Continuous digital hypothermia initiated after the onset of lameness prevents lamellar failure in the oligofructose laminitis model

Abstract

SummaryReasons for performing study: Prophylactic digital hypothermia reduces the severity of acute laminitis experimentally but there is no evidence for its efficacy as a treatment once lameness has already developed. Objectives: To investigate the therapeutic effects of digital hypothermia, applied after the onset of lameness, in an experimental acute laminitis model. Study design: Randomised, controlled (within subject), blinded, experimental trial. Methods: Eight Standardbred horses underwent laminitis induction using the oligofructose model. Once lameness was detected at the walk, one forelimb was continuously cooled (CRYO), with the other forelimb maintained at ambient temperature (NON-RX). Dorsal lamellar sections (proximal, middle and distal) harvested 36 h after the onset of lameness/initiation of cryotherapy were analysed by 2 blinded observers: laminitis pathology was scored . There was complete physical separation of lamellar dermis from epidermis (score of 4) in 4 of the NON-RX feet at one or more section level(s), which was not observed in any CRYO sections. Histomorphometry was thus limited to sections that remained intact; there was a trend of increased total (TELL) and secondary (SELL) epidermal lamellar length and decreased secondary epidermal lamellar width (SELW) in NON-RX limbs compared with CRYO at all 3 levels; differences were significant (P<0.05) for SELL and SELW in the distal sections. Conclusions: Digital hypothermia reduced the severity of lamellar injury and prevented lamellar structural failure (complete dermoepidermal separation) when initiated at the detection of lameness in an acute laminitis model. This study provides the first evidence to support the use of therapeutic digital hypothermia as a treatment for acute laminitis.

Effect of Delayed Digital Hypothermia on Lamellar Inflammatory Signaling in the Oligofructose Laminitis Model

“…As described in a previous publication by the same study subjects,13 8 Standardbred geldings were given an OF bolus (10 g/kg, maximum 4.2 kg) by nasogastric intubation, confined to stocks, and carefully monitored for the onset of lameness by gait evaluation, performed every 4 hours by 2 investigators and by pedometer counts. Pedometer counts were obtained to determine the amount of shifting back and forth between the forelimbs of the study subjects, an early sign of Obel Grade 1 (OG1) laminitis.…”

“…Hoof wall surface temperature was measured with hoof wall thermistors attached to data‐logging devices as described previously 16. At 36 hours after the development of OG2 laminitis (likely OG3 laminitis due to histologic changes observed),15 horses were euthanized and lamellar tissue samples were collected both from limbs that experienced digital hypothermia and limbs maintained at ambient temperature, as described previously 13. Tissue intended for gene expression analysis was immediately snap‐frozen in liquid nitrogen.…”

“…Initiation of cryotherapy at the time of oligofructose (OF) administration (before the onset of sepsis or lameness) has been linked to a marked decrease in expression of molecules associated with inflammatory signaling when compared to limbs maintained at ambient temperature 7. In a recent study that more closely simulated the time point at which hypothermia is initiated in many clinically affected horses, investigators did not initiate digital hypothermia until the study subjects exhibited lameness (OG2, Obel Grade 2) 13, 14. The first publication from this study established that, whereas the lamellae from the untreated limb had severe histologic changes consistent with Obel Grade 3–4 (OG3–4) lameness,15 there was minimal histologic evidence of lamellar injury in the limb treated by hypothermia 13.…”

“…In a recent study that more closely simulated the time point at which hypothermia is initiated in many clinically affected horses, investigators did not initiate digital hypothermia until the study subjects exhibited lameness (OG2, Obel Grade 2) 13, 14. The first publication from this study established that, whereas the lamellae from the untreated limb had severe histologic changes consistent with Obel Grade 3–4 (OG3–4) lameness,15 there was minimal histologic evidence of lamellar injury in the limb treated by hypothermia 13. This work confirms that digital hypothermia still has markedly protective effects even when initiated after the onset of lameness, but the mechanism of this lamellar protection at the later time point has yet to be elucidated.…”

“…Lamellar samples from the same study demonstrating the protective effect of hypothermia initiated at the onset of lameness were used to determine the gene expression of proinflammatory cytokines, chemokines, endothelial cell adhesion molecules, and COX‐2 13. As a result of finding a minimal effect of hypothermia on inflammatory signaling in this study, lamellar mRNA concentrations from the current study were compared to those of previous SRL studies in which lamellae were harvested at earlier time points to determine whether the lack of an anti‐inflammatory effect of hypothermia in this study was due to a lack of hypothermia‐induced inhibition of inflammatory mediators or simply the result of a generalized decrease in gene expression of inflammatory mediators in late‐stage laminitis 2, 4, 7…”