What is the effect of cutting the Recurrent Laryngeal Nerve (RLN) unilaterally during thyroidectomy?

Effects of Unilateral Recurrent Laryngeal Nerve Injury During Thyroidectomy

Unilateral recurrent laryngeal nerve (RLN) injury during thyroidectomy results in ipsilateral vocal cord paralysis with the affected cord in a paramedian position, leading to voice hoarseness. 1, 2

Pathophysiology of Unilateral RLN Injury

• The RLN is a mixed nerve with sensory and motor fibers that originates from the thoracic portion of the vagus nerve and innervates all intrinsic laryngeal muscles except the cricothyroid muscle 1, 2
• When cut unilaterally during thyroidectomy, the nerve can no longer provide motor function to the ipsilateral vocal cord muscles, particularly the posterior cricoarytenoid muscle (the primary abductor of the vocal cords) 1
• The affected vocal cord becomes paralyzed in a paramedian position (neither fully abducted nor adducted), resulting in incomplete glottic closure during phonation 2

Clinical Manifestations

• Hoarseness is the most common symptom of unilateral RLN palsy, occurring in 1.2%-5.0% of cases following thyroid surgery 2
• Voice changes include:
  • Breathy voice quality
  • Reduced volume
  • Voice fatigue
  • Difficulty with high-pitched sounds 2
• Aspiration of food and liquids may occur due to impaired airway protection during swallowing 2
• Unlike bilateral RLN injury, unilateral injury does not typically cause respiratory distress or airway obstruction 1, 2

Diagnosis

• Laryngoscopy is essential for diagnosis and should be performed in patients with persistent voice changes following thyroidectomy 2
• Routine laryngeal examination after surgery can detect vocal fold paralysis in twice as many patients compared to selective laryngoscopy 2
• The paralyzed cord will be visualized in a paramedian position (not at midline) 2

Prognosis and Recovery

• Most unilateral RLN injuries show compensation from the opposite vocal cord over time 2
• Recovery from RLN injury usually occurs within 1-3 months in temporary cases 2
• Complete recovery of vocal cord function is documented in up to 93% of patients with unintentional RLN injury 3
• The incidence of temporary and permanent cord palsy is approximately 5.2% and 1.4% respectively 3

Comparison with Bilateral RLN Injury

• Bilateral RLN injury is significantly more serious and presents with:
  • Stridor and acute airway obstruction requiring immediate intervention
  • Both vocal cords fixed in a paramedian position
  • Potential need for tracheostomy 1, 2
• Unilateral RLN injury has a variable damage rate of 6% to 25% according to health parameters, while bilateral damage is assigned a definitive biological damage rate of 25% 1

Risk Factors for RLN Injury

• Surgery for malignant neoplasms and recurrent substernal goiter is associated with an increased risk of permanent nerve palsy 3
• Re-operative procedures carry a risk for permanent RLN palsy of up to 30% 1
• Thyroid cancer increases the risk of intra-operative damage 3-8 times compared to benign disease 1
• Anatomical variations of the RLN, including abnormal trajectory patterns, are associated with higher risk of injury 4

Prevention Strategies

• Capsular dissection, visual identification, and intraoperative nerve monitoring can reduce the definitive RLN injury rate to 0.3-3% 1
• Routine recurrent nerve visualization and dissection is currently the surgical cornerstone for reducing nerve palsy 1
• When bilateral RLN dissection is planned, RLN monitoring is particularly useful to limit the risk of bilateral RLN paralysis 5

Based on the evidence presented, the correct answer to the question is B: Ipsilateral cord paralysis will be midline with voice hoarseness. However, it should be noted that the paralyzed cord is typically in a paramedian position rather than precisely at midline.