thoracodorsal nerve

**Origin and course:** The thoracodorsal nerve (also called the middle subscapular nerve or nerve to latissimus dorsi) arises from the posterior cord of the brachial plexus, with root values C6, C7, C8 (mainly C7). It runs between the upper and lower subscapular nerves. It descends along the posterior axillary wall on the subscapularis, then travels with the thoracodorsal artery (a branch of the subscapular artery) and vein along the lateral border of the scapula to enter the deep surface of the latissimus dorsi.

**Function:** It provides motor innervation to the latissimus dorsi only. The latissimus dorsi adducts, extends, and medially rotates the humerus. It also helps depress the shoulder girdle and, with the arms fixed, pulls the body upward (climbing).

**Clinical significance:** - **Axillary lymph node dissection:** The nerve lies near the subscapular (posterior) group of axillary nodes. It is at risk during breast cancer surgery. It is often cited with the long thoracic nerve (serratus anterior, "winged scapula") and the intercostobrachial nerve (sensory loss of the medial arm and axilla). Surgeons try to preserve the thoracodorsal neurovascular bundle. - **Injury:** It causes weakness of arm adduction, extension, and medial rotation. Patients have difficulty with pull-ups, climbing, rowing, or rising from a seated position using the arms. The deficit is usually subtle because other muscles compensate (teres major, pectoralis major, triceps). - **Reconstruction:** The thoracodorsal pedicle supplies the latissimus dorsi flap used in breast reconstruction. If the nerve is sacrificed, the muscle atrophies, but the flap remains viable through the vascular pedicle. - **Other causes:** Posterior cord injuries (e.g., crutch use, shoulder dislocation) can affect it along with the axillary nerve, radial nerve, and the upper and lower subscapular nerves.

**Posterior cord branches (mnemonic "ULTRA"):** - **U**pper subscapular → subscapularis (upper part) - **L**ower subscapular → subscapularis (lower part) and teres major - **T**horacodorsal → latissimus dorsi - **R**adial - **A**xillary

**Testing:** Have the patient adduct the abducted arm against resistance, or cough while you palpate the inferior scapular border. Contraction of the latissimus dorsi should be felt.

**High-yield contrast:** Do not confuse it with the long thoracic nerve (C5–C7, from roots, serratus anterior, winged scapula), the dorsal scapular nerve (C5, rhomboids and levator scapulae), or the thoracodorsal artery (branch of the subscapular artery from the third part of the axillary artery).