Stringhalt
Stringhalt is a condition which effects horses and is distinguished by involuntary, excessive and prolonged flexion of either one or both hindlimbs during movement (El-Hage et.al, 2017; Pasquini et al., 2005; Stashak, 2002). The condition causes the hind leg/s to spasm during walk, trot and moving backwards (Brockman, 2016). The extent of the hyperflexion of the hock may be minimal or as exaggerated to the level that the fetlock makes contact with the abdomen (Stashak, 2002). Classical stringhalt (only affecting one limb) most commonly appears following an injury to one dorsal tarsal/metatarsal region (El-Hage et.al, 2017). The cause of the condition is unknown however it is thought to be associated with peripheral nerve injury and/or trauma to the hindlimb with involvement from the lateral digital extensor (Brockman, 2016; Pasquini et al., 2005; Stashak, 2002). The lateral digital extensor travels in between the deep flexor and the long extensor on the lateral side of the limb. It originates from the lateral collateral ligament and adjacent parts of the fibula and tibia and inserts into the long extensor tendon. Its tendon is held in place by a retinaculum and a synovial sheath provides it with protection where it transverses the hock [Figure 1.] (Dyce et al., 2010).
Figure 2. Frames showing the varying degrees of hyperextension of the hindlimb during forward and backward movement (Draper et al., 2014).
References
Brockman, T. 2016, ‘A case study utilising myofascial release, acupressure and trigger point therapy to treat bilateral ‘stringhalt’ in a 12 year old Akhal-Teke horse’, Journal of bodywork and movement therapies
Draper, A. C. E., Trumble, T. N., Firshman, A. M., Baird, J.D., Reed, S., Mayhew, I.G. MacKay, R. and Valberg, S. J. 2014, ‘Movement characteristics of forward and backward walking in horses with shivering and acquired bilateral stringhalt’, Equine Veterinary Journal, doi: 10.1111/evj.12259
Dyce, K. M., Sack, W. O. and Wensing, C. J. G. 2010, Textbook of Veterinary Anatomy, 4th ed., Missouri, Saunders Elsevier
El-Hage, C.M., Huntington, P.J., Mayhew, I.G. & Slocombe, R.F. 2017, ‘Pasture-associated stringhalt: Contemporary appraisal of an enigmatic syndrome’, Equine Veterinary Education, doi 10.1111/eve.12751
Herbert, E. 2017, Stringhalt the movie 4.2.17, YouTube, 22/2/17, viewed 28/7/17, https://www.youtube.com/watch?v=F23qo2yxcTY
Pasquini, C., Jann, H., Pasquini, S. & Bahr, R. 2005, Guide to Equine Clinics Lameness, 2nd ed., Texas, Sudz Publishing
Slocombe, R. F., Huntington, P.J., Friend, S. C. E., Jeffcott, L. B., Luff, A. R. & Finkelstein, D. K. 1992, ‘Pathological Aspects of Australian Stringhalt’, Equine Veterinary Journal, doi: 0425-1644
Stashak, T. S. 2002, Adam’s Lameness in Horses, 5th ed., Maryland, Lippincott Williams & Wilkins
Figure 1. The annular ligaments and tendons of the left digit of a horse from the dorsolateral aspect. 2 on the figure corresponds with the lateral digital extensor (Dyce et al., 2010).
Pasture-associated stringhalt (Australian Stringhalt) develops following ingesting a plant-based neurotoxin/s and peripheral neuropathy results. Pasture associated stringhalt most commonly affects horses grazing drought-affected, poor quality pastures. Stringhalt is bilateral, however can on some occasions occur asymmetrically (El-Hage et al., 2017).
The plants H. radicata (flatweed), Taraxacum officinale (common dandelion) and Malva Parviflora (Marshmallow) have been found to be associated with the condition. Clinical signs of the condition may not appear until 1 to 3 weeks following grazing affected pastures (El-Hage et.al, 2017). In this type of stringhalt axonopathy of the long peripheral nerves occurs, with the recurrent laryngeal, tibial and peroneal nerves most greatly affected. This leads to laryngeal dysfunction and atrophy of the distal hindlimb muscles (particularly the type 2 muscle fibres). The affected muscles are the circoarytenoideus dorsalis, the lateral digital extensor, the long digital extensor and the gastrocnemius. It is believed that earlier the distal areas of the axon are affected sparing the nerve cell body which can allow for regeneration over time. It is believed that due to the nerve degeneration of the extensor muscles, the flexors (biceps femoris and seitendinosus) predominate (Stashak, 2002).
Clinical Signs & Diagnosis
Treatment & Prognosis
- Hyperflexion of one or both hindlimbs during movement (El-Hage 2017; Pasquini , 2005) [Figure 2].
- Diagnosed with a nerve block into the peroneal nerve (Pasquini , 2005)
- This hyperflexion can be exacerbated when walking downhill, following a sudden stop or following hard exercise (El-Hage 2017).
- In severe cases horses can exhibit a hopping action in which the hyperflexion becomes exaggerated and its duration increases. In these cases the fetlock may even touch the ventral abdomen during hyperflexion or the duration may be increased to the extent that both legs may be off the ground at the same time (El-Hage 2017).
- The horse may be unable to get up without assistance (El-Hage 2017).
- Hyperflexion is worse during cold weather or in stressed horses (El-Hage 2017).
- Muscle atrophy, particularly of the long and lateral digital extensor muscles in the early stages and the thighs, adductors and hindquarters in late stages (El-Hage 2017; Stashak, 2002).
- Pathologic lesions in horses with pasture associated stringhalt have been detected on the peripheral nervous system (El-Hage 2017).
- The most severely affected nerves are the superficial and deep peroneal, distal tibial, plantar digital, volar and recurrent laryngeal nerve. Shown by a loss of large diameter myelinated fibres with various degrees of demyelination, fibrosis and Schwann cell proliferation (El-Hage 2017).
- The most severely affected muscles with lesions are the long and lateral digital extensors, cranial tibial, dorsal circoarytenoid, gracillis and lateral deep digital flexor. (Slocombe, 1992)
- Spontaneous recovery over 6-18 months following the removal from the toxin source in pasture-associated stringhalt cases (El-Hage 2017).
- Severe cases may require euthanasia if the horse remains recumbent for a prolonged period of time (El-Hage 2017).
- Intra-articular steroids injected into the DIT & TMt jointsà Methylprednisolone (Pasquini et al., 2005; Stashak, 2002).
- Tenectomy and partial myectomy (if the steroid injections do not work) a section is cut out of the lateral digital extensor muscle and tendon (Pasquini et al., 2005)
- Phenylbutazone (Pasquini 2005, Stashak, 2002).
- The administration of muscle relaxants particularly Phenytoin (a sodium channel blocker) is able to modulate tetanic muscle contractions and abnormal nerve activity and reduce the severity of clinical signs (El-Hage 2017).
- Thiamine has anecdotally been proved to help treat pasture associated stringhalt, taurine and antioxidants have also been successful in a small number of cases (El-Hage et.al, 2017).
- Some horses may never recover fully (El-Hage 2017).
- Cutting the digital extensor muscles (Brockman, 2016).
- Iliacus myofascial release and trigger point release consistently decreased stringhalt symptoms (Brockman, 2016).
(Herbert, 2017)