Unilateral Laryngeal Nerve Injury

• Recurrent laryngeal nerve (RLN) injury:
  • Is one of the most serious complications in endocrine surgery:
    • It is related to significant morbidity and frequent malpractice litigation
• The recurrent laryngeal nerve:
  • Originates from the:
    • Trunk of the vagus nerve
  • Upon reaching the larynx:
    • It is renamed the inferior laryngeal nerve
  • It innervates all the intrinsic muscles of the same side:
    • With the exception of the cricothyroid muscles
  • It supplies sensory innervation to the laryngeal mucosa:
    • Below the true vocal folds
  • While ascending:
    • The nerve on the right and on the left side:
      • Delivers branches that supply the trachea and the esophagus
  • The morphologic appearance and course of the recurrent laryngeal nerve:
    • Are subject to great anatomic variability
  • In addition, it may often be overlooked:
    • That the nerve most frequently does not consist only of a single trunk but exhibits a network of smaller branches
  • On the right side:
    • It usually loops around and behind the subclavian artery and then ascends into the neck in the tracheoesophageal groove:
      • To enter the larynx distal to the inferior cornu of the thyroid cartilage
    • In instances of embryologic malformation of the aortic arch:
      • In terms of retroesophageal right subclavian artery:
        • The nerve passes with a more median course directly to the larynx (non-recurrent laryngeal nerve):
          • Although the reported incidence of non-recurrent laryngeal nerve is less than 1%:
            • The surgeon has to be aware of the existence of this rare anatomic condition
  • The left recurent laryngeal nerve:
    • Courses upward around the ligamentum arteriosum and the aortic arch and runs vertically toward the tracheoesophageal groove:
      • On their way to the cricothyroid muscle where they enter the larynx
  • Both nerves run close to the capsule of the lateral aspect of the thyroid and cross the inferior thyroid artery:
    • Several variations of the relationship between the nerve and the artery, particularly on the right side, can be observed:
      • The nerve may pass superficially to the artery, deep to it, or between the branches of the vessel
  • After running into the laryngeal wall:
    • The nerve separates into two branches that supply the innervation of various laryngeal muscles:
      • A third branch that serves as a connection with the superior laryngeal nerve
  • During cervical exploration the recurrent laryngeal nerve can be exposed at different levels:
    • Caudally:
      • At the crossing with the common carotid artery
      • In the neighborhood of the inferior thyroid artery
    • Cranially:
      • At Berry’s ligament:
        • A dense condensation of the posterior thyroid capsule near the cricoid cartilage and upper tracheal rings
  • In addition to visual identification:
    • The nerve can be located by direct palpation of the tracheal wall below the lower thyroid pole
    • Considerable debate has long existed concerning the necessity of deliberate exposure of the recurrent laryngeal nerve during thyroid surgery:
      • Kocher commented on postoperative hoarseness and stated that:
        • Following his technique of thyroid dissection, injury to the nerve can with certainty be avoided without the direct exposure
      • The first surgeon who advocated routine dissection and demonstration of the nerves in 1911 was:
        • August Bier of Berlin
      • He was followed by Frank Lahey of Boston in 1938
      • Others advocated that exposure itself:
        • Is a risk due to potential induction of local edema by dissection of adjacent tissues and hemorrhage
      • Following these initial experiences, several studies revealed that depending upon the skill of an individual surgeon principal identification of the nerve reduces the risk of permanent laryngeal nerve injuries from over 5% to less than 1%
      • Nowadays, the practice of visual identification of the nerve represents the gold standard
      • To alleviate the visual identification of the nerve and to provide an intraoperative tool to prove its functional integrity, diverse monitoring methods, i.e., intramuscular vocal cord electrodes inserted either through the cricothyroid membrane or placed endoscopically, endotracheal tube surface electrodes, endoscopic visualization of the vocal cords in combination with nerve stimulation, and palpation of the cricoarytenoid muscle with simultaneous neural stimulation, have been developed
      • Although intraoperative neuromonitoring might be of use in the presence of extended thyroid surgery, particularly in a patient with a preoperatively documented vocal cord paralysis or in difficult anatomic situations:
        • It does not further reduce the low risk of permanent recurrent nerve lesions and it fails to reliably predict the outcome:
          • This experience has been found not only in primary but also in reoperative thyroid and parathyroid procedures
    • Damage to the recurrent laryngeal nerve may be caused by different mechanisms:
      • Cutting
      • Clamping
      • Stretching of the nerve
      • Nerve skeletonization
      • Local compression of the nerve:
        • Due to edema or hematoma
      • Thermal injury by electrocoagulation
  • Transient cord paresis:
    • Which is often caused by edema or axon damage by excessive nerve stretching:
      • Seldom lasts more than 4 to 6 weeks
  • When no restitution of function is notable within 6 to 12 months postoperatively:
    • Permanent damage should be assumed
  • Accidental injury to the recurrent laryngeal nerve is not recognized during surgery in most of the cases:
    • If the surgeon is aware of this complication intraoperatively:
      • Primarily repair of the nerve using microsurgical techniques and epineural sutures or a cable graftfrom the greater auricular nerve can be attempted
      • Even if the nerve is reanastomosed, the dysfunctioning vocal cord will probably never completely recover
      • Delayed nerve repairs are virtually always ineffective in restoring cord function
    • When a paralyzed vocal cord stays in the paramedian position:
      • The patients frequently remain asymptomatic:
        • This phenomenon is due to compensatory overadduction of the intact cord and consecutive constriction of the glottic chink
      • The majority of asymptomatic patients need no special treatment but close observation
    • Unless routine indirect laryngoscopy or videostroboscopy is performed:
      • Many cases of vocal cord paresis will remain unrecognized
    • The some authors recommend preoperative and postoperative laryngoscopic examination of the vocal cord function:
      • Not only for medicolegal reasons but also to document potential preexisting pathologies and consecutively adapt the surgical approach
    • If the paretic cord moves to the lateralized position:
      • Hoarseness or aspiration can occur
        • The prognosis considering gain of normal function is favorable in cases of delayed onset of symptoms:
          • In symptomatic patients either treatment by a speech and language pathologist or invasive interventions such as injection laryngoplasty or medialization laryngoplasty are necessary
• References:
  • Beldi G, Kinsbergen T, Schlumpf R (2004) Evaluation of intraoperative recurrent nerve monitoring in thyroid surgery. World J Surg 28:589–591 
  • Hermann M, Hellebart C, Freissmuth M (2004) Neuromonitoring in thyroid surgery: prospective evaluation of intraoperative electrophysiological responses for the prediction of recurrent laryngeal nerve injury. Ann Surg 240:9–17
  • Dralle H, et al (2004) Risk factors of paralysis and func- tional outcome after recurrent laryngeal nerve monitoring in thyroid surgery. Surgery 136:1310–1322 

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