Pneumonia

Pneumonia is infection of the lung parenchyma, producing inflammation and consolidation of the alveolar spaces. It is a leading cause of infection related mortality worldwide.

It is classified as community acquired or hospital acquired according to the setting of acquisition. This distinction has important implications for the likely causative organisms and the empirical antibiotic choice.

Aetiology

Streptococcus pneumoniae remains the commonest identified cause of community acquired pneumonia across most age groups and severity levels. Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydophila pneumoniae and respiratory viruses (including influenza) contribute to the remainder. In specific clinical contexts, Legionella pneumophila and Staphylococcus aureus contribute.

Hospital acquired pneumonia has a different organism profile, including Gram negative bacteria and Staphylococcus aureus, including methicillin resistant strains. This reflects the healthcare environment and the frequently altered host defences of the affected population.

Pathophysiology

Organisms reach the lower respiratory tract through microaspiration of oropharyngeal secretions (the commonest route), inhalation of infectious droplets, or, less commonly, haematogenous spread. They overwhelm local host defences and produce an inflammatory response within the alveoli that impairs gas exchange. In more extensive disease this causes systemic sepsis.

Cough productive of purulent sputum, fever, pleuritic chest pain and breathlessness characterise the typical presentation. Examination shows tachypnoea, tachycardia and, over the affected area, focal crackles, dullness to percussion and bronchial breathing reflecting the consolidation.

Atypical presentations are common, particularly in the elderly. Confusion, falls or general functional decline may be the presenting feature without the classical respiratory symptoms.

Atypical organism infection (Mycoplasma, Chlamydophila) has a more gradual onset with prominent extrapulmonary symptoms (headache, myalgia, gastrointestinal upset). The chest examination is relatively unimpressive despite significant radiographic changes.

Severity assessment is central to management. It is guided by validated clinical scoring systems incorporating confusion, respiratory rate, blood pressure and age, alongside clinical judgement on oxygenation, comorbidity and the overall trajectory of the illness. It determines the setting of care (outpatient, ward admission or critical care) and the intensity of investigation and treatment.

Complications include parapneumonic effusion and empyema, lung abscess, sepsis and respiratory failure. Clinical deterioration, or failure to improve as expected on appropriate treatment, warrants reassessment for these possibilities.

Diagnosis combines the clinical presentation with chest radiography demonstrating new consolidation. Radiography is generally required to confirm the diagnosis in a compatible clinical picture. Radiographic changes can occasionally lag behind the clinical presentation, particularly early in the illness or in dehydrated patients.

Severity assessment uses a validated clinical scoring system (incorporating confusion, urea, respiratory rate, blood pressure and age). It guides the intensity of further investigation, the care setting, and the choice and route of antibiotic therapy.

Blood tests (full blood count, renal function, C reactive protein) support assessment of severity and the inflammatory response.

Consider blood cultures, sputum culture and urinary antigen testing for pneumococcus and Legionella for more severe disease, or where the result would meaningfully influence management. Do not perform them routinely for mild, low severity presentations managed in the community.

Pulse oximetry, and arterial or venous blood gas analysis for more significant hypoxia or respiratory distress, assess the degree of respiratory compromise and guide oxygen therapy and escalation decisions.

Consider further imaging (computed tomography of the chest) and specialist respiratory or pleural intervention where complications are suspected (empyema, abscess, failure to resolve as expected) or where the diagnosis remains uncertain.

Antibiotic choice and route are guided by the assessed severity and the clinical setting.

• Low severity community acquired pneumonia managed in the community: amoxicillin as typical first line oral treatment.
• Moderate to severe disease: broader spectrum combination therapy, commonly a beta lactam with a macrolide, covering both typical and atypical organisms. Give it intravenously.
• Most severe presentations: admission to a high dependency or intensive care setting.

Supportive care accompanies antibiotic treatment across all severity levels. It includes supplemental oxygen to maintain adequate saturation, adequate hydration and analgesia for pleuritic pain.

Duration of antibiotic treatment is generally five to seven days for uncomplicated community acquired pneumonia. Longer courses are reserved for specific organisms (Legionella, Staphylococcus aureus), complications (empyema, abscess) or slower clinical response.

Hospital acquired pneumonia is managed with broader spectrum empirical antibiotic cover reflecting the different likely organism profile. Guide the choice by local antimicrobial resistance patterns and, where available, individual risk factors for resistant organisms.

Reassess at forty eight to seventy two hours to confirm clinical improvement. If improvement is not as expected, reconsider the diagnosis, assess for complications (parapneumonic effusion, empyema), and review the antibiotic choice against any culture results now available.

Complete management with smoking cessation advice, and pneumococcal and influenza vaccination according to age and risk factor based eligibility. For patients with recurrent pneumonia or risk factors for aspiration, assess for an underlying predisposing cause. These measures support secondary prevention.

Follow up chest radiography, typically at six weeks, is recommended for patients with risk factors for underlying malignancy (smokers, older patients). It confirms radiographic resolution and excludes an obstructing lesion presenting as post obstructive pneumonia. This is an important safety net, because pneumonia can occasionally be the presenting feature of underlying lung cancer.