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The question asks about the most dangerous nerve injury in thyroid surgery, and the correct answer shown is (A) Bilateral RLN Partial.
Here is why this is the correct and most dangerous scenario:
Recurrent Laryngeal Nerve (RLN) Injuries - Comparison
| Type | Vocal Cord Position | Airway | Voice |
|---|---|---|---|
| Unilateral partial RLN | Cord moves to midline | Airway adequate | Hoarse voice |
| Unilateral complete RLN | Cord fixed (paramedian) | Airway adequate | Hoarse voice |
| Bilateral complete RLN | Both cords fixed (paramedian/lateral) | Airway adequate (cords are abducted/paramedian) | Aphonia |
| Bilateral partial RLN | Both cords pulled to midline | ACUTE AIRWAY OBSTRUCTION | Voice may be preserved |
Why Bilateral Partial RLN is Most Dangerous (Semon's/Simons' Law)
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In partial RLN injury, only the abductor fibers (to the posterior cricoarytenoid muscle - the only abductor of the vocal cord) are paralyzed.
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The adductor fibers remain intact, so both vocal cords are pulled to the midline.
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Bilateral midline cord position = complete airway obstruction, requiring emergency tracheostomy.
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In bilateral complete RLN injury, ALL fibers are paralyzed - both abductors AND adductors fail, leaving cords in a paramedian/cadaveric position with some airway preserved.
Clinical Pearl
Simons' (Semon's) Law: Abductor fibers of the RLN are more susceptible to injury than adductor fibers. Therefore partial injury = adduction (cords meet at midline) = danger.
This is why bilateral partial RLN injury is considered the most dangerous - it causes acute life-threatening airway obstruction, whereas bilateral complete injury, though severe, leaves a gap between the cords.
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