Differentiation of lameness diagnostic by a modified high 4 point nerve block

Abstract

The high 4-point block is an important diagnostic analgesia to localise pain in the proximal and mid metacarpal region. In most instances a simultaneous analgesia of the palmar nerves and the palmar metacarpal nerves is performed. However, the purpose of this study was to distinguish the painful region by a delayed anaesthesia of the palmar and metacarpal nerves. It was hypothesized that the modified high 4-point block is a more effective means to localize pain in the proximal and mid metacarpal region. In order to investigate this, the study was conducted in three parts (a-c). a) A retrospective analysis of data from horses with positive high 4-point block, b) An in vivo study to verify the diffusion characteristics of contrast medium at the two different localisations of the high 4-point block, c) An in vitro study on cadaveric limbs with a local dye anaesthetic solution to evaluate the diffusion at the locations of palmar nerve injection, a) Clinical and diagnostic imaging findings of 59 horses with positive analgesia of the palmar or palmar metacarpal nerves (modified high 4-point block) in the proximal metacarpal region were analysed. In all cases the lateral and medial palmar nerves were anaesthetized first with 2 ml of 2% Mepivacain solution (Scandicain®) on each side approximately 3 cm distal to the carpometacarpal joint. If this block did not improve the lameness, the lateral and medial palmar metacarpal nerves were anaesthesized 15 minutes later by applying 2 ml of 2% Mepivacain solution (Scandicain®) laterally and medially at the same level as before (representing the second step of the high 4-point block). In case of a positive response to the modified high 4-point block cases were worked up further using ultrasound and radiology. If analgesia of the medial and lateral palmar nerves was positive, the following diagnoses were confirmed: desmitis of the suspensory ligament branches, tendosynovitis of the digital flexor tendon sheath, desmitis of the accessory ligament of the deep digital flexor tendon, tendinitis of the superficial or deep digital flexor tendon. However, in 96% of the horses with a positive anaesthesia of the medial and lateral palmar metacarpal nerves, a desmitis at the origin of the suspensory ligament or an avulsion fracture in the proximal metacarpal region was stated as the cause of lameness, b) In the in vivo study 2 × 2 ml of radiopaque contrast medium were used to perform the high 4-point nerve block (palmar or palmar metacarpal nerves respectively) in both forelimbs of six nonlame horses. Lateromedial and dorsopalmar radiographs were obtained 0, 5, 10, 15, 20, 25 and 30 mins after injection and analysed subjectively and objectively. There was no significant diffusion after the specified timing from the medial and lateral palmar nerves site dorsally towards the palmar metacarpal nerves. c) The in vitro study was performed on cadaveric limbs to evaluate the diffusion of local dye anaesthetic solution in the region of the lateral and medial palmar nerves. The injection to the limbs was performed as in study b described above. 10 mins after the injection the cadaveric limbs were frozen at -20°C and subsequently cut transversally at one centimetre intervals starting at carpometacapal joint. The diffusion characteristics of the dye anaesthetic solution were analysed subjectively. There was no diffusion of the dye anaesthetic solution dorsally from the palmar nerves towards the metacarpal nerves. In conclusion, we could prove that local analgesia of the palmar nerves at the level of the high 4-point block does not diffuse towards the palmar metacarpal nerves. Therefore, a modified high 4-point block permits a differentiation between proximal suspensory desmitis and other soft tissue pathology in the proximal and mid metacarpal region.