I don't think many suffer to the level of regressing back to infants though...
I don't get the impression that her regression is a symptom of her PTSD, per se; rather, that she suffers from PTSD flashbacks even though she's lost most of her conscious memory from some other cause (head trauma would be my guess). This is something that does happen to some people who've suffered traumatic brain injuries.
Commander!Look at this!Kasumi!?Kasumi!Everything alright now...Everything's alright...Comm... an...I'm so scared...Well, aren't you just the little artist!Eheheh!So, see... This here is you, Comman-Ah!NOOOOOOO!!AAAAAAHHHHHH!!!Ah...Ah...Ahhh...!!Hey!Ahhghh...cough!That's amazing, Kasumi!
Good girl!