Which clinical finding best distinguishes recurrent laryngeal nerve (RLN) injury from superior laryngeal nerve (SLN) injury during thyroid surgery?

Explanation

## Distinguishing RLN from SLN Injury ### Superior Laryngeal Nerve (SLN) Injury **Key Point:** SLN innervates the cricothyroid muscle (via external branch) and provides sensory innervation above the vocal cords. - **Motor loss:** Cricothyroid paralysis → inability to lengthen and tense vocal cords - **Clinical presentation:** Loss of pitch control, voice fatigue, difficulty with high frequencies - **Vocal cord position:** Cord remains in paramedian position (NOT affected by SLN) - **Stridor:** Absent (airway patent) ### Recurrent Laryngeal Nerve (RLN) Injury **Key Point:** RLN innervates all intrinsic laryngeal muscles except cricothyroid (via internal and external branches). - **Motor loss:** All intrinsic muscles paralyzed → vocal cord abduction lost - **Clinical presentation:** Hoarseness, weak voice, aspiration risk - **Vocal cord position:** Paramedian (cadaveric position) — neither fully abducted nor adducted - **Stridor:** May occur if bilateral ### Comparison Table | Feature | RLN Injury | SLN Injury | |---------|-----------|----------| | **Cricothyroid function** | Preserved | **Lost** | | **Pitch control** | Maintained | **Impaired** | | **Vocal cord abduction** | **Lost** | Preserved | | **Voice quality** | Hoarse, weak | Fatigued, breathy in high tones | | **Aspiration** | Present | Absent | | **Stridor (unilateral)** | Absent | Absent | **High-Yield:** The **loss of pitch elevation** (inability to increase vocal frequency) is pathognomonic for SLN injury because cricothyroid function is essential for vocal cord tensioning. RLN injury causes hoarseness but preserves pitch control. **Clinical Pearl:** A patient post-thyroidectomy complaining of inability to sing high notes but with otherwise normal voice suggests SLN injury. Hoarseness with weak voice suggests RLN injury. [cite:Sabiston Textbook of Surgery 21e Ch 38]