History In November 2012, a previously healthy 31-year-old woman was admitted to our hospital with a 2-month history of right-sided numbness, diplopia, and intermittent nausea and dizziness. She did not have a history of fever, weight loss, headache, photophobia, seizure, or extremity weakness. Physical examination revealed left abduction limitation and right-sided hypoesthesia. Kernig and Brudzinski signs were absent, and pathergy test results were negative. Laboratory evaluation revealed normal complete and differential blood counts, normal serum chemistry, and normal immune function. Analysis of her serum was negative for antiaquaporin 4 antibody, rheumatism antibody profile, and paraneoplastic profile. Serum analysis was also negative for human immunodeficiency virus type 1 and 2 RNA, hepatitis B and C antigen or antibody profile, and fluorescent treponemal antibody absorption. Cerebrospinal fluid (CSF) analysis revealed clear fluid, a normal glucose level, an elevated protein level (45 mg/dL; normal range, 20-40 mg/dL), and an elevated white blood cell count (10/mm3 [0.01 × 109/L]; normal range, 0-8/mm3 [{0-0.008} ×109/L]; 81% lymphocytes, 19% monocytes). No CSF-specific oligoclonal bands were detected. Gram staining, acid-fast staining, and lactic acid and cryptococcal antigen test results were negative. CSF did not grow any bacteria, fungus, or acid-fast bacillus at culture. Spinal cord MRI, brain MR angiography, and CT of the chest, abdomen, and pelvis revealed normal findings (images not shown). Brain MRI and gadolinium-enhanced (20 mL gadopentetate dimeglumine, BeiLu Pharmaceutical, Beijing, China) MRI were performed. The patient's clinical symptoms and imaging findings responded to treatment with a high dose of steroids. However, the patient's symptoms exhibited clinical and radiologic progression as she attempted to taper the steroid dose. She arbitrarily stopped taking the steroids and started traditional Chinese treatment instead. However, her condition was not controlled. In November 2013, she was readmitted with worsening dizziness and diplopia accompanied by hearing loss, tinnitus, slurred speech, drinking-induced cough, walking instability, and involuntary outbursts of laughter and crying. Dysmetria, ataxia, brisk tendon reflexes, pathologic reflexes, and pseudobulbar signs were observed bilaterally. Repeated biochemical and immune tests did not yield positive findings. CSF analysis revealed mild lymphocytic pleocytosis (white blood cell count, 8/mm3 [0.008 × 109/L]; 83% lymphocytes, 17% monocytes) and a slightly elevated total protein level (46 mg/dL). Brain PET revealed diffuse high metabolism in the midbrain and pons (images not shown). Whole-body PET was negative for malignancy (images not shown). Brain MRI and gadolinium-enhanced MRI were performed. The patient's clinical symptoms and imaging findings improved after treatment with a high dose of steroids. Thereafter, intravenous cyclophosphamide therapy was added after her condition deteriorated again when the prednisone dose was tapered to 20 mg per day in March 2014. Her pontocerebral symptoms were relatively stable in the following year, with apparent diminishment of lesions in the brainstem and cerebellum observed at brain PET (images not shown). Follow-up MR images were obtained in July 2014. Subsequently, the patient exhibited clinical and radiologic aggravation. MR images were obtained again in July 2015 and February 2016. The patient underwent biopsy of the right frontal lobe, and a histopathologic examination was performed in August 2015. Afterward, her condition worsened, and she died in September 2016. © RSNA, 2018.