an apoplectic fit
The OED defines apoplexy:
A malady, very sudden in its attack, which arrests more or less completely the powers of sense and motion; it is usually caused by an effusion of blood or serum in the brain, and preceded by giddiness, partial loss of muscular power, etc.
In modern parlance, this would be termed a cerebral hemorrhage. It is not certain that the term was applied so specifically in Mary Shelley's day.