|This article needs additional citations for verification. (April 2008)|
Regional nerve blockade, or more commonly nerve block, is the interruption of signals traveling along a nerve, usually for the purpose of pain relief. A local anesthetic nerve block is a short-term block, usually lasting hours or days, involving the injection of an anesthetic and other drugs such as steroids onto or near a nerve. A neurolytic block (deliberate temporary damage to nerve fibers) and neurectomy (cutting through or removal of a nerve or a section of a nerve) produce blocks that may persist for weeks, months or indefinitely. Because neurectomy is often followed, months later, by the emergence of new, more intense pain, it is rarely used.
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. 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.
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.
Digital nerve block
Digital nerve blocks involve infiltrating local anesthesia around the two nerves that supply a digit. Historically this has been done with a local anesthetic without epinephrine however the use of a solution with epinephrine appears to be safe.
The use of peripheral nerve blockade offers several advantages when compared to general anesthesia or local anesthesia:
- The patient can remain awake and breathing on his/her own, thus protecting himself/herself from aspiration of stomach contents into the lungs. By avoiding general anesthesia, patients with adverse reactions to general anesthetics can be successfully treated. Similarly, patients who experience nuisance side effects from general anesthesia such as nausea and vomiting or excessive sleepiness can minimize these symptoms.
- There is no need to perform a tracheal intubation, with its attendant complications.
- The sympathetic nerves to the operative limb are anesthetized, leading to vasodilation. This improves blood flow to the affected limb and makes microvascular surgical procedures technically easier.
- The limb can remain numb for several hours after surgery, providing excellent pain relief.
- Deep and superficial structures of the limb are similarly anesthetized, allowing extensive surgical exploration and correction to occur. This is in contrast to locally injected local anesthetics, which tend only to numb superficial structures in the immediate vicinity of the injection.
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.
- Allergic reactions
- Increased pain*
-In some cases of a badly compressed nerve, particularly of the sciatic nerve, the nerve block process itself can be very painful. The injection of the medicine near the nerve root temporarily compresses it before dispersing, this compression can momentarily cause severe radicular pain until the anesthetizing agent takes effect.
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. When the painful area is blocked, the lameness should temporarily improve or resolve. 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.
Palmar digital nerve block - The palmar digital nerves (formerly known as the posterior digital nerves) run down the back of the pastern, one on each side of the deep flexor tendon. These nerves provide feeling to the back third of the horse's foot including:
- sensitive tissues of the hoof wall, sole, and frog
- coronet and heel bulbs
- lower half of the pastern (back part only)
- deeper structures of the foot (digital cushion, back third of the coffin bone, lateral cartilages, navicular bone and bursa, deep flexor tendon and its sheath)
This block is one technique used to diagnose navicular disease because the navicular bone and associated structures are typically desensitized though other structures desensitized by the block may be contributing to the lameness.
Low palmar nerve block - The low palmar (also known as the abaxial sesamoidian) block desensitizes the palmar nerves. Blocking both the medial and lateral palmar nerves near the base of the sesamoid bones desensitizes the entire foot and at least half of the pastern. Blocking above the sesamoid bones may desensitize part of the fetlock and sesamoid bones with variable success.
4-point nerve block - The 4-point nerve block involves the two palmar nerves and the palmar metacarpal nerves. Blocking these four nerves desensitizes the foot, pastern, and entire fetlock joint, including the sesamoid bones.
Other nerve blocks - Nerves above the knee or hock may be blocked to desensitize these joints and the entire lower leg though these blocks are more difficult to perform. Due to the large area that is desensitized these blocks are not very helpful in localizing lameness and may cause the horse to stumble.
The effectiveness of a nerve block is tested by checking the skin for loss of sensation. This is usually done with a pointed but dull object such as the tip of a pen. Some veterinarians use a needle though this may cause bleeding as well as unnecessary pain and irritation if the site is not completely blocked. Another nerve block must be performed if skin sensation has not been lost. Block failure is usually due to not enough anesthetic being absorbed which may be influenced by the following factors:
- injection too far away from the nerve
- not enough anesthetic injected for the size of the nerve
- testing the skin too soon after injection
Most commonly used local anesthetic drugs take 10-15 minutes to desensitize a nerve. The block may last 1-2 hours depending on the product used and volume injected.
- UCSD. Regional anesthesia
- Chowdhry S, Seidenstricker L, Cooney DS, Hazani R, Wilhelmi BJ (December 2010). "Do not use epinephrine in digital blocks: myth or truth? Part II. A retrospective review of 1111 cases". Plast. Reconstr. Surg. 126 (6): 2031–4. doi:10.1097/PRS.0b013e3181f44486. PMID 20697319.
- Bassage II, Lance; Ross MW (2003). "Diagnostic Analgesia". In Ross MW, Dyson SJ. Diagnosis and Management of Lameness in the Horse. St. Louis, MO: Saunders. pp. 93–124. ISBN 0-7216-8342-8.
- King, Christine, BVSc, MACVSc, and Mansmann, Richard, VDM, PhD. (1997). "Lameness Evaulation". Equine Lameness. Equine Research, Inc. pp. 32.
- Bassage II Diagnosis and Management of Lameness in the Horse p. 93–124.
- King Equine Lameness p. 33.
- King Equine Lameness p. 33.
- King Equine Lameness p. 33-34.
- King Equine Lameness p. 34.
- King Equine Lameness p. 34.
- King Equine Lameness p. 35.
- King Equine Lameness p. 36.
- Video demonstrating a Continuous Nerve Block Technique in Orthopaedics
- Video demonstrating ultrasound-guided interscalene nerve block using a probe positioning system