## Most Common Cause of Vocal Cord Paralysis **Key Point:** Recurrent laryngeal nerve (RLN) injury is the most common cause of vocal cord paralysis worldwide, accounting for 20–40% of all cases. In this clinical context (post-thyroid surgery), it is the overwhelmingly dominant cause. ### Mechanism of RLN Injury 1. **Anatomical course**: The RLN ascends obliquely on the left side, passing beneath the aortic arch; on the right, it loops under the subclavian artery. Both paths place it at risk during neck surgery. 2. **Intraoperative injury**: Traction, ligation, thermal injury, or direct transection during thyroid, parathyroid, or cardiac surgery. 3. **Postoperative neuropraxia**: Swelling, haematoma, or scar tissue compressing the nerve. ### Clinical Presentation of RLN Paralysis - **Unilateral RLN injury**: Hoarseness, weak voice, reduced vocal range, weak cough (vocal cord in paramedian or lateral position). - **Bilateral RLN injury**: Stridor, dyspnoea, weak cry (life-threatening; cords in midline). ### Why RLN Injury Dominates in Surgical Settings | Feature | RLN Injury | SLN Injury | |---------|-----------|----------| | **Incidence post-thyroid surgery** | 0.3–3% | 5–60% (but often subclinical) | | **Vocal cord position** | Paramedian (abducted) | Lateral (minimal visible change) | | **Voice change** | Hoarseness, weakness | Fatigue, loss of pitch control | | **Swallowing** | Normal | Normal | **High-Yield:** Post-thyroid surgery vocal cord paralysis = RLN injury until proven otherwise. The paramedian position of the cord is pathognomonic for RLN injury. **Clinical Pearl:** RLN injury may be immediate (intraoperative trauma) or delayed (weeks to months from scar formation). Many cases recover spontaneously within 3–6 months due to neuropraxia rather than transection. ### Other Causes of Vocal Cord Paralysis (Less Common) - **Idiopathic**: 25–50% (diagnosis of exclusion). - **Malignancy**: Lung, thyroid, oesophageal cancer compressing vagus or RLN (10–20%). - **Neurological**: Stroke, Parkinson's, myasthenia gravis (rare). - **Trauma**: Blunt neck injury, intubation injury. - **Infection**: Syphilis, tuberculosis, Lyme disease (rare in India). **Mnemonic: CHIME** — Carcinoma, Hypothyroidism, Idiopathic, Myasthenia, Endocrine (thyroid) — but this is for idiopathic paralysis; in a post-surgical patient, RLN injury dominates. [cite:Robbins and Cotran Pathologic Basis of Disease 10e Ch 16; Harrison's Principles of Internal Medicine 21e Ch 47]
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