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Regional anesthesia: Mech. of injury

Nerve damage tends to occur via two main mechanisms: pharmacologic toxicity and needle trauma. Both are uncommon and usually self-limited in nature. The risk of nerve injury from regional nerve blocks is less than that associated with neuraxial techniques and is often related to pain during needle placement or during injection of local anesthetic. It is unclear whether the use of ultrasound vs. nerve stimulation reduces the chance of nerve injury.

Paresthesias, which are shock-like sensations, can occur during the placement of regional nerve blocks and should not be deliberately produced. Local anesthetic should not be injected when the patient reports a persistent paresthesia as this could result in intraneural injection of local anesthetic and increased risk of permanent nerve injury. If damage to the perineurium is suspected from an unusually painful paresthesia, there should be no additional injection until the needle has been repositioned. The B-bevel tipped needle is more likely to “push” a nerve aside and is thought to be less likely to penetrate the nerve. However, when an injury does occur, it tends to be more severe.

Animal studies have linked high injection pressures to subsequent nerve injury but this has not been confirmed in humans. Research has also not demonstrated increased safety of one local anesthetic or additive over another. Patients with diseased or previously injured nerves (those with a history of diabetes, severe peripheral vascular disease or chemotherapy) may have an increased risk of nerve injury. Other risk factors for nerve injury include neural ischemia, traumatic injury to the nerves during needle placement, infection, and choice of local anesthetic solution. Case reports have discussed new or worsening neurologic deficits after regional blocks in patients with multiple sclerosis and it may be reasonable to avoid more potent local anesthetics or reduce the dose of local anesthetics in these patients. Most nerve injuries resolve completely within several days or weeks. If significant injury is suspected, a neurologic consultation with neurophysiologic testing, such as nerve conduction studies, may be in order.

Supraclavicular nerve blocks carry a small risk of pneumothorax (0.5-6%) and unintentional blockade of the recurrent laryngeal nerve, which can result in airway obstruction if the patient already has a vocal cord palsy on the opposite side. Given that the onset of symptoms is delayed (up to 24 hours), there is no role for routine chest x-ray. The phrenic nerve is blocked in 40-60% (or greater) of interscalene blocks due to its close location to the anterior scalene muscle and can result in a subjective sensation of shortness of breath. A phrenic nerve block can be decreased by using smaller volumes of local anesthetic or by blocking the brachial plexus at a lower level in the neck.

Hemorrhagic complications may also result from regional nerve blocks and can range from mild bruising to severe hematomas. Patient on anticoagulants are at the highest risk and placement of nerve blocks should be performed with caution, particularly in deep, noncompressible areas or where the hematoma could compress the airway (such as in the interscalene approach). Infection at the site of needle placement is an absolute contraindication to peripheral regional nerve blocks and caution should be used in patients with nearby cellulitis or bacteremia.

Severe systemic toxicity (seizures +/- cardiac arrest) occurs in roughly 1:1000 blocks and is typically due to intravascular injection or rapid systemic absorption of local anesthetic. Use of lipid infusions improves success during resuscitation from cardiac arrest.