The clinical presentation and the MRI features in a patient with a pre-existing pituitary macroadenoma are most consistent with pituitary apoplexy.

Pituitary apoplexy is an acute clinical syndrome caused by either hemorrhage or infarction of the pituitary gland. Although variable, it typically comprises of a headache, visual deficits, ophthalmoplegia, and altered mental status. An existing pituitary macroadenoma is usually present (60-90%) but it can occur with healthy glands in a few isolated cases. 

Additional contributor: ZE. Boudiaf, MD