A clinical approach to the diagnostic puzzle of misleading foot cases

The diagnostic challenge of cases with foot pain is mainly attributable to problems in the interpretation of clinical signs, and issues associated with diagnostic anesthesia.

Foot pain remains one of the main issues encountered in cases presented not only for lameness but also for poor performance; the clinical signs can vary from moderate to severe unilateral lameness to subtle unilateral or very commonly bilateral or even quadrupedal problems. The latter cases are most often presented for stiffness or unwillingness to work rather than lameness. It is not uncommon for a horse with severe navicular disease to be presented with a suspicion of laminitis; besides other clinical signs like digital pulsation that may allow differentiation, careful observation may reveal toe-landing in cases with navicular problems, in contrast with the characteristic increase in heel loading or pronounced heel landing in laminitis cases.

Toinvestigate cases with symmetrical lameness, it can be very helpful to include lungeing and ridden evaluation. Recently, a whole horse ridden ethogram has been developed, describing 24 behavioural markers reflecting musculoskeletal pain, and this system was applied consistently by veterinarians with differentiation between nonlame and most lame horses (Dyson et al., 2019a,b). It can be extremely helpful to apply unilateral local anesthesia to transform a ‘hidden’ bilateral problem into a unilateral lameness, thereby revealing the actual degree of pain. In normal, sound horses, unilateral perineural anesthesia does not result in relevant alterations in limb loading and hoof balance(Vande Water et al., 2016). Care must be taken not to misinterpret the natural asymmetry during lungeing, which may not be the same in both directions (Rhodin et al., 2016), and the asymmetry created by the rider during the rising trot, mimicking a push-off lameness in the hindlimb that is in stance when the rider sits down in the saddle (Persson-Sjodin et al., 2018).

The interpretation of diagnostic anesthesia of the foot can be complicated, especially in cases with multiple-limb lameness and/or incomplete resolution of lameness after diagnostic anesthesia. Repeating the blocking procedure on another occasion (the following day) or using a different order of techniques can be very rewarding. However, expectation bias after the execution of diagnostic anesthesia occurs (Arkell et al., 2006), and the inherent limitations in temporal and spatial resolution of the human eye may lead to erroneous conclusions. Using three-dimensional horse animations presenting with 0–60 per cent movement asymmetry, Starke and Oosterlinck (2018) showed that years of experience and exposure to a low, medium or high case load had no significant effect on correct assessment of lame or sound horses, with the exception of a significant effect of case load exposure on forelimb lameness assessment at 60 per cent asymmetry. The correct classification of sound horses as sound was significantly higher for forelimb (average 72% correct) than for hindlimb lameness assessment (average 28% correct): participants often saw hindlimb lameness where there was none. For subtle lameness, errors often resulted from not noticing forelimb lameness and from classifying the incorrect limb as lame for hindlimb lameness (Starke and Oosterlinck, 2018). [...]