The bat-wing appearance (also called the butterfly pattern) is bilateral, symmetrical airspace opacification centred on the perihilar and central lung, with sparing of the outer one-third of each lung (the “cortex”). The shape of the central opacity, fanning out from both hila, resembles the outstretched wings of a bat. It is a classic sign of severe, rapidly developing alveolar pulmonary edema, most often cardiogenic, but the same distribution can be produced by any process that floods the central alveoli. It is an uncommon presentation of pulmonary edema and, when present, usually signals acute and severe disease.
Spot diagnosis
A 62-year-old with acute breathlessness after a myocardial infarction has a portable chest radiograph showing dense, symmetrical perihilar opacity fanning out from both hila, sparing the lung periphery, with an enlarged cardiac silhouette. What single pattern is shown, and what does it most likely represent?
What is the bat-wing appearance?
The bat-wing appearance describes confluent alveolar (airspace) opacity that is concentrated in the central, perihilar lung and fades before it reaches the outer lung, so the peripheral cortex remains relatively lucent. The opacity is bilateral and broadly symmetrical, and often contains air bronchograms. It is the alveolar-filling counterpart of the more common, evenly graded pattern of pulmonary edema, and it tends to appear when fluid accumulates quickly and heavily.
- Modality: most often recognised on the frontal chest radiograph; the correlate on CT is central ground-glass or consolidation with peripheral and subpleural sparing.
- Distribution: central and perihilar, bilateral and symmetrical, sparing the outer third of the lungs.
- Finding: confluent airspace opacity, frequently with air bronchograms, that resolves rapidly when the underlying cause (typically fluid overload) is treated.
Why the central lung? Pathophysiology
In cardiogenic edema, a rise in left atrial and pulmonary venous pressure increases hydrostatic pressure in the pulmonary capillaries and drives fluid across the capillary membrane into the interstitium and then the alveoli. Several factors are thought to concentrate the fluid centrally in the bat-wing variant: the central lung carries a greater blood volume and is more distensible than the periphery; the peripheral cortex has more efficient lymphatic clearance and greater respiratory motion, which helps clear fluid; and when edema forms very rapidly, clearance mechanisms are overwhelmed centrally first. The result is dense central alveolar flooding with a relatively dry peripheral cortex. The exact mechanism remains debated, and the pattern is not specific to cardiac disease—any cause of rapid, heavy alveolar filling can reproduce it.
How to recognise it on the chest radiograph
- Check symmetry and centrality. The opacity should be bilateral, symmetrical and hug both hila, with a clear zone in the outer lung.
- Look for air bronchograms. These confirm an alveolar-filling process rather than a purely interstitial or pleural abnormality.
- Judge the heart and vascular pedicle. Cardiomegaly and a widened vascular pedicle push strongly towards a cardiogenic or overload cause.
- Correlate with time and treatment. Cardiogenic edema typically changes over hours and clears quickly with diuresis; opacity that persists for days or weeks argues for a non-edematous cause.
Causes of a bat-wing pattern
Although the bat-wing pattern is most strongly associated with cardiogenic pulmonary edema, any process that rapidly fills the central alveoli with fluid, blood, pus or other material can reproduce it. The clinical context and supporting radiographic features usually narrow the list.
| Cause | Why it produces a central pattern | Clue that favours it |
|---|---|---|
| Cardiogenic pulmonary edema (most common) | Rapid, severe rise in pulmonary capillary hydrostatic pressure with central alveolar flooding | Cardiomegaly, widened vascular pedicle, upper-lobe blood diversion, Kerley B lines, pleural effusions; clears with diuresis |
| Renal failure / fluid overload (uraemic edema) | Combined hydrostatic overload and increased capillary permeability | Known renal failure or fluid overload; heart size may be normal |
| Non-cardiogenic edema / ARDS | Diffuse alveolar damage with permeability edema; usually patchy and peripheral but can be central | Normal heart size, prominent air bronchograms, a precipitating insult (sepsis, aspiration, pancreatitis) |
| Diffuse alveolar haemorrhage | Blood fills the central alveolar spaces | Haemoptysis, falling haemoglobin, vasculitis, SLE or anticoagulation |
| Pneumocystis (PJP) pneumonia | Perihilar alveolar and ground-glass filling | Immunocompromise or HIV, subacute dyspnoea, raised LDH |
| Pulmonary alveolar proteinosis | Lipoproteinaceous material fills the alveoli in a perihilar distribution | Disproportionately mild symptoms, no cardiomegaly or effusion, crazy-paving on CT |
| Inhalation injury or near-drowning | Toxic or aspirated fluid causes permeability alveolar flooding | Clear exposure history |
| Invasive mucinous adenocarcinoma (formerly bronchioloalveolar carcinoma) | Tumour cells and mucin fill the alveoli, mimicking edema | Chronic and non-resolving; does not clear with diuresis; may cause bronchorrhoea |
Cardiogenic versus non-cardiogenic edema
When a bat-wing pattern is due to edema, the chest radiograph can often suggest whether the cause is cardiac, renal (overload) or a capillary-permeability injury. Milne and colleagues showed that a small set of features distinguishes these groups with high accuracy.
| Feature | Cardiogenic / overload edema | Permeability edema (ARDS) |
|---|---|---|
| Heart size | Usually enlarged | Usually normal |
| Vascular pedicle width | Widened | Normal or narrow |
| Distribution of blood flow | Balanced or inverted (upper-lobe diversion) | Normal or balanced |
| Distribution of edema | Even, central / perihilar | Patchy, often peripheral |
| Septal (Kerley B) lines | Common | Uncommon |
| Peribronchial cuffing | Common | Uncommon |
| Pleural effusions | Common | Uncommon |
| Air bronchograms | Uncommon | Common |
Reverse bat-wing appearance
The reverse (reversed) bat-wing pattern is the mirror image: peripheral, subpleural airspace opacity with relative sparing of the central perihilar lung. It is characteristically seen in chronic eosinophilic pneumonia (“photographic negative of pulmonary edema”) and cryptogenic organising pneumonia, and peripheral, reverse-bat-wing distributions have also been reported in COVID-19 pneumonia. Recognising the reversed pattern steers the differential away from hydrostatic edema and towards these organising and eosinophilic processes.
Clinical significance and pitfalls
- Signals severe, acute disease. A true bat-wing pattern usually reflects rapid, heavy alveolar flooding and demands prompt clinical correlation and treatment, not a wait-and-watch approach.
- Not synonymous with heart failure. Cardiogenic edema is the commonest cause, but haemorrhage, infection, proteinosis and even malignancy can look identical; always weigh the clinical context.
- Use the tempo. Edema changes over hours and clears with diuresis; a “bat-wing” opacity that persists for days to weeks should raise pneumonia, alveolar proteinosis, haemorrhage or mucinous adenocarcinoma.
- Correlate with the heart and pleura. Cardiomegaly, a widened vascular pedicle, septal lines and effusions support a cardiac or overload cause; their absence should prompt a non-cardiogenic differential.
Frequently asked questions
References
- Milne EN, Pistolesi M, Miniati M, Giuntini C. The radiologic distinction of cardiogenic and noncardiogenic edema. AJR Am J Roentgenol. 1985;144(5):879-894. PMID: 3872571.
- Gluecker T, Capasso P, Schnyder P, et al. Clinical and radiologic features of pulmonary edema. RadioGraphics. 1999;19(6):1507-1531. PMID: 10555672.
- Han J, Xiang H, Ridley WE, Ridley LJ. Bat wing or butterfly sign: pulmonary oedema. J Med Imaging Radiat Oncol. 2018;62(Suppl 1):18. PMID: 30309074.
- Kumar A, Kumar S, Katiyar V, et al. “Crazy-paving” pattern: a characteristic presentation of pulmonary alveolar proteinosis and a review of the literature from India. Lung India. 2016;33(3):335-342. PMID: 27186004.
- Ghosh S, Nandolia KK, Tale S, et al. Reverse Batwing sign in COVID-19 pneumonia. QJM. 2020;113(9):659-660. PMID: 32649770.
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