Susannah tells the reader that she woke Stephen up that night with low moans, squeaks, and then grunts. He thought that Susannah was just grinding her teeth, but rolled over to see her sitting with wide, unseeing eyes. Susannah didn't answer any of his questions and turned to face him, looking possessed. Her arms whipped out in front of her, her body tensed, and blood and foam spurted out of her mouth. Horrified, Stephen stared for a moment before laying Susannah down on her side and calling 911.
Note that Stephen's first thought is that something relatively normal is happening—Susannah grinding her teeth. This reinforces the idea that even if Susannah's family and friends knew that something was wrong prior to this seizure, they didn't consider that things were this wrong—this illness is a surprise for everyone.
Susannah explains that she never regained memories of this seizure, or the others to come. This seizure marks her line between sanity and insanity. Much of what follows is pieced together from outside sources, as she remembers little but hallucinations and paranoia.
Now Susannah's illness is directly inhibiting her ability to control her story. The fact that she relies on other sources to fill in the gaps shows that her relationships with these others remained strong.
Susannah says that this seizure was just the most dramatic of a number of seizures she'd been experiencing for days. She explains that when neurons misfire or fire all at once, seizures can result. She experienced a "tonic-clonic" seizure, which is characterized by a loss of consciousness, rigid muscles, and strange movements. Other people experience more subtle seizures that look like staring episodes or repetitive mouth or body movements. A seizure's type and severity depends on where in the brain the neurons are malfunctioning; i.e. if the visual cortex is to blame, the person experiences visual hallucinations.
When Cahalan offers these medical descriptions of what's actually going on inside her body or her brain, she's working towards her goal of spreading awareness about her disease, its symptoms, and the brain in general. This in turn can help patients advocate for themselves and use her memoir as a starting point for their own research. In terms of her personal narrative, this research also allows Cahalan the opportunity to piece together the story of her body on a chemical and functional level.
Susannah explains that she'd also been experiencing partial seizures from overstimulation in her temporal lobe, which explains her euphoria, mood swings, her sense of alienation in the bathroom at work, the expanding hallway, and her light sensitivity in Times Square. She says that some people who experience temporal lobe seizures also have out-of-body experiences.
For the reader, Susannah's strange behavior now makes more sense—but the next step is to see if her doctors take all it seriously, since the Susannah who's currently having a seizure cannot articulate any of this for her medical team.