A case of adult-onset diffuse midline glioma diagnosed by histone H3K27M immunostaining after death, which required differentiation from brainstem encephalitis and myelitis

H3K27M-mutant diffuse midline glioma is a malignant glioma that was newly added to the World Health Organization classification in 2016. In diffuse midline glioma, the H3K27M-mutant sometimes develops sub-acutely and requires a differential diagnosis from encephalomyelitis. Here, we report a case of adult-onset diffuse midline glioma that was diagnosed by immunostaining for histone H3K27M after death. The present case required a differential diagnosis of autoimmune cervical myelitis and brainstem encephalitis.