What is the likely cause of hoarseness of voice after thyroid surgery in an elderly patient with no visible pathology of vocal cord movement on indirect laryngoscopy?

External Laryngeal Nerve Injury as Cause of Post-Thyroidectomy Hoarseness

The most likely cause of hoarseness of voice after thyroid surgery in this elderly patient with no visible pathology of vocal cord movement on indirect laryngoscopy is external laryngeal nerve injury (option D). This diagnosis explains the clinical presentation and is consistent with the findings on laryngoscopy.

Pathophysiology and Clinical Presentation

The external laryngeal nerve (ELN), also known as the superior laryngeal nerve (SLN), is particularly vulnerable during thyroid surgery, especially in cases involving large multinodular goiters as in this patient. When injured, it causes:

• Hoarseness of voice
• Normal-appearing vocal cord movement on laryngoscopy
• Voice fatigue and inability to reach higher pitches
• Difficulty projecting voice

Unlike recurrent laryngeal nerve (RLN) injury, which would cause visible vocal cord paralysis or paresis on laryngoscopy, ELN injury preserves gross vocal cord movement while affecting voice quality 1.

Differential Diagnosis Analysis

1. External laryngeal nerve injury (Option D):

   • Most consistent with the clinical picture
   • Explains hoarseness with normal-appearing vocal cord movement
   • Common in surgeries for large multinodular goiters

2. Unilateral partial RLN injury (Option A):

   • Would typically show some visible impairment of vocal cord movement
   • Not consistent with the normal laryngoscopy findings

3. Bilateral partial RLN injury (Option B):

   • Would present with more severe symptoms including possible airway compromise
   • Would show visible abnormalities on laryngoscopy

4. Tracheomalacia (Option C):

   • Usually presents with stridor, respiratory distress, or dyspnea
   • Not typically associated with isolated hoarseness when vocal cords appear normal

Evidence-Based Rationale

The clinical practice guideline on improving voice outcomes after thyroid surgery notes that voice impairment following thyroid surgery can occur due to injury to the external laryngeal nerve, which innervates the cricothyroid muscle 1. This muscle is responsible for tensing the vocal cords and adjusting voice pitch. When damaged, it results in voice changes that may not be apparent on standard laryngoscopy.

According to the guideline, "Specific problems with fatigue or a weak voice can be addressed with injection laryngoplasty or medialization, but these treatments can also be incomplete. Cricothyroid approximation surgery is an option to increase pitch in patients who have had SLN injury following thyroidectomy" 1.

Risk Factors in This Case

Several factors increased this patient's risk for ELN injury:

• Advanced age
• Long-standing large multinodular goiter
• Extensive surgery required for such cases

Management Recommendations

1. Immediate management:

   • Reassurance that this is a recognized complication
   • Voice rest for 1-2 weeks
   • Adequate hydration

2. Follow-up care:

   • Referral to speech-language pathology for voice therapy
   • Consider video-stroboscopy for more detailed assessment
   • Re-evaluation in 4-6 weeks to assess for spontaneous recovery

3. Surgical options if no improvement after 6 months:

   • Cricothyroid approximation surgery
   • Injection laryngoplasty

Prognosis

The prognosis for external laryngeal nerve injury is variable:

• Some patients experience spontaneous recovery within 3-6 months
• Others may have permanent voice changes requiring adaptation or surgical intervention
• Voice therapy can significantly improve outcomes even in cases of permanent injury

The clinical practice guideline emphasizes that "All treatments of vocal fold mobility issues center on rehabilitation rather than restoration of preoperative normal vocal fold movement" 1.