Intercoastal nerve block. Clinical anatomy

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Intercostal Nerve Block - Clinical Anatomy

Overview of the Intercostal Nerves

The intercostal nerves are the anterior primary rami (ventral rami) of thoracic spinal nerves T1-T11. The 12th thoracic anterior ramus runs below the 12th rib as the subcostal nerve. Each nerve exits the spine through the intervertebral foramen, which lies midway between two adjacent ribs.

Upon exiting, each nerve immediately gives off:

Posterior (dorsal) cutaneous branch - supplies the skin and muscles of the paraspinal area
Lateral cutaneous branch - arises at the midaxillary line; supplies the anterior and posterior lateral chest wall
Anterior cutaneous branch - the terminal branch

Gray's Anatomy for Students, p. 197
Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 675

Neurovascular Bundle - The Key Anatomical Relationship

Each intercostal nerve travels with its corresponding intercostal artery and vein within the subcostal groove - a channel on the ventral (inferior) surface of each rib. The arrangement within the groove, from superior to inferior, is:

Position	Structure
Superior (most protected)	Intercostal Vein
Middle	Intercostal Artery
Inferior	Intercostal Nerve

The mnemonic VAN (Vein, Artery, Nerve) describes this top-to-bottom order. The vein and artery lie above the nerve and are therefore somewhat protected by the rib during a nerve block - but the nerve itself is the most inferiorly exposed structure.

Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 675

Muscular Layers of the Intercostal Space

The intercostal space is bound by three muscular layers:

External intercostal muscle - outermost; fibers run inferomedially
Internal intercostal muscle - middle layer
Innermost intercostal muscle - deepest; separated from the internal intercostal by the neurovascular bundle

The neurovascular bundle runs between the internal intercostal and innermost intercostal muscles, deep to the subcostal groove. The fasciae of the internal and external intercostal muscles form the walls of the intercostal groove.

From Miller's Anesthesia (10th ed.), the intercostal space is described as a triangular space with:

Medial border: posterior intercostal and innermost intercostal muscles, endothoracic fascia, and parietal pleura
Lateral border: internal and external intercostal muscles and intercostal membrane
Base: the lower rib

Deep to the innermost intercostal muscle lies the endothoracic fascia and then the parietal pleura - this is the critical structure to avoid.

Miller's Anesthesia, 10th ed., p. 11085
Barash Clinical Anesthesia, 9th ed., p. 2920

Why Block Position Matters

Intercostal nerve block - area of anesthesia (anterior and posterior views, T3-T12)

Cutaneous distribution of intercostal nerves T3-T12 - the shaded area shows the zone of anesthesia achieved (Roberts and Hedges)

The position of the block along the chest wall determines which branches are anesthetized:

Block Location	Branches Blocked	Clinical Effect
Paravertebral / angle of rib (5-7 cm from midline)	All branches (posterior cutaneous + lateral cutaneous + anterior cutaneous)	Full dermatomal anesthesia
Posterior axillary line	Lateral cutaneous + anterior cutaneous	Adequate for rib fractures; most practical with buffer from internal intercostals
Midaxillary line	Anterior cutaneous only (lateral branch already given off)	Incomplete - misses lateral chest wall
Anterior chest wall	Anterior cutaneous only	Very limited coverage

The nerve is blocked before the lateral cutaneous branch arises (i.e., proximal to the midaxillary line) to achieve complete dermatomal coverage. The posterior axillary line is the preferred site because the internal intercostal muscle here forms a buffer between the nerve and the pleura, reducing pneumothorax risk.

Roberts and Hedges', p. 675
Barash Clinical Anesthesia, 9th ed., p. 2990

Anatomical Changes Along the Rib

Posteriorly (near angle of rib, 5-7 cm from midline): The subcostal groove is well-defined and deep. The nerve is separated from the pleura only by the thin intercostal fascia here - careful technique required.
Anteriorly (beyond midaxillary line): The intercostal groove becomes progressively less well-defined. The nerves begin to move away from their protected position under the rib. This is why blocks are best performed posteriorly.
T1-T5 levels: These upper levels may be better approached via paravertebral block because the overlying scapula and bulky paraspinal musculature make intercostal access difficult.
Subcostal nerve (T12): Less closely associated with its rib, making it harder to identify and block with classic intercostal technique.
Barash Clinical Anesthesia, 9th ed., pp. 2990-2991

Ultrasound Anatomy

Ultrasound-guided intercostal nerve block - in-plane needle approach showing rib, pleura, and needle position

USG view: (1) needle contacts rib; (2) needle walks caudally off rib into the intercostal groove above the pleura (Miller's Anesthesia)

Under ultrasound, the key landmarks are:

The rib (hyperechoic curved structure with posterior acoustic shadow)
The parietal pleura (bright hyperechoic line below the rib, with sliding lung sign)
The neurovascular bundle in the subcostal groove just inferior to the rib

Clinical Applications

Indication	Levels Blocked
Rib fractures	Level of fractured rib ± 1-2 adjacent levels (for overlap)
Thoracotomy analgesia	Multiple levels covering incision
Cholecystectomy / upper abdominal surgery	T5-T12 (5-6 levels unilaterally)
Mastectomy	T2-T6
Chest tube insertion	Level of tube insertion
Percutaneous biliary drainage	T6-T10

Because adjacent intercostal nerves overlap significantly, the rib of interest plus the two ribs above and below must be blocked for reliable analgesia.

Complications (Anatomically Explained)

Pneumothorax (most serious): Needle penetrates the parietal pleura, especially when blocking posteriorly where only thin intercostal fascia separates nerve from pleura. True incidence is low (<0.1% per Moore; ~1.4% per Shanti et al.)
Intravascular injection: The intercostal artery and vein lie immediately above the nerve in the groove. Intercostal vessels have high systemic uptake - highest of any peripheral block site - making local anesthetic toxicity a real concern
Hemothorax: Laceration of intercostal vessels
Incomplete block: Overlapping dermatomal supply from adjacent nerves, or block placed too anteriorly (distal to lateral cutaneous branch)
Roberts and Hedges', p. 675-676
Gray's Anatomy for Students, p. 197

Summary: The intercostal nerve block targets the anterior primary ramus of each thoracic spinal nerve as it travels in the costal groove between the internal and innermost intercostal muscles, accompanied by the intercostal artery (above) and vein (most superior). The ideal injection site is at the posterior angle of the rib (5-7 cm lateral to midline) or at the posterior axillary line, where the nerve has already given off the posterior cutaneous branch but not yet the lateral cutaneous branch, and where the internal intercostal muscle provides a safety buffer between needle and pleura.

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