Recurrent laryngeal nerve
Anatomy and Structure
The Recurrent laryngeal nerve (RLN) is a branch of the vagus nerve, which is the tenth cranial nerve. It plays a critical role in the innervation of the larynx, which is essential for phonation, breathing, and protection of the airway. The RLN is unique due to its long and circuitous route, descending into the thorax before ascending back to the larynx. This nerve is responsible for motor control of most of the intrinsic muscles of the larynx, excluding the cricothyroid muscle, which is innervated by the external branch of the superior laryngeal nerve.
The RLN branches off from the vagus nerve at different locations on the right and left sides of the body. On the right side, it loops around the subclavian artery, while on the left, it loops around the aortic arch. This asymmetry is due to the embryological development of the great vessels. After looping, the RLN ascends in the tracheoesophageal groove, providing sensory and motor innervation to the larynx.
Function
The primary function of the RLN is to supply motor innervation to all intrinsic muscles of the larynx except the cricothyroid muscle. These muscles include the posterior cricoarytenoid, lateral cricoarytenoid, thyroarytenoid, and the interarytenoid muscles. The posterior cricoarytenoid muscle is particularly important as it is the only muscle responsible for abducting the vocal cords, allowing for breathing.
In addition to motor functions, the RLN provides sensory innervation to the mucosa of the larynx below the vocal cords. This sensory function is crucial for the cough reflex, which protects the airway from aspiration.
Clinical Significance
Pathology
Damage to the RLN can result in vocal cord paralysis, which may lead to hoarseness, loss of voice, or breathing difficulties. Unilateral paralysis often results in a breathy voice due to incomplete closure of the vocal cords during phonation. Bilateral paralysis can cause severe breathing difficulties and may necessitate surgical intervention.
The RLN is particularly vulnerable during surgical procedures in the neck and thorax, such as thyroidectomy, parathyroidectomy, and surgeries involving the esophagus or heart. Inadvertent injury to the RLN during these procedures is a well-known complication.
Diagnostic Evaluation
Diagnosis of RLN injury typically involves laryngoscopy, which allows direct visualization of the vocal cords. Electromyography (EMG) may also be used to assess the electrical activity of the laryngeal muscles and confirm nerve dysfunction.
Treatment
Management of RLN injury depends on the severity and cause of the dysfunction. Conservative management includes voice therapy with a speech-language pathologist. Surgical options, such as medialization thyroplasty or reinnervation procedures, may be considered in cases of persistent or severe vocal cord paralysis.
Embryology
The development of the RLN is closely linked to the embryological development of the pharyngeal arches and the great vessels. During embryogenesis, the RLN is initially positioned higher in the neck. As the heart descends into the thorax, the nerve is pulled downward, looping around the sixth aortic arch, which later develops into the ductus arteriosus on the left and the subclavian artery on the right. This descent results in the characteristic looping course of the RLN.
Evolutionary Perspective
The circuitous route of the RLN has been a subject of interest in evolutionary biology. It is often cited as an example of suboptimal design in vertebrate anatomy. The long path of the RLN is a consequence of the evolutionary history of vertebrates, tracing back to fish ancestors where the nerve had a more direct route. As vertebrates evolved and the neck elongated, the nerve maintained its original path, leading to its current configuration.