What causes Asymetric Clonus?

  Babinski reflex....normal.
  EMG....normal.
  Hyper - reflex
  Clonus in one leg normal, other leg abnormal (5b)
  One leg weak.
  What causes asymetric clonus?
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Clonus is the most exaggerated form of a reflex.
Think of a public address system. Talk into the microphone and hear yourself on the speakers. Turn the gain (amplification) down, and you can hardly hear yourself. Now turn it way up and you start to hear the squeal of feedback.
Deep tendon reflexes are controlled (by the nervous system) in a similar way. When we test reflexes in the clinic, we judge whether they are absent, diminished, just right, brisk, or abnormally brisk. Usually, there is a range of normal which depends on many factors, some of which might be definable from moment to moment.
In clonus, the movement of the limb around the joint (usually the foot at the ankle) not only jerks at the strike of the hammer, but keeps on jerking for a while. This reverberation (like feedback oscillation) may peter out after a few beats or it may just go on until everyone gets tired of looking at it. Usually, the examiner has put a small amount of tension on the foot at the ankle to optimize the response to the hammer strike.
Clonus usually implies injury in the spinal cord or brain. If hyper-reflexia is symmetric, it may mean nothing at all (behavioral state setting the point of sensitivity of the reflex). If you have clonus on one side but not on the other, weakness on one side, and so forth, there is probably damage to something in the central nervous system (cord or brain).
Note that missing or very diminished reflexes (particularly asymmetric) imply damage to the peripheral nervous system: nerve root, nerve, or even the muscle itself. But as above, sometimes you can't make much of symmetric reflexes - there are other factors which go into the interpretation of reflexes which don't fall in the range of "normal."
I expect the EMG would be normal, because that is a test of the peripheral nervous system and not of the central nervous system. Usually people with CENTRAL nervous system damage don't have PERIPHERAL nervous system damage, though there are certainly exceptions.
I hope this helps. CCF MD mdf.