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Serratus anterior block
Key Points
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Serratus anterior block is indicated for anterolateral chest wall incisions, targeting the lateral and postcutaneous branches of upper and middle thoracic dermatomes.
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The block has been used for different thoracoscopic and open chest wall incisions as well as chest tube placements and rib fractures.
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The long thoracic nerve is included in the serratus anterior block.
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Injection of the local anesthetic either superficial or deep to the serratus anterior muscle leads to similar results clinically.
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The optimal point of injection should be posterior to the midaxillary line at the level of fifth or sixth ribs. This allows better identification of the muscle as well as better spread of the medication.
Relevant anatomy
The serratus anterior muscles originate from the anterior surface of the first to eighth ribs at the lateral chest wall and insert along the entire anterior length of the medial border of the scapula, as well as the ventral aspect of its inferior angle. It consists of multiple serrated tendinous projections connecting the ribs, with the costal margin of the scapula pulling the scapula forward around the chest (protraction), especially when pushing forwards. It also helps with upward rotation of the scapula, adding to the trapezius muscle action. The muscular serrations are thinner at its origin (anteriorly) and get bigger along its course posteriorly.
The serratus anterior receives its nerve supply from the long thoracic nerve, also called nerve to serratus anterior; it originates from the roots of the brachial plexus (C5–C7). Its injury results in “winging” of the scapula. It has multiple sensory and motor branches and typically runs in a course between the middle and posterior axillary lines.
Anterior to the anterior axillary line, the serratus anterior muscle is covered by the pectoralis muscles. Along its dorsal course starting at the level of the fifth rib, the serratus anterior is covered by the latissimus dorsi muscle. Accordingly, posterior to the midaxillary lines, there are two potential fascial planes: superficial and deep to the serratus muscle. Superficial to the serratus anterior, the fascial plane is bounded by the deep surface of the latissimus dorsi, and deep to the serratus the plane is bounded by the ribs and intercostal muscles.
The lateral and posterior cutaneous branches of T2–T9 intercostal nerves, along with the long thoracic nerve, travel across those two fascial planes: superficial and deep to the serratus anterior muscle posterior to the midaxillary line. Clinical results indicate that injection of local anesthetic along both planes results in similar outcomes; mainly blocking upper and midthoracic dermatomes of the lateral chest wall ( Fig. 31.1 ).
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