Absent knee and ankle reflex with extensor plantar reflex

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