A singer is finding difficulty raising the pitch of her voice. On examination, her vocal cords were depressed and bowing. Which of the following muscles is involved?

Explanation

## Correct Answer: A. Cricothyroid The **cricothyroid muscle** is the sole intrinsic laryngeal muscle responsible for pitch control. It is the only muscle innervated by the **external branch of the superior laryngeal nerve** (a branch of the vagus), while all other intrinsic muscles are innervated by the recurrent laryngeal nerve. The cricothyroid muscle lengthens and tenses the vocal cords by tilting the thyroid cartilage anteriorly on the cricoid cartilage, increasing vocal cord tension and raising pitch. When the cricothyroid is paralyzed (as in this case), the vocal cords cannot be adequately tensioned, resulting in **depressed and bowing vocal cords**—a characteristic finding. The patient loses the ability to raise pitch because pitch is directly proportional to vocal cord tension. This is a classic presentation of **superior laryngeal nerve (SLN) palsy**, which may occur with thyroid surgery, carotid artery pathology, or skull base lesions. The depressed position reflects loss of the anterior tilting action, and bowing occurs because the cords sag without adequate tension. In Indian clinical practice, SLN injury is most commonly iatrogenic during thyroid surgery, particularly when the superior pole vessels are ligated carelessly. ## Why the other options are wrong **B. Lateral arytenoid** — The lateral arytenoid muscle **adducts the vocal cords** (brings them together) and is innervated by the recurrent laryngeal nerve. Its paralysis causes cord abduction and breathy voice, not depression and bowing. It has no role in pitch control or cord tensioning, making it incorrect for this pitch-raising difficulty. **C. Posterior cricoarytenoid** — The posterior cricoarytenoid is the **sole abductor of vocal cords** (innervated by recurrent laryngeal nerve) and is essential for breathing. Its paralysis causes stridor and airway compromise, not depressed/bowing cords. It does not control pitch and is not involved in cord tensioning. **D. Interarytenoid** — The interarytenoid muscle (both transverse and oblique parts) **adducts the vocal cords** and is innervated by the recurrent laryngeal nerve. Its paralysis affects voice quality and phonation but does not cause the characteristic depression and bowing seen with loss of cord tensioning. It plays no role in pitch elevation. ## High-Yield Facts - **Cricothyroid** is the only intrinsic laryngeal muscle innervated by the **external superior laryngeal nerve** (not recurrent laryngeal nerve). - **Pitch elevation** requires vocal cord tensioning, which is exclusively the function of the cricothyroid muscle. - **Depressed and bowing vocal cords** are pathognomonic for **superior laryngeal nerve (SLN) palsy**, not recurrent laryngeal nerve injury. - **Cricothyroid paralysis** causes loss of pitch control (inability to raise pitch) while preserving basic phonation and breathing. - **Thyroid surgery** is the most common cause of iatrogenic SLN injury in Indian clinical practice, especially with careless ligation of superior pole vessels. ## Mnemonics **SLN vs RLN Innervation** **SLN = Cricothyroid only** | **RLN = All other intrinsic muscles (PCA, LCA, IA, TA)**. Remember: SLN is the **exception**—it innervates only one muscle (cricothyroid), while RLN innervates the rest. **Pitch Control = Tension = Cricothyroid** **Pitch ∝ Tension**. Only cricothyroid lengthens and tenses cords. Loss of pitch = cricothyroid problem. Use this when you see 'difficulty raising pitch' in the stem. ## NBE Trap NBE often tests whether students confuse **SLN palsy (depressed/bowing cords, pitch loss)** with **RLN palsy (abducted cords, breathy voice, stridor)**. The "depressed and bowing" descriptor is the key discriminator—students who only remember "vocal cord paralysis" may pick the wrong muscle. ## Clinical Pearl A singer or professional voice user presenting with loss of pitch control and depressed cords should raise suspicion for **recent thyroid surgery or neck manipulation**. In Indian practice, this is a common post-thyroidectomy complication that may be missed if the patient is not specifically asked about pitch changes. Early recognition allows for voice therapy and possible nerve exploration before permanent fibrosis occurs. _Reference: Bailey & Love Ch. 38 (Larynx); Harrison Ch. 29 (Cranial Nerves); Robbins Ch. 16 (Nervous System)_