Hi Xmas,
90 mg/d of Pred is a pretty high dose,(1 mg/2.1 pounds of weight?) I hope your neuro gave you some advises for your food diet to cope with the steroids? Watch out also for you eyes, have them checked regularly for cataract (you will notice it!) and for glaucoma (you won't notice it!).
It is good therapy for generalized MG to take Mestinon, as Kate mentioned it, Pred ( fast acting ) for some time and an immunosuppressor (slow acting). After a while, Pred is tapered to a minimum (hopefully zero) and you're left with Mestinon with a dosage appropriate to your need and the immunosuppressor.
Maurice.