THE ALVEOLI AND THE INTERSTITIUM

Figure 3.1 illustrates the normal relationship between the alveoli and the interstitium.

Figure 3.1 Normal lung. The terminal bronchioles give rise to alveolar ducts which terminate in small air sacs (alveoli). The millions of thin walled alveoli are responsible for gas exchange…carbon dioxide out of the capillaries, oxygen into the capillaries. The interstitium of the lung surrounds both the alveoli and the terminal bronchioles. The interstitium has a most important mechanical function: it acts as a scaffold supporting the alveolar walls. It also has a dynamic function: fluid, cells and nutrients pass into and out of the interstitium.

ALVEOLAR AND INTERSTITIAL LUNG DISEASE^1,2

Some disease processes affect mainly the alveoli, or mainly the interstitium. Some diseases involve the alveoli and the interstitium just about equally.

The pathological process consequent on most lung injuries can be summarised as follows: the insult causes the cellular lining of the alveolar walls and/or the capillaries in the interstitium to become leaky. The oedema that leaks out accumulates in the alveoli or in the interstitium. Sometimes the injury continues to progress and causes extensive necrosis with the production of fibrin and cell debris. These necrotic products fill the alveolar air spaces or thicken the tissues in the interstitium.

Figure 3.2 Alveolar disease. The air in the alveoli has been replaced by blood, pus, water, protein or cell debris.

Figure 3.3 Interstitial disease. The scaffolding surrounding the alveoli is abnormal. The abnormal change can be due to oedema, inflammation or fibrotic thickening.

THE CXR: ALVEOLAR VERSUS INTERSTITIAL DISEASE^1-4

When a CXR shows an area of diffuse shadowing—whether localised or widespread—then the shadow pattern needs to be analysed carefully. It is worth emphasising: the CXR shadow pattern represents the underlying pathological process. The CXR features to look for are listed in Table 3.1.

Table 3.1 CXR features to look for.

	Alveolar changes	Interstitial changes
Usual shadows	Fluffy or blobby Ill-defined margins Coalescing/merging Segmental/lobar	Small nodules Linear/reticular Linear/reticular with septal lines Reticulo-nodular
Sometimes …additonally	Air bronchogram (see p. 227)	Reduced lung volume (extensive disease) Honeycomb pattern (end-stage disease)

Figure 3.4 Alveolar disease. The alveoli are (in this case) stuffed with pus, giving a fluffy homogeneous pattern which has coalesced. (Pneumonia.)

Table 3.2 Descriptive terms.

Fine or small nodules = tiny opacities. Reticular = mesh or basket-like. A pattern comprising fine or coarse lines. Reticulo-nodular = a combination of small nodules and basket-like linear opacities. Septal lines = fine thread-like lines produced by fluid or thickening of the septa between the lobules of the lung. Several different types of septal lines have been described1,2,4, but much the most common are those referred to as Kerley B lines. Kerley B lines = fine horizontal lines approximately 1 cm long, situated perpendicular to the lateral pleural surface. They are most commonly seen just above the costophrenic angles on a frontal CXR (see pp. 158–159).

Figure 3.5 Interstitial disease. The interstitium is fibrotic and thickened. (Sarcoidosis.)

INTERSTITIAL SHADOWING—ARE YOU SURE?

A regular problem when evaluating a CXR is determining whether the lungs are actually clear and normal. Sometimes the lung markings (i.e. the normal vessels) are rather prominent and the question is raised—is there anything else there? Could some of these markings be fine linear interstitial shadows? This usually relates to a lower zone.

Normal lungs on the CXR should show nothing other than vessels and occasionally a lung fissure. Normal bronchial walls are only visualised if they are seen end on. Normal alveoli, normal interstitium, normal lymphatics—none of these are visible.

So, is there anything there? When in doubt apply these two rules:

Rule 1	Look at the lung immediately adjacent to the costophrenic angle — it should be clear. If this particular part of the lung shows prominent markings then interstitial disease at the lung base is likely. This rule is particularly helpful in females when part of the lung projects just lateral to the overlying breast shadow because the breast density sometimes mimics lung pathology.
Rule 2	Take all of the lung markings and throw them away. Remember, it is just the vessels that you should throw away. In most cases this can be done, metaphorically speaking, fairly easily. Vessels thrown away: nothing left = clear lungs Vessels thrown away: fine shadows remain = interstitial disease likely

Figure 3.6 The CXR raises the possibility of increased interstitial shadows. This equivocal appearance is a common dilemma. Careful scrutiny and applying the “throw away the vessels” rule (see the magnified image) shows that the lung is clear.

THE CXR: NOT ALWAYS EASY TO ASSESS

Sometimes there is an “emperor’s new clothes” approach to the implied ease with which the distinction can be made between alveolar disease and interstitial disease on a CXR. Take such claims with a pinch of salt. In a number of instances you will find that it is very difficult to decide whether the classic features (Table 3.1) are actually present (Figs 3.9 and 3.10). All the same it is often possible, following a careful analysis, to recognise a particular pattern. If a particular pattern can be identified then a legitimate differential diagnosis can be suggested (Table 3.3).

Figure 3.9 The pattern is difficult to categorise. Neither classically alveolar nor classically interstitial. We are not able to pigeonhole the shadow pattern in this case.

Figure 3.10 The pattern is neither classically alveolar nor classically interstitial.

Table 3.3 Differential diagnosis.

Dominant alveolar/airspace pattern	Dominant interstitial pattern
Adults: Pulmonary oedema: cardiac non-cardiac Lobar pneumonia Haemorrhage Lymphoma Bronchioloalveolar cell carcinoma Adult respiratory distress syndrome (early) Aspiration pneumonia	Pulmonary oedema Pneumonia: viral Pneumocystis carinii—early Tuberculosis Sarcoid Idiopathic pulmonary fibrosis Rheumatoid lung Sclerodema Lymphangitis carcinomatosa Crack smoking
Infants: Hyaline membrane disease Transient tachypnoea of the newborn

Figure 3.7 Pneumonia. Classic alveolar pattern. Fluffy shadowing.

Figure 3.8 Interstitial fibrosis. Classic interstitial pattern. Reticulo-nodular shadowing.

DIFFERENTIAL DIAGNOSIS^1-4

Frequently, a classical alveolar or interstitial CXR pattern is dominant (Table 3.3). A dominant pattern allows a selective differential diagnosis to be considered. When linked to the clinical history, physical examination, and laboratory tests, then the likely diagnosis usually emerges (Figs 3.11-3.14).

Figure 3.11 Alveolar pulmonary oedema.

Figure 3.12 Interstitial pulmonary oedema.

Figure 3.13 Alveolar pattern. Pulmonary oedema. Patient using intravenous heroin.

Figure 3.14 Interstitial pattern. In this patient... sarcoidosis.

CONFUSION: INTERSTITIAL FIBROSIS & ALL THOSE ACRONYMS

Very often the cause of a chronic interstitial pattern (or a honeycomb appearance) on the CXR will be obvious because of a particular clinical history and the age-old principle: common things occur commonly.

When the diagnosis remains obscure then CT will frequently suggest the probable cause ^5–11. In some patients a lung biopsy will still be necessary.

The CXR

Various classifications/descriptions of interstitial fibrosis are in use. The numerous acronyms can be very confusing. The confusion started after the histopathologists separated out several of the different disease processes. Chest physicians and CT radiologists then sought to match and mirror the microscopic findings with the CT patterns. Some of the acronyms are those used by histopathologists, some are those used by radiologists, and some are used incorrectly.

Remember, the CXR is a plain, simple, and very modest examination. It recognises its limitations and leaves the sophisticated separation of the BIFs from the BANGs to CT or to a lung biopsy.

In a patient with dyspnoea and a chronic interstitial CXR pattern, we use the following approach. The CXR shadows will represent either (A) or (B):

(A) True interstitial fibrosis.

Either:

A complication or residuum of an already known disease or consequent on an extrinsic stimulus. Examples include: granulomatous diseases (tuberculosis, sarcoid); collagen-vascular diseases (rheumatoid arthritis, progressive systemic sclerosis); gastro-intestinal disease (aspiration pneumonitis); iatrogenic — radiotherapy or drug-related (Amiodorone, Nitrofurantoin, Methotrexate, Bleomycin); occupational disease (asbestosis, silicosis); extrinsic allergic alveolitis (bird fancier’s lung, farmer’s lung).

Or:

Aetiology unknown, i.e. cryptogenic fibrosing alveolitis (idiopathic pulmonary fibrosis). The histopathologists call this usual interstitial pneumonia (UIP).

(B) Lung disease that can mimic the CXR appearance of interstitial fibrosis.

This includes: lymphangitis carcinomatosa; eosinophilic pneumonia; Langerhan’s cell histiocytosis; bronchiolitis obliterans with organising pneumonitis (BOOP); lymphangioleiomyomatosis.