Pneumonia - Introduction, Pathogenesis, Clinical Symptoms, Typical, Atypical
Introduction to Pneumonia
Pneumonia is the inflammation of the lower tract involving the lung's airways and supporting structures. It is the major cause of illness and death. There are major categories of pneumonia -
Community-acquired pneumonia
Hospital-acquired pneumonia (within 2 days of admission)
Ventilator-associated (within 2 days of admission)
healthcare-associated – affects only patients hospitalized
* (in an acute care hospital for 2 or more days within 90 days of infection from a long-term care facility/patients who have received recent intravenous antibiotic therapy, chemotherapy/wound are within 30 days of current infection/who have attended a hospital or hemolysis clinic).
Once the micro-organisms have successfully invaded the lung, it affects the alveolar spaces and their supporting structure, the interstitial and the terminal bronchioles causing disease.
Pathogenesis of Pneumonia
Pathogens can cause infection of the lung by 4 possible routes.
By upper airway colonization or infection that subsequently extends to the lungs
By aspiration of organisms (avoiding URT defenses)
By inhalation of airborne droplets containing micro-organisms
By seeding of the lung through blood from a distant site of infection
Viruses cause primary infection of the respiratory tract, as well as inhibit host defenses leading to secondary bacterial infection. Eg: viruses destroy respiratory epithelium and disrupt normal ciliary activity encouraging an influx of non-specific immune effector cells exacerbating the damage and predisposing patients to secondary bacterial infection.
Aspiration of oropharyngeal contents is important in the pathogenesis of many types of pneumonia. Aspiration may occur during a loss of consciousness such as during anesthesia or a seizure, or after alcohol or drug abuse while geriatric (old) patients may also develop aspiration pneumonia.
Neurologic disease, esophageal pathology, periodontal disease, or gingivitis are important risk factors. Aided by gravity and often by loss of some host non-specific protective mechanisms, organisms reach lung tissue, where they multiply and attract host inflammatory cells. The buildup of cell debris and fluid contributes to the loss of lung function and this to the pathology.
Primary routes of bacterial entry into LRT are the aspiration of oropharyngeal organisms or leakage of secretions containing bacteria around an endotracheal tube. Thus, intubation and mechanical ventilation significantly increase the risk of pneumonia (6-21 fold).
Clinical manifestations
Symptoms/clinical manifestations include fever, chills, chest pain, and cough. In past, pneumonia was classified into 2 groups based on whether the cough was productive or non-productive for mucoid sputum.
1. Typical or acute pneumonia (eg: Streptococcus pneumoniae)
2. Atypical pneumonia
Atypical pneumonia pathogens include (Mycoplasma pneumoniae, Legionella pneumophila, and Chlamydophila pneumoniae) and symptoms are similar to that of typical bacterial pneumonia. Thus, it is important to consider all possible etiologies associated with patients' clinical presentation.
In some cases, no symptoms of pneumonia related to their respiratory tract (i.e. some only have fever) are exhibited. Therefore, physical examination of patients, chest radiograph results, patient history, and clinical laboratory findings are important references.
In addition to respiratory symptoms, 10-30% of patients with pneumonia complain of headache, nausea, vomiting, abdominal pain, diarrhea, and myalgias.
SARS (Severe acute respiratory syndrome) and influenzae outbreaks (H1N1) are typically associated with URI but may lead to serious lower respiratory infections in young, elderly, or immunocompromised patients.
Community-Acquired pneumonia
About 2 to 3 million cases of community-acquired pneumonia occur annually and 1/5th of these require hospitalization. The causative bacterial agents include S. pneumoniae, H. influenzae, Legionella spp., M pneumoniae, C. pneumonia, M. catarrhalis, etc.
The etiology of acute pneumonia is strongly dependent on age. More than 80% of pneumonia in infants and children are caused by viruses and 10-20% of pneumonia in adults.
Pneumonia in children
Determining the cause of pneumonia is difficult because the lungs are rarely sampled directly and sputum is difficult to obtain from children.
Children suffer less commonly from bacterial pneumonia usually caused by H. influenzae, S. pneumoniae, and S. aureus. Neonates may acquire LRT infection with C. trachomatis or P. jiroveci (indicating an immature immune system or immune defect).
Among children from 2 months – 5 years, RSV, human metapneumovirus parainfluenza, influenza, and adenovirus are the most common etiologic agents.
M. pneumoniae and C. pneumoniae are the most common causes of bacterial pneumoniae in school-age children (5-14 years).
Mixed viral and bacterial infections have been detected in 35% of patients.
The time of onset of hospital/ventilator/healthcare-associated pneumonia is an important variable. The early onset of pneumonia (within the first 4 days of hospitalization) is likely to be caused by antibiotic-sensitive bacteria.
The late onset of pneumonia (5 days or more) is more likely to be caused by MDR micro-organisms and is associated with increased patient morbidity and mortality.
Pneumonia in young adults
The most common etiological agent of LRTI among young adults (Aged <30) is Mycoplasma pneumoniae which is transmitted via close contact. Contact with secretion is much more potent for transmission and infection than aerosols in inhalation.
After contact with respiratory mucosa, Mycoplasma is able to colonize respiratory mucosal cells. Virulence includes adherence to protein and gliding motility. It attaches to the cilia of respiratory ciliary function. Cytotoxins produced by microorganisms induce cell damage.
Chlamydia pneumoniae is the third most common agent of LRTI in young adults following Mycoplasms and viruses (which also affect older people). Chlamydia spp. are intracellular pathogens capable of disrupting cellular functions and causing respiratory disease, similar to viral pathogens.
If patients have been hospitalized in the last 90 days, reside in a nursing home or long-term care facility, or have had recent intravenous antibiotic therapy, hemodialysis, etc, the patients must be classified as health-care-associated pneumonia (HCAP).
Patients with HCAP have a higher incidence of cardiopulmonary and neurodegenerative diseases, cancer, chronic kidney disease, chronic obstructive pulmonary disease (COPD), and immunosuppression than elderly patients with community-acquired pneumonia.
Healthcare-associated patients are generally colonized with gram-negative bacilli and other MDR pathogens. Other pathogens included in health-care-associated pneumonia are methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, Acinetobacter spp., and a variety of Enterobacteriaceae. Carbapentamase resistant Klebsiella pneumonia and extended spectrum beta-lactamase resistant Enterobacteriaceae (ESBL).
Oral anaerobes such as black-pigment Prevotella and Porphyromonas spp, Prevotella oralis, P. buccae, P. disiens, Bacteroides gracilis, fusobacteria, and anaerobic or microaerophilic streptococci are spread by gastric or oral secretions in a community setting.
Hospitalized patients and long-term care patients may experience asymptomatic colonization of URT which gradually aspirates LRT. These patients are prone to MDR bacteria (ESBLS or MRSA) including E.coli, Proteus mirabilis, K. pneumoniae, S. aureus, and Enterobacter spp.