Lameness Examination

Lameness Examination


Diagnostic Local Anesthesia



Basic Information image



Synonym(s)




Nerve block


Joint block



Overview and Goal(s)




• Diagnostic local anesthesia is often performed to localize lameness. Important considerations are:


image Understanding the anatomy to provide accurate placement of the anesthetic

image Being aware of different options for administering anesthetic to a site as clinical situations may dictate one approach over another

image Maintaining strict asepsis when penetrating synovial spaces

image Always using a fresh bottle of local anesthetic

• Technique


image The most painful portion of the injection is needle penetration of the skin and joint capsule.

image Confident, quick insertion of the needle in the correct anatomical site minimizes discomfort for the horse.

image A skin bleb of local anesthetic may be helpful in making the joint injection less uncomfortable, particularly when a larger-gauge needle must be used.

• The sequence of diagnostic local anesthesia:


image Begins with the most distal nerves being injected first and gradually working proximally until the lameness has been localized.

image Regional anesthesia usually results in less effective analgesia of intra-articular soreness than direct intra-articular anesthesia.

image If the lameness has been improved noticeably, but not completely, following regional nerve anesthesia, intra-articular anesthesia of the suspect joint should be conducted.


Indications


Localization of lameness



Contraindications


Do not inject through a potentially contaminated site such as a wound or a region of cellulitis.



Equipment, Anesthesia




• Mepivacaine HCl 2% (Carbocaine-V, Winthrop Laboratories) is the preferred local anesthetic for regional and joint anesthesia in horses.


image It is less irritating to tissues and has a comparable onset yet a longer duration of action than does lidocaine.

• Local anesthetics cause blockade of sodium channels that results in inhibition of nerve conduction. The effectiveness of local anesthesia depends on local tissue pH (local anesthetics are much less effective in acid pH), accuracy of deposition, and the size of nerves being blocked (small, unmyelinated fibers are more sensitive than large, myelinated fibers).


• The toxic dose of local anesthetic is approximately 13 mg/kg (~6 mg/lb) and may result in heart block, bradycardia, and convulsions. Toxicity is usually only a concern in smaller animals.


• Onset of action for regional nerve blocks is 10 to 25 minutes with smaller diameter nerves desensitized earlier than larger nerves.


• Onset of intra-articular analgesia is 5 to 10 minutes with a gradual increase in analgesia over that time.


• Check the effects of the local anesthetic in most joints at 20 minutes and in complex joints such as the stifle in 30 minutes. Mepivacaine inhibits nerve sensation for 90 to 180 minutes.



Preparation: Important Checkpoints




• The site for local peripheral nerve anesthesia does not need to be clipped and is prepared by wiping with 70% isopropyl alcohol until clean.


• The site for intra-articular anesthesia may be clipped, but clipping the site has not been found to improve results of skin surface bacteriologic cultures.


image A 7- to 10-minute scrub with povidone-iodine or chlorhexidine should be made with the injection site being carefully wiped with 70% isopropyl alcohol immediately prior to injection.

image The operator should wear sterile surgical gloves.

image Adequate assistance should be available for restraining the horse.


Possible Complications and Common Errors to be Avoided




• Local swelling at the injection site resolves following bandaging.


• Joint sepsis is unlikely but may occur due to a break in aseptic technique.



Procedure



Regional Diagnostic Anesthesia



• See Tables 1 and 2 for forelimb and hindlimb nerve blocks.


Intra-Articular Anesthesia



Navicular bursa


• Strict asepsis must be adhered to particularly at this site.

• A subcutaneous bleb of local anesthetic should be placed on the palmar aspect of the distal pastern in the mid-sagittal cleft immediately proximal to the heel bulbs.

• An 18-gauge, 3-inch (1.2 × 90 mm) spinal needle is advanced until it contacts the flexor surface of the navicular bone.

• The needle should be aimed at a point 1 cm distal to the coronary band and midway between its most dorsal and palmar extent.

• Correct placement of the needle should be confirmed with radiography or ultrasonography.

• 3 to 5 mL of local anesthetic is injected.

image Comparing the results of navicular bursa, distal interphalangeal joint and palmar digital nerve local anesthesia may aid localization of distal limb lameness. However, mepivacaine has been found to diffuse widely following injection in the distal equine limb.

Distal interphalangeal (DIP) joint


• The DIP joint has a large dorsal pouch that is easily entered with a needle.

• Anesthetic injected in the DIP joint will desensitize other nearby structures.

• Dorsal approach: This injection is best made with the horse fully weight-bearing using a 20- to 18-gauge, 1.5-inch (0.9–1.2 × 40 mm) needle.

image The needle is inserted 1.5 to 2 cm proximal to the coronary band and a like distance lateral or medial to the extensor process of the distal phalanx and directed toward the extensor process.

image A skin bleb may be made at this site to facilitate needle placement.

image Alternatively, the needle may be directed perpendicular to the dorsal cortex of the second phalanx and walked distally until the joint is entered.

image The needle usually penetrates the skin approximately 1 inch (25 mm) before entering the joint.

image 4 to 6 mL of anesthetic is injected.

image Hemorrhage often occurs from the coronary corium when the needle is withdrawn. Counter pressure and a light bandage will control any bleeding and help protect the region from contamination until the needle puncture seals.

• Lateral approaches: Two dorsolateral approaches have been described with the needle inserted either cranial or proximal to the collateral cartilage.

image The most cranial extent of the collateral cartilage is palpated and the needle is inserted at that point approximately 1 to 2 cm proximal to the coronary band with the horse fully weight bearing. The needle is first inserted parallel to the ground then immediately angled 30 degrees distally.

image Needle placement proximal to the collateral cartilage may be performed with the horse non–weight bearing. The needle is inserted just proximal to the collateral cartilage midway between the dorsal and palmar aspect of the second phalanx and is directed along the palmar surface of the second phalanx by aiming at the center of the frog.

Proximal interphalangeal (PIP) joint


• A dorsolateral or palmar approach may be used to enter the PIP joint.

• Dorsal approach: The horse should be weight bearing for this approach. A 20-gauge, 1.5-inch (0.9 × 40 mm) needle is directed through the skin approximately 1 to 2 cm distal to the distal eminence of the first phalanx and under the extensor tendon. Orient the needle parallel with the dorsal joint surface. Inject 4 to 5 mL of local anesthetic.
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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Lameness Examination

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