Vocal Fold Paresis

Anatomy & Pathophysiology:

Situated in the larynx at the level of the thyroid cartilage, the paired vocal folds (VFs) serve the dual function of phonation and protecting the airway from aspiration. For a detailed review of vocal fold anatomy and the physiology of speech, see The Larynx. Briefly, the vocal folds are attached posteriorly to the arytenoid cartilages. Anteriorly, they join together at the anterior commissure and attach to the thyroid cartilage. The actions of the intrinsic laryngeal muscles control vocal fold positioning and length (tension). These muscles include the posterior cricoarytenoid (PCA), lateral cricoarytenoid (LCA), thyroarytenoid (TA), and cricothyroid muscles. Of these, the PCA is the sole abductor of the vocal folds. Contraction of the TA and LCA muscles rotates the arytenoid cartilages medially, adducting (closing) the vocal folds. In addition, the TA also affects VF tension by shortening their length, while the cricothyroid muscle lengthens the vocal folds. Motor innervation to all of these muscles is via the recurrent laryngeal nerve (RLN, a branch of the vagus nerve), except for the cricothyroid muscle, which is innervated by the external branch of the superior laryngeal nerve (SLN, also a branch of the vagus nerve).

Injury to (or dysfunction of) the recurrent laryngeal nerve, superior laryngeal nerve, or proximal (high) vagal nerve disrupts intrinsic laryngeal muscle function and results in impairment of vocal fold mobility. The term "vocal fold paresis" is typically used to describe a partial immobility with some residual nerve function, whereas “vocal fold paralysis” generally denotes a complete lack of nerve function.

Most cases of vocal fold paralysis are acquired, although congenital etiologies are also possible. The most common cause of congenital VF paralysis is the Arnold-Chiari malformation, which can result in traction on the vagal nerve roots. Other congenital malformations, such as those involving the great vessels in the thorax, can also result in vocal fold paralysis.

Acquired VF paralysis may be a result of traumatic or iatrogenic denervation of the laryngeal muscles; involvement of the RLN, SLN, or vagus nerve by neoplasm; or neurologic disease (such as multiple sclerosis or brainstem infarction). In many cases, the nerve dysfunction is idiopathic in origin, particularly in cases of unilateral VF paralysis due to RLN dysfunction. Idiopathic cases are hypothesized to be caused by a viral neuropathy, usually following an upper respiratory infection.

Surgical procedures that carry a risk of RLN injury include cardiac, carotid, thyroid, esophageal, and cervical spinal surgeries. Additionally, tumors in the thorax (such as lung carcinoma) and neck can involve the RLN. Unilateral VF paralysis due to RLN dysfunction is more common on the left, due to the more complicated course of the left RLN (involving descent into the thorax and looping around the aortic arch). Unilateral RLN injury results in dysfunction of the PCA, LCA, and TA muscles. Since this causes action of the cricothyroid muscle to be unopposed, the paralyzed vocal fold assumes a paramedian position. Compensation by the unaffected vocal fold can occur over time in order to achieve sufficient glottic closure on phonation and during deglutition. In cases of bilateral RLN injury, both vocal folds are fixed in a paramedian position. In this configuration, the vocal folds are often sufficiently approximated to allow for normal or near-normal voice production. However, any further narrowing of the rima glottidis (such as with vocal fold edema) can rapidly result in airway compromise.

Isolated superior laryngeal nerve injury is typically a complication of thyroid surgery, but can be idiopathic as well. Because the SLN innervates only the cricothyroid muscle, SLN paralysis results in less pronounced effects on voice quality than RLN injury. Typically, patients experience difficulty with voice production at higher pitches or louder volumes. This is particularly noticeable for patients who sing. As the SLN also provides sensory innervation to the larynx (via its internal branch), proximal SLN injury can cause impairment of normal laryngeal reflexes, with severe cases of bilateral SLN paralysis resulting in aspiration.

High vagal injury can occur at the skull base (due to tumor or skull base surgery) or intracranially (due to tumor, infarction, or neurologic disease). Because the site of injury is proximal, both RLN and SLN paralysis result. This causes the paralyzed vocal fold to assume an intermediate position (abducted further as compared to the paramedian position. In unilateral vagal paralysis, the unaffected vocal fold may be unable to compensate sufficiently to achieve normal glottic closure during phonation. The resulting voice quality is weak and breathy. Additionally, incomplete glottic closure during swallowing and reduced pharyngeal and laryngeal sensation may result in aspiration. Bilateral high vagal paralysis is rare (may be due to neurologic conditions, such as infarction or motor neuron disease), but is invariably associated with aspiration.

Epidemiology:

Congenital vocal fold paralysis is the second most common congenital laryngeal abnormality, accounting for 10-20% of cases. It has no gender predilection. Recurrent laryngeal nerve injury has been reported as a postoperative complication in 2-8% of coronary artery bypass graft surgeries, 9% of patent ductus arteriosus ligations (in infants), and approximately 1-2% of thyroidectomies.

Natural History:

The likelihood of spontaneous resolution of VF paresis is dependent on the mechanism of nerve injury. In some cases of iatrogenic trauma, such as that sustained during surgical procedures, transient impairment of VF mobility occurs as a result of nerve compression, traction, or thermal injury from cauterization. In such instances, improvement in symptoms may be seen over time. Conservative treatment measures are usually employed for the first 6-12 months following iatrogenic or idiopathic injury to determine whether gradual improvement will take place before considering more invasive therapeutic options. In contrast, if the RLN, SLN, or vagus nerve is transected or infiltrated by tumor, symptoms are generally permanent and necessitate intervention if severe. As previously mentioned, unilateral RLN paralysis can often be relatively asymptomatic due to compensation by the unaffected vocal fold. Bilateral RLN paralysis may also have few symptoms under normal conditions at rest; however, any inflammatory process involving the laryngeal mucosa (such as may occur during an upper respiratory tract infection or with laryngopharyngeal reflux) can readily cause stridor and airway compromise due to obliteration of the already narrowed glottic aperture. Compensatory mechanisms are often ineffective for vagal nerve injury, due to the intermediate position of the paralyzed vocal fold(s). As aspiration frequently occurs due to the enlarged glottic opening, VF paralysis due to high vagal injury generally requires intervention.

Presentation:

Patients with symptomatic vocal fold paresis most commonly present with dysphonia. The voice may have a weak or breathy quality. Patients with isolated SLN injury may have relatively normal voice under most conditions, with symptoms noticeable only at high pitches. In cases where the internal branch of the SLN has been injured, patients may experience decreased or altered laryngeal sensation. This may manifest as cough, foreign body sensation in the throat, or aspiration. Episodes of aspiration are more common and more severe in patients with high vagal nerve injury, particularly those with bilateral vagal nerve paralysis.

Differential Diagnosis of Vocal Cord Paresis:

Evaluation:

History

The patient history should include the timing and duration of symptoms as well as any antecedent surgeries involving the neck or thorax.

Physical Examination

The physical examination should include cranial nerve assessment and palpation of the neck and thyroid gland. Flexible fiberoptic laryngoscopy should be performed to evaluate the extent of vocal fold paresis and to examine for any infiltrative lesions. The resting position of each vocal fold should be noted, along with degree of mobility when phonating (achieved by observing the vocal folds while the patient says the consonant "E" repeatedly). If the VF paresis is unilateral, it should be noted whether the unaffected vocal fold compensates sufficiently to achieve complete glottic closure. Note the degree of glottic closure during phonation and the dimensions of the glottic aperture at rest. In cases of a wide resting glottic aperture (as may be seen with bilateral vagal paralysis), frank aspiration of secretions may be observed.

Imaging Studies

Malignancy must be excluded in cases of vocal fold paralysis without an apparent cause. This may be done with computed tomography of the neck and chest, with or without magnetic resonance imaging of the skull base.

Other Studies

In cases where it is unclear whether the paralysis is due to nerve injury or VF fixation, laryngeal electromyography (EMG) can be performed. EMG employed between 3 weeks and 6 months post-injury can also be useful in determining prognosis in cases of denervation injury.

Treatment:

In the absence of malignancy, most cases of unilateral vocal fold paralysis due to RLN injury are approached conservatively with voice therapy. Some improvement in voice typically occurs over the first year after injury or onset due to reinnervation and compensation by the unaffected vocal fold. Unilateral VF paralysis due to high vagal injury is less likely to be compensated, due to the more abducted position of the paralyzed vocal fold, and commonly requires intervention to prevent aspiration and improve voice quality. Bilateral vocal fold paralysis due to either RLN or vagal injury generally necessitates aggressive measures to prevent aspiration (as is common in vagal paralysis) or airway compromise (as can occur with RLN paralysis exacerbated by vocal fold edema or inflammation).

Treatment options for unilateral RLN paralysis with dysphonia or dysphagia can be approached either with injection laryngoplasty or laryngeal framework surgery, most commonly medialization thyroplasty. The goal of these procedures is to move the paralyzed vocal fold medially to eliminate the glottic gap and facilitate complete glottic closure during phonation and swallowing. Injection thyroplasty utilizes filler substances, such as calcium hydroxyapatite, gelatin paste, collagen, or fat, which are injected in the lateral aspect of the paralyzed vocal fold, increasing its bulk and pushing the free edge medially. The injection can be performed transcervically or transorally under fiberoptic or rigid laryngoscopic visualization. Most clinicians will perform injection laryngoplasty with a temporary material during the first 6 to 12 months to improve voice and swallowing while the nerve is allowed to recover.

Medialization thyroplasty (also known as Isshiki type I thyroplasty) is a surgical procedure in which an implant is inserted through a window made in the thyroid cartilage, thereby displacing the vocal fold medially. This procedure is discussed in detail in Medialization Thyroplasty.

In cases of unilateral vagal nerve paralysis, injection laryngoplasty is usually insufficient to close the glottic gap, due to the intermediate (more abducted) position of the paralyzed vocal fold. Medialization thyroplasty is the preferred approach in such cases, as a greater amount of medial displacement can be achieved. Medialization thyroplasty can be performed in conjunction with arytenoid adduction, in which a suture is passed through the muscular process of the arytenoid cartilage and tacked to the thyroid cartilage. This pulls the muscular process of the arytenoid cartilage anteriorly and rotates the vocal process medially, approximating the function of the lateral cricoarytenoid muscle. Alternatively, adduction arytenopexy can be performed, in which the cricoarytenoid joint is disarticulated and the cartilage is sutured in position medially. These arytenoid procedures provide closure of any remaining posterior glottic gap that is not corrected by a thyroplasty implant.

Bilateral RLN paralysis presenting with acute airway obstruction requires emergent intervention and is often approached with tracheostomy. This may be a temporizing measure or can be permanent. In cases where some recovery of nerve function is expected, one of the paralyzed cords may be temporarily sutured in an abducted position (suture lateralization), allowing for removal of the tracheostomy (decannulation) during the recovery period. For patients with permanent bilateral RLN paralysis who do not wish to have a permanent tracheostomy, long-term treatment options are available, which involve removal of laryngeal tissue to prevent airway compromise. This can be achieved through arytenoidectomy or cordotomy. Arytenoidectomy may be partial or total and can be accomplished using laser. Cordotomy is generally done by excising the posterior portion of the vocal fold to minimize the impact on the voice. Removal of one or both vocal folds in their entirety (cordectomy) may also be performed; however, dysphonia and the risk of aspiration increase with increasing amounts of tissue removal.

Bilateral high vagal paralysis often requires tracheostomy to reduce aspiration. Bilateral medialization thyroplasty may also be considered for permanent paralysis, although sufficient medialization for adequate voice production must be balanced with the risk of airway compromise with overmedialization.

Complications, Prognosis & Follow-Up:

Complications of medialization procedures include insufficient medialization (failure to improve symptoms), airway obstruction, and migration of thyroplasty implant. Voice outcomes after medialization are generally quite good; phonation and the use of fiberoptic laryngoscopy in an awake patient during surgery allows for intraoperative assessment of voice quality and visualization of the vocal fold position with subsequent adjustment of implant size and placement. The rate of revision surgery for medialization thyroplasty has been reported between 5-16%.

Arytenoidectomy and cordotomy or cordectomy have been shown to be very effective at establishing a patent airway. Complications of lateralization procedures include increased risk of aspiration and dysphonia, as well as vocal fold scarring or granuloma formation.

Complications of tracheostomy are discussed in Tracheostomy. In bilateral RLN paralysis, over 50% of patients may experience some degree of recovery over time, which may allow for decannulation.