Fibrotic myopathy is a degenerative disorder in dogs and cats characterized by replacement of muscle tissue with dense fibrous connective tissue. It is chronic and progressive, affecting the semitendinosus, gracilis, quadriceps, infraspinatus, and supraspinatus muscles. The gracilis muscle is affected most often (86% of cases), either alone or in combination with other muscles.
Fibrotic myopathy occurs primarily in dogs, especially adult male German Shepherd Dogs (approximately 81% of cases). The cause is unknown; however, a prospective study suggests a genetic contribution to developing the disease (1). Age of onset ranges from 8 months to 9 years (mean, 5 years).
Typically, the only clinical sign is nonpainful pelvic limb lameness that is more obvious at a trot than at a walk. The lameness is characterized by an internal rotation of the stifle joint and external rotation of the hock as the limb is advanced, because the fibrous muscle prevents full extension of the pelvic limb during ambulation (see ). Affected muscle tissue may be visibly abnormal and palpably firm and hypertrophied. In approximately 50% of canine cases, there is bilateral involvement (1).
Diagnosisis made by recognition of the distinctive gait of fibrotic myopathy. Surgical release of affected tissues via tenotomy, myotenotomy, Z-plasty, or complete resection produces substantial but transient results that typically last only a few months. If the patient's lifestyle is not severely affected, no treatment is recommended.
The prognosis for dogs and cats with fibrotic myopathy is guarded because of recurrence.
For other degenerative myopathies in dogs and cats, see the Overview for this chapter.
For More Information
Also see pet owner content regarding fibrotic myopathy in dogs.
References
Wilson SA, Binversie EE, Kohler N, Patterson MM, Sample SJ, Muir P. Fibrotic myopathy and contracture of the caudal thigh musculature: a prospective study of 41 dogs (2019-2022). J Am Vet Med Assoc. 2023;261(11):1-6. doi:10.2460/javma.23.04.0183
