Pneumonia

Definition

Pneumonia is an inflammation of the lower respiratory tract and lung parenchyma. It is a leading cause of hospitalization and death worldwide. Pneumonia is epidemiologically categorized into community-acquired pneumonia (CAP), which is defined as pneumonia that is acquired outside the hospital and nosocomial pneumonia, which is defined as pneumonia that manifests at least 48 to 72 hours after admission. Nosocomial pneumonia includes nursing home-acquired pneumonia and hospital-acquired pneumonia (HAP).

Risk Factors

  • Age

  • Smoking

  • Chronic lung disease

  • Respiratory infection

  • Immunodeficiency

  • Prolonged ICU hospitalization

Pathogens

Typical Bacteria

  • Streptococcus pneumoniae

  • Haemophilus influenzae

  • Staphylococcus aureus

  • Moraxella catarrhalis

  • Enterobacteriaceae

  • Pseudomonas spp.

  • Anaerobes

Atypical Bacteria

  • Mycoplasma pneumoniae

  • Chlamydia pneumoniae

  • Legionella pneumophila

Viruses

  • Influenza viruses

  • Parainfluenza virus

  • Adenovirus

  • Respiratory syncytial virus

Fungi

  • Pneumocystis jirovecii

  • Histoplasma spp.

  • Coccidioides

  • Blastomyces

Pathophysiology

Pneumonia occurs when an organism’s ability to penetrate and infect the lung parenchyma overcomes the host’s defence mechanisms. The causative pathogens include bacteria, viruses, fungi, and parasites. Streptococcus pneumoniae is the most important pathogen for community-acquired pneumonia, accounting for about two-thirds of all cases of bacterial pneumonia.

Hospital-acquired pneumonia is typically caused by drug-resistant pathogens such as Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA), and Gram-negative Enterobacteriaceae. Lobar pneumonia is most commonly associated with CAP, while bronchopneumonia is more typical of HAP. Atypical bacteria, which usually have an intrinsic resistance towards beta-lactam-antibiotics, often cause interstitial pneumonia.  

What can facilitate an infection, are diseases or conditions, which come with an increased risk of aspiration which can favour the development of bronchopneumonia, due to colonized secretions from the trachea which can enter the lungs. This pathomechanism typically results in multifocal pneumonia centred in distal airways.

Signs and Symptoms

The most frequent signs and symptoms of pneumonia include:

  • Fever

  • Myalgia

  • Dyspnoea

  • Cough

  • Fatigue

In the elderly though, the absence of classic symptoms is frequent, and changes in awareness might be the only sign of pneumonia. Confusion, delirium, disorientation, or loss of appetite may also indicate pneumonia. Pneumonia should be suspected when there is an unexplained deterioration in general health, weakness, or exacerbation of underlying illnesses. Patients with pneumonia often also have a high respiratory rate (over 25 breaths per minute) and a low oxygen saturation (under 90%).

Diagnosis

Clinical Examination

Anamnesis and physical examination, including auscultation, can provide important information on the cause, location, and severity of pneumonia, and help decide whether the patient should be admitted to the hospital. Furthermore, measurements of the respiratory rate and oxygen saturation are important to evaluate the clinical condition of the patient.

Imaging

A chest x-ray is always indicated when pneumonia is suspected. It is essential for differentiating pneumonia from other respiratory illnesses such as upper respiratory tract infections, bronchitis, reactive airway disease, atelectasis, pulmonary embolism, and neoplasias. In cases where x-rays prove insufficient in diagnosing pneumonia, a CT scan can be performed to obtain more information, especially when complications are suspected, or when there is suspicion of further pathologies such as pulmonary embolism or tumor.

Laboratory Evaluation

Blood tests are indicated to evaluate inflammatory markers (white blood cell counts, CRP, procalcitonin). An arterial blood gas analysis is indicated to exclude a respiratory insufficiency.

Microbiologic Diagnosis

The identification of the pathogen can be obtained from the sputum, blood samples, bronchial secretion, Urin (antigen-test) or bronchoalveolar lavages. Even though there is no solid evidence that identifying the causative pathogen improves the patient’s outcome, finding it, allows to choose the optimal antibiotic therapy and the identification of severe diseases such as Legionnaires’ disease or tuberculosis. New rapid diagnostic tests, such as Multiplex-PCRs, have been shown to be useful in clinical situations.

Therapy

The majority of patients can be treated as outpatients. However, selected patients should be admitted to hospital if they require close observation, respiratory support, intravenous antibiotics, or other interventions. The CURB-65 score can help in deciding whether to admit a patient to the hospital. The acronym stands for the following criteria: Confusion, Urea (blood urea nitrogen > 7 mmol/L), Respiratory Rate (≥ 30/min), Blood pressure (diastolic ≤ 60 mm Hg or systolic < 90 mm Hg), and age ≥ 65 years. For each criterion met, one point is given. If the score is ≥ 2, hospital admission is recommended. Notably, a follow-up after 24 to 48 hours is advisable if the patient is treated as an outpatient.

Antibiotics are the mainstay of treatment for most patients with pneumonia. The emergence of resistant respiratory pathogens, particularly drug-resistant strains of Streptococcus pneumoniae, is a significant concern that has complicated the initial empirical management of pneumonia. According to the newest guidelines:

  • Patients with moderate to severe CAP should be treated with either a respiratory fluoroquinolone or a combination of a beta-lactam plus a macrolide.

  • For those needing ICU care, a combination of a beta-lactam with either a macrolide or a respiratory fluoroquinolone is recommended.

Generally, a treatment course of 5 to 7 days is sufficient. However, if patients have severe complications, do not reach clinical stability within 3 days, or are infected with certain pathogens such as Legionella pneumonia or MRSA longer course of treatment may be required. Additionally, in severe CAP, the use of corticosteroids may offer benefits, including a shorter ICU stay, lower rates of acute respiratory distress syndrome, and a shorter time to clinical stability.

References

1.      Lanks, C., Musani, A. I. & Hsia, D. (2019). Community-acquired Pneumonia and Hospital-acquired Pneumonia. Medical Clinics Of North America, 103(3), 487–501. https://doi.org/10.1016/j.mcna.2018.12.008

2.      Reynolds, J. H., McDonald, G. R., Alton, H. & Gordon, S. B. (2010). Pneumonia in the immunocompetent patient. British Journal Of Radiology, 83(996), 998–1009. https://doi.org/10.1259/bjr/31200593

3.      Chalmers, J. D., Taylor, J. H. M., Singanayagam, A., Fleming, G., Akram, A. R., Mandal, P., Choudhury, G. & Hill, A. T. (2011). Epidemiology, Antibiotic Therapy, and Clinical Outcomes in Health Care-Associated Pneumonia: A UK Cohort Study. Clinical Infectious Diseases, 53(2), 107–113. https://doi.org/10.1093/cid/cir274

4.      Almirall, J., Bolíbar, I., Balanzó, X. & Ca, G. (1999). Risk factors for community-acquired pneumonia in adults: a population-based case-control study. The European Respiratory Journal, 13(2), 349–355. https://doi.org/10.1183/09031936.99.13234999

5.      Henig, O. & Kaye, K. S. (2017). Bacterial pneumonia in older adults. Infectious Disease Clinics Of North America, 31(4), 689–713. https://doi.org/10.1016/j.idc.2017.07.015

6.      File, T. M. (2003). Community-acquired pneumonia. The Lancet, 362(9400), 1991–2001. https://doi.org/10.1016/s0140-6736(03)15021-0