Parainfluenza

Background

Human parainfluenza virus (HPIV) is an enveloped, negative-sense, single-stranded RNA virus that is most commonly known as a causative agent of croup (laryngotracheobronchitis). There are 4 known serotypes of the parainfluenza virus and it is capable to causing both upper and lower airway disease. Though often thought of as a relevant primary to young children (under the age of five) because of their relatively narrow airway passages, immunocompromised adults and those with underlying chronic cardiac or respiratory conditions are also at risk of serious complications from infection. The virus itself is related to but of a different family classification as the influenza virus.

Clinical Features

HPIV infection can result in a large number of varied presentations.
Pediatric Patients:

• Croup: Peak age is 1-2 years, with boys slightly more likely to present than girls. Infection begins in upper airway but often lower airway signs (wheezing, air trapping)
• Bronchiolitis: Causative agent in 10-20% of diagnosed bronchiolitis. Initially fever and congestion for 1-3 days followed by lower airway symptoms, including cough and wheezing. Most recover within 21 days, though children with underlying pulmonary conditions are a risk of prolonged course and greater complications
• Pneumonia: Indistinct from other presentations of viral pneumonia with a similar overall disease course as bronchiolitis.
• Tracheobronchitis: Inflammation of only the large airways in the absence of evidence of croup or pneumonia.

Up to half of the above are complicated by concurrent otitis media

Immunocompetent Adult Patients:

• Generally mild URI cold symptoms, though can also initiate exacerbation of COPD, asthma, or CHF

Immunocompromised Adult Patients:

• Often intially URI cold symptoms but high risk to progress to pneumonia

Differential Diagnosis

The presentation of HPIV shares many characteristics with other viruses including RSV and influenza virus. It must also be distinguished from bacterial causes of upper and lower respiratory infections. The following is a non-exhaustive list for conditions that may share characteristics with HPIV:

Pediatric Patients

• Foreign object airway obstruction
• Other viral infection (including RSV, influenza virus, adenovirus, rhinovirus, COVID)
• Bacterial respiratory infection

Adult Patients

• COPD exacerbation
• Asthma exacerbation
• Other viral infection
• Bacterial respiratory infection

Influenza-Like Illness

• Influenza
• Parainfluenza
• URI
• Pneumonia
• Sinusitis
• Toxic exposure
• Pyelonephritis
• Bronchitis
• Coronavirus
  • MERS-CoV
  • SARS-CoV
  • 2019-nCoV (COVID-19)

Evaluation

Initial evaluation of the patient with HPIV infection centers around assuring the stability of the airway. Any signs of airway compromise should be addressed aggressively up to and including intubation. While wheezing is often a hallmark of HPIV infection, as with other airway conditions the absence of wheezing may suggest significant airway obstruction and should not necessarily be viewed as a reassuring sign.

Once emergent airway concerns are addressed, the patient should be examined for signs of mechanical airway obstruction rather than infection, as this will change management. In the absence of known ingestion/aspiration, these signs may include diminished breath sounds on one side or unilateral wheezing.

Workup

Chest X-Ray is often helpful in confirming diagnosis of a viral syndrome, the canonic "steeple sign" is not a unique finding to HPIV infection. In the absence of signs of dehydration or other concurrent conditions (immunocompromised, COPD exacerbation, asthma exacerbation, CHF exacerbation), additional workup is not typically needed.

Diagnosis

In general, in the absence of warning signs in the unhospitalized patient it is not especially important to specifically diagnose a parainfluenza virus infection. In the emergency department context, PCR assays are the most likely confirmatory testing to yield results in a reasonable timeframe.

Management

There are not currently any antiviral agents for the treatment of HPIV. Therefore, management of the patient with HPIV infection is largely symptomatic. In the case of croup, racemic epi may provide symptomatic relief while early corticosteroids may reduce the need for intubation. ~8-15% of children with croup require hospitalization while 1-3% may require intubation.

Disposition

Generally, pediatric patients without signs of respiratory distress may be discharged and managed symptomatically in the outpatient setting. Immunocompromised adults and those with concurrent COPD exacerbation, asthma exacerbation, or CHF exacerbation typically require admission for monitoring and management of symptoms.