Last updated on May 12, 2019 at 12:36
Lower respiratory tract infections
Lower respiratory tract infections are a very common source of death. This isn’t surprising, as the lung is exposed to many litres of microbe-contaminated air every minute, and that smoking-related lung disorders predispose the lungs to get infected. Pneumonia is rare in cases where there are no impairments of the normal defences. It usually occur is people who have some predisposition, like:
- Suppression of the cough reflex, which can occur in anaesthesia and coma
- Decreased mucociliary clearance, which occurs in smokers
- Accumulation of mucus, which occurs in bronchiectasis and airway obstruction
- Decreased immune system activity, which decreases the lung’s ability to defend itself against microbes
- Congestion and oedema
- Already present lower respiratory tract infection
We differentiate between four types of lower respiratory tract infections:
- Bronchitis, inflammation of the bronchi. Commonly associated with coughing up mucus
- Bronchiolitis, inflammation of the bronchioli.
- Pneumonia, where there’s exudate in the alveoli
- Pneumonitis, where the inflammation is of the lung interstitium, the alveolar septa
We distinguish three types of pneumonia:
- Bronchopneumonia, where the intraalveolar exudate associated with the inflammation has a patchy distribution along the whole lung
- Lobar pneumonia, where the exudate distribution is limited at a lobe-by-lobe basis, so that one lobe is affected but the others may not be. It’s almost always caused by streptococcus pneumoniae. The affected lobe undergoes hepatization, where the consistency changes to be similar to that of the liver.
- Interstitial pneumonia, or pneumonitis, where the alveolar septa are inflamed. It’s often caused by other pathogens than for the other two types.
Clinically we divide bacterial pneumonia into two types based of how they were acquired, the community acquired pneumonias and hospital-acquired pneumonias (nosocomial pneumonias). They’re distinguished because different pathogens cause pneumonia in and out of the hospital and those that occur in a hospital setting.
Community acquired pneumonias are commonly caused by:
- Streptococcus pneumoniae
- Haemophilus influenzae
- Staphylococcus aureus
Nosocomial pneumonias are commonly caused by:
- Klebsiella pneumoniae
- Pseudomonas aerunginosa
Legionnaire disease is a disease caused by legionella pneumophila. It’s usually found in stale waters. Legionnaire pneumonia is usually quite severe and requires hospitalization.
Viral lower respiratory tract infection are more common in children than in adults. They can be caused by:
- Parainfluenza virus
- Influenza virus
- Respiratory syncytial virus
- Cytomegalovirus (CMV)
The viruses invade and kill the infected cells. This alveolar epithelial necrosis can lead to diffuse alveolar damage (DAD), which causes hyaline membrane formation.
Viral infections can often depress the immune system so that secondary infections can also infect the lung. These secondary infections are often bacterial.
The influenza virus is the most frequent cause of viral pneumonia in adults. This infection is often called the “flu” and occurs most commonly during the winter. The biggest problem with influenza is that the virus changes its surface antigens constantly, meaning that the body can never develop immunity against all types, because new types occur all the time. The influenza virus causes degenerative changes in the bronchial epithelium and mucosa, destroying them and predisposing to secondary bacterial infection. The virus most commonly affects infects the pharynx and tracheobronchial tree, and only rarely causes outright pneumonia.
Severe acute respiratory distress syndrome (SARS) is a relatively recently discovered airborne disease. It was discovered in 2002, when it had an outbreak in China and many people died. It’s caused by a virus called coronavirus, which causes diffuse alveolar damage.
Immunosuppression-related cytomegalovirus infection occurs in immunosuppressed people, often in people who have received transplants or have AIDS. Recipients of organs like the heart, liver and kidney may receive the virus from the donor organ, as the virus can ride along with the organ. The disease mainly causes pneumonitis and colitis.
For histology of cytomegalovirus infection, see the slide.
Aspergillosis is a disease caused by the fungus aspergillus that causes necrotizing pneumonia. It almost exclusively infects immunocompromised people. An aspergilloma, also known as a fungus ball, is a clump of aspergilloma that exist in a body cavity, like bronchi, cavities after tuberculosis or abscesses. It rarely invades the lung parenchyme.
Pneumocystis jirovecii is a fungus that causes pneumocystis pneumonia. It too almost exclusively affects immunocompromised people, especially people with AIDS.
Aspiration pneumonia affect people with decreased gag and swallowing reflexes, like debilitated patients, drunk people and stroke patients. They can the aspire gastric contents, which will irritate the alveoli. Bacteria from the oropharynx infect the lung, causing necrotizing pneumonia.
Many types of chronic pneumonia exist, however tuberculosis is the most important. Most types of chronic pneumonia are granulomatous inflammations.
Lung abscess is a potential complication of several conditions:
- Aspiration of infective material, from carious teeth or such
- Aspiration of gastric content
- In necrotizing pneumonias
- In bronchial obstruction
- Septic embolism, an embolism that carries bacteria, like from an infective endocarditis of the heart
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