Patrick McIlheran, age 13, of Burlington, Wis., for his question:

WHAT IS MYASTHENIA GRAVIS?

A disease characterized by progressive weakness of the skeletal muscles is called myasthenia gravis. It affects all age groups and both sexes but usually can be found in women between the ages of 15 and 30 and among men from 40 to 70.

Myasthenia gravis primarily affects the muscles of the face, neck, arms and legs. Drooping eyelids, weakness of the arms and legs and difficulty in chewing and breathing are among the symptoms. The exact cause for the defect is not known but many doctors say it might originate from a virus.

The disease is treated with a drug called anticholinesterases. With medication, most victims of myasthenia gravis can live almost completely normal lives.