The horse with acute manifestation of classic midmetacarpal SDFT tendinopathy will have variable lameness, depending on lesion severity, and overt lameness may disappear rapidly. The horse should be observed for local swelling characteristic of a “bowed” tendon (Figure 190-2, A). The palmar structures of the metacarpus should be palpated both during weight bearing and with the limb unloaded. Horses with an active injury often resent palpation of the affected region, which may feel soft compared with normal tendon. More chronic lesions may be nonpainful and firm on palpation. A complete lameness examination is warranted because SDFT tendinopathy is often bilateral or may be a secondary overstrain injury arising secondary to lameness of a different cause in another limb. Diagnostic analgesia may be performed if confounding lameness exists, but information from the history, clinical signs, and ultrasound examination is often sufficient for diagnosis of tendinopathy.

Ultrasonography is the primary imaging modality for diagnosis and monitoring of tendon lesions. The palmar or plantar aspect of the distal part of the limb should be clipped, cleaned if necessary, and imaged with a 7.5- to 14-MHz linear probe with a standoff pad. The typical findings of acute tendinopathy include a hypoechoic core lesion with loss of linear fiber pattern, increased cross-sectional area of the tendon, and change in shape of the tendon (Figure 190-2, B). Lesions along the margins of the tendon are also possible. A tendon that is healing appropriately will have an increase in lesion echogenicity, improvement in linear fiber pattern, and decrease in tendon cross-sectional area. The timing of the first ultrasound evaluation and impact of lesion expansion on the ultrasonographic appearance of acute tendon lesions must always be considered. Initially, core lesions may not be evident but will appear several days later. Ultrasound examinations performed within the first week after injury should be repeated in 2 to 4 weeks to allow full assessment of lesion size and severity.

Standardbreds may have SDFT tendinopathy as a part of the “curb” complex of plantar hock soft tissue injury. An atypical form of proximal SDFT tendinopathy was recently described. The lesions were detected in older horses performing lower-level work; horses had an acute onset of lameness, with prolonged lameness thereafter. Horses reacted positively to carpal flexion, and lameness was abated following ulnar nerve block. Diagnosis was confirmed by ultrasonography of the SDFT at the level of the carpal canal, and effusion within the carpal sheath was evident. Prognosis for return to previous use was poor.