Fibrotic and Ossifying Myopathy
Fibrotic and ossifying myopathy is a mechanical lameness by which the horse exhibits a ‘goose-step’. This is where the hoof is suddenly pulled caudally just before it is placed on the ground resulting in it being slapped or jerked down (Hinchcliff et.al, 2014; Pasquini et al., 2005). This lameness is most easily seen at the walk and is most commonly unilateral. This lameness is most commonly seen in the hindlimbs and vary rarely can affect a forelimb (Stashak, 2002). Although this condition is rare it mostly affects western performance or rodeo horses which perform ‘sliding stops’ (Pasquini et al., 2005). In the condition there is fibrosis or ossification of the muscles in the crus which leads to adhesions between the semimembranosus, semitendinosus, biceps femoris or gracillis (Pasquini et al., 2005; Stashak, 2002). The adhesions formed between the muscles limit the action of the semitendinosus which results in the lameness (Stashak, 2002). The vertebral head of semitendinosus originates from the sacrum and adjacent area of sacrosciatic ligament and merges with the pelvic head. The semitendinosus inserts on the medial aspect of the tibia and crural fascia [Figure 1.] (Dyce et al., 2010). The condition is caused either by trauma, a poorly administered intramuscular injection or it is congenital (Hinchcliff et.al, 2014; Pasquini et al., 2005; Stashak, 2002). It is thought that the congenital form of the condition is caused by periparturient muscle trauma which causes rupture of the semitendinosus muscle.
Figure 2. Frame from a video of a horse affected by fibrotic ossifying myopathy at a walk depicting the ‘goose step’ characteristic of the condition (Ultrazipped, 2011).
References
Dyce, K. M., Sack, W. O. and Wensing, C. J. G. 2010, Textbook of Veterinary Anatomy, 4th ed., Missouri, Saunders Elsevier
Hinchcliff, K.W., Kaneps, A. J. & Geor, R. J. 2014, Equine Sports Medicine and Surgery E-book, 2nd ed., Saunders Elesevier
Janicek, J., Lopes, M. A. F., Wilson, D. A., Reed, S. & Keegan, K. G. 2012, ‘Hindlimb kinematics before and after laser fibrotomy in horses with fibrotic myopathy’, Equine Veterinary Journal, 44 (43), pp. 126-131, doi 10.1111/j.2042-3306.2012.00681.x
Paden, M. 2013, June fibrotic myopathy, YouTube, 25/7/13, viewed 28/7/17, https://www.youtube.com/watch?v=b85I52Sae_w
Pasquini, C., Jann, H., Pasquini, S. & Bahr, R. 2005, Guide to Equine Clinics Lameness, 2nd ed., Texas, Sudz Publishing
Stashak, T. S. 2002, Adam’s Lameness in Horses, 5th ed., Maryland, Lippincott Williams & Wilkins
Ultrazipped 2011, 'Mo- Fibrotic myopathy', Video, Youtube.
Figure 1. Proximal hindlimb of a horse from the lateral aspect. 6 depicts the semitendinosus muscle (Dyce et al., 2010).
- The characteristic ‘goose step’ [Figure 2.] (Pasquini , 2005).
- The cranial phase of the stride is shortened which results in the caudal phase of the stride being lengthened (Stashak, 2002).
- If the semitendinosus is the muscle which is most greatly affected the jerking downward of the hindlimb before footfall is cranial to caudal while if the gracillis is most greatly affected the hindlimb is jerked down laterally to medially (Stashak, 2002).
- In semitendinosus affected horses the hindlimb is placed on the ground more vertically resulting in the toe making contact with the ground first followed by the heel ‘slapping down’ (Stashak, 2002).
- A fibrous mass is able to be palpated being and proximal to the stifle (Hinchcliff al, 2014; Pasquini , 2005; Stashak, 2002).
- Ossification shown on radiographs and ultrasound if ossification has occurred (Hinchcliff al, 2014; Pasquini , 2005).
- A history of trauma, injury or an intramuscular injection in the affected leg (Hinchcliff al, 2014).
- Does not respond to analgesics (Hinchcliff al, 2014; Pasquini 2005).
- In congenital cases of this condition, foals are born exhibiting the ‘goose step’ and upon palpation a tightening in the semitendinosus muscle can be detected however there is no firm thickening of the muscle which is characteristic of the condition (Stashak, 2002).
Clinical Signs & Diagnosis
Treatment & Prognosis
- A partial tenotomy of the semitendinosus can be undertaken. In this procedure the semitendinosus insertion is transected from the caudomedial side of the proximal tibia (Pasquini , 2005).
- A partial myectomy of the semitendinosus can be undertaken. In this procedure a four-inch segment of the semitendinosus is removed. This is the traditional treatment for the condition (Pasquini , 2005).
- Complete removal of the affected muscle tissue can be undertaken during the partial myectomy. This procedure is carried out by placing the horse in lateral recumbency with the affected limb on the top side. A vertical incision is made over the caudal aspect of the semitendinosus tendon and the tendon is identified and the adhesions from the biceps femoris and semitendinosus muscles are removed. The belly of the affected muscles are isolated and a four inch portion removed (this should include 4cm of muscle and 4cm of tendon). If ossification has occurred the skin should be reflected away over the ossified area and the muscle dissected free of the bony cover (Stashak, 2002).
- Laser transection of the fibrotic mass can be undertaken followed by early post-operative exercise can reduce the severity of the abnormal gait (Janicek 2012).
- The prognosis for the traditional treatment is guarded as complications such as bleeding, dehiscence, disfigurement, recurrence of lameness and scar formation can occur (Pasquini , 2005). It may take 3-7 days for the benefit of the operation to appear (Stashak, 2002).
- The prognosis for the partial tetomy is good although has been only performed a number of times. This procedure may take several days to weeks to show the full benefit of the operation (Stashak, 2002).
- Transection of the fibrotic mass can be performed either under a general anaesthetic or in a standing horse under local anaesthetic. The fibrotic mass is transected out using a bistoury knife. Light exercise is undertaken the day following surgery (Hinchcliff al, 2014).
(Paden, 2013)