We have observed focal skeletal muscle uptake of 99mTechnetium-hydroxymethylene diphosphonate (Tc-HDP), which could mimic a tibial lesion, in horses following peroneal nerve blocks. To characterize this observation further, 45 bone phase scintigrams were performed in 12 horses undergoing peroneal nerve blocks. Scans were performed before, and 1, 3, 7, and 14 days postblock. The superficial and deep branches of the peroneal nerve were blocked by injecting 10 ml of 2% mepivacaine in one limb and 20 ml in the other. Images were evaluated for uptake at the block site and uptake likely to mimic a tibial lesion. Regions of interest were placed over the block site and distal tibia. Count density ratios were used to estimate change in uptake intensity over time. The overall proportion affected was 0.52 (95% confidence interval [CI], 0.36–0.68; P<0.001) 1 day postblock and 0.24 (95% CI, 0.13–0.40; P=0.005) 3 days postblock. The overall proportion likely to mimic a tibial lesion was 0.19 (95% CI, 0.09–0.33; P<0.001) 1 day postblock and 0.21 (95% CI, 0.09–0.40; P=0.005) 3 days postblock. Focal skeletal muscle uptake was seen in only one horse 7 days postblock. Increased uptake intensity was associated with higher local anesthetic dose (P=0.042). Peroneal nerve blocks cause focal skeletal muscle uptake of 99mTc-HDP on bone phase scintigraphy. This occurs in approximately 50% of blocked limbs and can mimic a tibial lesion on the lateral view in approximately 20% of blocked limbs.
Veterinary Radiology & Ultrasound