To describe a pediatric case presenting with subacute progressive hemiplegia due to a lesion involving the thalamus and posterior limb of the internal capsule with associated midline shift, later diagnosed as tumefactive neuromyelitis optica spectrum disorder (NMOSD).

Tumefactive demyelinating lesions (TDLs) are uncommon even in multiple sclerosis (MS) and occur even more rarely in NMOSD. These lesions pose significant diagnostic challenges, and in atypical cases, accurate diagnosis often requires multidisciplinary evaluation.

The true prevalence of TDLs remains uncertain, occurring in approximately 2% of MS cases. In NMOSD, TDLs are exceptionally rare and reported only in isolated cases. Our case is notable for its pediatric presentation. These lesions often carry a poor prognosis, and it remains unclear whether they represent a distinct demyelinating subtype or share similar treatment responses with non-tumefactive forms. Potential differences in histopathologic and biomarker profiles warrant further study, highlighting the importance of multidisciplinary collaboration among neurology, neuroradiology, and neuropathology teams.