About Nerve Blocks

Regional nerve blockade, or more commonly nerve block, is a general term used to refer to the injection of local anesthetic onto or near nerves for temporary control of pain. It can also be used as a diagnostic tool to identify specific nerves as pain generators. Permanent nerve block can be produced by destruction of nerve tissue.

Therapeutic Uses

Nerve blocks can help humans and livestock who suffer from lower back pain, neck pain, sciatica (from a herniated disc), spinal stenosis, reflex sympathetic dystrophy (a complex regional pain syndrome), shingles, cancer, and painful peripheral vascular disease. It can also be used in the treatment of migraine headaches. Proper patient selection is important.

Methods

Temporary nerve blocks are achieved by combining a local anesthetic (such as lidocaine) with epinephrine, a steroid (corticosteroid), and/or opioids. Epinephrine produces constriction of the blood vessels which delays the diffusion of the anesthetic. Steroids can help to reduce inflammation. Opioids are painkillers. Injection nerve blocks can be either single treatments, multiple injections over a period of time, or continuous infusions.

Permanent nerve block can be effected using other drugs or methods including alcohol or phenol to selectively destroy nerve tissue, cryoanalgesia to freeze nerves, and Radiofrequency ablation to destroy nerve tissue using heat.

Nerve blocks are sterile procedures that are usually performed in an outpatient facility or hospital. The procedure can be performed with the help of ultrasound, fluoroscopy (a live X-ray) or CT. Use of any one of these imaging modalities enables the physician to view the placement of the needle. A probe positioning system can be used to hold the ultrasound transducer steady.

In addition, electrical stimulation can provide feedback on the proximity of the needle to the target nerve.

Complications

Nerve blocks, like other medical procedures, are not risk-free. There is a possibility of side effects and complications from the procedure, needle puncture, and medications used. The most critical factor in the efficacy of a nerve block is the proper location of the target nerve.

Complications include: Infection, allergy reactions and/increased pain

Short-term nerve blocks are useful in regionalizing the source of pain during lameness examinations in horses. A local anesthetic alone is injected perineurally to desensitize tissues beyond the site of injection. For example, the palmar digital nerves can be blocked to investigate pain arising from the sole and heel area of the hoof. If pain is arising from this area, the horse will stop limping. If the source of lameness arises from higher up in the leg, the horse will continue to limp, and a nerve block can be performed higher up the leg. Because there are essentially no motor neurons in the lower limbs of horses, the risk of weakness or ataxia is negligible.

When a veterinarian performs a lameness examination nerve blocks are often used to help in determining the location of the problem. The areas are "blocked" so that they become numb to pain, revealing which structures are involved in causing lameness.

Equine Lower Limb

• Two distinct peripheral nerves: sensory (afferent) and motor (efferent) nerves.
• Motor nerves: conduct information from brain/spinal cord to limbs. Causes limb movement.
• Sensory nerves: conduct pain, pressure and temperature information from limbs to brain/spinal cord.
• Sensory nerves originate in the hoof.
• Sensory nerves gather information.
• Sensory nerves conduct information.
• Use of analgesia can selectively block portions of the limb.
• By blocking specific sensory nerves vets can isolate the area causing the lameness in the horse.

Local Anesthetics

• Drugs used to block nerve impulse.
• Only block specific location where deposited.
• Mepivacaine and lidocaine are two drugs used in horses for nerve block.
• Fast acting, good efficacy, long duration of action, very few side affects.
• Injection site may swell.

How Is This Done?

• Minimal restraint is used.
• Hair is shaved at point of injection.
• Area is cleansed with an antiseptic like povidone-iodine scrub or isopropyl alcohol.
• Needle is inserted at specific location.
• Takes about 15 minutes for block to work.
• Vet will test to see if area is blocked by pricking area.
• No response by horse means area is blocked.

Four Major Nerve Block Areas

• Palmar Digital Nerve Block - Affects the heel bulbs, frog, bars, navicular bone and bursa, palmar aspect of the coffin bone and joint, and part of the second phalanx.
• Abaxial Sesamoid Nerve Block - Blocks everything below the fetlock joint. (Foot, second phalanx and pastern joint, and first phalanx)

Low Palmar Nerve Block

High Palmar Nerve Block or High Four-point Block

High Palmar Nerve Block at the Accessory Carpal Bone

Because the high four-point block does not anesthetize the top of the cannon bone or the origin of the suspensory ligament, two additional diagnostic nerve blocks were developed--the high suspensory block and the high palmar nerve block at the accessory carpal bone.

• Anesthetizes all structures distal to carpus.
• Anesthetizes origin of suspensory.
• Does not anesthetize the lower carpal joints.

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