Equis ISSN 2398-2977

Neurology: stringhalt

Synonym(s): Australian stringhalt, Classic stringhalt

Contributor(s): Steve Adair, Caroline Hahn, Nicola Menzies-Gow

Introduction

  • Two forms classic/sporadic stringhalt and Australian/epidemic stringhalt.
  • Cause: etiology obscure, although ingested plant toxins have been implicated in Australian stringhalt.
  • Signs: involuntary delayed protraction and excessive flexion of one or both pelvic limbs during movement.
  • Diagnosis: characteristic clinical signs.
  • Treatment: surgery is required for the classic form; removal from inciting pasture may be adequate in the Australian form.
  • Prognosis: classic stringhalt rarely improves without surgery. Australian stringhalt frequently resolves spontaneously, although this may take weeks to years.

Pathogenesis

Etiology

Classic stringhalt
  • Obscure, but the following have been implicated:
    • Trauma to dorsal and lateral tarsal and proximal metatarsal region of hindlimb. Cases sometimes occur sometime after trauma to the pelvic limbs.
    • Some cases associated with tarsal arthropathy.
    • A degenerative neurogenic disorder may be involved in some cases, but has not been well documented.
    • Some cases appear to be idiopathic.
    • No proven cause.
Australian stringhalt
  • Is seen late summer and autumn following long spells of dry weather in horses grazing on poor quality pasture containing high concentrations of certain weeds including flatweed (Hypochaeris radicata), dandelion (Taraxacum officinale) and Malva parviflora.
  • Plant toxins have therefore been implicated in the possible etiology.
  • Mycotoxins have also been implicated.
  • Other preconditions may be necessary for disease to develop as not all horses on affected pasture develop disease, commonly only one will.
  • Vitamin deficiency has been postulated as a possible precondition.

Predisposing factors

Specific

  • Weather and pasture conditions for Australian stringhalt.

Pathophysiology

  • Nerves most severely affected are:
    • Superficial and deep peroneal.
    • Distal tibial and plantar digital nerve.
    • Recurrent laryngeal; left and right.
  • Affected nerves exhibit selective loss of large diameter myelinated fibers with various degrees of demyelination, fibrosis, Schwann cell proliferation and onion bulb formation.
  • The muscles affected to the greatest degree include the long and lateral digital extensors, cranial tibial, cricoarytenoideus dorsalis, gracilis and long digital flexor.
  • Muscle changes include extensive atrophy of fibers and a diffuse fibrosis. There is a reduction in type 2 fibers and an abnormally wide distribution in fiber size due to hypertrophy of fibers which remain innervated.
Australian stringhalt
  • An example of a distal axonopathy → neurogenic muscle atrophy. Why this results in hypermetria and not paresis, is unknown.
  • The lesions are confined to the peripheral nerves and muscles.
  • Longer, larger myelinated nerve fibers are more susceptible to damage thus explaining the distribution of neuromuscular lesions.
  • Damage to the long peripheral nerves may interfere with the reflex arc that maintains postural tone and co-ordinates muscle contraction → exaggerated flexion.
Classic stringhalt
  • There may be damage to other parts of the reflex arc in classic stringhalt, eg injury → damage to the muscular component.
  • Central damage, eg equine protozoal myeloencephalitis and spinal cord diseases → stringhalt-type gait.
Lathyrism
  • A distal axonopathy due to chronic intoxication of horses with sweet pea plants.
  • Identical clinical syndrome to Australian stringhalt but differs pathologically in that the primary lesion is a myelin degeneration of the caudal spinal nerves.

Timecourse

  • Incubation period is unknown.
  • Classic stringhalt may occur some time after a traumatic insult to the pelvic limb and is usually permanent.
  • Australian stringhalt frequently resolves spontaneously over a varying time period, from weeks to months, and resolution has been recorded up to 3 years.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hahn C (2008) Common peripheral nerve disorders in the horse. In Pract 30 (6), 322-329 VetMedResource.
  • Gardner S Yet al (2005) Stringhalt associated with a pasture infested withHypochoeris radicata. Equine Vet Educ 17 (3), 118-122 VetMedResource.
  • Crabill M R, Honnas C M, Taylor D S, Schumacher J, Watkins J P & Snyder J R (1994) Stringhalt secondary to trauma to the dorsoproximal region of the metatarsus in horses; 10 cases (1986-1991). JAVMA 205 (6), 867-869 PubMed.
  • Cahill, J I & Goulden B E (1992) Stringhalt - current thoughts on etiology and pathogenesis. Equine Vet J 24 (3), 161-162 PubMed.
  • Slocombe R F, Huntington P J, Friend S C E, Jeffcott L B, Luff A R & Finkelstein D K (1992) Pathologic aspects of Australian stringhalt. Equine Vet J 24 (3), 174-183 PubMed.
  • Kannegieter N J & Malik R (1992) The use of baclofen in the treatment of stringhalt. Aust Equine Vet 10 (2), 90 VetMedResource.
  • Huntington P J, Jeffcott L B, Friend S C E, Luff A R, Finkelstein D I & Flynn R J (1989) Australian stringhalt - epidemiologic, clinical and neurologic investigations. Equine Vet J 21 (4), 266-273 WileyOnline.


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