12th January 2024

Pneumonia

Background

Pneumonia is inflammation of the lung parenchyma caused by infection, which results in alveoli of the affected areas becoming filled with pus or fluid. In hospital, the diagnosis is made based on symptoms and signs of an acute lower respiratory tract infection and chest X‑ray findings of new shadowing that is not due to another cause. Pneumonia can be classified in multiple ways, including:

• The causative organism which can be bacterial, viral, fungal or parasitic
• The severity of infection, most commonly using the CURB-65 score
• The pattern of consolidation on the chest X-ray. Pneumonia can be split into lobar pneumonia, where inflammation is confined to one lobe, and bronchopneumonia, where there are patchy foci of consolidation with no lobar localisation
• The place where infection occurred, e.g. community-acquired vs hospital-acquired. Pneumonia is defined as hospital-acquired if it occurs >48 hours after hospital admission

Epidemiology

Between 0.5-1% of adults in the United Kingdom (UK) develop community-acquired pneumonia (CAP) every year, with the condition particularly common in the elderly population. Pneumonia carries significant morbidity and mortality; over 100,000 patients per year are admitted to an NHS hospital in England with a primary diagnosis of pneumonia, and approximately 30,000 deaths per year in the UK are due to pneumonia.

Causative organisms

Pneumonia is most commonly caused by bacterial infection. Streptococcus pneumoniae is the most common cause of CAP, accounting for approximately 80% of cases. Other common organisms include Haemophilus influenzae, Moraxella catarrhalis and Staphylococcus aureus, which is classically seen after influenza infection. Klebsiella pneumoniae is a less common cause of pneumonia except in alcoholics and can present with cavitating lesions. “Atypical” causes of pneumonia are those which are challenging to detect through standard bacterial methods and include Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae and Chlamydia psittaci, which is commonly seen in bird owners.

The pathogens typically associated with hospital-acquired pneumonia (HAP) differ from those in CAP, with Gram-negative enterobacteria (e.g. Enterobacter spp, Escherichia coli and Proteus spp) and Staphylococcus aureus being considered the ‘core’ pathogens. Pseudomonas aeruginosa is a cause of severe HAP and is classically seen in patients on mechanical ventilation. Aspiration pneumonia, typically seen in individuals with swallowing difficulties, often has a mixed picture with anaerobic species involved.

Presentation

Pneumonia typically presents with a relatively short history of symptoms over hours to days. Common symptoms include:

• Fever
• Shortness of breath
• Productive cough
• Pleuritic chest pain
• Malaise

Risk factors for pneumonia are smoking, immunosuppression, impaired swallowing, and concurrent lung conditions such as bronchiectasis.

Atypical pneumonias typically present with a slightly longer history of flu-like symptoms, including headache, diarrhoea, and malaise. Mycoplasma pneumoniae infection is commonly associated with dermatological manifestations, such as erythema multiforme or Stevens-Johnson syndrome. Legionella can colonise building water systems and classically occurs in outbreaks in hotels.

Signs on examination can include:

• Pyrexia
• Tachycardia
• Tachypnoea
• Hypoxia
• Reduced chest expansion on the affected side
• Dull percussion note over the area of consolidation
• Coarse crackles and bronchial breathing on auscultation over the area of consolidation
• Increased vocal resonance over the area of consolidation

Investigations

Investigations in suspected pneumonia aim to confirm the diagnosis, establish the causative pathogen and determine the severity.

Investigations to consider include:

• Blood tests
  • Full blood count – to assess the white cell count and differential. Mycoplasma infection can also cause cold autoimmune haemolytic anaemia. If this is suspected, the full blood count should be followed up with a blood film showing spherocytes and reticulocytosis and direct antiglobulin test (DAT).
  • Urea & electrolytes – urea is included as part of the CURB-65 score. Legionella pneumonia is typically associated with hyponatraemia.
  • Liver function tests – Legionella can cause deranged liver function tests
  • C-reactive protein
  • Blood cultures
  • Venous blood gas for lactate
  • Serology for atypical organisms, e.g. Mycoplasma
• Arterial blood gas – if hypoxic
• Chest X-ray to assess for consolidation
• Sputum culture
• Urine for Legionella and Pneumococcal antigens

Image Source: Wikimedia Commons (2017).  X-ray_of_lobar_pneumonia, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/5/51/X-ray_of_lobar_pneumonia.jpg (Accessed: 22 February 2022).

Management

The management of pneumonia depends on the severity of the condition and the causative organism. The CURB-65 score is a predictive score of mortality from CAP, which is commonly used to guide treatment. The score is as follows:

• Confusion (AMTS ≤ 8) – 1 point
• Urea ＞7mmol/L – 1 point
• Respiratory rate ≥ 30 breaths per minute – 1 point
• Blood pressure: systolic ＜90mmHg or diastolic ≤ 60mmHg – 1 point
• Age ≥ 65 years – 1 point

Patients with a score of 0 or 1 are categorised as low severity (less than 3% mortality risk), and these individuals can often be managed as outpatients. A score of 2 is moderate severity (3‑15% mortality risk), which often requires hospital admission, while a score of 3 or above is categorised as severe (more than 15% mortality risk), which should prompt a consideration of discussion with the intensive care team.

Antibiotics are the mainstay of treatment for bacterial pneumonia. If possible, low-severity CAP should be managed with oral amoxicillin, with doxycycline or clarithromycin being alternatives if amoxicillin is unsuitable. Moderate severity CAP is typically treated with amoxicillin and clarithromycin, while severe CAP is managed with a combination of co-amoxiclav (typically IV initially) and clarithromycin.

If specific causative organisms for the pneumonia are identified, the treatment may be modified following discussion with the microbiology team. For instance, the first-line treatment for Legionella pneumophila would typically be a fluoroquinolone such as levofloxacin or a macrolide such as clarithromycin, while Mycoplasma is often managed with a macrolide.

Antibiotic guidelines differ for HAP, with NICE recommending oral co-amoxiclav for patients with non‑severe symptoms or signs and not at higher risk of resistance or a choice of either piperacillin with tazobactam (Tazocin), ceftazidime, ceftriaxone, cefuroxime, meropenem or ceftazidime with avibactam IV for patients with severe symptoms or signs (for example, symptoms or signs of sepsis) or at higher risk of resistance. In patients with aspiration pneumonia, metronidazole is typically added to provide anaerobic cover, and a referral to Speech and Language Therapy should be strongly considered to assess the safety of the swallow.

A follow-up chest X-ray should be arranged after 6 weeks to ensure resolution of consolidation (if it has not resolved, this should raise suspicion for an underlying abnormality, such as a lung tumour) for adults with either symptoms and signs that persist despite treatment, and/or who are at higher risk of underlying malignancy (particularly smokers and people aged more than 50 years). It is important to advise patients that symptoms may persist long beyond the end of their antibiotic course. For instance, it is expected that by three months, most of their symptoms should have resolved, yet fatigue may still be present.

Complications

A number of complications can result from pneumonia. Type 1 respiratory failure, with a PₐO₂ < 8, is common due to the impedance of oxygen diffusion across pus/fluid in the alveolar spaces. Inflammation of the pleura due to the infection can cause an exudative pleural effusion to develop. Small pleural effusions may be of no consequence; however, drainage may be required if the effusion becomes large and leads to symptoms. Additionally, if the effusion becomes infected with pus in the pleural space (empyema), this must also be drained. An empyema should be suspected in patients with evidence of pleural effusion on a chest X-ray and recurrent fevers despite treatment of their pneumonia. Aspirated pleural fluid is classically yellow and turbid, with a pH < 7.2 being diagnostic of empyema. Additional complications of pneumonia include sepsis and the development of lung abscesses, which typically cause a swinging fever.

Image Source: Wikimedia Commons (2017). Left-sided Pleural Effusion, Licensed under a creative commons licence. Available for free at :https://upload.wikimedia.org/wikipedia/commons/5/57/Left-sided_Pleural_Effusion.jpg (Accessed: 22 February  2022).

References

• Regunath H, Oba Y. Community-Acquired Pneumonia. StatPearls. 2021 Aug 11; [accessed 21 Feb 2022] Available from: https://www.ncbi.nlm.nih.gov/books/NBK430749/
• Prince SE, Dominger KA, Cunha BA, Klein NC. Klebsiella pneumoniae pneumonia. Heart & Lung. 1997 Sep 1;26(5):413–7. doi: 10.1016/S0147-9563(97)90028-5
• Pneumonia in adults Quality standard. 2016; [accessed 21 Feb 2022] Available from: www.nice.org.uk/guidance/qs110
• Pneumonia statistics | British Lung Foundation. [accessed 21 Feb 2022] Available from: https://statistics.blf.org.uk/pneumonia#statistic175
• Mackenzie G. The definition and classification of pneumonia. Pneumonia (Nathan Qld). 2016 Dec;8(1). [accessed 21 Feb 2022] Available from: https://pubmed.ncbi.nlm.nih.gov/28702293/
• Recommendations | Pneumonia (hospital-acquired): antimicrobial prescribing | Guidance | NICE. [accessed 21 Feb 2022] Available from: https://www.nice.org.uk/guidance/ng139/chapter/Recommendations#treatment-for-adults-young-people-and-children
• Recommendations | Pneumonia (community-acquired): antimicrobial prescribing | Guidance | NICE. [accessed 21 Feb 2022] Available from: https://www.nice.org.uk/guidance/ng138/chapter/Recommendations
• Atypical Pneumonia | CDC. [accessed 21 Feb 2022] Available from: https://www.cdc.gov/pneumonia/atypical/index.html
• Abdulhadi B, Kiel J. Mycoplasma Pneumonia. PCR for Clinical Microbiology: An Australian and International Perspective. 2021 Jul 25;179–82. [accessed 21 Feb 2022] Available from: https://www.ncbi.nlm.nih.gov/books/NBK430780/
• Trotter CL, Stuart JM, George R, Miller E. Increasing Hospital Admissions for Pneumonia, England – Volume 14, Number 5—May 2008 – Emerging Infectious Diseases journal – CDC. Emerging Infectious Diseases. 2008;14(5):727–33. [accessed 21 Feb 2022] Available from: https://wwwnc.cdc.gov/eid/article/14/5/07-1011_article
• Chahin A, Opal SM. Severe Pneumonia Caused by Legionella pneumophila: Differential Diagnosis and Therapeutic Considerations. Infectious Disease Clinics of North America. 2017 Mar 1;31(1):111. [accessed 21 Feb 2022] Available from: /pmc/articles/PMC7135102/
• McEachern R, Campbell J. HOSPITAL-ACQUIRED PNEUMONIA: EPIDEMIOLOGY, ETIOLOGY, AND TREATMENT. Infectious Disease Clinics of North America. 1998 Sep 1;12(3):761–79. doi: 10.1016/S0891-5520(05)70209-9