my son is eight yrs old and for the past 2 months i have notice a tremendous difference in him my child is adhd and runs around like a energizer bunny full of energy then one day just started slurring when talked, his eyes are very droopy, and randomly falls he says his legs and arms get weak the doctors hv done so many test on him and cant find out what it is and now they drew blood for myasthenia gravis can some explain what this is? please