A 37-year-old man presented with a 2- month history of ptosis, diplopia, and right-sided decreased muscle strength. Those symptoms were rapidly progressing within 1 week. Instead, he didn’t complain of fever, headache, vomiting, neck stiffness, weight changes, night sweats, respiratory, or gastrointestinal symptoms recently. Patient has suffered from ankylosing spondylitis for almost 20 years, and was well-controlled by oral administration of prednisolone. Results of laboratory studies didn’t show any evidence of immune deficiency or human immunodeficiency virus infection. Brain magnetic resonance imaging (MRI) demonstrated a homogeneously enhanced solid lesion locating in the dorsal mesencephalon, with severe peripheral edema. Besides, no obvious hypertensive diffusion weighted imaging signal was found inside the lesion (Figure 1). Routine computed tomography scan showed an iso-hyper density brainstem mass without any hemorrhage, calcification or necrotic signals.