According to Semon's law, in a slowly progressive unilateral recurrent laryngeal nerve palsy, the vocal cord comes to lie near the midline. This happens because:

• A Fibrosis of the cricoarytenoid joint fixes the cord in adduction
• B The contralateral cord crosses the midline to compensate
• C The cricothyroid muscle pulls the ipsilateral cord medially
• D The abductor fibres of the nerve are affected earlier than the adductor fibres ✓

Explanation

Semon's law states that in progressive lesions of the recurrent laryngeal nerve, the abductor fibres are paralysed before the adductor fibres, so the cord assumes a paramedian position while remaining mobile in adduction. The unopposed action of adductors keeps the cord near the midline, preserving a serviceable voice. Cricothyroid acts via the superior laryngeal nerve and tenses rather than medially displaces the cord, and joint fibrosis describes cricoarytenoid fixation, a different entity.

Reference: Dhingra's Diseases of Ear, Nose and Throat, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.