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absent knee and ankle reflex with extensor plantar reflex
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Causes of an absent knee and ankle reflex with extensor plantars implies a mixed upper and lower motor neurone lesion, and causes include:

  • subacute combined degeneration of the spinal cord
  • motor neurone disease
  • syphilitic tabo-paresis
  • Friedreich's ataxia
  • conus medullaris or cauda equina lesion - at the conus medullaris the spinal root entry zones and the pyramidal tracts are in close proximity - they may be damaged by a small lesion such as a neurofibroma
  • multiple sclerosis - involvement of the corticospinal tract leads to hyperreflexia and extensor plantars. Severe sensory involvement may break the reflex arc, and the reflexes disappear
  • pellagra

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