intercostal nerve

**Anatomy** - Origin: Anterior (ventral) rami of T1–T11. T12's ramus is the subcostal nerve. T1 largely joins the brachial plexus (via the T1 root), and only a small branch continues as the 1st intercostal nerve. T2 also gives the intercostobrachial nerve. - Course: Each nerve runs in the costal groove, between the internal intercostal and innermost intercostal muscles. Mnemonic: **VAN** (superior to inferior): Vein, Artery, Nerve. The neurovascular bundle lies along the inferior border of each rib. - Branches: (1) Rami communicantes to the sympathetic chain (white and gray). (2) Muscular branches to the intercostals, subcostals, transversus thoracis, serratus posterior, and levator costarum. (3) Lateral cutaneous branches, arising at the midaxillary line, supply the lateral thoracic and abdominal wall. (4) Anterior cutaneous branches supply the anterior chest and abdomen. (5) Collateral branches. (6) Pleural and peritoneal sensory branches. - Atypical intercostal nerves: T1 (mostly to the brachial plexus), T2 (intercostobrachial nerve to the medial arm and axilla), and T7–T11 (leave the intercostal space and continue onto the anterior abdominal wall as thoracoabdominal nerves, supplying the abdominal muscles, skin, and parietal peritoneum). T12 is the subcostal nerve.

**Function** - Motor: Intercostal muscles (respiration, trunk stability). The external intercostals elevate the ribs (inspiration), the internal intercostals depress them (forced expiration), and the innermost intercostals act similarly to the internal intercostals. - Sensory: Skin of the thorax and abdomen (dermatomes), parietal pleura (costal and peripheral diaphragmatic), and parietal peritoneum. - Sympathetic: Carried via the rami communicantes to the body wall (sweat glands, arrector pili, vessels).

**Dermatomal landmarks (high-yield)** - T4: nipple - T7: xiphoid process - T10: umbilicus - (L1: inguinal ligament/groin)

**Clinical correlations** - **Chest tube/thoracentesis:** Insert **above the upper border of the rib** (the 5th intercostal space, midaxillary line, "safety triangle" for chest tubes; the 7th–9th space, midscapular line, for thoracentesis) to avoid injuring the neurovascular bundle on the inferior rib border. - **Intercostal nerve block:** Used for rib fractures, post-thoracotomy pain, and herpes zoster/postherpetic neuralgia. Inject at the inferior rib border, posterior to the angle. - **Herpes zoster (shingles):** Reactivation of VZV in dorsal root ganglia commonly produces a unilateral dermatomal vesicular eruption in thoracic dermatomes. Postherpetic neuralgia can follow. - **Referred pain:** Pleural irritation (pneumonia, pleurisy) can refer pain to the abdomen via shared lower thoracic intercostal nerves (mimics an acute abdomen). Parietal pleura pain is sharp and localized (intercostal nerves), but central diaphragmatic pleura pain is referred to the shoulder via the phrenic nerve (C3–C5). - **Intercostal neuralgia/rib fracture:** Sharp, band-like thoracic pain, worse with breathing. - **Intercostobrachial nerve (T2):** Can be injured during axillary lymph node dissection (mastectomy), causing numbness of the medial upper arm and axilla. Also contributes to referred pain of angina (medial arm pain, along with T1). - **Thoracic outlet/Pancoast tumor:** An apical lung tumor can involve T1 and the sympathetic chain (Horner syndrome) and cause ulnar-distribution arm pain/weakness. - **Spinal cord level:** A T4 sensory level suggests nipple-line loss, and a T10 level suggests umbilical-level loss. Useful in spinal cord lesion localization. - **Breathing:** Intercostal paralysis (e.g., high thoracic cord injury) spares diaphragm function (C3–C5), so patients can still breathe by diaphragmatic breathing, with paradoxical chest movement.

**Do not confuse:** The phrenic nerve (C3–C5) supplies the diaphragm. Intercostal nerves do NOT innervate the diaphragm motor function, but they do supply sensation to the peripheral diaphragm.