Kathryn B. Wulster

Background Increasing accessibility and advancements in computed tomographic (CT) imaging improve understanding of the contributors to poor performance in the Thoroughbred racehorse. Objectives To characterise an unreported site of tarsal subchondral bone injury (SBI) in Thoroughbred racehorses. Study design Retrospective, cross‐sectional analytical study. Methods Tarsal CT scans of 108 horses were reviewed for evidence of SBI in the dorsodistolateral calcaneus (DDLC). Signalment, including age, breed, sex, and discipline of horses, was recorded. The association of DDLC SBI with Thoroughbred racehorse compared to other breeds/disciplines was calculated. Nuclear scintigraphic scans of the hindlimbs of Thoroughbreds between 2007 and 2022 were also reviewed for increased radiopharmaceutical uptake (IRU) suggestive of DDLC SBI. Results Tarsal CT scans of 108 horses were analysed and lesion location and type were recorded. DDLC SBI was identified in 8/108 (7.3%) horses. All lesions were found in racing Thoroughbreds (Fischer's exact test p = 0.002) and in 20% (8/40) of racing Thoroughbreds that underwent tarsal CT. DDLC SBI was determined to be the primary cause of lameness in 3/8. Third tarsal bone fracture was considered the primary cause of lameness in 3/8 horses. A total of 1663 nuclear scintigraphic scans of 1603 Thoroughbred horses were reviewed. Increased IRU in the DDLC was present in 13 horses (0.9%). Main limitations Study limitations include the retrospective nature of the study, incomplete medical records, lack of sensitivity/specificity data for lesion identification on various imaging modalities or comparison to histopathology, and limited substantive follow‐up information. Conclusions The DDLC is a previously unreported site of SBI in the Thoroughbred racehorse. The lesion can be the primary source of lameness but may also be found in horses with other clinically relevant osseous injuries. This lesion may be underestimated in the Thoroughbred population. Further investigation is required to determine the true prevalence, clinical relevance, and prognosis in Thoroughbred racehorses with this injury.

Summary This clinical case report details the use of a radio‐guided parathyroidectomy technique using a hand‐held gamma detector and technetium Tc 99m sestamibi scintigraphy for detection and removal of a functional parathyroid adenoma in a horse. An 18‐year‐old Arabian gelding was diagnosed with primary hyperparathyroidism based on persistent hypercalcaemia and increased parathyroid hormone concentration. Single gland disease was suspected after technetium Tc 99m sestamibi scintigraphy showed focal increased radiopharmaceutical uptake (IRU) at the thoracic inlet. Ultrasonography failed to identify a discrete mass in the region of IRU. Surgical excision was recommended; however, due to unreliable preoperative localisation, a radio‐guided parathyroidectomy technique was elected. A radio‐guided parathyroidectomy technique was performed under general anaesthesia using a hand‐held gamma detector and technetium Tc 99m sestamibi scintigraphy to identify the parathyroid adenoma, which was subsequently excised. The horse recovered from the procedure without complication. Successful removal of the parathyroid adenoma was confirmed based on histopathological and serum bioch findings. Following the procedure, the horse showed no evidence of hyperparathyroidism and returned to light pleasure riding. Three years postoperatively, the gelding died of unrelated natural causes; a post‐mortem examination was not performed. This radio‐guided parathyroidectomy technique was effective in facilitating the removal of a parathyroid adenoma in this gelding and might be useful in other equids that have primary hyperparathyroidism IRU on technetium Tc 99m sestamibi scintigraphy, particularly if a discrete mass cannot be identified ultrasonographically.