Which nerve injury during thyroid surgery results in loss of cricothyroid muscle function and causes voice fatigue and difficulty with high-pitched sounds?

Explanation

## Superior Laryngeal Nerve Injury in Thyroid Surgery ### Anatomy and Function | Nerve Branch | Motor Supply | Function | Injury Consequence | |---|---|---|---| | **External SLN** | Cricothyroid muscle | Vocal cord tension, pitch control | Voice fatigue, loss of high-pitched sounds | | **Internal SLN** | Sensory to larynx above vocal cords | Laryngeal sensation, cough reflex | Loss of laryngeal sensation, aspiration risk | | **RLN** | All intrinsic laryngeal muscles except cricothyroid | Vocal cord abduction and adduction | Vocal cord paralysis, hoarseness, stridor | ### External SLN Injury **Key Point:** The external branch of the superior laryngeal nerve (ESLN) innervates the cricothyroid muscle, which is responsible for tensioning the vocal cords and controlling pitch. Injury results in a characteristic voice profile. ### Clinical Features of ESLN Injury 1. **Voice changes:** Loss of vocal range, difficulty with high-pitched sounds 2. **Voice fatigue:** Voice becomes fatigued with prolonged speaking 3. **Loss of vocal projection:** Difficulty projecting voice 4. **Subtle presentation:** Often overlooked because the voice is not hoarse (unlike RLN injury) **High-Yield:** ESLN injury is often missed because patients retain normal phonation at rest; the deficit becomes apparent with voice stress or singing. ### Anatomical Vulnerability - The ESLN runs in close proximity to the superior pole of the thyroid - It lies medial to the superior thyroid vessels - Risk is highest during ligation of superior pole vessels - Incidence: 5–28% (often transient) **Clinical Pearl:** To avoid ESLN injury, ligate the superior thyroid vessels individually close to the thyroid capsule rather than in a mass ligature, and identify the nerve before ligating. ### Mnemonic: SLN Branches **SLN = Superior Laryngeal Nerve** - **E**xternal branch → **E**ncricothyroid muscle → **E**levates pitch - **I**nternal branch → **I**nnervates sensory fibers → **I**nhibits aspiration ### Comparison with RLN Injury - **RLN injury:** Hoarseness, vocal cord paralysis (more obvious) - **ESLN injury:** Voice fatigue, loss of pitch control (subtle, easily missed)