Chronic interstitial lung diseases are characterized by inflammation and fibrosis
in alveolar walls.
Several conditions can be grouped together on the basis of a common process of
inflammation in the walls of alveoli (chronic interstitial pneumonitis),leading to
progressive diffuse fibrosis in the lung interstitium.
These are termed interstitial lung diseases, as the primary pathology is within
walls rather than air spaces.
Despite this grouping on the basis of a common lung response to damage, there are
widely differing causes since the disease process can be initiated by a wide
range of extrinsic factors, both inhaled (e.g. industrial dust disease) and
non-inhaled (drugs, radiation), as well as intrinsic disease such as scleroderma
and rheumatoid disease.
It is notable that, as well as causing a chronic fibrosing syndrome, many of these
conditions can also cause an acute exudative phase of diffuse alveolar damage and may
present as ARDS.
• Idiopathic interstitial pneumonitis (also called 'usual interstitial pneumonia')
• Connective tissue diseases (rheumatoid disease and scleroderma)
• Atypical pneumonias (Chlamydia, Mycoplasma,some viruses)
• Pneumoconiosis (disease caused by inhaling mineral dusts)
• Extrinsic allergic alveolitis (disease caused by immune reactions to inhaled
organic dusts)
The end-result of long-standing interstitial fibrosis, from whatever cause,
is conversion of the lung into a mass of cystic air spaces separated by
areas of dense collagenous scarring.
This is termed 'honeycomb lung', as the cut surface has been thought to resemble
a honeycomb.
This is an end-stage process resulting from any of the causes
of chronic interstitial lung disease and is the result of the
sequence of damaging stimuli and reparative changes.
Idiopathic interstitial pneumonitis
Interstitial pneumonitis with no apparent cause on investigation is termed 'idiopathic'.
The disease causes chronic fibrosis in the lung interstitium and is also
termed cryptogenic fibrosing alveolitis.
It presents with insidious onset of dyspnoea and tachycardia, most commonly in the
sixth decade.
A restrictive pattern of respiratory disturbance develops,with impaired diffusion of
gas across alveolar walls.
These pathological findings are also referred to as usual interstitial pneumonitis
(UIP). In patients whose disease progresses, honeycomb lung develops.
Desquamative interstitial pneumonitis (DIP) is a phase of UIP characterized by numerous
macrophages in the alveoli.
Some workers distinguish this as a separate entity, as it may respond to
steroid therapy.
Extrinsic allergic pneumonitis
Extrinsic allergic pneumonitis, also called hypersensitivity pneumonitis,
is caused by immune reaction in the lung to inhaled antigens.
There are two main groups of allergen:
animal proteins, e.g. proteins in bird droppings; and microbial agents that
contaminate vegetable-derived material.
The latter are thermophilic actinomycetes and fungi that colonize
rotting crops (e.g. hay, compost,sugar cane, maple bark).
Many clinical syndromes result from this process, their names reflecting the
circumstances of allergen exposure, e.g. farmers' lung (Actinomyces in mouldy hay),
bird fanciers' lung (budgerigar and pigeon faeces), bagassosis (Actinomyces
in mouldy sugar cane, called 'bagasse').
There are two types of clinical problem caused by inhalation of allergens:
• Symptoms resulting from acute exposure to antigen.
After inhalation, there is a Type III hypersensitivity response. Immune complexes
generated at the site of allergen entry in the lung activate complement and
there is inflammation. Those affected have dyspnoea,fever and cough 4-8 hours
after exposure to antigen,resolving after 12-24 hours.
• Symptoms resulting from chronic pulmonary fibrosis.
Repeated exposure to antigen results in a Type IV cell mediated hypersensitivity
reaction, with small granulomas visible histologically.
This causes interstitial fibrosis and insidious onset of cough and dyspnoea,
eventually leading to honeycomb lung in about 5% of cases.
