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

Explanation

## Distinguishing RLN vs SLN Injury ### Anatomical Basis The recurrent laryngeal nerve (RLN) and superior laryngeal nerve (SLN) innervate different laryngeal muscles, resulting in distinct clinical presentations. ### Comparison Table | Feature | RLN Injury | SLN Injury | |---------|-----------|----------| | **Innervation** | All intrinsic laryngeal muscles except cricothyroid | Cricothyroid muscle only | | **Voice quality** | Hoarse, breathy, weak | High-pitched, fatigable, loss of projection | | **Pitch control** | Preserved initially | Loss of ability to raise pitch | | **Vocal cord position** | Paramedian (abducted) | Normal position | | **Aspiration** | Present (RLN supplies vocal cord closure) | Absent (cricothyroid alone cannot cause aspiration) | | **Cry in infants** | Weak cry | Normal cry | ### Key Point: **Loss of cricothyroid function with inability to raise pitch is the hallmark of SLN injury.** The cricothyroid muscle is responsible for vocal cord tension and pitch modulation. When damaged, patients cannot raise the pitch of their voice and experience voice fatigue. ### Clinical Pearl: **RLN injury causes hoarseness due to vocal cord paralysis in a paramedian position, preventing complete glottic closure.** SLN injury causes a high-pitched, weak voice with loss of vocal projection — the patient cannot shout or sing high notes. ### High-Yield: - **SLN injury = loss of pitch control** (cricothyroid paralysis) - **RLN injury = hoarseness + aspiration risk** (multiple intrinsic muscle paralysis) - SLN injury is often missed clinically because voice changes are subtle ### Mnemonic: **CRISP** — CRIcothyroid = Superior laryngeal nerve Pitch control