Jennifer Taintor

Effect of anaesthesia of the palmar digital nerves on proximal interphalangeal joint pain in the horse

REASONS FOR PERFORMING STUDY Anaesthesia of the palmar digital nerves is claimed to attenuate lameness in some horses that are lame because of pain in the proximal interphalangeal (PIP) joint. OBJECTIVE To determine the response of horses with pain in the PIP joint to anaesthesia of the palmar digital nerves. METHODS Horses were video recorded trotting before and after induction of pain in the PIP joint and 10 mins after anaesthesia of the palmar digital nerves. The palmar digital nerves were anaesthetised 3 times at different sites, and the video recorded gaits were scored subjectively. RESULTS The median lameness score of gaits after administration of 2% mepivacaine 1 cm proximal to the cartilages of the foot was not significantly different from the median lameness score before anaesthesia of the palmar digital nerves (P > or = 0.05), although that of 1 of 6 horses improved markedly. The median lameness score was significantly (P < or = 0.05) improved after mepivacaine was administered 2 and 3 cm proximal to the cartilages of the foot. CONCLUSIONS The PIP joint is unlikely to be anaesthetised when the palmar digital nerves are anaesthetised at the proximal margin of the cartilages of the foot. POTENTIAL RELEVANCE Pain within the PIP joint cannot be excluded as a cause of lameness when lameness is attenuated by anaesthesia of the palmar digital nerves at any site proximal to the proximal margin of the cartilages of the foot.

Pharmacokinetics of amikacin in plasma and selected body fluids of healthy horses after a single intravenous dose

REASONS FOR PERFORMING STUDY No studies have determined the pharmacokinetics of low-dose amikacin in the mature horse. OBJECTIVES To determine if a single i.v. dose of amikacin (10 mg/kg bwt) will reach therapeutic concentrations in plasma, synovial, peritoneal and interstitial fluid of mature horses (n=6). METHODS Drug concentrations of amikacin were measured across time in mature horses (n=6); plasma, synovial, peritoneal and interstitial fluid were collected after a single i.v. dose of amikacin (10 mg/kg bwt). RESULTS The mean±s.d. of selected parameters were: extrapolated plasma concentration of amikacin at time zero 144±21.8 µg/ml; extrapolated plasma concentration for the elimination phase 67.8±7.44 µg/ml, area under the curve 139±34.0 µg*h/ml, elimination half-life 1.34±0.408 h, total body clearance 1.25±0.281 ml/min/kg bwt; and mean residence time (MRT) 1.81±0.561 h. At 24 h, the plasma concentration of amikacin for all horses was below the minimum detectable concentration for the assay. Selected parameters in synovial and peritoneal fluid were maximum concentration (Cmax) 19.7±7.14 µg/ml and 21.4±4.39 µg/ml and time to maximum concentration 65±12.2 min and 115±12.2 min, respectively. Amikacin in the interstitial fluid reached a mean peak concentration of 12.7±5.34 µg/ml and after 24 h the mean concentration was 3.31±1.69 µg/ml. Based on a minimal inhibitory concentration (MIC) of 4 µg/ml, the mean Cmax:MIC ratio was 16.9±1.80 in plasma, 4.95±1.78 in synovial fluid, 5.36±1.10 in peritoneal fluid and 3.18±1.33 in interstitial fluid. CONCLUSIONS Amikacin dosed at 10 mg/kg bwt i.v. once a day in mature horses is anticipated to be effective for treatment of infection caused by most Gram-negative bacteria. POTENTIAL RELEVANCE Low dose amikacin (10 mg/kg bwt) administered once a day in mature horses may be efficacious against susceptible microorganisms.

Function of the ramus communicans of the medial and lateral palmar nerves of the horse.

REASONS FOR PERFORMING STUDY The role of the communicating branch between the medial and lateral palmar nerves of horses (i.e. the ramus communicans) in conveying sensory impulses proximally should be determined to avoid errors in interpreting diagnostic anaesthesia of the palmar nerves. HYPOTHESIS Sensory nerve fibres in the ramus communicans of horses pass proximally from the lateral palmar nerve to merge with the medial palmar nerve, but not vice versa. OBJECTIVE To determine the direction of sensory impulses through the ramus communicans between lateral and medial palmar nerves. METHODS Pain in a thoracic foot was created with set-screw pressure applied to either the medial or lateral aspect of the sole of each forelimb of 6 horses. The palmar nerve on the side of the sole in which pain was created was anaesthetised proximal to the ramus communicans with local anaesthetic. Lameness was evaluated objectively by using a wireless, inertial, sensor-based, motion analysis system (Lameness Locator). Lameness was also evaluated subjectively by using a graded scoring system. Local anaesthetic was then administered adjacent to the ramus communicans to determine the effect of anaesthesia of the ramus communicans on residual lameness. RESULTS When pain originated from the medial or the lateral aspect of the sole, anaesthesia of the ipsilateral palmar nerve proximal to the ramus communicans did not entirely resolve lameness. Anaesthesia of the ramus communicans further attenuated or resolved lameness. CONCLUSIONS Sensory fibres pass in both directions in the ramus communicans to connect the medial and lateral palmar nerves. POTENTIAL RELEVANCE When administering a low palmar nerve block, both palmar nerves should be anaesthetised distal to the ramus communicans to avoid leaving nondesensitised sensory nerve fibres passing through this neural connection. Alternatively, local anaesthetic could also be deposited adjacent to the ramus communicans when anaesthetising the palmar nerves.

Comparison of 2 methods of centesis of the bursa of the biceps brachii tendon of horses.

REASONS FOR PERFORMING STUDY Centesis of the bicipital bursa using an 8.9 cm long spinal needle has been reported but the alternative of employing a 3.8 cm long hypodermic needle requires validation. OBJECTIVE To compare the efficacy of 2 different methods of centesis of the bicipital bursa and to evaluate the usefulness of ultrasonographic imaging to determine the location of solution administered when centesis of the bursa is attempted. METHODS For Trial 1, 6 clinicians, who had no previous experience of centesis of the bicipital bursa, attempted to inject a solution composed of an aqueous radiopaque contrast medium and physiological saline solution (PSS) into the bicipital bursae of 2/12 horses using the previously described distal approach to inject one bursa and a proximal approach to inject the contralateral bursa. The bicipital tendon and bursa were examined ultrasonographically before and after injection; and both shoulders were examined radiographically to identify the location of the medium. In Trial 2, another 6 clinicians, also with no previous experience of centesis, repeated Trial 1, using 6 horses, but the radiopaque contrast medium was mixed with air instead of PSS. RESULTS Accuracy of centesis using the proximal approach was 39% and that of the distal approach 28%. Ultrasonographic examination of the shoulder allowed the location of solution and air to be accurately predicted in all 12 shoulders examined. CONCLUSIONS Clinicians who have had no previous experience performing centesis of the bicipital bursa are unlikely to be successful in centesis using either approach. Radiographic examination after injecting a radiopaque contrast medium may be necessary to assess the success of centesis especially if bursal fluid is not obtained during centesis. Injecting air along with the radiopaque contrast medium provides more accurate ultrasonographic confirmation of centesis and better radiographic definition than does injection without air.