Overview
Definition:
External branch of the superior laryngeal nerve (EBSLN) preservation refers to surgical techniques employed during thyroidectomy and other neck procedures to prevent iatrogenic injury to this nerve
The EBSLN innervates the cricothyroid muscle, which is crucial for vocal cord tension and pitch modulation.
Epidemiology:
The incidence of EBSLN injury is variable, reported from 1% to over 20% depending on surgical volume, technique, and surgeon experience
Risk is higher in revision surgeries or with extensive goiters.
Clinical Significance:
Injury to the EBSLN can lead to significant morbidity, including voice changes (hoarseness, reduced vocal pitch, vocal fatigue), difficulty with prolonged speaking, and sometimes aspiration, impacting a patient's quality of life and professional capabilities
Preservation is a critical aspect of meticulous thyroid surgery.
Surgical Anatomy
Origin And Course:
The superior laryngeal nerve originates from the vagus nerve in the neck
It divides into superior and inferior (external) branches
The EBSLN typically courses anteriorly and laterally, often near the superior thyroid artery, and descends towards the cricothyroid muscle.
Relationships To Thyroid Gland:
The EBSLN has a close anatomical relationship with the superior pole of the thyroid gland and the superior thyroid artery
It is vulnerable during ligation of the superior thyroid vessels and dissection of the upper thyroid pole.
Variations:
Anatomical variations exist, including dual EBSLNs, absence of EBSLN, or different branching patterns
Awareness of these variations is crucial for intraoperative identification and preservation.
Risk Factors And Injury Mechanisms
Surgical Techniques:
Direct traction, thermal injury from electrocautery, inadvertent ligation of the nerve with vessels, and aggressive dissection near the superior thyroid pole are common mechanisms of injury.
Patient Factors:
Large goiters, retrosternal extension of the thyroid, previous neck surgery (revision thyroidectomy), and inflammatory conditions can increase the risk of injury due to distorted anatomy.
Surgeon Factors:
Lack of familiarity with detailed anatomy, insufficient experience, and suboptimal surgical technique contribute significantly to EBSLN injury rates.
Identification And Preservation Techniques
Intraoperative Nerve Monitoring:
Continuous or intermittent intraoperative nerve monitoring (IONM) using electromyography (EMG) can aid in identifying the EBSLN and assessing its integrity, although its role is debated compared to meticulous visual identification.
Visual Identification:
Systematic dissection, careful retraction of the thyroid lobe, and identification of the EBSLN from its origin or as it approaches the cricothyroid muscle are paramount
Often found traveling with the superior thyroid artery or just lateral to it.
Ligature Strategy:
Ligating the superior thyroid artery and veins at a distance from the thyroid capsule, or ligating them individually after identifying and preserving the EBSLN, is a safer approach
The "skeletonization" technique involves dissecting the nerve away from the vessels.
No Touch Policy:
Minimizing manipulation of the nerve, avoiding direct cautery, and using gentle dissection techniques are essential for preventing injury.
Clinical Manifestations Of Injury
Voice Changes:
Dysphonia (hoarseness), decreased vocal loudness, inability to sustain high notes, vocal fatigue, and difficulty with prolonged speech are common complaints
The affected individual may feel their voice is breathy or weak.
Swallowing Difficulties:
While less common than RLN injury, significant EBSLN injury can sometimes contribute to mild dysphagia, particularly with swallowing liquids, due to altered laryngeal sensation or minor cricothyroid dysfunction.
Diagnostic Evaluation:
Laryngoscopy and stroboscopy can assess vocal cord tension and mobility
Acoustic analysis of voice and subjective voice handicap indices (e.g., VHI-10) are valuable for quantification and follow-up.
Management Of Ebsln Injury
Conservative Management:
For mild injuries with temporary symptoms, voice rest, speech therapy focusing on vocal cord strengthening and compensatory strategies, and symptomatic relief are often sufficient
Most temporary injuries resolve within weeks to months.
Surgical Reconstruction:
For persistent and functionally significant EBSLN injury, surgical options like nerve grafting or end-to-end neurorrhaphy are rarely performed due to technical challenges and limited evidence of efficacy
More commonly, functional voice restoration is addressed through other means if needed.
Prevention As Primary Treatment:
The most effective management is meticulous intraoperative prevention, emphasizing anatomical knowledge and careful surgical technique to avoid injury in the first place.
Key Points
Exam Focus:
The EBSLN innervates the cricothyroid muscle, crucial for vocal pitch
It is at risk during thyroidectomy, especially near the superior thyroid pole
Visual identification and careful dissection are key to preservation.
Clinical Pearls:
Always trace the superior thyroid artery pedicle to identify the EBSLN
Consider ligation of the superior thyroid vessels away from the nerve
The "no-touch" technique and judicious use of electrocautery are paramount.
Common Mistakes:
Mistaking the EBSLN for a small vessel, aggressive dissection close to the superior pole, excessive electrocautery use, and failing to identify anatomical variations can lead to injury
Over-reliance on IONM without visual confirmation is also a pitfall.