To our knowledge, this is the first large study to describe the improvement pattern from the clinical signs of suspected HAL and the results show that the Meier severity scoring method, developed specifically for monitoring the signs of HAL, is suitable to evaluate laminitis improvement, not only in a research setting, but also in clinical studies. The strength of the Meier method is that it allows veterinarians to allocate a severity score over a wide range, between 0 to 12, according to individual clinical criteria that are present in the majority of HAL cases. A broader scoring system improves the ability to observe and monitor a change in scoring and hence clinical improvement and prognosis. In comparison, the strength of the Obel method appears to be in the diagnosis and monitoring of sepsis-related and supporting-limb laminitis, where its utility has been proven in laminitis induction studies [16, 17], with Obel grades showing a good correlation with the severity of histopathological damage to the lamellae [18]. The authors suggest the two scoring methods might be used according to the type of laminitis being studied, because although laminitis shares the common clinical sign of foot pain, it is acknowledged that there are differences in the clinical presentation, diagnosis, treatment, and management, depending on the cause [9, 18].
In the present study, laminitis improvement followed an exponential decay pattern in most of the horses. The initially rapid rate of improvement was due to the early resolution of particular clinical signs, especially digital pulses, weight shifting, and resistance to foot lift. This coincided with a period of dietary modification and forced confinement designed to lower insulin concentrations and minimize further damage to the foot, respectively. Together with the use of supportive pads or bandages, this initial management appears to play a key role in the rate of improvement during the initial phase of acute HAL. There seems to be scope, however, to increase the rate of improvement of the locomotor variables, as it took more than 6 times longer (25 d vs 4 d) for median scores to reach 0 for gait at the circle, compared with weight shifting, or resistance to a foot lift. Furthermore, by d 42 more than 25% of the horses were still not able to circle without showing some degree of lameness. The slow improvement in these signs is expected, as during the examination, locomotion would often elicit the most foot pain. In fact, in a recent study, the loading pattern and the kinetics of the hoof were still different in recovering horses 6 to 12 weeks after laminitis treatment began, compared with healthy controls [19].
Across the full cohort, our data are consistent with a previous survey of horse owners, where the median time to return to soundness was estimated to be 30 days, with an IQR of 14 to 91 days [20]. The large range of improvement rates reported in that survey could have been due to several factors, such as the inclusion of more severe laminitis cases, causes other than HAL, subjective lameness assessments made by the owners, and a greater variation in management. In contrast, in the present study the management during the improvement period was standardised and conformed to best practice. Nevertheless, as the present study demonstrates, improvement from laminitis is not an ‘all or nothing’ event, but a multi-faceted continuum. In this respect, the Meier method has an advantage by incorporating a wider range of possible scores and the ability to identify changes in specific clinical signs over time.
A second area in which there is clearly room for improvement, is in addressing the significant cohort of horses that did not show an exponential improvement, but which demonstrated clinical signs that waxed and waned over the monitoring period. This study demonstrated that variation in improvement pattern was not associated with initial severity of laminitis, but studies to investigate the physical and metabolic variables that may explain why some horses do not improve quickly are ongoing, with the hope that we can inform further intervention strategies. Meanwhile, these cases confirm that improvement from HAL can be slow and variable. Indeed, it should be acknowledged that if significant rotation of the distal phalanx occurs, a horse may never recover fully, and unless the underlying ID is addressed, the horse will continue to be at risk of further laminitis bouts throughout its life [21]. Determining the reasons for slow improvement and/or recurrence is particularly important, as these are associated with a high euthanasia rate [1]. A recent survey indicated that factors such as a previous history of laminitis and the degree of insulin dysregulation are predictive of laminitis recurrence [22], and further detailed investigations are currently underway to determine the association of these and other factors with improvement rates.
Our observation that weight shifting had a lower prevalence (64%) on the day of diagnosis than the locomotive criteria and digital pulse (98–100%), is similar to that seen in an earlier study [2]. This criterion could be confounded by the presence of a different observer, to whom the horse is not accustomed, and/or the removal of a horse from its environment during the examination. As such, it is recommended that stage 1 of the examination should occur prior to removing the horse from its stable or paddock. Nevertheless, there is evidence that stance is a good distinguisher of laminitis-associated lameness [13], and the inclusion of a clinical sign that resolves rapidly may help to differentiate horses that do not recover quickly, allowing early intervention. Similarly, resistance to lifting the forelimbs also provides useful information on the degree of foot pain and was present in a significant proportion of cases (88%) on d 0. This sign has previously been reported in a cross-sectional study to have a prevalence of 52.7% (CI: 48.6–56.8), which is consistent with our results [13].
The fact that 25 veterinarians were used in the present study may be seen as a limitation of the study as it introduced an additional source of variation. Conversely, as this represented the situation in practice, it could be argued that our findings are more representative of real-world circumstances as a result. Furthermore, we have also previously shown that both the Obel and Meier laminitis scoring methods have excellent inter-observer agreement [12]. It is important to note though, that all participating veterinarians received training in both scoring methods prior to the study, and this would not be the situation in normal practice.
Another potential limitation of the present study is the variation in case management beyond d 9, particularly in terms of diet, exercise and ancillary treatments. This was compensated for in part by the large sample size of 80 horses, and further by the fact that all pain medications were withdrawn 24 h prior to each examination. Nevertheless, this was an inherent limitation of this field study, which did not allow management conditions to be replicated exactly for the entire observation period.