Usual interstitial pneumonia

Usual interstitial pneumonia (UIP) refers to a morphological pattern of interstitial lung disease. In the past the term UIP was used to sometimes synonymously used 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 non specific interstitial pneumonia (NSIP) and desquamative interstitial pneumonia DIP ¹.

Pathology

The histologic diagnosis of UIP is based on temporal heterogeneity : which is the identification of fibrotic lesions at different stages (fibroblastic foci, mature fibrosis, and honeycombing) within the same biopsy specimen ¹. On a typical biopsy, there are areas of normal lung alternate with interstitial inflammation and honeycombing .

Associations

• 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 : can have either a UIP or NSIP (commoner) pattern ⁴.
  • polymyositis / dermatomyositis : those with these can have a UIP, NSIP or BOOP pattern ⁴.
  • mixed connective tissue disease : can have either a UIP or NSIP pattern ⁴.
• asbestos related interstitial lung disease : asbestosis ¹

Location - distribution

The distribution of UIP on CT images is typically characteristically with an apico-basal gradient with basal and peripheral predominance, although it is often patchy.

Radiographic features

Plain film - Chest radiograph

Plain film features are non specific. While chest radiographs can be even be normal in patients with very early disease, in advanced disease, it may show decreased lung volumes and sub-pleural reticular opacities that increase from the apex to the bases of the lungs.

HRCT 

When describing imaging features, the term UIP type pattern is often used. 

The positive predictive value of CT in the diagnosis of UIP is high and ranges from ~ 70 - 100 ¹.

As observed on a pathology specimen, cross sectional imaging also reveals heterogeneous, with areas of fibrosis alternating with areas of normal lung ⁵.

Typical features include ^1,5

• the presence of reticular opacities, often associated with traction bronchiectasis (considered a very good differentiating feature from patients with NSIP and concurrent emphysema ⁶. 
• honeycombing : common. 
• ground-glass opacities : common but are usually less extensive than the reticular pattern. 
• architectural distortion, which reflects lung fibrosis, is often prominent. 
• lobar volume loss is seen in cases of more advanced fibrosis.

Treatment and prognosis

In patients with UIP, areas of ground-glass attenuation tend to increase in extent or progress to fibrosis despite treatment ⁸.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 .

Differential diagnosis

A key imaging differential on cross sectional imaging would be non specific interstitial pneumonia pattern (especially fibrotic non specific interstitial pneumonia)

If there is added upper lobe predominant emphysematous change consider

• combined pulmonary fibrosis and emphysema (CPFE)