Pneumonia

Overview

Typical vs Atypical Pneumonia

Feature	Typical (S. pneumoniae)	Atypical (Mycoplasma)	Atypical (Legionella)
Onset	Acute (<3 days)	Insidious (>1 week)	Subacute
Cough	Productive, purulent	Dry, persistent	Dry
CXR	Lobar consolidation	Bilateral interstitial infiltrates	Patchy/lobar
Key extra-pulmonary features	Herpes labialis	Erythema multiforme, haemolytic anaemia, bullous myringitis, meningoencephalitis, pericarditis	Confusion, raised ALT, haematuria, severe pneumonia
Hyponatraemia	Uncommon	Yes	Yes - prominent
Abdominal symptoms	Uncommon	Yes - pain, nausea	Yes - diarrhoea
Epidemiology	Any age	Younger adults; epidemics every 3-4 years	Contaminated water (cooling towers, hotel showers) - travel history clue
Diagnosis	Blood cultures, sputum culture	Mycoplasma serology/PCR; cold agglutinins	Urinary Legionella antigen
Beta-lactam efficacy	Yes	No (no cell wall)	No
Treatment	Amoxicillin	Clarithromycin or doxycycline for 10-14 days	Clarithromycin or fluoroquinolone

🎯

Mycoplasma pneumoniae exam clues: young adult, insidious dry cough >1 week, erythema multiforme (target lesions), hyponatraemia, abdominal pain, bilateral interstitial infiltrates - beta-lactams are ineffective. Cold agglutinins cause autoimmune haemolytic anaemia.

Investigations

🥇 First-line

•CXR - lobar consolidation (typical), bilateral interstitial shadowing (atypical/PCP), cavitation (Staph aureus, Klebsiella, TB)

•U&E - urea for CURB-65; hyponatraemia points to atypical organism

•Blood cultures x2 before antibiotics in moderate/severe CAP

•Mycoplasma serology/PCR - if atypical features; cold agglutinin test supports diagnosis

•Urinary Legionella antigen - rapid; high sensitivity for serogroup 1

•ABG - if SpO2 <92% or severe disease

Management

•Low severity CAP (CURB-65 0-1): amoxicillin 500 mg oral three times daily for 5 days

•Low severity, penicillin allergy: doxycycline 200 mg loading then 100 mg once daily, or clarithromycin 500 mg twice daily

•Moderate severity CAP (CURB-65 2): amoxicillin 500 mg oral three times daily PLUS clarithromycin 500 mg oral twice daily for 5-7 days

•High severity CAP (CURB-65 3-5): co-amoxiclav 1.2 g IV three times daily PLUS clarithromycin 500 mg IV/oral twice daily

•Confirmed Mycoplasma/atypical: clarithromycin 500 mg twice daily or doxycycline 100 mg twice daily for 10-14 days - beta-lactams ineffective (no cell wall)

•PCP (immunocompromised): co-trimoxazole high-dose first-line; add corticosteroids if pO2 <9.3 kPa; IV pentamidine for severe/refractory cases

•PCP prophylaxis: co-trimoxazole for all HIV-positive patients with CD4 <200 cells/µL

💡

PCP (Pneumocystis jirovecii) presents in immunocompromised patients with exertional breathlessness and desaturation on walking. CXR shows bilateral interstitial shadowing. Always consider in HIV patients - check CD4 count.

Follow-up

•Repeat CXR at 6 weeks post-discharge to confirm radiological resolution - persistent opacity must be investigated to exclude underlying malignancy

•Earlier follow-up CXR for patients aged >50 years or smokers given higher risk of underlying lung cancer presenting as pneumonia

Complications

•Empyema - infected pleural effusion; suspect if fever persists despite antibiotics; requires chest drain

•Lung abscess - cavitating necrotic infection; Staph aureus, Klebsiella, anaerobes (aspiration); CXR shows cavity with air-fluid level

•Mycoplasma-specific: autoimmune haemolytic anaemia (cold agglutinin disease), erythema multiforme, Stevens-Johnson syndrome, meningoencephalitis, Guillain-Barré syndrome, pericarditis

Severity - CURB-65

•C - Confusion (new disorientation)

•U - Urea >7 mmol/L

•R - Respiratory rate ≥30 breaths/minute

•B - Blood pressure: systolic <90 mmHg or diastolic ≤60 mmHg

•65 - Age ≥65 years

CURB-65 score and management

Score	Severity	Action
0-1	Low	Consider home treatment, oral antibiotics
2	Moderate	Consider hospital admission, oral or IV antibiotics
3-5	High	Hospital admission; consider ICU if score 4-5