RESUMEN
INTRODUCTION: We describe a case of intrathecal methotrexate toxicity and perform a literature review of existing cases. CASE PRESENTATION: A 23-year-old man who received diagnosis of acute lymphoblastic leukemia and started chemotherapy according to the LAL1913 protocol underwent CNS prophylaxis with intrathecal methotrexate. About 1 month after, he developed a flaccid paraparesis. CSF analysis showed albumin/cytological dissociation. Spinal MRI showed thickening of the ventral roots of the cauda equina with contrast enhancement. Nerve conduction studies showed severe lower limb motor axonal neuropathy. Needle examination showed acute denervation involving L3-S1 roots. Methotrexate was stopped, and the patient was treated with intravenous immunoglobulins, followed by high-dose intravenous methylprednisolone, with a gradual improvement. Three months later, the spine MRI was normal. Electrophysiological and imaging findings were indicative of pure motor L3-S1 polyradiculopathy. DISCUSSION: Literature review of existing cases confirm the relatively selective involvement of lumbosacral ventral roots in intrathecal methotrexate toxicity. Pathophysiologic mechanisms suggest either a direct toxicity with localized folate deficiency or an immune-mediated mechanism, the latter consistent, in our patient, with the albumin/cytological dissociation and response to immunomodulatory treatments. Pure motor polyradiculopathy of the lower limbs is rare but predictable complication of intrathecal methotrexate, which can benefit from early withdrawal and immunomodulatory treatments.