Thyroarytenoid Muscle (Answer: B)
The thyroarytenoid muscle is most likely involved in causing vocal cord immobility in this patient with a laryngeal mass, as it forms the bulk of the vocal cord itself and is the primary muscle affected when a mass directly invades or compresses the vocal cord structures.
Anatomical Reasoning
The thyroarytenoid muscle comprises the body of the vocal cord and is innervated by the recurrent laryngeal nerve (RLN). 1, 2 When a mass directly involves the vocal cords as described in this case, the thyroarytenoid muscle is the most anatomically relevant structure affected because:
- It constitutes the vocal cord tissue itself - any mass "involving the vocal cords" would directly infiltrate or compress this muscle 1
- The RLN innervates all intrinsic laryngeal muscles except the cricothyroid - including the thyroarytenoid, lateral cricoarytenoid, interarytenoid, and posterior cricoarytenoid muscles 1, 2, 3
- Direct tumor invasion causes mechanical immobility through muscle infiltration, not just neural compromise 1
Why Not the Other Options
Posterior Cricoarytenoid (Option C)
While the posterior cricoarytenoid (PCA) muscle is the sole abductor of the vocal cords and would show atrophy in RLN palsy 4, it is located posteriorly in the larynx, not within the vocal cord itself. A mass described as "involving the vocal cords" would not primarily affect this muscle unless there was extensive posterior extension. 4
Cricothyroid (Option A)
The cricothyroid muscle is innervated by the external branch of the superior laryngeal nerve, not the RLN 1. It functions primarily in vocal cord tensing and pitch control, not adduction/abduction. 1
Interarytenoid (Option D)
The interarytenoid muscle functions in vocal cord adduction but is located between the arytenoid cartilages posteriorly, not within the vocal cord body itself. 1
Clinical Context: Mass Effect vs. Neural Injury
This patient's presentation suggests laryngeal carcinoma given the chronic smoking history, hoarseness, and weight loss. 2 The guideline literature distinguishes between:
- Vocal fold paralysis/paresis: neurological impairment from RLN or muscular dysfunction 1
- Vocal fold immobility: when the basis is unknown or results from mechanical limitations such as bulk effect of cancer or joint pathology 1
In this case with a visible mass on laryngoscopy, the immobility likely represents both mechanical invasion of the thyroarytenoid muscle and potential RLN involvement by tumor. 1, 2
Key Pitfall to Avoid
Do not assume all vocal cord immobility is due to isolated RLN injury affecting the PCA muscle. 1 When a mass directly involves the vocal cords, the thyroarytenoid muscle—which forms the vocal cord body—is the primary structure affected through direct tumor infiltration, making it the most likely answer. 1, 2