Nerve block

Nerve block or regional nerve blockade is any deliberate interruption of signals traveling along a nerve, often for the purpose of pain relief. Local anesthetic nerve block is a short-term block, usually lasting hours or days, involving the injection of an anesthetic, a corticosteroid, and other agents onto or near a nerve. Neurolytic block, the deliberate temporary degeneration of nerve fibers through the application of chemicals, heat, or freezing, produces a block that may persist for weeks, months, or indefinitely. Neurectomy, the cutting through or removal of a nerve or a section of a nerve, usually produces a permanent block. Because neurectomy of a sensory nerve is often followed, months later, by the emergence of new, more intense pain, sensory nerve neurectomy is rarely performed.
The concept of nerve block sometimes includes central nerve block, which includes epidural and spinal anaesthesia.

Local anesthetic nerve block

Local anesthetic nerve block is a short-term nerve block involving the injection of local anesthetic as close to the nerve as possible for pain relief. The local anesthetic bathes the nerve and numbs the area of the body that is supplied by that nerve. The goal of the nerve block is to prevent pain by blocking the transmission of pain signals from the affected area. Local anesthetic is often combined with other drugs to potentiate or prolong the analgesia produced by the nerve block. These adjuvants may include epinephrine, corticosteroids, opioids, or ketamine. These blocks can be either single treatments, multiple injections over a period of time, or continuous infusions. A continuous peripheral nerve block can be introduced into a limb undergoing surgery – for example, a femoral nerve block to prevent pain in knee replacement.

Uses

Regional analgesia

Regional blocks can be used for procedural anesthesia, post-operative analgesia, and treatment of acute pain in the emergency room. Consequently they can be an alternative to general anesthesia as well as oral pain medications. An advantage over oral pain medications is that regional blocks can provide complete relief of pain along a nerve distribution. This can lead to a reduction in the amount of opiates needed. Advantages over general anesthesia include faster recovery and less need for monitoring.

Diagnostic blocks

Nerve blocks can be used for the diagnosis of surgically treatable chronic pain, such as nerve compression syndrome. Advances in surgical techniques such as minimally invasive surgery have made virtually all peripheral nerves surgically accessible since the invention of open surgery. Any nerve that can be blocked can now be treated with a nerve decompression. Imaging such as MRI has poor correlation with clinical diagnosis of nerve entrapment as well as intraoperative findings of decompression surgeries and so diagnostic blocks are used for surgical planning.

Technique

Local anesthetic nerve blocks are sterile procedures usually performed in an outpatient facility or hospital. The procedure can be performed with the help of ultrasound, fluoroscopy, CT, or MRI/MRN to guide the practitioner in the placement of the needle. The various imaging modalities differ in their availability, cost, spatial resolution, soft tissue resolution, bone resolution, radiation exposure, accuracy, real-time imaging capabilities, and ability to visualize small or deep nerves.

Landmark-guided peripheral nerve block

Landmark-guided nerve blocks use palpable anatomical landmarks and a working knowledge of the superficial and deep anatomy to determine where to place the needle. Although a peripheral nerve stimulator can be used to facilitate placement of the block, it is designed to elicit a motor response rather than creating a paresthesia, making it less effective for identifying purely sensory nerves. Landmark-guided injections have largely been replaced with image guidance due to increased accuracy, but there are some nerves for which landmark-guidance still has comparable accuracy, such as the pudendal nerve.

Fluoroscopy-guided peripheral nerve block

is an imaging technique that uses X-rays to obtain real-time moving planar images of the interior of an object. In this sense, fluoroscopy is a continuous x-ray. Fluoroscopy is broadly similar to landmark-guided injections except that the landmarks are based on radiographic anatomy. However, there is poor soft tissue contrast, meaning that nerves cannot be clearly visualized. Nerves that are situated by bony landmarks can be good candidates, such as epidural steroid injections, which target the spinal nerves.
The radiation involved is higher than an x-ray, but lower than a CT-guided injection. One study found about 0.40 mSv exposure per minute of fluoroscopy for up to 3 minutes and another found that 3711 epidural injections took a maximum of 47 seconds.

Ultrasound-guided peripheral nerve block

-guided peripheral nerve block is a procedure that allows real-time imaging of the positions of the targeted nerve, needle, and surrounding vasculature and other anatomical structures. This visual aid increases the success rate of the block and may reduce the risk of complications. It may also reduce the amount of local anesthetic required, while reducing the onset time of blocks. Ultrasound has also resulted in an exponential rise in fascial plane blocks. Ultrasound is particularly well-suited for regional anesthesia, since many of the anesthesia targets have large blood vessels that travel with the target nerves. Direct visualization of nerves is not just important for localization, but also to ensure that the injected material surrounds the nerve. Likewise, visualization of blood vessels is important to ensure that needle placement avoids blood vessels, which often travel directly parallel to nerves.
Ultrasound machine is generally portable and inexpensive in comparison to the CT scanner, fluoroscopic machine, and MRI scanner. The relatively low cost of an ultrasound machine compared to other imaging machines allows for its widespread availability.
Ultrasound has a few limitations. First an acoustic window is required, and certain tissue types such as bone can interfere with image acquisition. Next hand-operated probe can make the images challenging for surgical planning when the exact needle location must be known. CT and MRI have standard slicing orientations, but for ultrasound the orientation of the 2D image depends on the position and orientation of a probe held by the operator. Lastly ultrasound has a tradeoff between penetration depth and resolution. Higher frequencies provide better resolution but have less penetration depth. You may be able to acquire good resolution at shallow depths or see deep structures only with poor resolution. The limited penetration depth and resolution tends to make ultrasound a poor choice in particular for deeply situated pelvic nerves.

CT-guided peripheral nerve block

The use of CT guidance is largely predicated on the limitations of lower cost image-guided injections such as fluoroscopy and ultrasound, as well as the cost considerations and availability of more precise imaging such as MRI-guidance.
CT provides excellent spatial resolution and good soft-tissue contrast. This makes it easy to verify the anatomic level. While the use of CT does expose the patient to radiation, the amount of radiation is less than a full scan. For example, the radiation from a lumbar spine CT is approximately 7.5 mSv, but the radiation from standard protocols for CT-guided epidurals is about 1.3-1.5 mSv. A low-dose CT protocol may still provide the required resolution, and if used can reduce the radiation exposure by another 85%, bringing the radiation exposure to about 0.2 mSv.
The machine cost of CT is a barrier to availability and more widespread use, though still more cost-effective than an MRI. CT machine costs can range from $415,000 to $615,000 USD.

MRI-guided peripheral nerve block

provides excellent visualization of soft tissues, but the detail is not usually enough to see the small nerves that are often entrapped. Newer technology, however, has increased the level of nerve details seen and allowed for more accurate MRI-directed injections. The ability to visualize nerves is important for localization and also for ensuring that injected material properly surrounds the nerve. The good soft tissue contrast also makes it easier to avoid injuring other tissue structures such as blood vessels and in the case of pelvic injections, the large intestines.
MRN-guided blocks are especially effective for deep, small nerves which are otherwise difficult to visualize with ultrasound and CT. The use of radiation-free MRI complies with ALARP practice mandate and can be a better choice for radiation-sensitive patients such as children and pregnant women. However, due to the expense of MRI machines, MRN-guidance is not a substitute for other imaging modalities but rather a specialized tool which higher accuracy is required. The cost of an MRI machine limits more widespread use and is significant, at about $1,000,000 USD per Tesla. Often a 1.5T machine with a wide bore will be used, but a 3T machine should provide the highest resolution.

Common local anesthetics

Local anesthetics are broken down into two categories: ester-linked and amide-linked. The esters include benzocaine, procaine, tetracaine, and chloroprocaine. The amides include lidocaine, mepivacaine, prilocaine, bupivacaine, ropivacaine, and levobupivacaine. Chloroprocaine is a short-acting drug, lidocaine and mepivacaine are intermediate duration, and bupivacaine, levobupivacaine, and ropivacaine are long-acting. Drugs commonly used for peripheral nerve blocks include lidocaine, ropivacaine, bupivacaine, and mepivacaine.

Mechanism of action

Local anesthetics act on the voltage-gated sodium channels that conduct electrical impulses and mediate fast depolarization along nerves. Local anesthetics also act on potassium channels, but they block sodium channels more.
Lidocaine preferentially binds to the inactivated state of voltage-gated sodium channels, but has also been found to bind potassium channels, G protein-coupled receptors, NMDA receptors, and calcium channels in vitro. The duration of the block is mostly influenced by the amount of time the anesthetic is near the nerve. Lipid solubility, blood flow in the tissue, and presence of vasoconstrictors with the anesthetic all play a role in this. A higher lipid solubility makes the anesthetic more potent and have a longer duration of action; however, it also increases the toxicity of the drug.