Less than two weeks later, Susannah returns to the hospital for a week of plasma-exchange treatment. The exchange happens through a catheter inserted directly into Susannah's jugular. During the treatment, the patient's blood is separated from the plasma that contains harmful antibodies, and the plasma is replaced with protein-rich fluid. Several of Susannah's friends visit during this stay. When Angela visits, she's startled by Susannah's appearance. She later emails Paul and says that Susannah is pale, thin, and out of it.
Just as was the case with Jeff, Angela's shock comes directly from the fact that her memory of Susannah doesn't at all match up with the Susannah she sees in front of her. As Susannah undergoes these treatments to filter her blood, more of her old self also filters in: the plasma exchange is a poignant metaphor for Susannah's returning identity throughout her recovery.
On Susannah's last night in the hospital, she listens to her roommate pray with the nurses. When Susannah is released the next morning, Stephen takes her on a drive through Summit. He turns on the CD player, and out of the blue, both he and Susannah belt out the chorus of a song by the Mamas and the Papas. Stephen is thrilled at this clue that Susannah is still herself.
Even if Cahalan doesn't remember this version of herself, this Susannah still possesses some memories—and with treatment, these memories are now able to bypass her body and make their way out. Stephen's excitement shows how important these moments are, as they suggest that she will recover eventually.