Muscle disorder

  My mother has been diagnosed with parkinson's for about 4 years now and
  is being treated with Sinemet and Permax. Her primary symptom is muscle
  weakness.  When her medication is not working she gets so weak she
  cannot feed or dress herself. Her left foot drags during those times and she
  must "run" in order to walk.  But she does not have palsy nor "freezing" like
  classic parkinson's patients.  Another feature about her is droopy eyes.
  She was given a blood test (?) for mysthenia gravis that came back negative.
  I am concerned that she might have another muscle disorder.  Do you have any
  opinions on other possible disorders?  
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As you know, Parkinson's disease does not affect the muscles per se. Muscle control, yes - the brain becomes progressively less able to control the muscles adequately. But the muscles themselves remain strong with no loss of power or true paralysis, though some strength is often lost as a consequence of lack of exercise.
Eyelid drooping is not uncommon in parkinsonism.
The symptoms you describe fit the description of fluctuations in the motor response to the medications. That is, when they work everything is fine (or nearly so), and when they wear off there is a big decline in mobility and dexterity. Foot dragging, freezing, and so forth, are all appropriate symptoms.
Parkinson's disease changes with time. That is, the medications which once worked well don't work at the same doses with the same time schedule after some time. However, in general, a patient successfully treated for Parkinson's at the first stages will still be able to benefit from medications as time goes on. It just gets tricky, with motor fluctuations and side effects.
You are welcome to ask for a second opinion. I doubt that there is a second disease process going on, but it has been known to happen, and a good systematic thorough examination can reveal that. For Parkinson's disease, there are three new medications which were introduced just in the past 12 months. Nothing miraculous, but definitely useful particularly for controlling fluctuations.
At CCF, we have a movement disorders clinic within the department of neurology, staffed by experienced neurologists who treat a lot of complicated patients with parkinsonism. Call 800 223-2273 and ask for neurology appointments.
As above, this information is provided for your medical education. Specific comments regarding diagnosis, treatment options, and prognosis must come from your doctor after appropriate evaluation. CCF MD mdf.