Description
Desquamative Interstitial Pneumonia (DIP) is a form of idiopathic interstitial pneumonia characterised by the accumulation of numerous pigmented macrophages within the alveolar spaces. It is one of the less severe forms of interstitial lung disease and is associated with a more favourable prognosis compared to other interstitial pneumonias. However, it is the most severe form of the continuum of smoking-related ILD which includes respiratory bronchiolitis associated ILD.
Pathogenesis
The exact cause of DIP is unknown. However, it is strongly associated with cigarette smoking, and indeed the majority of patients with DIP are current or former smokers. The proposed mechanism is that smoke induces inflammation and injury to the small airways, leading to an influx of macrophages into the alveolar spaces.
Epidemiology, Risk Factors & Associations
- DIP primarily affects adults, typically presenting in the 4th to 5th decade of life (mean age at diagnosis 39-56 years).
- There is a strong association with cigarette smoking (high pack-years), with up to 90% of affected individuals being smokers.
- However, DIP has also been reported in non-smokers (19%) who have exposure to inorganic particles or mycotoxins (alfatoxins), connective tissue diseases, infection (cytomegalovirus, Aspergillus).
Clinical Features
- Chronic, non-productive cough and progressive exertional dyspnoea (common)
- Less commonly, they may have constitutional symptoms such as weight loss and fatigue.
- Physical examination findings are often non-specific, but may include crackles on lung auscultation.
Complications
- Progression to respiratory failure
- Pulmonary hypertension
- Acute exacerbation, particularly following viral infections
Subtypes
DIP is a distinct clinical entity and does not have subtypes.
Pathological Features
Morphology
- Characterised by the accumulation of pigmented macrophages within the alveolar spaces.
- Can be seen in other conditions, such as respiratory bronchiolitis-associated interstitial lung disease (RB-ILD) and in smokers’ lungs, albeit to a lesser extent.
- Interstitial tissue and alveolar walls are generally preserved.
Histopathology
- Exhibits a uniform involvement with minor architectural distortion.
- Alveolar spaces are filled with macrophages, and alveolar septa may be thickened due to mild fibrosis and/or inflammatory cell infiltrate.
Radiological Features
HRCT
- Ground-glass opacities (GGOs): Seen in virtually all DIP patients (100%), they predominantly affect the lower lobes (73%) and peripheral lung fields (59%), but may be patchy (23%) and diffuse (18%). May be associated with consolidation.
- Thickening of the interlobular septa (up to 30%): Subpleural predominance. May exhibit crazy paving appearance when superimposed on GGOs.
- Absence of honeycombing in approximately 95% of cases (unlike other forms of interstitial pneumonia)
X-ray
- CXR non-specific – May show diffuse reticular or ground-glass opacities.
Grading and Staging
There is no specific grading or staging system for DIP.
Differential Diagnosis
- Respiratory Bronchiolitis-Associated Interstitial Lung Disease (RB-ILD): Ground-glass opacities and is associated with smoking. RBL-ILD demonstrates centrilobular and upper lobe predominance
Management
- The primary management of DIP is smoking cessation, which can lead to significant improvement or even resolution of the disease.
- Corticosteroids may be used in severe or progressive cases, although the evidence base is limited.
- In refractory cases, lung transplantation may be considered.
- As with all interstitial lung diseases, patients should be managed in a multidisciplinary setting with input from pulmonology, radiology, and pathology.