Isolated Flaccid Limb Weakness in a Toddler: The Timing of Spinal MRI Matters.

Acute flaccid myelitis (AFM) and Guillain-Barré syndrome (GBS) share overlapping clinical features, making differentiation challenging. Although characteristic spinal cord gray matter lesions are typically present in AFM, magnetic resonance imaging (MRI) findings may evolve during the disease course. In later stages, isolated ventral nerve root enhancement can be observed without visible gray matter lesions, mimicking the radiologic features of GBS. We report a child in whom AFM was ultimately favored, although the initial radiologic impression suggested GBS. A 23-month-old boy developed acute flaccid weakness of the left leg shortly after a febrile illness, which persisted for one month before presentation. Spinal MRI revealed ventral nerve root enhancement without gray matter lesions, a pattern suggestive of GBS; however, the clinical presentation was more consistent with AFM. Over two years of follow-up, persistent unilateral weakness and marked limb atrophy supported AFM as the more likely diagnosis. This case highlights the temporal evolution of imaging findings in AFM and emphasizes that delayed imaging may obscure characteristic features, potentially leading to diagnostic confusion with GBS.