How is reactivation of acetylcholinesterases by Pralidoxime (PAM) after action of echothiophate?

PAM reactivates acetylcholinesterase but is unable to penetrate into CNS, charged group allows it to reach anionic site of enzyme, displaces phosphate group of organophosphate and regenerate the enzyme, given before aging of alkylated enzymes occur, can reverse the effects of echothiophate except in CNS