Digital Sheath Tenosynovitis
by Carol Gillis, DVM, PhD,DACVSMR
The digital sheath is a complex synovial structure which surrounds the superficial (SDF) and deep (DDF) flexor tendons from proximal to the fetlock joint distally to mid-pastern. Normally, the sheath contains a small amount of synovial fluid, which serves to promote gliding of the flexor tendons around the palmar/plantar aspect of the fetlock joint. With severe inflammation the sheath can become greatly distended, reaching to mid-metacarpus proximally, and/or ”herniating” palmar to the SDF tendon on midline. Digital sheath swelling is frequently dismissed as a blemish rather than a lameness issue. In fact, in more than 20% of cases with swelling, lameness will eventually occur and can be career ending. Early diagnosis using ultrasound, followed by appropriate therapy, can avoid future performance problems.
Diagnosis Clinical signs of digital sheath synovitis reflect the degree of inflammation, which can be categorized into 3 stages. It is not uncommon for athletic horses to present with swelling (Stage 1 synovitis) of unilateral or bilateral forelimb or hind limb digital sheaths. Often, sheath distension decreases following exercise. The distension is fluidly fluctuant on palpation. The patient is sound, non-painful or minimally painful to sheath palpation, and mildly to moderately positive to fetlock flexion. Ultrasonographic examination reveals a moderate amount of fluid in the affected sheath(s), with no evidence of synovial proliferation or adhesions between the tendons and sheath walls. It is important to fully evaluate a horse presented at this stage to determine the cause of digital sheath distension, particularly if only one limb is affected as this strongly indicates overload from lameness elsewhere. Foot balance can also be a contributory factor, in particular uneven heel height between paired limbs or long-toed, low-heeled foot shape.