Croup, medically known as laryngotracheobronchitis, is the inflammation and swelling of the voice box (larynx) and windpipe (trachea). This swelling narrows the upper airway, causing the characteristic “seal-like” barking cough and noisy, high-pitched breathing called stridor. While croup is most common in young children, it can affect adults. Because the adult airway is larger, a greater degree of inflammation is required to produce noticeable symptoms. When croup does occur in an adult, the symptoms are often severe and may require intensive medical care.
Primary Viral and Bacterial Causative Agents
Viral infection is the most frequent cause of croup in both children and adults. The Parainfluenza Viruses (HPIV), particularly types 1 and 2, are the most common viral culprits, accounting for the majority of cases. These viruses initiate infection in the nasopharynx before spreading to the tracheal mucosa, causing edema that constricts the subglottic region.
Other respiratory viruses implicated in adult croup include Influenza A and B, Respiratory Syncytial Virus (RSV), and Adenovirus. Influenza-associated croup tends to be more severe than cases caused by Parainfluenza and is more likely to be complicated by a secondary infection.
Bacterial causes are less common but generally result in a more severe presentation known as bacterial tracheitis. This can occur as a primary infection or as a secondary complication following a viral episode. Specific agents, such as Mycoplasma pneumoniae, are rare causes of croup. Secondary bacterial infections, often caused by Staphylococcus aureus, can also invade inflamed tissue, worsening the condition and requiring antibiotic intervention.
Non-Infectious Triggers and Environmental Irritants
While infectious agents are the dominant cause, non-pathogenic factors can also induce the laryngeal and tracheal swelling characteristic of croup in adults. Exposure to harsh environmental irritants directly inflames the airway lining. Examples include heavy exposure to smoke (such as from wildfires or industrial sources), chemical fumes, or high concentrations of dust and particulates.
Mechanical trauma or irritation to the throat and neck region can also trigger localized swelling. Recent medical procedures, such as intubation for surgery, can cause temporary irritation that mimics croup symptoms. Conditions like Laryngopharyngeal Reflux (LPR) or Gastroesophageal Reflux Disease (GERD) can predispose individuals to croup-like symptoms. Stomach acid refluxing into the upper airway causes persistent irritation, making the tissue more susceptible to swelling.
Adult-Specific Factors Increasing Susceptibility
Smoking is a primary factor, as nicotine and tar damage the protective mucosal lining and impair the immune response within the respiratory tract. This chronic injury increases the likelihood that a common virus will cause the severe, localized inflammation required to obstruct the larger adult airway.
Immunosuppression also increases susceptibility, as a weakened immune system cannot mount an effective defense against common viruses. This can be due to medications taken for autoimmune diseases or organ transplants, or from chronic conditions like HIV. Even typical respiratory pathogens can lead to unusually severe upper airway disease.
Underlying chronic respiratory conditions, such as Chronic Obstructive Pulmonary Disease (COPD) or asthma, compromise the airway’s ability to clear pathogens and manage inflammation. Pre-existing anatomical factors, such as narrowing from prior radiation therapy, airway surgery, or structural abnormalities, reduce the functional size of the trachea. When these narrower passages swell due to infection, obstruction becomes severe more quickly than in a healthy airway.