Adult H3 K27-altered, H3.3 K27-mutant diffuse midline glioma affecting the conus medullaris: illustrative case.

backgroundDiffuse midline glioma (DMG) H3 K27-altered is a rare CNS tumor that predominantly affects midline structures in children. A relatively new subtype of glioma, it was first classified in 2016 and was further expanded in 2021 to include 4 molecular subtypes. While reported on in children, this is the first reported case of an H3.3 K27-mutant subtype of DMG affecting the conus medullaris in adults. OBSERVATIONS: The authors report the case of a 62-year-old man with gradual onset of bladder and lower-limb dysfunction over a 6-month period. Because of the patient's synchronous diagnosis of lymphoma, CSF sampling and positron emission tomography (PET) were initially utilized. Ultimately, an open biopsy was required to yield a diagnosis of H3 K27-altered DMG. After multiple disciplinary team discussions and discussions with the patient, radiotherapy was commenced. LESSONS: H3 K27-altered DMG affecting the conus medullaris is a very rare tumor that can present with gradual-onset lower-limb dysfunction and can be difficult to diagnose on traditional imaging alone. This case emphasized the continued importance of tissue sampling, with further research required on the utility of PET, CSF sampling, and the significance of molecular subtyping on treatment response and prognosis. https://thejns.org/doi/10.3171/CASE24879.