Achieving a definitive diagnosis of pain in the foot in the horse

Introduction

Reaching a diagnosis of general foot pain in the horse is usually straight forward; however, the bigger challenge is to reach a specific diagnosis of which structure or structures within the foot are causing the pain. The most specific diagnosis possible is important to develop a targeted treatment plan and to provide the client with a prognosis. Some conditions such as a foot abscess or injuries with clear external clinical abnormalities of the foot or pastern regions are easy to diagnose, compared to lameness without any definitive clinical exam findings apart from the lameness. This session will concentrate on lameness cases without any obvious clinical abnormalities which easily localise lameness to a particular area or structure within the foot.

Discerning pain in the foot from pain in the more proximal limb is usually straight forward; however, discerning the exact structure in the foot responsible for the pain can be more challenging. Diagnostic nerve blocks remain the gold standard for diagnosis; however, the perineural and intra-synovial anaesthetic techniques lack anatomic specificity so nerve blocks require careful interpretation.

History and clinical examination

Information regarding the lameness is important such as: suspected limb(s), duration and severity of lameness, any known traumatic incidents and any alterations in shoeing. Identification of the lame limb or limbs is essential, so evaluation of the horse in a straight line and in a circle on a hard and soft surface in each direction is recommended. The degree and character of the lameness provides important clues to the potential differential diagnoses. Moderate unilateral fore foot lameness would have a different list of differential diagnoses compared to a mild bilateral lameness.

Examination of the digit

A careful examination of the digit involves observation of the external structures, from the fetlock distally. Since the horn of the hoof capsule conceals many of the important structures of the equine digit, a clear knowledge of the underlying internal structures is essential so that any external abnormalities detected can be interpreted related to the potential dysfunction occurring within the foot. As two examples, swelling dorsally proximal to the coronary band would indicate effusion of the distal interphalangeal joint and an increased digital pulse would be indicative of an abnormality within the hoof. A sensible routine for clinical examination includes: observation of the horse’s stance, comparison of the right and left digits (anatomy, foot balance, shoeing), palpation of the pastern and coronary band regions with the limb weight-bearing and non-weight-bearing, palpation of the hoof capsule, examination of the solar surface of the foot and the application of hoof testers.

Diagnostic anaesthesia

The role of diagnostic anaesthesia cannot be underestimated in the diagnosis of foot lameness; however, the veterinary surgeon must remain aware of the potential for diffusion of the local anaesthetic to surrounding structure or proximal to the site of injection (Nagy et al. 2015). Excellent detailed reviews of the techniques for perineural and intra-synovial diagnostic techniques are available (Bassage and Ross 2011, Schumacher et al. 2013). In addition, the equine literature contains multiple articles that detail the potential diffusion distances and locations for the perineural and intra-synovial diagnostic nerve blocks of the equine digit (Schumacher et al. 2013, Pilsworth and Dyson 2015).

Perineural (local) anaesthesia

The palmar digital and abaxial sesamoid nerve blocks are the main perineural techniques used to diagnose general foot pain in the horse. These blocks should be performed with the smallest amount of local anaesthetic needed, in an effort to limit diffusion of the local anaesthetic proximally; however, some diffusion may still occur. In the majority of cases, a positive response the palmar digital nerve blocks (significant improvement or abolition of lameness) would indicate that pain associated with lameness was within the structures contained in the hoof capsule. When a positive response is found to the abaxial sesamoid nerve block, the lameness may also include structures of the palmar pastern region as well as the foot and the coffin joint. On occasion, these blocks can also desensitize the fetlock joint region, leading to a conclusion of foot pain when a problem is present proximal to that region.

Intra-synovial anaesthesia

Once the lameness is localised to the digit, intra-synovial diagnostic anaesthesia can help to further localise the lameness; however, these intra-synovial techniques are not easy to interpret. Diagnostic anaesthetic placed in distal the interphalangeal (DIP, coffin joint) can diffuse into the navicular bursa (NB), leading to desensitisation of both structures (DIPJ and navicular bursa). In addition, the nerves adjacent to the collateral sesamoidean ligaments may be in contact with the local anaesthetic, so desensitisation of the palmar/plantar extra-synovial structures may occur. Duration of time between placing intra-articular diagnostic anaesthesia in the DIPJ and observation of the horse could influence the conclusions, therefore the general recommendation is to reassess the lameness within 5 minutes following the block. A rapid response has been thought to indicate pain originating from the DIPJ.

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