## Distinguishing RLN vs SLN Paralysis ### Anatomy & Innervation **Key Point:** The recurrent laryngeal nerve (RLN) innervates all intrinsic laryngeal muscles except the cricothyroid, while the superior laryngeal nerve (SLN) innervates only the cricothyroid muscle. ### Clinical Feature Comparison | Feature | RLN Paralysis | SLN Paralysis | |---------|---------------|---------------| | **Vocal cord position** | Paramedian (adducted) | Lateral (abducted) | | **Voice quality** | Breathy, weak, hoarse | Weak, fatigable, loss of pitch control | | **High-pitched sounds** | Preserved initially | **Lost or severely impaired** | | **Pitch control** | Maintained | Absent | | **Stridor** | Inspiratory (if bilateral) | Absent | | **Glottic closure** | Poor | Adequate | | **Cricothyroid function** | Normal | Paralyzed | ### Mechanism of Correct Answer **High-Yield:** The cricothyroid muscle (SLN-innervated) is responsible for vocal cord tension and pitch modulation. Paralysis of this muscle causes: - Loss of ability to produce high-pitched sounds - Inability to vary pitch (monotone voice) - Fatigue with prolonged phonation - Preserved but weak voice quality In contrast, RLN paralysis causes breathy voice due to incomplete glottic closure, but pitch control remains relatively intact because the cricothyroid is spared. **Clinical Pearl:** A patient with SLN paralysis complains of "voice fatigue" and inability to sing or speak loudly, whereas RLN paralysis presents with persistent hoarseness and breathy voice from the onset. ### Why Other Options Are Incorrect - **Loss of vocal cord abduction** → This occurs in RLN paralysis (cord stays paramedian), not SLN paralysis - **Inspiratory stridor** → Seen in bilateral RLN paralysis or severe RLN cases; not typical of SLN paralysis - **Complete voice loss with breathy quality** → Characteristic of RLN paralysis, not SLN paralysis [cite:Scott-Brown's Otolaryngology Ch 45] 
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