Clinical reasoning: an unexpected diagnosis in a 4-month-old infant with lethargy and H1N1 influenza.

Ann E. Hyslop, MD Brian S. Droker, MD Laura A. Jansen, MD, PhD SECTION 1 A 4 month-old former full-term male infant presented with 4 days of lethargy, fussiness, low-grade fever, and dehydration, prompting hospital admission. His mother had tested positive for H1N1 influenza the week before, and treatment with oseltamivir was started. Oseltamivir treatment was started empirically in the infant, but he was brought in for emergency care for progressively worsening symptoms, including poor breastfeeding and decreased urine output. He was otherwise previously healthy and developmentally normal and lived with his parents and 4 siblings on a farm in rural Washington. On admission, the patient was febrile, lethargic, and dehydrated. After an episode of emesis and subsequent increased respiratory effort, he was intubated and transferred to the intensive care unit. At the time of the initial neurology service consultation, on hospital day 2, he had recently received doses of morphine and fentanyl but was responsive to gentle physical stimulation. Although he did not track visual stimuli, he had intact horizontal eye movements on oculocephalic testing and 4-mm briskly reactive pupils bilaterally. His gag reflex was weak, and he appeared to have mild facial diplegia, but this was difficult to assess because of endotracheal tube taping. Spontaneous extremity movements were infrequent, and although his resting position was normal, appendicular tone was mildly low. His reflexes were 1 at the brachioradialis, biceps, knees, and ankles. Plantar responses were upgoing. Results of initial electrolyte analyses and complete blood count were normal, and C-reactive protein level was 3.7 mg/dL (normal 0.7 mg/dL). Results of a head CT scan and routine CSF studies were normal. Blood and CSF cultures remained negative. CSF herpes simplex virus 1 and 2, enterovirus, and parechovirus PCR assays yielded no detectable copies. Results of nasal wash fluorescent antibody testing and PCR were positive for influenza A, subtype H1N1. Brain MRI showed normal results. The serum creatine kinase level was 20 IU/L, and evaluation for metabolic disorders was unremarkable. His lethargy was attributed to an influenzaassociated encephalopathy.