Rosen & Barkin's 5-Minute Emergency Medicine Consult (395 page)

BASICS

• Acute, usually self-limited, viral infection
  
• Transmission: By dispersion in small-particle aerosols created by sneezing, coughing, and talking
  
• Virus is deposited on respiratory tract epithelium and absorbed.
  
• Incubation period: 1–4 days (avg. 2)
  
• Mean duration in adults: 4 days
  
• Seasonal outbreaks most common in February.
  
• 2009 novel H1N1 pandemic peaked in fall and early winter of that year. Children and pregnant women had particularly high complication rates.
  
• Complications:
  
  • Primary influenza viral pneumonia
    
  • Secondary bacterial pneumonia
    
  • Exacerbations of COPD
    
  • Otitis and sinusitis in children
    
  • Reactive airway disease
    
  • Rare complications: Myositis, myocarditis, pericarditis, Guillain–Barré syndrome, and aseptic meningitis
    
  • ARDS and multisystem organ failure
    
• Key features:
  
  • Seasonal epidemics are spread by high attack rates in immunologically naive children.
    
  • Intermittent unpredictable pandemics
    
  • Mortality results largely from pulmonary complications.
    

• Children exhibit more lower respiratory involvement (croup, bronchitis, bronchiolitis, pneumonitis) and higher temperatures than adults.
  
• Children were particularly susceptible to complications of novel H1N1 influenza virus.
  
• Myalgias in the calf muscle
  
• Febrile convulsions occur in ∼10% of children <5 yr of age with influenza infection.
  
• Reye syndrome:
  
  • Influenza may be a predisposing factor.
    
  • Rare and severe complication associated with salicylate use (children taking aspirin chronically are recommended to get vaccinated)
    
  • Acute liver and brain injury
    

• Caused usually by 1 of 2 influenza types, A or B, the latter usually less severe.
  
• Influenza A subtypes are classified by hemagglutinin antigens H1, H2, or H3 and less importantly by the neuraminidase subtype.
  
• Vaccine targets the subtype antigen, which is also the target of natural immunity.
  
• Annual epidemics are seasonal:
  
  • Caused by
    antigenic drift
    —new variants from minor changes in surface protein
    
  • Duration of epidemic <6 wk
    
• Pandemics:
  
  • Unpredictable
    
  • Caused by
    antigenic shift
    —major changes in virus structure
    
• Waterfowl reservoir of influenza virus
  
• Avian flu has proven difficult to transmit to humans and between humans, but infection is often very severe.
  
• The 3 most common strains in 2012 (most vaccinations cover) were influenza B viruses, influenza A (H1N1), and influenza A (H3N2)
  

DIAGNOSIS

• Complicated by similar acute infections caused by other respiratory viruses
  
• CDC defines influenza-like illness (ILI) as cough or sore throat in a patient with fever >100°F and no alternative diagnosis.
  

• Local status of the epidemic (see CDC weekly status update
  http://www.cdc.gov/flu/weekly/
  ) is by far the most important predictor of influenza in a patient with ILI.
  
• Despite poor discriminating properties of specific symptoms, a rise in ILI cases accurately predicts onset of the seasonal influenza epidemic.
  

• No single finding on history has much predictive power. Influenza can be asymptomatic or fatal.
  
• Fever and cough together is somewhat specific for influenza but insensitive.
  
• Specificity of findings depends on prevalence of other circulating viruses. E.g., RSV epidemics are also accompanied by high frequency of fever.
  

• Fever: Degree of fever is correlated with likelihood of influenza in randomized trials of persons with ILI.
  
• There is no consistent relationship between physical findings and influenza positivity across multiple studies, but there are very few studies of ED patients.
  
• Many patients have evidence of reactive airway disease with bronchoconstriction.
  

Elderly may present with high fever, lassitude, and confusion without pulmonary complications.

Clinical diagnosis based on the signs and symptoms of influenza during the winter months in the setting of a known outbreak

• CBC (optional):
  
  • WBC: Normal to mildly decreased
    
• Pulse oximetry/arterial blood gas for significant pulmonary symptoms
  

CXR for prominent lower respiratory signs or symptoms:

• Normal (50–90%)
  
• Bilateral interstitial infiltration
  

• Culture of nasopharyngeal swab or aspirate is more sensitive than pharynx.
  
• Yield declines rapidly with duration of symptoms. Infrequently positive after day 2.
  
• Rapid influenza diagnostic tests and direct fluorescent antibody tests are inexpensive, rapid, and specific but often of very low sensitivity. Some are able to discriminate between A and B, but not subtypes of A.
  
• Polymerase chain reaction (PCR) tests have short turnaround time, are both sensitive and specific, and can discriminate H1 from H2 antigens; a combination of H1 negative and H2 negative is very specific for 2009 H1N1.
  
• Viral culture: Turnaround time too long for ED use, although OK for local surveillance.
  

• Other respiratory viruses
  
• Bronchitis
  
• Atypical pneumonia
  
• Epstein–Barr infection (infectious mononucleosis)
  
• Anthrax is very rare and much more likely to include dyspnea and nausea.
  

TREATMENT

Vaccination and respiratory hygiene for EMS personnel during outbreaks

Aggressive fluid resuscitation, supplemental oxygen, and positive-pressure ventilation as clinical circumstances dictate

• Supportive and symptomatic:
  
  • Antipyretics (acetaminophen or NSAIDs)—avoid aspirin
    
  • Cough suppressants (rarely useful)
    
  • Rehydration
    
• Antivirals are effective if given within 48 hr of symptom onset:
  
  • Antiviral resistance patterns vary each season; confirm at CDC update page.
    
  • The neuraminidase inhibitors (NI) zanamivir and oseltamivir are generally active against types A and B.
    
  • The adamantanes amantadine and rimantadine are only effective against influenza A, but not current strains (currently not recommended for use due to resistance).
    
  • Antivirals reduce symptom duration by less then 1 day. Indirect evidence of benefit in severe disease.
    
  • Costly, except for amantadine
    
  • Recommended for:
    
    • Patient with severe illness
      
    • Immunocompromised patients
      
    • Patients at high risk for complications
      
• PREVENTION:
  
  • Inactivated, polyvalent influenza vaccine recommended annually for:
    
    • Adults >50 yr
      
    • Residents of nursing homes and long-term care facilities
      
    • Children of age 6–4 yr
      
    • Children of age 6 mo–18 yr on chronic aspirin therapy
      
    • High-risk individuals (asthma, COPD, cardiovascular disease, immunocompromised, diabetics)
      
    • Health care workers
      
    • Morbidly obese (BMI >40)
      
    • American Indians/Alaska natives
      
    • Caretakers of children <6 mo old
      
    • Women who will be pregnant during influenza season
      
  • Attenuated-live intranasal vaccine (FluMist) is currently approved for healthy people of age 2–49 yr.
    
  • Contraindicated for:
    
    • Pregnant women
      
    • Close contacts and health care workers for severely immunocompromised patients
      
    • Children <5 yr with h/o recurrent wheezing
      
    • Children/adolescents receiving aspirin
      
    • Severe allergy to chicken eggs
      
    • People with comorbidities placing them in high risk for influenza complications
      
  • Chemoprophylaxis in the following settings:
    
    • Postexposure prophylaxis for exposed family members, especially high risk
      
    • Short-term prophylaxis during outbreak of influenza A in high-risk patients who did not receive vaccine
      
    • In conjunction with vaccine in high-risk patients (including those with HIV infections) expected to respond poorly to vaccine
      
    • In lieu of vaccine when vaccine is contraindicated in high-risk individuals
      
    • In individuals providing care for high-risk persons
      
    • Extended duration, season-long prophylaxis of health care workers is effective but consumes large quantity of stockpiled drug.
      
    • Exceptions:
      
      • Could interfere with live-virus vaccine
        
      • Should not be started for at least 2 wk after inoculation
        
• Patients with evidence of bronchoconstriction and reduced breath sounds may benefit from bronchodilators such as albuterol.
  

• Inactivated vaccine is recommended for women expected to be pregnant during influenza season.
  
• Live-attenuated virus is contraindicated in pregnancy.
  

• Oseltamivir: 75 mg PO BID for 3–4 days:
  
  • Postexposure prophylaxis: 75 mg PO daily for 7–10 days
    
• Zanamivir: 10 mg nasal insufflation (2 inhalations) q12h for 3–5 days
  
• Amantadine: 200 mg PO initially, then 100 mg PO BID for 3–5 days (halve dose if age >65 yr)
  
• Rimantadine: 200 mg PO initially, then 100 mg PO BID for 3–5 days
  
• Albuterol 2.5 mg in 3 mL by nebulizer or metered-dose inhaler with spacer