Clinical Reasoning: Rapidly progressive gait disorder and cranial nerves involvement in a 9-year-old boy.

A previously healthy 9-year-old boy presented with a history of fatigue for 3 weeks, pain in both calves for 2 days, and a mild cough, without fever. His medical history was negative and vaccinations were up to date. Examination by his pediatrician only showed decreased deep tendon reflexes (DTR) in both lower limbs. Blood count (white blood cells [WBC] 5.4 G/L, hemoglobin [Hb] 138 g/L, thrombocytes [Tc] 251 G/L), thyroid-stimulating hormone (1.76 mU/L), creatine kinase (CK) level (104 U/L), and erythrocyte sedimentation rate (3 mm/h) were all normal. Epstein-Barr virus monotest was negative and blood serology suggested a past infection.