External Branch Of The Superior Laryngeal Nerve

The external branch of the superior laryngeal nerve (EBSLN) is a small but vital nerve responsible for innervating the cricothyroid muscle, a key player in vocal cord tension and high-pitched voice production. Damage to this nerve, though often overlooked, can lead to significant voice changes, particularly impacting singing and professional speaking. Understanding its anatomy, function, and potential injuries is crucial for surgeons, otolaryngologists, and anyone interested in the intricate workings of the human voice.

Anatomy of the External Branch of the Superior Laryngeal Nerve

The EBSLN originates from the superior laryngeal nerve (SLN), itself a branch of the vagus nerve (cranial nerve X). Here's a detailed breakdown of its anatomical course:

1. Origin: The vagus nerve exits the skull through the jugular foramen. Shortly after, it gives off the superior laryngeal nerve.

2. Division: The SLN then divides into two branches:

   • Internal Branch: This branch is primarily sensory. It pierces the thyrohyoid membrane, providing sensation to the supraglottic larynx (the area above the vocal cords).
   • External Branch: This is the motor branch, our main focus. It continues inferiorly, running close to the superior thyroid artery.

3. Course and Relations: The EBSLN's path relative to the superior thyroid artery is clinically significant due to the risk of injury during thyroid surgery. There are generally three recognized patterns:

   • Type 1: The EBSLN crosses the superior thyroid artery high above the superior pole of the thyroid gland. This is the safest configuration for surgeons.
   • Type 2a: The EBSLN crosses the superior thyroid artery close to the superior pole. This poses a moderate risk during surgery.
   • Type 2b: The EBSLN crosses below the superior thyroid artery, making it highly vulnerable to injury during ligation of the superior thyroid artery. This is the riskiest configuration.

4. Innervation: The EBSLN terminates by innervating the cricothyroid muscle. In some individuals, it may also send a small branch to the inferior constrictor muscle of the pharynx.

5. Variations: It's important to note that anatomical variations in the EBSLN's course are common. These variations are a key reason why intraoperative nerve monitoring is increasingly used during thyroid surgery.

Microscopic Anatomy

The EBSLN is primarily composed of motor nerve fibers (axons) responsible for transmitting signals to the cricothyroid muscle. These fibers are surrounded by a protective sheath called myelin, which helps to speed up nerve conduction. Within the nerve, there are also supporting cells called Schwann cells, which produce the myelin. Histological examination of the EBSLN reveals a typical peripheral nerve structure.

Relationship to Surrounding Structures

Understanding the EBSLN's relationship to nearby structures is critical for surgeons:

• Superior Thyroid Artery and Vein: As mentioned, the EBSLN's proximity to the superior thyroid artery is paramount. The artery and vein often run in close proximity to each other, and both can pose a risk to the nerve during thyroid surgery.
• Thyroid Gland: The EBSLN's position relative to the superior pole of the thyroid gland dictates the risk of injury during thyroidectomy.
• Cricothyroid Muscle: This is the target muscle of the EBSLN. The nerve enters the muscle on its lateral surface.
• Inferior Constrictor Muscle: In some cases, the EBSLN may have a branch that innervates this pharyngeal muscle, contributing to swallowing function.
• Carotid Sheath: The vagus nerve, from which the SLN originates, is located within the carotid sheath along with the carotid artery and internal jugular vein.

Function of the External Branch of the Superior Laryngeal Nerve

The primary function of the EBSLN is to innervate the cricothyroid muscle. This muscle plays a crucial role in voice production by:

• Tensing the Vocal Cords: The cricothyroid muscle acts to tilt the cricoid cartilage backward, which in turn lengthens and tenses the vocal cords. This tension is essential for producing high-pitched sounds. Think of it like tightening the strings of a guitar.
• Pitch Control: By varying the tension on the vocal cords, the cricothyroid muscle allows us to modulate our pitch. This is essential for singing, speaking with inflection, and avoiding a monotone voice.
• Vocal Projection: The cricothyroid muscle contributes to vocal projection, allowing us to speak loudly and clearly.

Consequences of EBSLN Injury

Damage to the EBSLN can have a significant impact on voice quality and function. The most common consequence is:

• Voice Weakness: Patients may experience a weakening of their voice, particularly in the higher registers.
• Difficulty with High Pitches: Singing high notes becomes challenging or impossible.
• Vocal Fatigue: The voice may tire easily with prolonged use.
• Loss of Vocal Projection: Difficulty projecting the voice, making it hard to be heard in noisy environments.
• Change in Vocal Quality: The voice may sound breathy, hoarse, or strained.
• Inability to Shout: A noticeable difficulty in raising the voice to a shout.
• Subtle Voice Changes: In some cases, the changes may be subtle and only noticeable to singers or professional voice users.

Compensation Mechanisms

Following EBSLN injury, the body may attempt to compensate for the weakened cricothyroid muscle. The vocalis muscle (the main body of the vocal cord) and other laryngeal muscles can try to take over some of the cricothyroid's function. However, this compensation is often incomplete, and the voice may still be noticeably affected. Voice therapy can help patients learn to optimize these compensatory mechanisms.

Etiology of External Branch of the Superior Laryngeal Nerve Injury

The most common cause of EBSLN injury is thyroid surgery. However, other causes include:

• Thyroidectomy: Surgical removal of the thyroid gland, particularly total thyroidectomy, carries the highest risk due to the nerve's proximity to the superior thyroid artery.
• Parathyroid Surgery: Surgery on the parathyroid glands, which are located near the thyroid, can also potentially injure the EBSLN.
• Anterior Cervical Spine Surgery: Procedures involving the front of the neck, such as spinal fusion, can sometimes damage the nerve.
• Neck Trauma: Blunt or penetrating trauma to the neck can directly injure the EBSLN.
• Tumors: Tumors in the neck, particularly those involving the thyroid gland or larynx, can compress or invade the nerve.
• Viral Infections: In rare cases, viral infections can cause inflammation and damage to the nerve.
• Idiopathic: Sometimes, the cause of EBSLN injury is unknown.

Risk Factors for Injury During Thyroid Surgery

Several factors increase the risk of EBSLN injury during thyroid surgery:

• Surgeon Experience: Less experienced surgeons may be less familiar with the nerve's anatomy and more likely to injure it.
• Large Goiters: Large thyroid glands can distort the anatomy and make it more difficult to identify the nerve.
• Reoperative Surgery: Previous neck surgery increases the risk of injury due to scarring and altered anatomy.
• Malignant Tumors: Thyroid cancer can invade the surrounding tissues, making nerve dissection more challenging.
• Failure to Identify the Nerve: Not identifying and carefully dissecting the nerve increases the risk of inadvertent injury.

Diagnosis of External Branch of the Superior Laryngeal Nerve Injury

Diagnosing EBSLN injury can be challenging, as the symptoms can be subtle and may overlap with other voice disorders. A comprehensive evaluation typically includes:

1. Patient History: A detailed history of the patient's symptoms, including the onset, duration, and severity of voice changes. It's important to inquire about any recent surgeries, trauma, or illnesses.

2. Physical Examination:

   • Indirect Laryngoscopy or Flexible Laryngoscopy: These procedures allow the doctor to visualize the larynx and vocal cords using a mirror or a flexible endoscope. They can help identify vocal cord paralysis or paresis (weakness).
   • Palpation of the Cricothyroid Space: Assessing for tenderness or asymmetry in the cricothyroid space.

3. Laryngeal Electromyography (LEMG): This is the gold standard for diagnosing EBSLN injury. LEMG involves inserting a small needle electrode into the cricothyroid muscle to measure its electrical activity. Reduced or absent activity indicates nerve damage.

4. Acoustic Analysis: This involves recording the patient's voice and analyzing its acoustic properties, such as pitch, intensity, and frequency. Acoustic analysis can help quantify the severity of the voice impairment.

5. Stroboscopy: This technique uses a strobe light to visualize the vocal cords in slow motion, allowing the doctor to assess their vibratory pattern. EBSLN injury can cause subtle changes in vocal cord vibration.

6. Perceptual Voice Assessment: This involves having a trained speech-language pathologist listen to the patient's voice and rate its quality using standardized scales.

Differential Diagnosis

It's important to differentiate EBSLN injury from other conditions that can cause similar symptoms, such as:

• Vocal Cord Paralysis: This can be caused by damage to the recurrent laryngeal nerve (RLN), another branch of the vagus nerve that innervates most of the laryngeal muscles.
• Vocal Cord Nodules or Polyps: These growths on the vocal cords can cause hoarseness and voice changes.
• Laryngitis: Inflammation of the larynx can cause temporary voice changes.
• Muscle Tension Dysphonia: This is a voice disorder caused by excessive tension in the laryngeal muscles.

Prevention of External Branch of the Superior Laryngeal Nerve Injury

Preventing EBSLN injury, especially during thyroid surgery, is crucial. Here are some key strategies:

• Preoperative Laryngoscopy: Performing laryngoscopy before surgery to assess vocal cord function and identify any pre-existing vocal cord abnormalities.
• Meticulous Surgical Technique: Using a careful and precise surgical technique to minimize trauma to the nerve.
• Identification of the EBSLN: Identifying and carefully dissecting the EBSLN during surgery. This can be challenging due to anatomical variations.
• Intraoperative Nerve Monitoring (IONM): Using IONM to continuously monitor the function of the EBSLN during surgery. This involves placing electrodes on the cricothyroid muscle and stimulating the vagus nerve to check for a response. A loss of signal indicates potential nerve damage.
• Ligature Placement: If the superior thyroid artery needs to be ligated (tied off), doing so as far away from the thyroid gland as possible to avoid injuring the EBSLN.
• Energy Devices: Using energy devices (such as ultrasonic scalpels or electrocautery) carefully and avoiding their use near the EBSLN.
• Minimally Invasive Techniques: Considering minimally invasive surgical techniques, which may reduce the risk of nerve injury.
• Surgeon Experience: Choosing an experienced surgeon who is familiar with the anatomy of the EBSLN and the techniques to avoid injury.
• Knowledge of Anatomical Variations: Surgeons should be aware of the common anatomical variations of the EBSLN and be prepared to adapt their technique accordingly.
• Capsular Dissection: Employing capsular dissection techniques to stay close to the thyroid capsule and away from the nerve.

Intraoperative Nerve Monitoring (IONM)

IONM has become increasingly popular in thyroid surgery as a way to reduce the risk of EBSLN injury. IONM works by:

1. Placement of Electrodes: Electrodes are placed on the cricothyroid muscle.
2. Vagus Nerve Stimulation: The vagus nerve is stimulated with a small electrical current.
3. Monitoring Muscle Response: The electrodes on the cricothyroid muscle detect the electrical activity of the muscle.
4. Auditory and Visual Feedback: The surgeon receives auditory and visual feedback indicating whether the EBSLN is functioning properly.
5. Warning Signals: If the signal decreases or disappears, it indicates potential nerve damage, and the surgeon can take steps to avoid further injury.

While IONM is a valuable tool, it's important to remember that it's not foolproof. False negatives and false positives can occur. Therefore, it's essential to combine IONM with meticulous surgical technique.

Treatment of External Branch of the Superior Laryngeal Nerve Injury

The treatment of EBSLN injury depends on the severity of the injury and the patient's symptoms.

• Observation: In some cases, if the injury is mild and the symptoms are minimal, observation may be the only treatment needed. The nerve may recover spontaneously over time.

• Voice Therapy: Voice therapy is often the first-line treatment for EBSLN injury. A speech-language pathologist can teach the patient techniques to:

  • Strengthen the Remaining Laryngeal Muscles: Exercises to strengthen the vocalis and other laryngeal muscles to compensate for the weakened cricothyroid muscle.
  • Improve Vocal Cord Closure: Techniques to improve vocal cord closure and reduce breathiness.
  • Optimize Vocal Technique: Strategies to improve vocal technique and reduce vocal strain.
  • Compensatory Strategies: Learning compensatory strategies to improve voice projection and pitch control.

• Surgical Intervention: In some cases, surgery may be necessary to improve voice quality. Surgical options include:

  • Thyroplasty: This procedure involves placing an implant in the larynx to medialize (move towards the midline) the vocal cord. This can improve vocal cord closure and voice quality.
  • Arytenoid Adduction: This procedure involves surgically repositioning the arytenoid cartilage to improve vocal cord closure.
  • Laryngeal Reinnervation: In rare cases, nerve grafting or other reinnervation techniques may be used to restore function to the cricothyroid muscle.

• Botulinum Toxin (Botox) Injection: In some cases of compensatory muscle tension dysphonia, Botox injections into the laryngeal muscles can help to relax the muscles and improve voice quality.

Prognosis

The prognosis for EBSLN injury varies depending on the severity of the injury and the treatment received.

• Mild Injuries: Mild injuries may recover spontaneously within a few months.
• More Severe Injuries: More severe injuries may require voice therapy and/or surgery to improve voice quality.
• Long-Term Outcomes: Even with treatment, some patients may experience persistent voice changes. However, most patients can achieve significant improvement in their voice with appropriate management.

Conclusion

The external branch of the superior laryngeal nerve plays a critical role in voice production, particularly in pitch control and vocal projection. Damage to this nerve, most commonly during thyroid surgery, can lead to significant voice changes. A thorough understanding of the nerve's anatomy, function, and potential injuries is essential for surgeons, otolaryngologists, and speech-language pathologists. Prevention strategies, such as meticulous surgical technique and intraoperative nerve monitoring, are crucial to minimizing the risk of injury. With appropriate diagnosis and treatment, most patients with EBSLN injury can achieve significant improvement in their voice quality and function. Further research is ongoing to develop even more effective methods for preventing and treating EBSLN injury.