Analysis of Terri Schiavo's brain revealed that it was mush.

To put it more analytically

"She had developed hydrocephalus ex vacuo, a condition marked by enlarged ventricles filled with cerebrospinal fluid, because of this profound loss of cortical volume"

Half her brain was gone and it was the part we have identified as the seat of identity.

Her brain was so far gone that believing she still existed requires dualism.