Complex sentence: When Mrs. Frederic realizes that Leena was being controlled by MacPherson and she removes the Pearl of Wisdom from Leena's mind, some residual energy trapped in her brain causes her to suffer from headaches and vertigo.
Split into simpler sentences: When Mrs. Frederic realizes that Leena was being controlled by MacPherson, she removes the Pearl of Wisdom from Leena's mind. However, there is still some residual energy trapped in her brain that causes her to suffer from headaches and vertigo.