Different layers of intercostal muscle. (Reprinted with permission from Philip Peng Educational Series)
The intercostal nerves are made up of twelve pairs of thoracic spinal nerves (T1–T12) and pass through the intervertebral foramina where they are divided into the ventral and dorsal rami. The ventral rami of T1–T11 form the intercostal nerves entering the intercostal spaces (Fig. 5.1). The ventral ramus of T12 forms the subcostal nerve that is located inferior to the corresponding 12th rib. The dorsal rami of T1–T12 travel posteriorly and innervate sensation to the skin, muscles, and bones of the back.
Each intercostal nerve is associated with an artery and vein of the corresponding level (Fig. 5.2). The aorta derives the intercostal arteries, while the azygous and hemiazygous veins derive the intercostal veins. The intercostal nerve courses inferior to both the vein and artery of the same segment.
The intercostal nerves are composed of dorsal horn sensory fibers, ventral horn motor efferent fibers, and postganglionic sympathetic nerves. The intercostal nerves divide into the anterior and lateral cutaneous branches that innervate the skin and intercostal muscles of each individual segment (Fig. 5.3). It is important to note that there is variable collateral innervation of the adjacent segment; therefore, it is necessary to block both the level above and below the desired level when performing an intercostal nerve block.
The subcostal nerve is a unique T12 intercostal nerve; it does not run as closely to the intercostal groove of its accompanying rib as other intercostal nerves do. The nerve supplies the lower abdominal wall and is not closely associated with the 12th rib. Instead, the subcostal nerve makes up the anterior ramus of spinal nerve T12 and is approximately 3 mm in diameter. It passes over the iliac crest by joining the ventral ramus of L1, forming the iliohypogastric, ilioinguinal, and genitofemoral nerves. The remaining portion transects the transverse abdominis muscle and lies between that and the internal oblique muscle.
Indications for intercostal nerve blocks
Intercostal nerve block mechanism
Diagnostic nerve block
Initial single-injection block to determine efficacy of temporary pain relief. A subsequent neurolytic block, cryoablation, radiofrequency ablation, or chemical neurolysis is then performed to relieve pain for an extended period.
Chest wall surgery
Relieves pain after upper abdominal or flank surgery
Chest wall trauma
Controls pain resulting from fractured ribs and other chest wall trauma
Shingles or postherpetic neuralgia
Acute herpes zoster infection results in inflammation of the intercostal nerves and dorsal root ganglion.
Manages pain associated with chest wall tumors, nerve entrapment syndromes, thoracic spine pain, and intercostal neuralgia
The only absolute contraindications to intercostal nerve blocks are patient refusal of the procedure and active infection over the site of injection.
Extra caution must be taken in patients to whom a pneumothorax may be a life-threatening complication. These patients include those that have respiratory decompensation, single lung on the side of planned procedure, mechanically ventilated with positive-pressure ventilation, prior nerve injury, or neuromuscular disease involving area to be injected or postsurgical patients.
General relative contraindications include patients with blood dyscrasias, local or uncontrolled systemic infection, or unknown anatomical changes (i.e., unknown rib resection).
Contraindications to intercostal nerve block
Patient refusal or active infection over site of injection
Extra caution/special consideration
Respiratory decompensation, single lung on side of planned procedure, positive-pressure ventilation, postsurgical patients, prior nerve injury, neuromuscular disease involving area to be injected
Blood dyscrasias, local or uncontrolled systemic infection, unknown anatomical changes (unknown rib resection), allergy to local anesthetics, prior nerve injury or damage, inability of patient to consent to procedure, on anticoagulants or diagnosed with coagulopathy
The advantages of intercostal nerve blocks are associated with decreased need for parenteral or oral opioids. Decreasing use of oral and/or parental opioids may reduce the incidence of nausea, vomiting, urinary retention, itching, and hypotension. In thoracic surgery, the use of intercostal nerve blocks can lead to improved respiratory function in FEV1 and peak expiratory flow rate.
While complication rates vary, performing intercostal nerve block with ultrasound guidance may reduce some of the associated risks. These risks include bleeding along the path of the needle, hematoma, infection, toxicity to injectate (local anesthetic), pneumothorax, nerve damage, spinal cord injury due to epidural or spinal injection or uptake of alcohol from a neurolytic alcohol injection, and muscle trauma.
Patient Position: Prone; lateral decubitus, or sitting
Probe: Linear high frequency
It is important to know your anatomic landmarks. By palpating the spine of the scapula or the inferior edge of the scapula, the physician can then infer spinous levels T4 and T7, respectively. However, T1 spinous process may be more reliably palpated (as compared to C7 spinous process which moves with rotation and flexion of the cervical spine), and thus thoracic vertebral levels can be more accurately determined.
The ultrasound probe is placed in the longitudinal axis parallel to the posterior midclavicular line, optimally at the angle of the rib. This approach allows for views of two ribs in a cross-sectional view (Fig. 5.4a, b). Note the orientation of the probe to align with the orientation of the internal intercostal muscle. With this view, one can identify external intercostal (EI) and internal intercostal (II) muscles.