I have Salt Wasting Congenital Adrenal Hyperplasia. During an server illness the dr's couldn't get blood so cut my ankles open. Why did they do this? Thanks

IV access. It sounds like they had to have reliable access to a superficial vein to put a needle or thread a catheter into your ankle vein to replenish your body with fluids and electrolytes that it lost because of your illness.
Venous access. I would assume that when you said they "couldn't get blood" you also meant that they couldn't get an IV. The large veins at the ankle have traditionally been used to obtain emergency venous access. It's called a venous cut down. Typically in the US we use ultrasound to place difficult IVs. In an emergency situation we sometimes drill a needle directly into a bone. .