Equis ISSN 2398-2977

Neurology: motor neuron disease

Contributor(s): Frank Andrews, Tom Divers, Caroline Hahn, Catherine McGowan, Catherine McGowan, Ruth Morgan, Graham Munroe

Introduction

  • Sporadic, neurodegenerative disease causing weakness and muscle atrophy (first described in 1990).
  • Incidence: most prevalent in North America, but has been shown to account for up to 15% of neurology admissions in the UK.
  • Cause: unclear, oxidative damage to motor neurons, associated with vitamin E/selenium deficiency.
  • Diagnosis: tailhead (sacrocaudalis dorsalis medialis) muscle biopsy or EMG.
  • Treatment: none proven, vitamin E therapy used.
  • Prognosis: approximately one third of cases will deteriorate rapidly and require euthanasia, one third remain stable with obvious signs of disease and cannot be ridden, and one third improve in response to therapy.

Pathogenesis

Etiology

  • Non-inflammatory neuronal degeneration and neuronal loss at all levels of the spinal cord, including the brainstem.
  • The ventral horn grey matter, ie the lower motor neurons, is particularly targeted.
  • Denervation of the muscle results in atrophy of the type I and II fibers abut EMND predominantly affects type I fibers. At least 30% of motor axons must be destroyed before there is clinical evidence of atrophy. Postural muscles are most severely affected because of their high percentage of type I fibers.
  • Motor neurons are particularly susceptible to oxidative stress due to their very long axons and resulting in high energy requirements.
  • The etiopathogenesis of EMND is not fully understood but it is thought that neurons undergo free radical damage. The histopathologic identification of lipopigments deposition in the capillaries of the spinal cord is suggestive of oxidative injury.

Predisposing factors

General
  • Restricted pasture access, for at least 1 year typical, however EMND has recently been described in horses kept at pasture.
  • Affected horses have been found to have consistently low plasma Vitamin E concentrations.
  • Inflammatory bowel disease and/or liver disease has been found in as many as 5% of horses with EMND as have abnormal glucose absorption tests these horses could potentially therefore have abnormal absorption or metabolism of Vitamin E. 
  • Ration rich in grain/poor-medium quality grass hay or straw/no alfalfa.

Specific

  • Motor neurons with high oxidative activity, ie those supplying type I muscle fibers.
  • Lack of dietary antioxidants.
  • Periods of oxidative stress - short-term neurotoxin exposure - increased consumption of intrinsically produced, free radical reducing enzymes.

Pathophysiology

  • Degeneration of somatic motor neurons in ventral horns of spinal cord and selected brainstem nuclei.
  • Death of motor neurons is accompanied by degenerative axonal changes in ventral roots/peripheral nerves   →   neurogenic muscle atrophy.
  • Type I muscle fibers appear most affected.
  • Cervicothoracic and lumbosacral intumescences of spinal cord are most severely affected.
  • >30% need to be dysfunctional   →   acute onset of clinical disease.
  • Subclinical cases exist.
  • Scattered muscle fiber necrosis and lipopigment accumulation in capillaries of spinal cord and retina are characteristic of lipid peroxidation and vitamin E deficiency.

Timecourse

  • Unclear.

Epidemiology

  • No evidence of contagion.

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.
  • Bedford H Eet al(2013)Histopathologic findings in the sacrocaudalis dorsalis medialis muscle of horses with vitamin E-responsive muscle atrophy and weakness.JAVMA242(8), 1127-1137 PubMed.
  • McGowan C M, McGowan T W & Patterson-Kane J C (2009)Prevalence of equine polysaccharide storage myopathy and other myopathies in two equine populations in the United Kingdom.Vet J180(3), 330-336 PubMed.
  • Syrja Pet al(2006)Equine motor neuron disease (EMND) in a horse without vitamin E deficiency: a sequela of iron excess?Equine Vet Educ18(3), 122-126 VetMedResource.
  • Wijnberg I D (2006)Equine motor neuron disease.Equine Vet Educ18(3), 126-129 VetMedResource
  • McGorum B Cet al(2006)Horses on pasture may be affected by equine motor neuron disease.Equine Vet J38(1), 47-51 PubMed.
  • Benders N A, Wijnberg I D & Van der Kolk J H (2001)Equine motor neuron disease: a review based on a case report.  Tijdschr Diergeneeskd126(11), 376-380 PubMed.
  • Kyles K W J (2001)Electromyography under caudal epidural anaesthesia as an aid to diagnosis of equine motor neuron disease.Vet Rec148(17), 536-538 PubMed.
  • Verhulst D, Barnett K C & Mayhew I G (2001)Equine motor neuron disease and retinal degeneration.Equine Vet Educ13, 59-61 VetMedResource.
  • Divers T J, deLahunta A, Hintz H F, Riis R C, Hackson C A & Mohammed H O (2001)Equine motor neuron disease.Equine Vet Educ13, 63-67 VetMedResource.
  • Valentine B A, Divers T J, Murphy D J & Todhunter P G (1998)Muscle biopsy diagnosis of equine motor neuron disease and equine polysaccharide storage myopathy.Equine Vet Educ10, 42-50 VetMedResource.
  • Divers T J, Mohammed H O & Cimmings J F (1997)Equine motor neuron disease.Vet Clin North Am Equine Pract13(1), 97-105 PubMed.
  • Divers T Jet al(1994)Equine Motor Neuron Disease - Findings in 28 horses and proposal of a pathophysiological mechanism for the disease.Equine Vet J26(5), 409-415 PubMed.

Other sources of information

  • Divers T J, Mohammed H O, Hintz H F & De Lahunta A (2003)Equine Motor Neuron Disease: A Reivew of Clinical and Experimental Studies.In:Proc 49th AAEP Convention.pp 230-232.


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