Usual interstitial pneumonia (UIP) is one of the morphological and pathological patterns of interstitial lung disease.
On imaging, it usually presents with a patchy craniocaudal gradient of peripheral septal thickening, bronchiectasis, and honeycombing.
In the past, the term UIP was used synonymously with idiopathic pulmonary fibrosis (IPF) while more lately the term idiopathic pulmonary fibrosis is applied solely to the clinical syndrome associated with the morphologic pattern of UIP and specifically excludes entities such as nonspecific interstitial pneumonia (NSIP) and desquamative interstitial pneumonia (DIP) 1. Differentiation of UIP is important as it carries the worst prognosis and treatment varies widely from the other types of ILDs.
The clinical presentation can be variable and can overlap with other entities such as NSIP which can make the diagnosis challenging 11. Most patients present with progressive exertional shortness of breath and chronic dry cough usually over a period of 24 months before diagnosis 12. Physical examination usually reveals fine end-inspiratory crackles and in severe cases finger clubbing. Lung function test shows a restrictive pattern with decreased DLCO.
The histological diagnosis of UIP is based on temporal and spatial heterogeneity, which is the identification of fibrotic lesions at different stages (fibroblastic infiltrates, mature fibrosis, and honeycombing) within the same biopsy specimen and architectural distortion. Honeycombing particularly if it is more than 5% of the lung volume is an almost 100% specific finding. On a typical biopsy, there are areas of normal lung alternating with interstitial fibrosis and honeycombing. The distribution of UIP characteristically is with an apicobasal gradient with basal and peripheral (subpleural) predominance, although it is often patchy.
Inflammation is absent or mild and mostly limited to the areas of honeycombing 1-12.
UIP pattern of ILD can be seen in idiopathic pulmonary fibrosis or secondary to underlying systemic diseases. These would include:
- connective tissue disorders
- rheumatoid arthritis: UIP is considered to be the dominant pattern in those with rheumatoid arthritis who have concurrent interstitial lung disease 3
- systemic sclerosis (scleroderma): can have either a UIP or NSIP (commoner) pattern 4
- polymyositis/dermatomyositis: those with these can have a UIP, NSIP or COP pattern 4
- mixed connective tissue disease: can have either a UIP or NSIP pattern 4
- asbestos related interstitial lung disease: asbestosis 1
- chronic hypersensitivity pneumonitis
- medications/drug toxicity: amiodarone lung
- Hermansky-Pudlak syndrome (very rare)
In practice, the diagnosis is usually made in a multidisciplinary approach involving chest physicians, radiologists and pathologists with expertise in interstitial lung disease (ILD) 12.
Plain film features are nonspecific. While chest radiographs can be even normal in patients with a very early disease, in advanced disease, it may show decreased lung volumes and basal fine to coarse reticulation. Usually, due to the more extensive involvement of the lower lobes, the major fissure is shifted inferiorly which is best seen on the lateral chest radiograph.
When describing imaging features, the term UIP pattern is often used, which has specific diagnostic criteria on HRCT 16. The positive predictive value of CT in the diagnosis of UIP is high and ranges from 70-100% 1. Similar to the pathology specimen, cross-sectional imaging also reveals heterogeneity, with patchy areas of fibrosis alternating with areas of normal lung 5.
Typical features include 1,5:
- honeycombing: particularly if it is more than 8 % of lung parenchyma is highly specific for UIP. In general, UIP can be divided to two groups, those with less than 5% honeycombing and those with more than 5% honeycombing. It mainly reflects the stage and severity of the disease. Those with less than 5% honeycombing may pose diagnostic difficulty as differentiation with NSIP on imaging can be impossible; however, these still follow similar prognosis as other UIP patients 2
- reticular opacities: in the immediate subpleural lung, often associated with honeycombing and traction bronchiectasis, with peripheral and lower lobe predominance is considered a very good differentiating feature from patients with NSIP and concurrent emphysema 2
- reticular opacity to ground gland glass opacity ratio: one or greater
- ground-glass opacities: usually less extensive than the reticular pattern and almost never seen in isolation - usually happens in areas of reticulation or honeycombing
- lung architectural distortion: which reflects lung fibrosis and is often prominent
- lobar volume loss (predominantly lower lobes) is seen in cases of more advanced fibrosis
See article: UIP pattern: diagnostic HRCT criteria.
Treatment and prognosis
In patients with UIP, areas of ground-glass attenuation tend to increase in extent or progress to fibrosis despite treatment 8,13. In those with more active inflammation involving the pulmonary interstitium, there is a faster progression of honeycombing in long-term follow-up 10. The average rate of progression of honeycombing in patients with idiopathic usual interstitial pneumonia according to one study was 0.4% of lung volume per month 7.
A key imaging differential on cross-sectional imaging would be:
- nonspecific interstitial pneumonia pattern (especially fibrotic non-specific interstitial pneumonia)
chronic hypersensitivity pneumonitis
- HP usually involves the mid and upper zones of the lung, and also the presence of centrilobular nodules and areas of air trapping are very useful hints to differentiate it from UIP
- UIP cases are also thought to have honeycombing and peripheral or lower lung zone predominance of disease, and less likely to have micronodules
- amiodarone lung fibrosis: helpful clues are the presence of hyperdense pulmonary nodules or hyperdense liver on a non-contrast CT
- systemic sclerosis: presence of patulous oesophagus and correlation with hand radiographs if available can be helpful
- asbestosis: bilateral pleural plaques with or without calcification or peritoneal calcification are helpful in diagnosis
- combined pulmonary fibrosis and emphysema (CPFE): especially if there are added upper lobe predominant emphysematous
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Interstitial lung disease
interstitial lung disease
- drug-induced interstitial lung disease
- hypersensitivity pneumonitis
idiopathic interstitial pneumonia (mnemonic)
- acute interstitial pneumonia (AIP)
- cryptogenic organising pneumonia (COP)
- desquamative interstitial pneumonia (DIP)
- idiopathic nonspecific interstitial pneumonia (NSIP)
- idiopathic pleuroparenchymal fibroelastosis
- lymphoid interstitial pneumonia (LIP)
- respiratory bronchiolitis–associated interstitial lung disease (RB-ILD)
- usual interstitial pneumonia / idiopathic pulmonary fibrosis (UIP/IPF)