Lung abscess and necrotising pneumonia

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Tissue necrosis changes an ordinary pneumonia into a source-control problem. A lung abscess typically has a thick irregular wall and may develop an air-fluid level after communication with a bronchus. Necrotising pneumonia consists of devitalised, poorly perfused lung, which can limit antibiotic penetration and generate pneumatoceles, bronchopleural fistula, empyema or haemorrhage. The syndromes overlap and cannot be managed by a fixed antibiotic-duration rule alone.

Aspiration of polymicrobial oropharyngeal material remains a common pathway, especially with dental sepsis, alcohol or sedative exposure, seizures, neurological dysphagia or reflux. Staphylococcus aureus, including post-influenza disease, streptococci and Gram-negative organisms may produce aggressive necrosis. Septic pulmonary emboli suggest right-sided endocarditis, infected venous access or septic thrombophlebitis. Upper-lobe or persistent cavities also demand evaluation for tuberculosis, NTM, fungi, vasculitis and cavitating malignancy.

Most uncomplicated abscesses drain internally through a bronchus and respond to appropriate antibiotics plus management of aspiration and dental sources. Percutaneous or endoscopic drainage is selective because traversing normal lung can cause bleeding, pneumothorax or bronchopleural fistula. Surgery is reserved for uncontrolled sepsis, major haemorrhage, extensive gangrene, a remediable obstruction, failure of less invasive management or unresolved concern about cancer.

Key points

A lung abscess is a localised pus-containing cavity; necrotising pneumonia is more diffuse parenchymal destruction with multiple small cavities and may progress rapidly.
Aspiration, poor dentition, impaired consciousness, swallowing dysfunction, an obstructing tumour, immunosuppression and septic emboli are high-yield causes or contributors.
CT with contrast usually characterises cavitation, distinguishes pleural empyema and looks for obstruction, infarction, foreign body or malignancy.
Collect blood and good lower-respiratory samples before antibiotics where safe, but never delay sepsis treatment or stabilisation for sampling.
Empirical therapy normally covers aerobic and anaerobic oral flora; recent healthcare exposure, influenza, resistant colonisation and immune compromise alter the spectrum.
Clinical response is judged over days, but radiological resolution often takes weeks; total antibiotic duration is individualised to response, organism and source control.
Persistent fever should trigger a diagnostic reset: inadequate coverage, empyema, blocked drainage, resistant organism, TB, fungus, cancer or non-infective cavitation.
Massive haemoptysis, shock, respiratory failure or a tension-like expanding cavity requires immediate escalation and coordinated critical, respiratory, interventional and surgical care.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Aspiration and oral flora

Impaired consciousness, swallowing dysfunction and poor dentition permit aspiration of polymicrobial secretions that can necrose dependent lung.

Virulent pulmonary infection

Selected bacterial pathogens cause tissue destruction within severe pneumonia, sometimes after influenza or in immune-suppressed hosts.

Bronchial obstruction

Lung cancer, an inhaled foreign body or stenosis traps secretions and prevents drainage, producing persistent focal infection and cavitation.

Haematogenous seeding

Septic emboli from right-sided endocarditis, infected venous thrombosis or catheters can create multiple peripheral necrotic nodules or abscesses.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Local inoculation
A high microbial burden reaches poorly clearing distal lung through aspiration, airway obstruction or the bloodstream.
2
Suppurative inflammation
Neutrophils and bacterial toxins destroy alveolar walls and small vessels, creating liquefactive necrosis, contributing to the resulting loss of respiratory reserve.
3
Cavity formation
Necrotic material drains into a bronchus, leaving an air-containing cavity that may show an air-fluid level.
4
Persistent source
Poor drainage, devitalised tissue or continuing obstruction sustains infection despite systemic antimicrobial exposure, thereby altering ventilation, gas transfer or respiratory mechanics.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Subacute abscess

Days to weeks of fever, sweats, weight loss, pleuritic pain, productive cough and offensive sputum, sometimes with halitosis or poor dentition. A cavity with an air-fluid level supports but does not prove the diagnosis.

Fulminant necrotising pneumoniaRed flag

Rapid hypoxaemia, shock, high inflammatory burden, multilobar consolidation and evolving small cavities, sometimes after influenza. Escalate early because deterioration and organ failure can be abrupt.

Haemorrhage or fistulaRed flag

Large-volume haemoptysis, falling haemoglobin, new pneumothorax, persistent air leak or expectoration of copious purulent material suggests vascular erosion, rupture or bronchopleural communication.

Obstructing lesion

A focal recurrent pneumonia in the same lobe, older age, smoking history, haemoptysis, lobar collapse or failure to resolve should raise concern for bronchogenic cancer or an inhaled foreign body.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Chest radiographFirst step
Why
Confirm consolidation or a cavity, identify an air-fluid level and detect pneumothorax or pleural fluid rapidly.
Interpretation and limitations
A normal or equivocal film does not exclude early necrosis. A lenticular pleural collection may be empyema rather than an intrapulmonary abscess.
02
Contrast-enhanced CT chest
Why
Map necrotic lung and vessels, define cavity wall and relationship to pleura, identify empyema, obstruction, tumour, foreign body, infarction or septic embolic pattern.
Interpretation and limitations
Loss of parenchymal enhancement and multiple small cavities favour necrotising pneumonia. A split-pleura collection requires the pleural-infection pathway.
03
Blood cultures and lower-respiratory microbiology
Why
Take blood cultures and sputum for Gram stain and culture before antibiotics when feasible; seek anaerobic processing or specialist tests when the phenotype warrants.
Interpretation and limitations
Expectorated sputum can be contaminated by oral flora. A blood, pleural or protected lower-airway isolate has greater weight, but prior antibiotics reduce yield.
04
Pleural ultrasound and diagnostic aspiration
Why
Investigate an accompanying effusion and obtain immediate pH plus microbiology when pleural infection is possible.
Interpretation and limitations
Frank pus or biochemical evidence of complicated infection calls for prompt drainage; pleural infection will not resolve through the bronchial drainage of a lung abscess.
05
Bronchoscopy
Why
Consider for suspected endobronchial obstruction, foreign body, unusual pathogen, significant haemoptysis or failure despite appropriate therapy.
Interpretation and limitations
Visualisation and directed samples may reveal tumour or obstruction, but bronchoscopy is not automatically required in a typical, improving aspiration abscess.
06
Cause-directed tests
Why
Use echocardiography and repeat blood cultures for septic emboli; TB or fungal studies, HIV testing with consent, ANCA or tissue sampling when supported by the presentation.
Interpretation and limitations
Testing should follow the differential rather than a scattergun panel; a positive colonising organism still requires clinicoradiological correlation.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Cavitating lung cancer

Irregular thick-walled cavity, persistent focal disease, smoking history or failure to resolve raises squamous or other malignancy with secondary infection.

Pulmonary tuberculosis

Chronic constitutional symptoms, epidemiological risk and upper-zone cavitation require acid-fast microbiology and infection-control measures, while chronology and directed testing distinguish the competing explanation.

Fungal or non-tuberculous mycobacterial disease

Immune status, structural lung disease, chronic tempo and species-specific cultures distinguish these infections from ordinary bacterial abscess.

Empyema

A loculated pleural collection may mimic a peripheral cavity; CT and ultrasound define whether pus lies within lung or pleural space.

Granulomatosis with polyangiitis

Multiple cavitating nodules with renal, upper-airway or vasculitic features suggest inflammatory vascular destruction rather than primary infection.

Additional chapter-specific clues

Alternative cavitary syndromes

Multiple peripheral nodules suggest septic emboli; apical chronic cavities suggest TB or NTM; a mobile intracavitary mass may indicate an aspergilloma; renal or ENT features may indicate vasculitis.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01StabiliseSevere necrotising infectionFirst stepHypoxaemia, shock, altered mental state, major haemoptysis or rapidly progressive cavitation.

1Use an ABCDE approach, controlled oxygen to an individual target, blood cultures and lactate, prompt sepsis antimicrobials and early critical-care review.
2Obtain urgent contrast CT when the patient can safely transfer; involve respiratory medicine, microbiology and radiology, and call thoracic surgery or interventional radiology early for haemorrhage, fistula or non-viable lung.
3Address pleural sepsis separately with ultrasound-guided drainage and protect the airway if bleeding or reduced consciousness threatens ventilation.
4Reassess within hours for organ support, antimicrobial adequacy and evolving source-control needs.

02TreatPresumed aspiration-related lung abscessCompatible cavity, aspiration or dental risk and no immediate surgical complication.

1Collect blood and respiratory samples, assess dentition and swallowing, and start locally recommended antibiotics with oral anaerobic and aerobic coverage; broaden only for defined resistance or healthcare risks.
2Optimise hydration, analgesia, physiotherapy or airway clearance and oral care; obtain speech-and-language and dental input when indicated.
3Review physiology and inflammatory trajectory daily. Step down to an active oral agent once improving and able to absorb, with duration individualised over weeks rather than copied from uncomplicated pneumonia.
4Arrange interval imaging and a named review to confirm resolution and exclude an obstructing lesion.

03ResetFailure to improvePersistent fever, sepsis or radiological progression after an appropriate early treatment interval.

1Recheck adherence, dose, absorption, renal adjustment, culture results and whether resistant staphylococci, Gram-negatives, TB, fungi or non-infective disease were missed.
2Repeat contrast CT to identify empyema, enlarging cavity, gangrene, vascular complication or obstruction; sample pleural or deep material when feasible.
3Discuss percutaneous, endoscopic or surgical source control in a multidisciplinary team, balancing cavity location and procedural risks.
4Do not simply extend the same antibiotic indefinitely without a revised diagnosis and documented endpoint.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Common empirical option for aspiration-associated abscess because it covers many oral aerobes and anaerobes.

Co-amoxiclav

For a stable adult, a locally approved oral or intravenous regimen selected for severity and renal function; verify current BNF and antimicrobial policy rather than copying a generic dose.

Penicillin allergy, hepatic injury, renal adjustment and C. difficile risk. It does not reliably cover MRSA or all resistant hospital Gram-negatives.

Broader initial cover for severe sepsis, recent healthcare exposure or resistant Gram-negative risk, then narrow promptly.

Piperacillin with tazobactam

Intravenous hospital regimen adjusted for renal function and local extended-infusion policy; confirm current BNF and microbiology advice.

Beta-lactam allergy, renal injury, sodium load, cytopenia with prolonged use and ecological harm. Broader is not automatically better.

Adds anaerobic cover where dental disease, putrid sputum or another strong anaerobic phenotype exists.

Metronidazole as an add-on

Use only with an agent lacking adequate anaerobic activity, following the current local dose and duration.

Do not duplicate anaerobic cover unnecessarily. Alcohol interaction advice, neuropathy with prolonged exposure, hepatic adjustment and warfarin interaction are relevant.

Reserved for credible MRSA risk, culture evidence or a severe compatible post-influenza necrotising syndrome.

MRSA-active therapy

Agent and dose, such as intravenous glycopeptide or linezolid, chosen with microbiology according to local susceptibility, renal function and site.

Glycopeptide levels and renal toxicity or linezolid cytopenia, neuropathy and serotonergic interaction require active monitoring. De-escalate when MRSA is not supported.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Sepsis and shock

Uncontrolled necrotic infection can cause bacteraemia, haemodynamic instability and multi-organ dysfunction, and potentially prolonging treatment and functional recovery.

Massive haemoptysis

Tissue and vessel erosion can produce life-threatening airway bleeding, particularly in a cavity adjacent to major vasculature.

Empyema or bronchopleural fistula

Rupture into pleura spreads infection and may create a persistent airway-pleural connection with difficult source control.

Metastatic infection

Bacteraemia may seed brain, joints or other organs, while septic embolic disease implies an ongoing extrapulmonary source.

Chronic structural damage

Healing can leave fibrosis, bronchiectasis and a residual cavity that predisposes to recurrent infection or fungal colonisation.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record temperature, respiratory rate, oxygen requirement, haemodynamics, mental state and urine output frequently during acute instability.
Trend FBC, CRP and renal and hepatic function, interpreting biomarkers alongside clinical change rather than demanding normalisation before step-down.
Review cultures at 24 to 72 hours and after final identification; document narrowing, oral switch and intended review date.
Watch for haemoptysis, new pleural pain, subcutaneous emphysema, pneumothorax or persistent air leak as signals of erosion or fistula.
Repeat imaging sooner if deterioration occurs and at an agreed interval after recovery to demonstrate resolution and exclude cancer or foreign body.
During prolonged antibiotics, monitor toxicity, adherence, gastrointestinal tolerance, interactions and C. difficile risk.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

An air-fluid level localises imperfectly

A lung abscess tends to be round within parenchyma, whereas empyema is pleural and lenticular, but CT and ultrasound are often needed because treatment differs.

Bad-smelling sputum is useful but not required

Putrid sputum supports anaerobic infection, yet many aspiration abscesses lack it and prior antibiotics alter flora.

Radiology lags behind physiology

A cavity can persist while fever and oxygenation improve. Conversely, a smaller cavity does not reassure if shock, haemorrhage or pleural sepsis worsens.

The mouth may be the source

Dental assessment and twice-daily oral hygiene address recurrent aspiration of pathogenic biofilm; antibiotics alone leave that driver intact.

Drain the right compartment

Pleural pus requires an intercostal drain. An intrapulmonary cavity often drains via a bronchus and is only invasively drained after expert review.

11Common pitfallsFrequent interpretation and management errors.

01
Mistaking empyema for a lung abscess and omitting pleural drainage.
02
Ignoring an obstructing lung cancer because infection initially improves.
03
Assuming every cavity is bacterial and missing TB, fungus, septic emboli or vasculitis.
04
Adding metronidazole to an agent that already has good anaerobic activity without a reason.
05
Waiting for bronchoscopy or CT before giving urgent antibiotics to a shocked patient.
06
Prolonging broad intravenous therapy without culture review, oral-switch criteria or a follow-up image.