Continuous digital hypothermia in the prevention and treatment of acute equine laminitis

s 68 23 28 8 2 7 Total relevant papers when duplicates removed 7 CONFLICT OF INTEREST The authors declare no conflict of interest.

All horses had retired from racing within the preceding 4 weeks, and were reported to be sound, with no gross or radiographic abnormalities of the feet.

Sample size: Eight horses
Intervention details: • All horses were stabled for 1 week prior to study and fed lucerne (alfalfa) hay • All horses were confined to stocks (a crush) and received ad libitum access to lucerne hay and water • 72 hour experimental model: o 24 hour control period initially, followed immediately by laminitis induction via euglycaemic hyperinsulinaemic clamp (EHC) model [intravenous bolus of insulin (45 mIU/kg bodyweight (bwt)] diluted in 50 ml of saline (0.9% Sodium Chloride (NaCl)) immediately followed by a continuous intravenous infusion of insulin [ o all horses received one dose of phenylbutazone (4 mg/kg bwt IV) between 52-58 hours when Obel Grade 1 laminitis (Obel, 1948) became clinically apparent • 5/8 horses received a second dose of phenylbutazone [8][9][10] hours later. Reasons for this were not given. • Histology, histomorphometry and immunohistochemistry o histology performed by single blinded veterinary pathologist on randomised sections from the proximal, middle and distal regions of the forefeet; histological severity of each section was graded using a previously published 0-3 scale (Pollitt, 1996) based on basement membrane (BM) separation from the lamellae in increasing severity: ▪ Grade 0: normal ▪ Grade 1: mild changes (BM lifted to form teat shaped bubbles) ▪ Grade 2: moderate changes (absence of BM at the tips of secondary dermal lamellae) ▪ Grade 3: severe and extensive changes (complete separation of primary epidermal and primary dermal lamellae) o histomorphometry performed by single blinded observer on randomised sections from the proximal, middle and distal regions and mean of 5 measurements of primary epidermal lamellae (PEL) length (measured from abaxial to axial margins) used for data analysis o immunohistochemical staining of middle region sections for the cellular proliferation marker targeting protein for xenopus kinesin-like protein 2 (TPX2), and TPX2-positive cell count performed by single blinded observer with the mean cell counts from both the abaxial and axial regions of 5 PEL used for data analysis Study design: Randomised, controlled (within subject), blinded, experimental Outcome studied: • Pedometer count of each forelimb independently • Hoof temperature monitored using hoof wall thermistors attached to data logging devices • Lameness evaluation via Obel grading system (Obel, 1948) 48 hours post EHC initiation by videoing each horse walking in a straight line for 20 metres, evaluated by a blinded clinician experienced in evaluation of lameness due to laminitis • Lateral radiographs of each foot to assess changes during study and between groups • Histology of dorsal hoof lamellae to measure lamellar length

Clinical parameters
• Heart rate significantly higher in Groups 2 and 3 vs Group 1 and significantly higher in Group 3 between 44-60 hours, vs Group 2 • Clinical signs of laminitis first noted in Group 3 at 24 hours (4/6), and 36 hours (2/6). No signs noted in Groups 1 and 2 Lameness: • Horses in Groups 1 and 2 were judged to be non-lame (by a single observer) at all time periods between 72 hours and 7 days. • Median Obel Grade lameness scores at 7 days were significantly less in Group 1 (range 0-1) and Group 2 (range 0-2), compared with Group 3 (range 2.5-4) • No significant difference in Obel lameness scores between Groups 1 and 2 Forelimb internal hoof temperature: • There was no significant difference between mean internal hoof temperatures of Group 2 (3.8â•° ±6â•°C) and Group 1 (3.9â•° ± 0.9â•°C) at any time point Radiographs: • Rotation of the distal phalanx relative to the dorsal hoof wall was not detected in any radiographs • A small but significant increase in the dorsal hoof wall to distal phalanx distance was noted in Group 3 at 7 days, compared to their baseline radiographs • The laminitis preventive effect of distal limb CDH lasted beyond the 72 hours of application

Main findings: (relevant to PICO question):
• Mean hoof temperature of CDH treated limbs (Group 1: 3.9° ± 9°C, Group 2: 3.8° ± 0.6°C) was significantly less than all the ambient limbs by 2 hours • Lameness evaluations revealed all horses were obviously lame in the ambient forelimb at walk. Lameness was not observed in the CDH limb of any horse Histology: • Histological scores of the ambient forefeet (median scores 1-3) were significantly greater than that of the CDH forefeet (median scores 0-0.5) (p<0.05) • Detachment of the BM from the secondary epidermal lamellae was present in all ambient feet except hind feet of one subject • Detachment of the BM was not seen in any of the CDH feet

Limitations:
• All horses had the left forelimb CDH treated • Lameness evaluation was not blinded • Different devices were used to measure temperature in forelimbs and hindlimbs • Only Standardbred horses included • Small sample size

Appraisal, application and reflection
Laminitis is a common problem affecting equids seen in practice ( The study by van Eps & Pollitt (2009) reported that CDH, when performed for 72 hours on six horses that had not undergone laminitis induction, did not produce any significant lameness or other ill-effects. The horses appeared to tolerate CDH well, although distal limb oedema was reported in all 18 CDH treated horses, which resolved by 7 days following cessation of treatment (van Eps & Pollitt, 2009). A hypothesis provided by the study's authors was that this was a consequence of the cryotherapy in conjunction with restricted ambulation for 72 hours. All other included studies reported using CDH for a minimum of 36 hours with no significant sideeffects, demonstrating that CDH is safe to use in horses. The main limitation to clinical application in this study is that CDH was initiated at the time of laminitis induction, before recognition of clinical laminitis. However, the results of the study indicate that overall the protective effects of CDH were similar to those noted in the OF studies, with a reduction in the severity of laminitis, both clinically and histologically (Stokes et al. 2019 As a retrospective analysis of clinical records, Kullmann et al. (2014) falls much lower on the hierarchy of evidence but was included here as it provides evidence of CDH being used in a clinically relevant scenario. In this case series, CDH was performed by submerging the limbs in ice just proximal to the metacarpophalangeal joint, as opposed to the level of the proximal metacarpus which was used in all other studies. There were many limitations to this study which must be borne in mind, particularly surrounding the recording and standardisation of why CDH treatment was initiated, how the cases were selected for CDH treatment, the lack of information regarding when CDH treatment was commenced, and whether CDH continued in any cases beyond the minimum 48 hours specified. The criteria stated that horses admitted to the hospital with acute or chronic laminitis or diagnosed with laminitis at the time of admission were excluded, so an assumption is made that CDH was started after they had arrived at hospital -similar to the timing used in the experimental study by van Eps et al. (2014). The authors note however that lameness at time of admission may have been masked in some cases by the administration of analgesics by the referring vet (Kullmann et al. 2014).
The histopathology results from all the studies must be used to try to extrapolate clinical outcome given that all of the horses in the experimental studies were euthanised before clinical outcome could be assessed. All studies where histological examination was undertaken (Stokes et Pollitt, 2004) reported significant reduction in histological scores for laminitic feet treated with CDH compared to untreated laminitic feet. As histological changes due to laminitis are assumed to be non-reversible and the CDH reduced histological lesion progression, the significant difference observed due to the CDH may subsequently improve clinical outcome. Clinical significance of the different histology gradings is still unknown and a major limitation to these studies is the severity of laminitis caused by the induction models. In many natural cases, the onset of laminitis is insidious and may not be spotted until well after the time period where CDH was initiated in these studies. All of the horses studied also had no history of laminitis and it would be interesting to compare effects of CDH on horses with pre-existing chronic laminitis.
The study by van Eps & Pollitt (2009) provides the best evidence that the protective effects of CDH in acute laminitis continue after the CDH is discontinued. Horses were subjected to 72 hours of CDH immediately following laminitis induction before being euthanised 7 days after induction. Results of blinded Obel lameness grading just prior to euthanasia showed no significant difference in control horses in Group 1 (no laminitis induction but CDH performed) and horses in Group 2 (laminitis induction and CDH). However, Obel lameness scores in Group 3 (laminitis induction but no CDH) were significantly higher than in either Group 1 or 2. Given that one limb was encased in a cumbersome ice boot, the subjective data may be subject to significant bias but the objective pedometer data may prove useful for further research into clinically relevant time periods to initiate CDH. Reduced frequency of weight shifting was also reported in CDH treated limbs compared to ambient temperature limbs in the two other experimental studies where this was measured objectively using pedometers (Dern et  Given the PICO question related to CDH and supportive treatment compared to supportive treatment alone, it should be noted that there are other forms of supportive treatment in the management of laminitis than NSAIDs. Commonly foot supports or corrective foot trimming, deep bedding, dietary management, and other pharmaceuticals such as paracetamol or acepromazine are often employed in the treatment of laminitis. Due to the experimental nature of six of the studies, including these variables within the studies would have been difficult to achieve but this should be borne in mind when thinking about the applicability to general clinical practice.
Further research is required to determine whether the conclusions from the experimental studies included in this Knowledge Summary are applicable to all equidae and underlying causes of laminitis. It is reasonable to assume that these findings would translate to other large size breeds but future work needs to be done to evaluate the effectiveness of CDH in clinical situations, in equids other than Standardbred horses such as ponies or donkeys, and on the effectiveness in animals with a previous history of laminitis, or a history of equine metabolic syndrome and/or pituitary pars intermedia dysfunction. Prospective cohort studies of clinical cases utilising strict inclusion criteria could help evaluate application of CDH for treatment of acute laminitis in clinical practice.
In conclusion, there is moderate evidence demonstrating that CDH does reduce the severity and/or frequency of clinical signs of pain, such as weight shifting and lameness in experimentally induced laminitis in Standardbred horses. Histological examination of the lamellae in cases of experimentally induced laminitis demonstrates that CDH reduces epithelial inflammatory events and protects against lamellar separation, especially if initiated before clinical signs are apparent. There is weak evidence to show that CDH provides protective effects in true clinical situations and a lack of evidence to show it improves clinical outcome compared to supportive treatment alone.

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