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Background Mycosis fungoides is the most common cutaneous T-cell lymphoma. Central nervous system (CNS) involvement of this disease is very rare and has a poor prognosis (median 9 weeks from CNS involvement to death). Features include ataxia, confusion, and distal weakness however there are limited cases described in literature.Case JW is a 62-year-old male presenting with a six-week history of confusion, fatigue and new urinary incontinence. This is on a relevant background of cutaneous T-cell lymphoma diagnosed four years earlier, from which he was thought to be in remission following treatment with steroids, narrow band UVB and peginterferion-alfa-2a injections. Initial examination revealed disorientation and widespread rash but no focal neurology. Despite a normal initial CT-brain, PET brain revealed diffusely increased metabolism in the cerebellum, brain stem and cerebrum. Lumbar puncture revealed T lymphocytes but no definitive evidence of clonality or malignant cells. Over a period of 2 weeks, JW developed progressive right-sided weakness and reduced level of consciousness requiring intubation. A cerebellar biopsy was performed, showing histology and flow cytometry consistent with a large cell transformation of mycosis fungoides. Unfortunately his disease had progressed by this time and he passed away a short time later. He did not receive any disease-specific treatment due to disease extent at the time of diagnosis.Discussion CNS involvement in mycosis fungoides is rare and aggressive, highlighting the importance of early disease recognition. Treatment typically involves methotrexate-based chemotherapy with or without radiotherapy, however there is no established standard of care due to its rarity.