It's very cute, but — so she desperately needs surgery, because the unchecked condition would kill her, right? I can't think of any procedure that'd both fit those criteria and leave her with retrograde amnesia that would still let her keep all her hair.
(I mean, the obvious thing would be a benign/low-grade brain tumor that would need to be removed, but wouldn't require further radio- or chemotherapies. I guess it could be near the brainstem, which could be gotten to at the back, removed all at once, and the bald spot could be hidden by her other hair.)
(Actually, looking at Nico, and consulting Dr. Google, it could be a juvenile pilocytic astrocytoma near the hypothalamus, at the base if the brain. One website says, "a JPA in the hypothalamic region may be associated with ... diencephalic syndrome, which is characterized by failure to thrive, abnormal thinness, irritability, and eye abnormalities." Hmmm. The standard treatment is a surgical removal, and if they can get the whole thing, there's no continuing treatment required.)