The Anastomoses of the Recurrent Laryngeal Nerve in the Larynx

• The vagus nerve:
  • Also known as the 10th cranial nerve
  • Gives rise to the superior laryngeal nerve (SLN) and recurrent laryngeal nerve (RLN) in the neck
  • After descending toward the larynx:
    • The SLN divides into:
      • The internal laryngeal nerve (ILN)
      • External laryngeal nerve (ELN)

• The RLN innervates all of the intrinsic muscles of the larynx:
  • Except the cricothyroid muscle:
    • This muscle, tenses the vocal cords and adducts the vocal cords:
      • Is innervated by the ELN
• The other branch of the SLN, the ILN:
  • Provides sensory innervation to the laryngeal mucosa
• There are many exceptions to the normal innervation of the laryngeal muscles:
  • Which can influence the interpretation of laryngoscopy results or voice changes after thyroid surgery
• A neural anastomosis:
  • That provides additional motor innervation to the muscles of the larynx normally innervated by the injured nerve:
    • Can contribute to an incorrect interpretation of injury during laryngoscopy or stroboscopy
• According to recent clinical studies:
  • Electrical stimulation of the RLN can cause contraction of the cricothyroid muscle:
    • This suggests that extra-laryngeal branches and or other communications of the RLN:
      • Can sometimes contribute to innervation of this muscle
• The laryngeal nerves:
  • Can form a great variety of anastomoses:
    • These various connections among the ILN, ELN, and RLN have been investigated by many anatomists over the centuries
• Claudius Galen:
  • Was the first to describe the communication between the ILN and RLN
  • Currently, Galen’s anastomosis:
    • Is most commonly defined as:
      • The direct communication between the posterior branches of the ILN and the RLN
    • It can occur as:
      • A single trunk
      • A double trunk
      • A plexus

• Besides Galen’s anastomosis, other communications have been observed and described as follows:
  • The arytenoid plexus:
    • Which links the anterior branch of the RLN with the arytenoid branch of the ILN

• The cricoid communication:
  • Which connects branches originating bilaterally from the RLNs with the superior branch from the deep portion of the arytenoid plexus
• The thyroarythenoid communication:
  • Which is formed by the ascending branch of the RLN and the descending branch from the anterior branch of the ILN

• The communication between the ELN and RLN:
  • Human communicating nerve
• The communication between the ILN and ELN
• The communication between the RLN and the sympathetic trunk

• Despite progress in the development of new techniques:
  • Such as intra-operative nerve monitoring:
    • Which help to reduce the risk of iatrogenic injuries during thyroid surgeries and other procedures conducted in close proximity to the laryngeal nerves:
      • The laryngeal muscles are often paralyzed postoperatively due to iatrogenic injury to the laryngeal nerves
• In view of the complexity and variability of the anatomy in this region:
  • Detailed anatomical knowledge is crucial if surgery is to be both successful and safe, and to reduce the risk of nerve injury
• Observing an intra-laryngeal anastomosis during laryngeal surgery or an extra-laryngeal communication between laryngeal nerves during thyroid surgery:
  • Can lead to confusion, misidentification, and an increased risk of iatrogenic injury
• A thorough understanding of the complex anastomoses between the laryngeal nerves is crucial in patients with laryngeal muscle paralysis
  • Paralyzed laryngeal muscles can be spontaneously reinnervated from an anastomosis between laryngeal nerves
  • Additionally, in cases in which surgical reinnervation is required, some of the nerves that form anastomoses can be used as grafts to restore damaged nerve connections
  • On the other hand, variations in the normal anatomy of the laryngeal nerves can disrupt selective surgical laryngeal reinnervation:
    • A procedure based on the assumption that each laryngeal muscle is supplied by only one nerve branch originating from the RLN
  • Anastomoses among laryngeal nerves can result in exceptions to this rule
  • Such anastomoses have been widely described in the literature:
    • However, there is still no consensus about their prevalence and functionality
  • The significant heterogeneity among studies reporting data on anastomoses between the laryngeal nerves is noteworthy:
    • For example, the reported prevalence of the most common communication, Galen’ s anastomosis:
      • Ranges from 25% to 100%
• Prevalence of Galen’s anastomosis:
  • A total of 14 studies (n = 890 hemilarynges) presented data on the prevalence of Galen’s anastomosis
  • The overall pooled prevalence rate:
    • Was 76.7% (95% confidence interval [CI]: 59.0– 90.0)
  • Subgroup analysis revealed no significant difference in the prevalence of Galen’s anastomosis between the right and left sides
  • Subgroup analyses by gender (males vs females) and geographical origin, and the sensitivity analysis, also revealed no significant differences:
    • However, although the difference was not significant:
      • The prevalence of the anastomosis was highest in Europeans (88.2%) and lowest in North Americans (44.8%)
  • Analysis of the different types of Galen’s anastomosis (two studies, n = 261 anastomoses):
    • Showed a significant difference in the prevalence between single versus double trunk and plexus formation
    • But no significant difference between double trunk and plexus formation
    • The most common type of Galen’s anastomosis was a single trunk:
      • With a pooled prevalence rate 92.3% (95% CI: 84.1–97.5)
    • This was followed by the double trunk anastomosis type:
      • With a pooled prevalence of 4.2% (95% CI: 0.5– 10.7)
    • The plexus formation type with a pooled prevalence of 3.5% (95% CI: 0.2–9.5) (I2: 70.4%, 95% CI: 0–93.3; Co- chran’s Q, P value = 0.066
• Prevalence of a communication between the ELN and RLN
  • A total of eight studies (n = 639 hemilarynges) provided data on the prevalence of the communication between the ELN and RLN
  • The overall meta-analysis revealed that this communication was present in:
    • 21.3% of hemilarynges (95% CI: 3.8–46.0)
  • A subgroup analysis showed no significant difference between left and right sides
  • Although the difference was not statistically significant:
    • The pooled prevalence rate calculated for the North American subgroup (32.0%) was twice that for the European subgroup (14.4%)
• Prevalence of the arytenoid plexus:
  • Five studies (n = 478 hemilarynges) included data on the prevalence of the arytenoid plexus:
    • The pooled prevalence rate was 79.7% (95% CI: 41.1–100)
  • In the European subgroup:
    • The arytenoid plexus was observed in 96.9% of hemilarynges (95% CI: 83.6–100)
  • Subgroup analysis revealed no significant difference with respect to side
• Prevalence of the cricoid communication:
  • Two studies (n = 120 hemilarynges) reported prevalence data for the cricoid communication:
    • The pooled prevalence rate was 19.7% (95% CI: 0–100)
  • There was no significant difference in sub- group analysis based on side
• Prevalence of the thyroarytenoid communication:
  • A total of three studies (n = 430 hemilarynges) presented data on the prevalence of the thyroarytenoid communication:
    • The overall pooled prevalence was 6.3% (95% CI: 0.4–16.9)
  • In subgroup analysis, there was no significant difference in pooled prevalence between the right and left sides:
    • The calculated pooled rate for the left side (15.9%) was almost twice that for the right side (8.8%)
• Communication between the ILN and ELN:
  • A total of two studies (n = 280 hemilarynges) reported data on a communication between the ILN and ELN
    • The pooled prevalence estimate of this communication in hemilarynges was 8.8% (95% CI: 0–35.3; I2 97.0%, 95% CI: 92.2–98.8; Cochran’s Q, P value <0.001)
• References:
  • Journal of Voice, Vol. 31, No. 4, 2017

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