathology, and damage to the nerve anywhere along its path that can cause impaired vocal function. One of the more commonly cited injuries is secondary to surgical intervention. One study reviewing over 800 patients showed that surgical intervention was the most important cause.[3] This can include any surgical interventions of the chest, neck, or skull base. However, the most frequently studied interventions are thyroidectomies and parathyroidectomies. One study showed that permanent, operation-related vocal cord palsies from injury to the recurrent laryngeal nerve had an incidence of approximately 1%. They also reported the incidence of RLN injuries after thyroid surgery and parathyroid surgery to be 14% and 7%, respectively.[4]

Another common cause of RLN injury is secondary to tumors. In fact, one study found that non-laryngeal malignancy accounted for 24.7% of all unilateral vocal cord paralysis, 80% of those being pulmonary or mediastinal.[5] Therefore, it is important to assess for malignancy before RLN injury is labeled as idiopathic.

Endotracheal intubation is also responsible for a significant number of RLN injuries. When RLN injury after intubation is suspected, it is necessary to also consider arytenoid dislocation as a possible cause.[6] Although masses, surgery, and idiopathic are among the most common causes, viral illness, diabetic neuropathy, and trauma have also been reported less frequently.

In an article reviewing multiple studies on unilateral RLN injury, surgery is cited as the most common cause with most studies putting it as the cause of 30 to 40% of all RLN injuries. Tumors are the second leading cause accounting for approximately 17% to 32% of injuries. Idiopathic causes are cited around 10% to 27% of all RLN injuries with endotracheal intubations last at around 7% to 11% of all causes.[7]

In another article reviewing 2,267 cases of unilateral vocal cord palsy, the top three causes were surgery, cancer, and idiopathic accounting for 36.9%, 29.7%, and 20.9% of cases, respectively.[8] Therefore recurrent laryngeal nerve injury and unilateral vocal cord palsy seem to have similar etiologies, as expected.

Epidemiology

It is important to note that there is poor epidemiological data on RLN injuries, and additional studies are needed to further elucidate this. This is likely secondary to the multiple causes of vocal cord palsy, with RLN injuries being only a portion of these as well as the relative difficulty in diagnosing RLN injuries. One study that followed a cohort of 325 patients found that males were twice as likely to present with laryngeal nerve palsy. They also found the mean age to be 55 years old.[9] Another study reported the incidence of vocal cord paralysis, a common presenting symptom of RLN injury, to be 0.42% of new patients seen. However, they reported that males were three times more likely to be affected than females. They also reported a similar age group stating that most patients were in their 5th and 6th decades of life.[10]

History and Physical

Injury to the recurrent laryngeal nerve has the potential to cause unilateral vocal cord paralysis. Patients with this typically complain of new-onset hoarseness, changes in vocal pitch, or noisy breathing. Bilateral vocal cord paralysis is much less common due to both left and right recurrent laryngeal nerves being affected but can present with more serious symptoms, including significant difficulties breathing and swallowing. Recent surgeries of the head/neck or recent intubation should prompt suspicion of possible injury to the recurrent laryngeal nerve. Signs of underlying malignancy should also be investigated, such as hemoptysis, severe coughing, unexplained weight loss, tobacco/alcohol use, or dysphagia. Less common causes, including recent viral illness or neck trauma, may also point towards an underlying RLN injury.

Physical examination of the head and neck may show lymphadenopathy secondary to underlying malignancy. Examination of the thyroid may also show thyroid nodules or irregularities, prompting further investigation for malignancy as a cause. Unilaterally decreased breath sounds in the lung apex may also point towards a tumor of the pulmonary apex, also known as a Pancoast tumor. In this case, clinical symptoms of Horner syndrome, thoracic outlet syndrome, or superior vena cava syndrome may also be seen on examination.

Evaluation

As usual, a thorough history and physical examination should be performed to guide clinical evaluation. Once recurrent laryngeal nerve injury is suspected, imaging can be considered. It is important to consider that the RLN travels from the base of the skull to the thorax, and imaging can involve any or all of these areas. For instance, a screening chest x-ray can be considered if a pulmonary cause is suspected. In general, evaluation with CT is the most common imaging modality and usually involves the entire length of the recurrent laryngeal nerves. Evaluation with a CT scan can also show signs of possible vocal cord paralysis.[11]

However, when patients present with vocal cord paralysis, direct laryngoscopy is usually considered before CT, and imaging is generally preferred as an adjunct study.[12] Flexible laryngoscopy has been shown to have excellent reliability when evaluating vocal fold motion.[13] Strobolaryngoscopy is an additional tool that can be utilized to evaluate vocal fold vibrations.[14]

Laryngeal ultrasonography is also a newer technique that can be considered when evaluating recurrent laryngeal nerve injury. One study evaluated 112 patients for vocal cord palsy using ultrasonography and compared this with laryngoscopy, the current gold standard. It showed that laryngeal ultrasonography was 83.3% sensitive and 97.2% specific for detecting vocal cord palsy and had a negative predictive value of 99%.[15]

Treatment / Management

The primary treatment options for recurrent laryngeal nerve injury include voice therapy or surgery. In general, early reinnervation techniques are based on the extent of nerve injury and the disease course. Less serious RLN injuries in which there is no definite transection of the nerve can usually be monitored for around six months with voice therapy as needed. If the recurrent laryngeal nerve becomes separated during surgical intervention, end-to-end anastomosis is performed to repair the nerve.[16]

After a period of conservative treatment, vocal fold medialization techniques can be implemented. This moves the affected vocal cord closer to the unaffected vocal cord, creating improved contact. Vocal fold medialization techniques can include medialization thyroplasties, injection laryngoplasty, arytenoid adduction, and laryngeal reinnervation.

Type 1 thyroplasty involves making an external incision to place an implant that permanently moves the affected vocal cord medially. Overall this is a safe procedure that has a low major complication rate, lower than outpatient thyroidectomy.[17]

Injection laryngoplasty is when the affected vocal cord is injected with a material, filling the vocal cord and moving it medially. These injectables can include carboxymethylcellulose, hyaluronic acid derivatives, collagen derivatives, calcium hydroxyapatite, or autologous fat/fascia. However, a Cochrane review has shown no definitive advantage or disadvantage for any specific material.[18](A1)

Arytenoid adduction is a procedure that involves placing a permanent suture through the muscular portion of the arytenoid cartilage. This pulls the affected vocal cord medial to correct vocal cord paralysis secondary to RLN injury. The procedure is often utilized in conjunction with other corrective procedures.

Differential Diagnosis

The differential for recurrent laryngeal nerve injury with resulting vocal cord paralysis can include:

• Iatrogenic – Endotracheal intubation or during surgical procedures of the skull base, neck, or chest
• Malignancy – Especially of the skull base, neck, or chest
• Trauma – Including the neck and chest in addition to the larynx
• Neurological – Including stroke (specifically lateral medullary syndrome), bulbar palsies, and demyelinating diseases
• Idiopathic – Diagnosis of exclusion

Prognosis

In general, recurrent laryngeal nerve injuries can be temporary or permanent, and prognosis can vary greatly based on a variety of factors including mechanism of injury and extent of the injury, to name a few. In addition, recovery can be complete or incomplete, highlighting the complexity of nerve injuries and prognosis.

Neuropraxia is the mildest injury. With this injury, the axon remains intact, and nerve function returns in 6-8 weeks. Axonotmesis involves damage to the axon and has varying degrees of severity and prognosis.[19] In one study that reviewed patients undergoing total thyroidectomy secondary to malignancy, 9.5% of patients had recurrent laryngeal nerve injuries resulting in vocal cord paresis with 22% of those becoming permanent, requiring additional interventions.[20] In general, patient prognosis is a complex topic that requires an individual approach for each patient.

Complications

One of the most serious complications is respiratory distress occurring from bilateral recurrent laryngeal nerve injury. However, this is rare, as unilateral RLN injury is much more common. Aspiration with resulting aspiration pneumonia is a substantial complication to consider in susceptible patients. In addition, the resulting dysphagia or voice changes from unilateral injury can still pose substantial morbidity to patients and affect the overall quality of life. This is especially true for patients who have a career based on the use of their vocal cords, such as speakers or singers.