• The superior laryngeal nerve (SLN) provides sensory innervation to a portion of the oral pharynx and motor innervation to the cricothyroid muscle, whose primary function is to tense the vocal cords.
• The SLN is an important nerve to block in order to achieve adequate anesthesia for instrumentation of the airway (e.g., awake fiberoptic intubation).

• Branch of the vagus nerve
• The SLN, itself, is made up of two branches:
  • Internal: Provides sensation to the base of the tongue, epiglottis, and arytenoids
  • External: Provides motor innervation solely to the cricothyroid muscle

• The SLN separates from the vagus nerve at the base of the skull and travels along with the internal carotid artery to the superior pole of the thyroid gland (2).
• It divides into the internal and external branches at the level of the hyoid bone.
• The internal branch of the SLN is usually larger and enters the thyrohyoid membrane before innervating the larynx.
• The smaller external branch of the SLN travels along the lateral border of the inferior pharyngeal constrictor muscle. It then descends in an anterior-medial direction and courses along with the superior thyroid artery. As the external branch of the SLN and the superior thyroid artery enter the thyroid capsule, the external branch takes a more medial direction to innervate the cricothyroid muscle.

• Because of its sometimes discreet location under the inferior pharyngeal muscle fascia, the external branch of the SLN may be severed during (3)
  • Thyroid lobectomies
  • Parathyroidectomies
  • Anterior approaches to the cervical spine
  • Neck dissections
  • Carotid endarterectomies
• Since the majority of motor innervation to the vocal cords is by the recurrent laryngeal nerve, damage to the external branch of the SNL does not result in complete vocal cord dysfunction. It could however result in
  • Decreased voice quality
  • Decreased voice strength
  • Easy tiring
  • Hoarseness
  • Possible increased risk for aspiration; the cricothyroid muscle functions to maintain tension and elongation of the vocal cords.

• The SLN provides sensation from the inferior portion of the epiglottis to the vocal cords. Thus, blocking the nerve with local anesthetic agents can be useful for:
  • Awake fiberoptic intubation
  • Awake bronchoscopy or esophagoscopy
  • Transesophageal echocardiography in critically ill patients who cannot be sedated
  • Treatment or prevention of laryngospasm or stridor especially in the pediatric patient population
  • Supplement to total IV anesthesia for airway procedures such as rigid bronchoscopy.
• SLN block technique (1):

  • The hyoid bone, located superior to the thyroid cartilage and inferior to the angle of the mandible, is palpated and displaced laterally to identify the greater cornu at each end while the neck is extended (see figure).

    

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    FIGURE 1. Anatomical depiction of superior laryngeal nerve block technique.

  • A 25-gauge needle is advanced toward the cornu of the hyoid with one hand, while the other hand applies contralateral pressure at the opposite end.
  • The needle is advanced until the tip contacts the hyoid bone and then is walked off laterally and posteriorly passing the thyrohyoid membrane to a depth of 1–2 cm.
  • The needle is aspirated to ensure that the placement is not intravascular or in the pharynx
  • 1–3 cc of local anesthetic is injected. The identical procedure is repeated on the other side.
• Alternate approach:
  • Identify the superior ala of the thyroid cartilage and insert the needle toward the superior margin of the cartilage.
  • The needle is then walked superiorly off the cartilage through the thyrohyoid membrane.
  • The needle is aspirated and ~2–3 cc of local anesthetic is injected. This is then repeated on the opposite side.
• Risks of SLN blocks (4):
  • Hypotension and bradycardia have been reported.
  • Inability to protect the airway and potential for aspiration
  • Local anesthetic systemic toxicity; small amounts of intravascular injections can readily cause CNS side effects due to the close proximity to the brain.
  • Edema or airway obstruction if inserted too deeply; insertion into the thyroid cartilage at the level of the vocal cords may cause edema and airway obstruction.
  • Hematoma formation; maintain pressure should this occur.
• Laryngospasm is an exaggeration of the normal glottic closure as a result of stimulation of the vocal cords via the SLN. It is triggered by the presence of blood, secretions, or surgical debris during light anesthesia. It is more common during upper airway procedures and in children.
  • Manifests initially as stridor and can lead to complete airway obstruction.
  • Management involves removing the offending stimulus with suctioning, positive pressure ventilation to open the airway, and/or small doses of IV anesthetic or muscle relaxant (typically succinylcholine due to its quick onset and very short duration of action).

• Recurrent Laryngeal Nerve
• Laryngospasm

Peter K. Yi , MD