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What are the causes of absent of knee jerk ?

Absent knee jerk  signifies the lesion at the level of L4 spinal segment.
  • Absence of the knee jerk signify an abnormality anywhere within the reflex arc, including the muscle spindle, the Ia afferent nerve fibers, or the motor neurons to the quadriceps muscle. 
  • In general the afferent loop is much more critical for reflex function than the efferent loop, hence that unless the muscle is almost paralysed, loss of reflexes suggests a sensory nerve or root lesion rather than a motor nerve lesion.
  • The most common cause is a peripheral neuropathy from such things as diabetes, alcoholism, and toxins.
Causes of absent ankle jerk
  • Non-cooperative patient or faulty technique.
  • Acute anterior poliomyelitis.
  • Peripheral neuropathy.
  • Myelopathy in shock stage.
  • Progressive musclar atrophy (MND).
  • Subacute combined degeneral ion.
  • Friedreich's ataxia
  • Tabes dorsalis
  • Sometimes in diseases of myoneural Junction (myasthenic crisis, hypokalaemia etc).
  • Muscular dystrophy in late stages.
If there is  loss of deep reflex restricted to one limb only, you have to rule out the following conditions
  • Acute anterior poliomyelitis, 
  • Entrapment neuropathy, 
  • Leprosy 
  • Diabetic mononeuropathy.
Always perform the Jendrassik's manoeuvre before declaring a tendon reflex absent