Updated: 12/25/2021

Influenza

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  • Snapshot
    • A 50-year-old man presents to the emergency room for altered mental status, high fevers, chills, muscles aches, and headaches for 1 week. He was diagnosed with flu at an urgent care center but declined oseltamivir at the time. He had been improving until 2 days ago, when he started having fevers and chills again along with a productive cough. On physical exam, his oxygen saturation is at 90% on room air and is put on supplemental oxygen. Infectious workup, including blood cultures, are pending. A chest radiograph shows a right lower lung consolidation, and he is started on empiric antibiotics.
  • Introduction
    • Classification
      • influenza virus
        • an enveloped, segmented, negative-sense, single-stranded RNA virus with a helical capsid
        • an orthomyxovirus
        • transmission via respiratory droplets
    • Prevention
      • annual flu vaccine for those 6 months or older
        • contains multiple killed viral strains that are thought to be likely to appear during flu season
          • intramuscular
        • live-attenuated vaccine
          • intranasal
  • Epidemiology
    • Incidence
      • annual epidemics, often occurring in the winter
    • Demographics
      • affects both adults and children, though with higher frequency in children
    • Risk factors
      • advanced or young age
      • pregnancy
      • immunosuppression
      • morbid obesity
      • pulmonary disease
  • Etiology
    • Influenza A and B are causes of seasonal epidemics
    • Can originate from animal strains
      • avian flu A
      • swine flu A (H1N1)
    • Pathogenesis
      • surface protein hemagglutinin (H) and neuraminidase (N)
      • hemagglutinin binds to sialic acid and allows for viral entry into cells
      • neuraminidase allows for progeny virion release from cells
  • Presentation
    • Symptoms
      • systemic symptoms predominate
        • high fevers and chills
        • myalgias and malaise
        • headaches
        • nausea and vomiting
      • may have cough or sore throat
    • Physical exam
      • may have cervical lymphadenopathy
  • Imaging
    • Chest radiography
      • indication
        • if pneumonia is suspected
      • findings
        • consolidation or infiltrate
        • ground-glass opacities
  • Studies
    • Labs
      • rapid antigen detection tests (RADTS)
        • most commonly used
      • detection of RNA on reverse transcriptase polymerase chain reaction
  • Differential
    • Upper respiratory infection
      • distinguishing factor
        • upper respiratory symptoms (e.g., cough, congestion, etc.) often predominates rather than systemic symptoms (e.g., high fevers, chills, malaise, etc.)
  • DIAGNOSIS
    • Making the diagnosis
      • based on clinical presentation and confirmed with laboratory studies
  • Treatment
    • Management approach
      • mainstay of treatment is supportive care
      • avoid aspirin in children
        • can cause Reye disease, leading to hepatorenal failure
    • Conservative
      • supportive care
        • indication
          • all patients
        • modalities
          • hydration
          • antipyretics
    • Medical
      • oseltamivir/zanamivir
        • indication
          • patients with severe disease or at high risk of severe disease
  • Complications
    • Secondary bacterial infection
    • Myositis or myocarditis
    • Central nervous system involvement
  • Prognosis
    • May lead to severe bacterial superinfections
      • most commonly S. aureus, S. pneumoniae, and H. influenzae
    • May be fatal

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