Desquamative interstitial pneumonia

Desquamative interstitial pneumonia (DIP) is an interstitial pneumonia closely related to and thought to represent the end stage of respiratory bronchiolitis interstitial lung disease (RB-ILD). It is associated with heavy smoking.

Epidemiology

It is considered one of the rarest of idiopathic interstitial pneumonias ¹¹. There is a recognized male predilection (M:F = 2:1). 

Clinical presentation

Presentation tends to be in middle age (30-60 years of age) with progressive shortness of breath and chronic cough ⁴.

The vast majority of patients are heavy smokers (90%) with an average smoking history of 18 pack-years. However, other predisposing factors include:

• autoimmune/systemic disorders, e.g. rheumatoid arthritis, scleroderma ⁷
• infection, e.g. HIV ⁴
• toxin, occupational or environmental exposure, e.g. asbestos ^6,7
• drugs

Pathology

Its name is misleading as no desquamation of alveolar epithelium is present, but rather the cells that fill the alveoli are pigment-laden macrophages. This is more pronounced than in RB-ILD.

Pulmonary function tests

Pulmonary restriction is usually absent or slight, although diffusion is impaired in most patients ⁹. 

Radiographic features

Plain radiograph

• nonspecific
• may show bilateral interstitial infiltrates ⁸: they may have a greater predilection for the lower-lung zones and, sometimes, peripheral predominance ⁹
• has been classically described as one of triangular-shaped regions of increased density radiating from the hilar regions to the periphery; although, this is seen in only a minority of cases ¹⁰

CT

DIP is usually characterized by diffuse ground-glass opacities (sometimes has a granular or nodular texture), which correlate histologically with the spatially homogeneous intra-alveolar accumulation of macrophages and thickening of alveolar septa:

• bilateral and symmetric (86%) ⁷
• basal and peripheral (60%)
• patchy (20%)
• diffuse (20%) ⁴

Other frequent CT findings include spatially limited, irregular linear opacities and small cystic spaces, which are indicative of fibrotic change (50% of patients ⁷).

Other changes related to background smoking-related lung disease are often seen, e.g. bronchial wall thickening and centrilobular emphysema.

Despite differences in the CT appearance of RB-ILD and DIP, imaging findings may overlap and may be indistinguishable from each other. To improve diagnostic accuracy, lung biopsy is required in all cases of suspected RB-ILD or DIP.

Honeycombing may be seen in less than one-third of cases, and if present tends to be usually peripheral and limited in extent  ⁹.

Treatment and prognosis

With smoking cessation and corticosteroid therapy, the prognosis is good. The overall prognosis of has been described as being significantly better than that of fibrotic NSIP ¹¹. Nevertheless, progressive disease with eventual death can occur, notably in patients with continued cigarette smoking.

Differential diagnosis

For diffuse ground glass changes on CT consider the differential for ground glass opacities.