What is community-acquired pneumonia? Etiopathogenesis, diagnosis and management

Definition. Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside hospital, or within 48 hours of admission, characterised by new symptoms and signs of lower respiratory tract infection together with a new radiographic infiltrate. The presence of a parenchymal focus on imaging distinguishes pneumonia from bronchitis. About two out of three hospitalisations for pneumonia are in patients over 70 years of age.

Aetiology by setting (high-yield).

Pathogenesis. Infection follows micro-aspiration of oropharyngeal secretions colonised by pathogens, and less often inhalation of aerosols or haematogenous spread. Ageing impairs every defence layer: reduced cough and glottic reflexes with silent aspiration, impaired mucociliary clearance, reduced elastic recoil and lung "inflammageing", immunosenescence with poor IgM response to pneumococcus, malnutrition, sarcopenia and reduced ventilatory reserve. Structural changes — bronchiectasis, tracheobronchomalacia, old TB scarring, sabre-sheath trachea — impair mucus clearance and predispose to infection.

Risk factors. Age itself; dysphagia and stroke, Parkinson's disease, dementia, sedatives and antipsychotics, GORD, poor oral hygiene and edentulism, tube feeding, COPD, heart failure, diabetes, chronic kidney disease, malignancy, inhaled corticosteroids, proton pump inhibitors, alcohol, smoking, institutionalisation, and immobility.

Clinical presentation — the geriatric pitfall. The classical picture of malaise, high fever, cough and pleuritic chest pain is rarely complete in the elderly. Fever and cough are absent in 30–40% of cases, and onset is often gradual with less-marked signs, leading to delayed diagnosis. Presentation may be with delirium, falls, functional decline, anorexia, incontinence or decompensation of a comorbidity. Warning signs on examination: tachypnoea > 30/min, tachycardia > 110/min, fever or hypothermia, hypotension and hypoxaemia.

Imaging. A chest radiograph (PA and lateral) is recommended in every patient with suspected pneumonia, mainly to distinguish pneumonia from bronchitis. Look for alveolar consolidation, mixed alveolar–interstitial shadowing, atelectasis, and hidden retrocardiac or perihilar opacities. The radiographic pattern is poorly related to the causative organism. A chest CT is indicated when the film is normal but pneumonia is still suspected, in very early disease, or to detect pulmonary embolism. Point-of-care lung ultrasound shows B-lines with excellent sensitivity and specificity, and later "hepatisation" of consolidated lung — very useful in nursing homes and home care where radiography is difficult.

Laboratory and microbiological tests. Not required for patients managed at home. In hospitalised patients: full blood count (leucocytosis suggests bacterial infection but may be absent in older patients), CRP, urea, electrolytes, creatinine and glucose (dehydration, renal impairment and hyperglycaemia are common), pulse oximetry, and arterial blood gas if there is chronic respiratory failure, cyanosis, sweating or reduced consciousness. Blood cultures and sputum examination in all patients admitted with pneumonia; add urinary pneumococcal and Legionella antigens, respiratory viral PCR, and — in India — sputum for AFB/CBNAAT where the course is subacute.

Severity assessment and the decision to admit. Use CURB-65 (Confusion, Urea > 7 mmol/L, Respiratory rate ≥ 30, Blood pressure < 90/60, age ≥ 65) or the Pneumonia Severity Index. Three reasons prompt hospital referral: (i) the diagnosis is unclear and a serious alternative such as pulmonary embolism or heart failure is suspected; (ii) pneumonia is likely but the patient cannot be safely cared for at home (help with ADLs, prevention of pressure sores and dehydration, follow-up); (iii) severity signs are present. Remember that scores under-estimate risk in the frail — always add a functional and social assessment.

Antibiotic therapy. Initial therapy is probabilistic and must cover S. pneumoniae and H. influenzae. For outpatients with comorbidities (which applies to most older patients), a single oral agent may be used — amoxicillin, amoxicillin-clavulanate, an advanced macrolide (azithromycin or clarithromycin), doxycycline, or a respiratory fluoroquinolone. Per ATS/IDSA 2019: a β-lactam plus a macrolide or doxycycline, or a respiratory fluoroquinolone alone. In nursing home patients the regimen must also cover anaerobes because of aspiration. For severe CAP use a broad-spectrum combination, and cover Pseudomonas if there is bronchiectasis or recent β-lactam/fluoroquinolone use within three months. In older people reduce doses for renal function, and beware fluoroquinolone-associated QT prolongation, tendinopathy, delirium, hypoglycaemia and C. difficile.

Supportive care, the 48-hour review and prevention. Give oxygen (target 88–92% if at risk of CO retention), 2 fluids, early mobilisation, chest physiotherapy, VTE prophylaxis, nutritional support, delirium prevention and swallow assessment. Assessment at 48 hours is the key step — defervescence and clinical stability indicate a favourable response, allowing IV-to-oral switch. If the response is unfavourable, consider resistant organism, tuberculosis or virus, empyema or encysted effusion, non-infectious lung disease, drug adverse effect, or worsening comorbidity, and repeat imaging including CT. Persistence of the radiological shadow at 12 weeks, marked weight loss or heavy tobacco/asbestos exposure must raise suspicion of lung cancer and prompt bronchoscopy or CT. Prevention: influenza, pneumococcal and COVID-19 vaccination, smoking cessation, oral hygiene, dysphagia management, and review of sedatives and PPIs.

Corticosteroids in severe CAP. The CAPE COD trial (NEJM 2023) showed that intravenous hydrocortisone 200 mg/day started within 24 hours reduced 28-day mortality in patients admitted to intensive care with severe CAP. Current international guidance therefore recommends adjunctive corticosteroids in severe CAP (but not in influenza or suspected aspergillosis), with careful monitoring for hyperglycaemia and delirium in older patients.

2024 ACIP pneumococcal vaccination update. The routine age for pneumococcal vaccination in the United States was lowered from 65 to 50 years, and a single dose of a higher-valency conjugate vaccine (PCV20 or PCV21) is now recommended for pneumococcal-naïve adults, simplifying the older PCV13-then-PPSV23 sequence. This reflects the substantial residual burden of pneumococcal disease from age 50 onward.