Medicine – Usual Interstitial Pneumonia (UIP) / Cryptogenic Fibrosing Alveolitis

Usual interstitial pneumonia (UIP) is a characteristic pattern of chronic interstitial lung injury associated with progressive pulmonary fibrosis. The older term cryptogenic fibrosing alveolitis (CFA) was historically used for what is now usually called idiopathic pulmonary fibrosis (IPF) when no underlying cause is identified.

UIP is defined mainly by its radiological and histopathological pattern, whereas IPF is the clinical diagnosis made when that UIP pattern occurs without another identifiable cause.

1. Pathology

UIP is characterised by patchy interstitial fibrosis affecting the lung parenchyma, particularly in the subpleural and basal regions.

A key pathological feature is temporal heterogeneity, meaning that different areas of the lung show different stages of disease at the same time.

Alternating Areas of Lung Injury

Microscopically, there may be alternating zones of:

Relatively normal lung,

Active fibroblastic change,

Established fibrosis, and

Honeycomb destruction.

This patchwork appearance helps distinguish UIP from some other interstitial lung diseases that produce a more uniform pattern of inflammation or fibrosis.

2. Epidemiology

UIP/IPF is generally more common in older adults and has historically been reported more frequently in men than women.

It is also strongly associated with a history of cigarette smoking, although smoking is not required for the diagnosis.

3. Dry Cough

A persistent dry, non-productive cough is one of the most common presenting symptoms.

The cough is usually chronic and may become troublesome as fibrosis progresses.

4. Progressive Breathlessness

Patients commonly develop progressive exertional dyspnoea.

Initially, breathlessness may occur only with significant activity, but as lung compliance and gas transfer deteriorate, symptoms may progress to breathlessness during routine activities or even at rest.

5. Finger Clubbing

Digital clubbing is a recognised physical sign in UIP/IPF.

It reflects chronic pulmonary disease and may be present even before very advanced respiratory failure develops.

6. Cyanosis

Cyanosis may occur in advanced disease when gas exchange becomes severely impaired.

It usually reflects significant arterial hypoxaemia and therefore tends to be a later clinical feature.

7. Fine Inspiratory Crepitations

A very characteristic examination finding is fine, late-inspiratory bibasal crackles, often described as “Velcro” crackles.

These result from the sudden opening of small fibrotic airways and alveolar units during inspiration.

Diagnosis

8. Pulmonary Function Tests

Pulmonary function testing typically shows a restrictive pattern.

There is usually a reduction in:

Forced vital capacity (FVC) and

Total lung capacity (TLC).

The FEV1/FVC ratio is usually normal or increased, helping distinguish restriction from obstructive airway disease.

9. Reduced Gas Transfer

Gas transfer is commonly impaired.

The DLCO/TLCO is reduced, often significantly, because fibrosis thickens and disrupts the alveolar-capillary interface.

This contributes to exertional oxygen desaturation.

10. Chest Radiograph

The chest radiograph may show bilateral bibasal interstitial or reticular shadowing.

As the disease advances, lung volumes may become reduced and coarse reticular change may become more obvious.

However, chest radiography is less sensitive and less specific than high-resolution CT.

11. High-Resolution CT

High-resolution CT (HRCT) is the key imaging investigation.

A typical UIP pattern includes:

Subpleural and basal predominance,

Reticular abnormalities,

Honeycombing, and

Traction bronchiectasis or bronchiolectasis.

Marked ground-glass change is usually less prominent than the fibrotic reticular and honeycomb abnormalities.

12. Honeycomb Change

Honeycombing refers to clustered, cystic air spaces caused by severe destruction and remodelling of normal lung architecture.

It is a hallmark of advanced fibrosis and strongly supports the diagnosis of UIP when present in the appropriate distribution.

13. Type I Respiratory Failure

Advanced disease may produce type I respiratory failure, characterised by hypoxaemia without primary hypercapnia.

This occurs because severe fibrosis impairs diffusion and creates ventilation-perfusion mismatch.

Carbon dioxide retention is usually a late feature because patients initially compensate by increasing ventilation.

14. Excluding Secondary Causes

Before diagnosing idiopathic pulmonary fibrosis, other causes of a UIP pattern must be excluded.

These include connective tissue disease, occupational or environmental exposure, certain drugs, and chronic hypersensitivity pneumonitis.

Therefore, a careful clinical history and appropriate autoimmune and exposure assessment are essential.

Key Clinical Pattern

Think of UIP/IPF in an older patient with progressive exertional breathlessness, persistent dry cough, finger clubbing, and fine bibasal late-inspiratory “Velcro” crackles.

The typical investigation pattern is restrictive spirometry, reduced gas transfer, bibasal interstitial shadowing, and HRCT showing basal subpleural reticulation, traction bronchiectasis, and honeycombing.

The most important modern distinction is that UIP is the radiological/pathological pattern, while idiopathic pulmonary fibrosis is the clinical disease diagnosed when UIP occurs without an identifiable underlying cause.