A 15-month-old boy was admitted to our hospital due to apathetic facial expressions since 2 months, recurrent cough since 1 month, and developmental regression since half a month. Two months previously (at the age of 13 months), the patient appeared to show a lack of proper responses to stimulations, including dull eyes, decreased crying and laughing, and decreased verbal communication with his parents. Then, he developed a monophonic cough accompanied by a slight fever and runny nose. Half a month ago (at the age of 14 months), he exhibited a gradual regression in motor function, and was unable to walk or sit steadily. He was also unable to express himself and could only speak in one- or two-word responses compared with two- to three-word phrases before. The patient's milk intake had decreased from a feed of 150 ml to a feed of 30–40 ml. The patient had no history of other illnesses and no family history of genetic diseases, CNS diseases, autoimmune diseases, developmental delay, or other chronic diseases. A physical examination showed that the patient was awake with apathetic facial expressions. A chest examination revealed rough breath sounds without rales or wheezing. The patient could raise his hands and legs but could not stand or walk steadily due to hypotonia. The Babinski sign was positive on both sides, and the meningeal irritation sign was negative. Laboratory tests performed on admission showed the following: white blood cell count, 19.21 × 109/L (normal range, 4–9 × 109/L); procalcitonin, 0.74 ng/ml (normal range, <0.15 ng/ml); and blood lactate, 4.2 mmol/L (normal range, 0.5–1.7 mmol/L) (). Blood cultures were positive for Staphylococcus hominis subsp., which was susceptible to vancomycin and levofloxacin. A CSF examination revealed a lactate level of 3.5 mmol/L (normal range, 0.999–2.775 mmol/L); the results of CSF cytology, biochemistry, and culture examinations were all normal. Cell-based assays were performed to assess the serum levels of anti-AQP4–IgG, anti-myelin oligodendrocyte glycoprotein, anti-glial fibrillary acidic protein, and anti-myelin basic protein antibodies: only the serum anti-AQP4–IgG test was positive (), with a titer of 1:10 (initial titer, 1:1). Identical oligoclonal bands in the serum and CSF were observed (“mirror pattern,” type 4). Antibodies associated with autoimmune encephalitis were not detected. The auditory brainstem response test showed bilateral prolongation of wave V, and III–V and I–V interwave latencies. The wave amplitudes were decreased. Visual evoked potential testing showed that the P100 wave had a lower amplitude and a latency delay in both eyes. Cranial magnetic resonance imaging (MRI) revealed bilateral symmetrical lesions of abnormal signal intensity around the third ventricle, pons, dorsal medulla oblongata, and in the gray matter around the dorsal aqueduct on T2-weighted images and fluid attenuated inversion recovery (FLAIR) images (). Spinal MRI showed no abnormalities in the cervical, thoracic, and lumbar spine. A chest x-ray revealed bilateral pneumonia. An antibiotic treatment regimen of vancomycin was initiated according to the identified bacteria (Staphylococcus hominis subsp.) and drug-sensitivity results.