Motor neuropathy

Central neuropathy • Spinal muscular atrophy

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Introduction

  • Affects motor neurones with their cell bodies in the spinal cord and/or motor nucleus of cranial nerves in the brain-stem.
  • Cause : inherited condition in the Brittany [Peripheral neuropathies] and the Swedish Lapland dog.
  • Also occurs in the Pointer, the Cairn Terrier, the Rottweiler, the German Shepherd dog, the Griffon Briquet Vendeen dog and giant-breed crosses.
  • Several isolated reports of motor neuron disease in puppies of several other breeds, which are believed to be inherited.
  • Signs : progressive neuropathy in pelvic and thoracic limbs.
  • Diagnosis : signs, EMG and nerve conduction studies, nerve and muscle biopsy. Definitive diagnosis is made postmortem.
  • Treatment : none.
  • Prognosis : guarded or poor.

Diagnosis

Clinical signs


Signs of diffuse lower motor neurone lesion
  • Weakness or paralysis.
  • Decreased or absent muscle tone.
  • Decreased or absent tendon reflexes (although this tends to occur late in the course of the disease, because reflexes can be elicited while there are surviving functional motor neurons).
  • Hindlimbs first, then forelimbs, progressing to tetraparesis (forelimbs in German Shepherd dogs).
  • Progressive muscular atrophy:
    • Distribution of atrophy depends on entity, severity depends on degree of axonal degeneration.
    • Respiratory distress (involvement of intercostal muscles).
    • Dysphonia and dysphagia can result if there is involvement of brainstem motor nuclei.
    • Deformities in distal limbs (muscle wasting and contracted tendons).

Other signs, depending on breed
  • Head tremor.
  • Muscle fasciculations.
  • Weakness of muscles of mastication and the tongue.
  • Reduced gag reflex.
  • Dysphonia.
  • Proprioceptive deficits due to profound weakness.
  • Because the sensory system is unaffected, ataxia is not a component of the disease.
  • Carpal valgus deformity.
  • Carpal flexion due to contracted tendons.
  • Intention tremor/limb tremor after exercise.
  • Reduced postural reactions.
  • Megaesophagus.
  • Pelvic limb extensor rigidity.
  • Nystagmus.
  • Episodic weakness.
  • Cataplexy.

Differential diagnosis


Causes of progressive paresis/paralysis starting with pelvic limb(s)
  • Any cervical cord disease.
Causes of progressive paresis/paralysis starting with thoracic limb(s)
  • Cervico thoracic tumor.

Outcomes

Prognosis

  • Grave.
  • Life expectancy depends on the individual entity, but tends to be longer the later the onset of the disease.

Expected response to treatment

  • In most instances, clinical signs are inexorably progressive until animals are unable to stand/walk.
  • Mild forms of motor neuron disease may be seen, eg unilateral forelimb involvement in German Shepherd dogs in which signs may not progress.
  • In Brittany spaniels with the 'chronic' disease, clinical signs may stabilize.

Reasons for treatment failure

  • Incorrect diagnosis.
  • Inadequate nursing care.
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