Chest X-ray manifestationsof pneumoniaDebasis Das

David C Howlett

 Abstract Pneumonia is a leading cause of morbidity and mortality in the UK, and

chest X-rays are the initial modality of investigation in most cases.

Broad categories of infective change can be recognized on chest X-ray,

and are associated with different aetiological organisms. These chest

X-ray patterns, including lobar pneumonia, bronchopneumonia, nodular 

consolidation, interstitial consolidation, atypical pneumonia, and lung

abscesses, are reviewed and the commonest micro-organisms that are

likely to be responsible discussed. The sequelae of pneumonia, and the

differential diagnoses that pneumonia is frequently mistaken for are

also discussed. Knowledge of the chest X-ray manifestations of 

pneumonia will help readers to guide appropriate therapy in the future,

and anticipate any complications that may arise.

Keywords  consolidation; chest X-ray (CXR); pneumonia

Pneumonia can be defined as a respiratory infection that

produces consolidation of the lungs. It is one of the commonest

causes of mortality and morbidity within the UK, and chest X-ray

(CXR) represents the initial investigation of choice in most cases.

A working knowledge of the appearances of pneumonia on CXR

is therefore, important for the successful management of this

commonly occurring condition.

What is consolidation?

Consolidation is essentially inflammatory exudate within the

lung tissue. This results in the normally lucent (black), air-filled

pulmonary tissue appearing opaque (white) on CXR. Different

infective organisms tend to produce consolidation with differing

distribution around the lung and, when taken together with

clinical information, recognition of these different ‘patterns’ of 

infection on CXR can narrow down the list of likely causative

organisms and help guide appropriate therapy.

Patterns of consolidation on CXR

 Air-space consolidation

The air spaces of the lungs comprise the alveoli, alveolar ducts,

and respiratory bronchioles. Infection that originates in the air

spaces produces ‘Lobar pneumonia’ (see Figure 1).1 This pattern

of consolidation is characterized by:

  a solitary, peripheral focus of dense opacity

  sparing of the larger airways   to produce ‘air bronchograms’

(air-filled bronchi surrounded by opacity)

  progressive spread to adjacent areas and eventually towards

the hilum

Lobar pneumonia is classically associated with community

acquired infection, and is most commonly due to  Streptococcus

 pneumoniae (Gram-positive bacterium).

  Less commonly, community-acquired lobar pneumonia may

be caused by   Legionella pneumoniae   (Gram-negative

organism). The diagnosis is suggested by associated non-

respiratory signs and symptoms (e.g. diarrhoea, neurological

dysfunction, raised liver function tests).

  Tuberculosis (TB) can also cause homogeneous consolidation

similar to lobar pneumonia. Upper and middle lobe prefer-

ence, associated enlarged lymph nodes, and a compatible

history (e.g. patient from Indian subcontinent, or immuno-

compromized) are all supportive.

Figure 1 Lobar pneumonia: homogeneous consolidation in the left upper 

zone with air bronchograms (black arrows). Concomitant bronchopneu-

monia is also evident in the right mid zone  e  patchy consolidation

without air bronchograms (white arrows).

Learning points

C Recognition of different CXR patterns in respiratory sepsis can

help to narrow the list of differential causes

C Pneumonia is associated with complications such as pleural

effusion, abscess formation and pneumothorax e look out for 

them!

C Alternative organisms, or non-infectious causes of consolida-

tion (including cancer), should be considered if there is limited

clinical improvement in the patient, or in the appearance of 

the CXR following treatment.

Debasis Das  MRCS(Eng) is a Specialist Registrar within the Department of 

Radiology at Eastbourne District General Hospital, East Sussex, UK.

Conflicts of interest: none declared.

David C Howlett  MRCP(UK) FRCR  is a Consultant Radiologist within the

Department of Radiology at Eastbourne District General Hospital, East 

Sussex, UK. Conflicts of interest: none declared.

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SURGERY 27:10   453    2009 Published by Elsevier Ltd.

 

Infection that originates in the airways and then spreads to the

air spaces, however, produces a slightly different pattern of 

consolidation termed ‘Bronchopneumonia’ (see   Figure 1).

Characteristic features include:

  multiple areas of patchy consolidation, often bilaterally

  lack of air bronchograms (as the airways are consolidated)

  progressive coalescence of the patchy consolidation with time.

Bronchopneumonia is typically associated with hospital

acquired (nosocomial) organisms such as  Escherichia coli  and

 Pseudomonas aeruginosa  (Gram-negative bacteria). Community

acquired bronchopneumonia does occur and is classically caused

by Staphylococcus aureus  (Gram-positive bacteria), though it too

frequently causes nosocomial infection as well. Remember:

  Modern treatment regimens for nosocomial respiratory sepsis

cover both Gram-positive and Gram-negative bacteria, so

aetiological origin is not as important as simply identifying

the consolidation.2

It is important to understand that the patterns described above

detail the established changes of pneumonia on CXR. The

features of early infection may be more subtle, and looking for

the ‘silhouette sign’ may be useful in such circumstances:

  normally distinct borders of opaque structures such as the heart,

aorta, and diaphragm appear unclear/irregular (commonly

referred to as ‘loss of the hemidiaphragm/heart border’)3

  the appearance is caused by a small focus of consolidation

(which is also opaque) lying adjacent to these structures  e

the similar densities of these tissues prevents a clear differ-

entiation between them on CXR.

Nodular consolidation

Nodules are small, rounded foci of air-space opacity, and are

usually associated with non-bacterial or uncommon bacterial

infections.1,2,4 Examples include:

  Varicella zoster  (chicken pox virus), which produces wide-

spread, bilateral nodular pneumonia (see  Figure 2)

  Mycoplasma pneumoniae (atypical bacterium), which usually

causes nodular pneumonia within one lung. These nodules

can coalesce as the infection progresses and produce a patchy

bronchopneumonia-type pattern (nodules and denser

consolidation may be seen together within one lung at this

stage).

  TB also commonly produces nodular opacification, though

predominantly within the upper portions of the lungs.

Progression of infection can also lead to coalescence of the

nodules and a patchy, bronchopneumonia-type pattern.

Interstitial consolidation

As well as being divided into lobes and segments, the lungs are

further subdivided into millions of microscopic lobules which

contain the alveoli, or air spaces, described before. The lobules

are separated by interlobular septa, and the space between

individual lobules is called the interstitial space. Respiratory

infections do not commonly produce opacification of the inter-

stitial space, but when they do, the appearances are quite distinct

from the air-space opacification:1,2,4

  numerous short, opaque lines (reticulations)

  symmetrically distributed in both lungs

  frequent association with small, opaque nodules, which often

requires close inspection of the CXR in order to differentiate

them from the reticulations (the CXR is described as having

a ‘reticulo-nodular’ pattern in these circumstances)

Organisms producing interstitial consolidation are usually associ-

ated with immunocompromized patients, including those with AIDS.

 Pneumocystis carinii (protozoa) is a good example (see Figure 3).

 Atypical pneumonia

A non-specific term that is used to describe:

  pneumonias   with common (lobar or bronchopneumonic)

CXR patterns but unusual clinical history, such as  Legionella

infection (which can present with gastrointestinal and

neurological symptoms)

  pneumonias with unusual CXR patterns, namely nodular and

interstitial consolidation, such as  Mycoplasma  and  Pneumo-

cystis infections, respectively

Figure 2 Nodular consolidation in a patient with chicken pox: multiple

rounded opacities can be seen throughout both lungs.

Figure 3 Interstitial pneumonia secondary to  Pneumocystis carinii :

bilateral and symmetrical opacities can be seen in the mid zones. Close

examination of the film will reveal the presence of innumerable, thin

opaque lines that are superimposed upon one another and deceptively

appear as homogeneous areas of consolidation.

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SURGERY 27:10   454    2009 Published by Elsevier Ltd.

 

The relevance of broadly categorising such pneumonias as

‘atypical’ is due to the fact that they frequently require non-

standard therapy, including uncommon antimicrobial agents.

  Most hospitals have antibiotic prescription protocols in place

to manage such atypical infections empirically prior to

specific diagnosis.

Lung abscess

The difference between a lung abscess and consolidation is the

presence of an epithelial wall around the former. This is a histo-

pathological observation and is not perceptible on CXR, resulting

in most lung abscesses appearing identical to focal areas of 

consolidation.1 Unlike regular consolidation, however, abscesses

cause necrosis of the lung tissue involved and the subsequent

formation of a cavity, surrounded by consolidation. If this results

in communication with an airway, the cavity fills with air and

produces an airefluid level (the fluid represents necrotic debris,

see Figure 4).

  Anaerobic bacteria, normally found as commensals within

the oropharynx and gastrointestinal tract, can cause abscess

formation.5 The bacteria gain entry to the lungs in aspirated

secretions or vomitus, leading to what is termed ‘aspiration

pneumonia’. This typically manifests as patchy consolidation

in the mid zones of the lungs, with or without cavitation (see

Figure 4).

  S. aureus and Klebsiella pneumoniae  (Gram negative) are two

more examples of bacteria that cause cavitation within areas

of consolidation, usually as part of a widespread

bronchopneumonia.1,2

  TB frequently causes cavitation as well, predominantly within

the upper and mid zone opacities mentioned previously.

Secondary complications of pneumonia

  Pleural effusion  e   common feature, correctly termed ‘para-

pneumonic’ effusion, occurs in association with many different

organisms. Should resolve with resolution of pneumonia.

  Empyema e  an abscess localized within the pleura, requires

prompt drainage. Usually occurs secondary to an effusion that

fails to resolve and often located in unusual locations on CXR,

e.g. effusion fluid localized along the lateral thoracic wall (as

opposed to the costophrenic angles).

  Pneumatocele  e  air-filled cyst, resembles an abscess (air-fil-

led area surrounded by consolidation, see Figure 4) but more

spherical and without airefluid level. Classically associated

with   Staphylococcus aureus   and paediatric pneumonias.

Following resolution of pneumonia, pneumatoceles appears

as circular, air-filled areas surrounded by an extremely thin

opaque wall. This appearance is classically seen following

 Pneumocystis pneumonia.

  Pneumothorax  e  rare association with pneumonia, usually

secondary to ruptured pneumatocele or extension of air-filled

lung abscess into pleura.

 Alternative causes of consolidation on CXR

  Blood e  especially in trauma settings or secondary to condi-

tions known to cause pulmonary haemorrhage, e.g. Good-

pasture’s syndrome. Typically causes patchy consolidation

and may be associated with rib fractures in trauma.

  Oedema  e  causes an interstitial pattern in early stages, and

dense, air-space consolidation when severe. Classical features

include bilateral mid zone (‘peri-hilar’) consolidation, Kerly B

lines (fine reticulations at the lateral edges of the lungs), and

cardiomegaly (cardio-thoracic ratio >50%).

  Non-infectious inflammation  e   includes adverse drug reac-

tions and idiopathic conditions, e.g. non-specific interstitial

pneumonia (NSIP). Produces a range of appearances ranging

from homogenous or patchy consolidation, to a reticulo-

nodular pattern.

  Cancer   e   bronchoalveolar carcinoma can cause patchy

consolidation that does not resolve with antibiotics.

As the last example suggests, all pneumonias must be followed

up with repeat CXR during and/or following treatment. Failure to

respond to therapy, denoted primarily by a lack of clinical

improvement in the patient and persistence/progression of the

initial CXR appearances, can suggest an alternative diagnosis.

Remember:

  consolidation takes up to 6 weeks to resolve on CXR and often

lags behind clinical resolution of the infection.   A

REFERENCES

1   Wilson AG, Armstrong P. Pulmonary infection in adults. In: Grainger RG,

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Livingstone; 1992. p. 213e47.

2   Herold CJ, Sailer JG. Community-acquired and nosocomial pneumonia.

Eur Radiol  2004;  14:  E2e20.

3   Wilson AG. Interpreting the chest radiograph. In: Grainger RG,

Allison DJ, eds. Diagnostic radiology. 2nd edn. London: Churchill

Livingstone; 1992. p. 149e61.

4   Ketai L, Washington S. Radiology of acute diffuse lung disease in

the immunocompetent host.  Semin Roentgenol  2002 Jan;   37:

25e36.

5   Marik PE. Aspiration pneumonitis and aspiration pneumonia.  N Engl J 

Med  2001;  344(9):  665e71.

Figure 4 Lung abscess secondary to aspiration pneumonia. Note the

opacity (air efluid level) within the area of consolidation in the left mid

zone (black arrow).

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SURGERY 27:10   455    2009 Published by Elsevier Ltd.