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Pathogenesis, Clinical Features and Diagnosis

Complications of influenza

Influenza-associated pneumonia

In otherwise healthy individuals, influenza infection normally results in an uncomplicated URTI that resolves within 1–2 weeks. However, pneumonia is a relatively common complication (5–38% with influenza A and 10% with influenza B), predominantly in the elderly, patients with chronic cardiopulmonary disease, pregnant women and immunocompromised individuals. The aetiology may be viral, bacterial or mixed viral–bacterial. 12 x TR Cate. Clinical manifestations and consequences of influenza. Am J Med 82 (1987) (15 - 19) Crossref. Such patients can deteriorate rapidly and mortality can be close to 50%. 13 x MG Ison, FG Hayden. Viral infections in immunocompromised patients: what's new with respiratory viruses?. Curr Opin Infect Dis 15 (2002) (355 - 367) Crossref.

In primary viral pneumonia, typical influenza is followed by a rapid progression (over 2–3 days) of fever, cough, dyspnoea, chest pain and cyanosis. Physical examination and chest X-ray disclose diffuse bilateral infiltrates consistent with acute respiratory distress syndrome. If fatal, death usually occurs within 4–5 days of first symptoms.

Combined viral–bacterial pneumonia is more common than primary viral pneumonia. Of patients with a severe pneumonia, 75% will have secondary bacterial infection. In these cases, the individual will appear to be recovering from the influenza illness and then have a recurrence of the respiratory symptoms. The bacteria most commonly involved are Staphylococcus aureus and Streptococcus pneumoniae, with Haemophilus influenzae being less common. There is evidence that influenza infection actively facilitates the pathogenicity of bacteria and the impact of illness, causing immunosuppression. In the 1957–58 pandemic, mortality from staphylococcal pneumonia (28%) was similar in all age groups and twice as high as mortality from other pneumonias (12%), which tended to occur in the elderly and those with chronic medical conditions. Once the responsible pathogen has been identified, appropriate antibiotic treatment should be initiated promptly.

Bacterial pneumonia as a complication of influenza also has a different presentation from primary viral pneumonia. Again, patients initially show clinical improvement from illness and then develop worsening respiratory symptoms. In this case, physical and chest X-ray examinations are more likely to show localized signs of consolidation.

Influenza B virus can cause the same spectrum of disease as that seen after influenza A virus infection, and severe illness can occur, particularly in the elderly.

Other respiratory complications

Exacerbations of asthma, chronic obstructive pulmonary disease and cystic fibrosis are common complications of influenza illness. Acute bronchitis develops in 30% of cases and, less commonly, lung abscess and empyema may be observed. Three-quarters of children with asthma will suffer an exacerbation. In hospitalized children, influenza accounts for 16% of asthmatic exacerbations and 4–13% of exacerbations of cystic fibrosis. The most common respiratory complications in hospitalized children are acute bronchitis (12–26%), croup (5–15%) and pneumonia (5–8%).

Non-respiratory complications

Myositis is reported more frequently in children with influenza B, but adults may also be affected and may develop rhabdomyolysis with acute renal failure. 14 x L Berry, S Braude. Influenza A infection with rhabdomyolysis and acute renal failure – a potentially fatal complication. Postgrad Med J 67 (1991) (389 - 390) Crossref. Cardiac complications, specifically myocarditis, have been described in patients with influenza A and B, but these complications are mostly asymptomatic. 15 x M Miura, Y Asaumi, Y Wada, et al.. A case of influenza subtype A virus-induced fulminant myocarditis: an experience of percutaneous cardio-pulmonary support (PCPS) treatment and immunohistochemical analysis. Tohoku J Exp Med 195 (2001) (11 - 19) Crossref. Pericarditis has been reported rarely. ECG abnormalities are common and usually transient (81% of hospitalized patients, 43% of community patients), but underlying conditions can cause fatal arrhythmias or cardiomyopathies. CNS complications are rare and range from irritability and confusion to psychosis and severe encephalopathy due to a variety of inflammatory processes, including Reye's syndrome. 16 x S Kimura, N Ohtuki, A Nezu, M Tanaka, S Takeshita. Clinical and radiological variability of influenza-related encephalopathy or encephalitis. Acta Paediatr Jpn 40 (1998) (264 - 270) Crossref. Recovery is usually complete. The link between the 1918–20 pandemic and encephalitis lethargica remains unproven. A small increase in the incidence of Guillain–Barré syndrome (GBS) (1–2 excess cases per million against a background of 10–20 cases per million) has been reported after influenza vaccination. 17 x T Lasky, GJ Terracciano, L Magder, et al.. The Guillain–Barré syndrome and the 1992–1993 and 1993–1994 influenza vaccines. New Engl J Med 339 (1998) (1797 - 1802) Crossref. However, it is unclear whether this marginal increase is truly associated with influenza vaccination, while, even if it is, the risk is considerably smaller than the risk of death from influenza or its complications, particularly in high-risk groups (see below). Febrile convulsions are common in hospitalized children (20–50%). 18 x U Heininger. An update on the prevention of influenza in children and adolescents. Eur J Pediatr 162 (2003) (828 - 836) Crossref.

References

Label Authors Title Source Year
12

References in context

  • The clinical presentation of influenza ranges from an asymptomatic infection or a self-limiting upper respiratory tract infection (URTI) to a severe illness, often with serious, potentially fatal, complications.12 The incubation period is 1–4 days, with an average of 2 days.
    Go to context

  • In otherwise healthy individuals, influenza infection normally results in an uncomplicated URTI that resolves within 1–2 weeks.
    Go to context

TR Cate. Clinical manifestations and consequences of influenza. Crossref. Am J Med 82 (1987) (15 - 19) 1987
13

References in context

  • In otherwise healthy individuals, influenza infection normally results in an uncomplicated URTI that resolves within 1–2 weeks.
    Go to context

MG Ison, FG Hayden. Viral infections in immunocompromised patients: what's new with respiratory viruses?. Crossref. Curr Opin Infect Dis 15 (2002) (355 - 367) 2002
14

References in context

  • Myositis is reported more frequently in children with influenza B, but adults may also be affected and may develop rhabdomyolysis with acute renal failure.14 Cardiac complications, specifically myocarditis, have been described in patients with influenza A and B, but these complications are mostly asymptomatic.15 Pericarditis has been reported rarely.
    Go to context

L Berry, S Braude. Influenza A infection with rhabdomyolysis and acute renal failure – a potentially fatal complication. Crossref. Postgrad Med J 67 (1991) (389 - 390) 1991
15

References in context

  • Myositis is reported more frequently in children with influenza B, but adults may also be affected and may develop rhabdomyolysis with acute renal failure.14 Cardiac complications, specifically myocarditis, have been described in patients with influenza A and B, but these complications are mostly asymptomatic.15 Pericarditis has been reported rarely.
    Go to context

M Miura, Y Asaumi, Y Wada, et al.. A case of influenza subtype A virus-induced fulminant myocarditis: an experience of percutaneous cardio-pulmonary support (PCPS) treatment and immunohistochemical analysis. Crossref. Tohoku J Exp Med 195 (2001) (11 - 19) 2001
16

References in context

  • CNS complications are rare and range from irritability and confusion to psychosis and severe encephalopathy due to a variety of inflammatory processes, including Reye's syndrome.16 Recovery is usually complete.
    Go to context

S Kimura, N Ohtuki, A Nezu, M Tanaka, S Takeshita. Clinical and radiological variability of influenza-related encephalopathy or encephalitis. Crossref. Acta Paediatr Jpn 40 (1998) (264 - 270) 1998
17

References in context

  • A small increase in the incidence of Guillain–Barré syndrome (GBS) (1–2 excess cases per million against a background of 10–20 cases per million) has been reported after influenza vaccination.17 However, it is unclear whether this marginal increase is truly associated with influenza vaccination, while, even if it is, the risk is considerably smaller than the risk of death from influenza or its complications, particularly in high-risk groups (see below).
    Go to context

T Lasky, GJ Terracciano, L Magder, et al.. The Guillain–Barré syndrome and the 1992–1993 and 1993–1994 influenza vaccines. Crossref. New Engl J Med 339 (1998) (1797 - 1802) 1998
18

References in context

  • Myositis is reported more frequently in children with influenza B, but adults may also be affected and may develop rhabdomyolysis with acute renal failure.14 Cardiac complications, specifically myocarditis, have been described in patients with influenza A and B, but these complications are mostly asymptomatic.15 Pericarditis has been reported rarely.
    Go to context

U Heininger. An update on the prevention of influenza in children and adolescents. Crossref. Eur J Pediatr 162 (2003) (828 - 836) 2003

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