During thyroid surgery, injury to the recurrent laryngeal nerve (RLN) results in vocal cord paralysis in the paramedian position. Which muscle is responsible for this characteristic positioning?

Explanation

## Mechanism of RLN Injury and Vocal Cord Positioning ### Innervation and Function The recurrent laryngeal nerve (RLN) innervates all intrinsic laryngeal muscles **EXCEPT the cricothyroid muscle**, which is supplied by the external branch of the superior laryngeal nerve (EBSLN). ### Why Paramedian Position Occurs After RLN Injury When the RLN is injured: - All intrinsic laryngeal muscles (adductors AND abductors) lose their innervation — **except the cricothyroid muscle** - The **cricothyroid muscle** (EBSLN-innervated) remains functional and continues to act on the vocal cord - The cricothyroid muscle tenses and slightly adducts the vocal cord, pulling it into a **paramedian (slightly adducted) position** - This is why RLN injury causes a **paramedian** vocal cord position, NOT a fully lateral (cadaverous) position ### The Key Muscle: Cricothyroid The **cricothyroid muscle** is the muscle responsible for the characteristic paramedian positioning after RLN injury. Because it is the only intrinsic laryngeal muscle that escapes RLN denervation, its **unopposed action** draws the paralyzed cord medially toward the paramedian position. ### Comparison with Complete Denervation (Cadaveric Position) If both RLN and EBSLN are injured (or in a long-standing RLN palsy where the cricothyroid fatigues/atrophies): - All laryngeal muscles are paralyzed - The vocal cord assumes a fully **lateral (cadaveric) position** due to elastic recoil of the cord ### Summary Table | Nerve Injured | Muscle Paralyzed | Vocal Cord Position | |---|---|---| | RLN only | All intrinsic muscles except cricothyroid | **Paramedian** (cricothyroid unopposed) | | EBSLN only | Cricothyroid | Near-median (subtle change) | | Both RLN + EBSLN | All intrinsic muscles | **Lateral/cadaveric** | **Key Point:** Paramedian position after RLN injury = **cricothyroid muscle** (EBSLN-innervated) acting unopposed on the denervated cord. *(Scott-Brown's Otolaryngology; Cummings Otolaryngology)* **High-Yield:** This anatomical principle is fundamental to understanding laryngeal nerve injuries and is frequently tested in thyroid surgery questions. **Clinical Pearl:** Unilateral RLN injury causes hoarseness; bilateral RLN injury causes stridor and airway obstruction requiring urgent intervention.