Clonus in one leg

  For the past month, my son has not been able to bear weight on his right leg.  He is 3 years old and has a central nervous system dysfunction.  He began to ambulate right before his 3rd birthday in March.  Since May, he started to slack off, which we thought was due to chronic otitis media.  His PT and I thought that his actions were due to a lack of security standing on his feet- he walks on his knees now.  Last night after I gave him a bath, his right leg started to shake, more like vibrate.  I decided to take him to see his pediatrician the next day.  His doctor suggested we go see his pediatric neurologist.  We also saw his PT today and I told her about his actions last night.  She stretched out his leg then told me he had 3b clonus.  What stumped her was that he only had it in one leg, and he had never had it before.  She strongly suggested that we see his p. neurologist, with whom we are meeting with next week.  But being a concerned Mom, I want some answers now.  I am very concerned about my son's neurological make-up because no one can give me a definitive answer about his condition.  Any answer is better than "...Well, we don't know for sure, it could be a number of things...".  So please, if anyone could help me I would really appreciate it.
                                       Thank You!
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Clonus is an exaggerated reflex response. In general, deep tendon reflexes (knee jerks, ankle jerks, etc when tapped by a hammer) have a certain degree of vigor which can vary somewhat in different individuals or in the same person over time. Neurologists grade the vigor of a reflex and determine whether it is too little (hypo-reflexic) or too much (hyper-reflexic). Clonus is at the too much end of the spectrum: multiple or reverberating jerks.
If one side of the brain or cord is damaged, it is possible for reflexes to be asymmetric. The fact that your son has clonus on one side may be a clue about what the underlying problem is. I would expect that if your son didn't start to walk until now, that you've been aware of a problem and your pediatric neurologist is too.
Clonus usually goes along with spasticity. Spasticity is increased muscle tone, usually yielding certain types of postures. For example, spastic legs are stiff and straight, with a tendency to scissoring and plantar flexion (foot pointing down at the ankle).
One of the most common causes of spasticity and clonus of the legs is cerebral palsy. CP does not have to be symmetric. If your child is normal on one side (including all aspects of use of that limb, not just when tone and reflexes are tested) but abnormal on the other, then there may have been a stroke at some time in the past.
I hope by now most of these possibilities have been raised, investigated, and discussed by your pediatric neurologist. My intent here is to fill in gaps in your knowledge in case some of that discussion hasn't been clear. Of course, this forum can't substitute for medical advice, so you'll have to wait until the doctor visit to get a definitive answer.
By the way, clonus and spasticity take a bit of time to develop. That is, they usually are not signs that some damage has developed acutely just now. Rather, they are signs of chronic or longstanding motor dysfunction.
I hope this helps. CCF MD mdf.