9. Brown induration of the lung

Overview of the slide.

Staining: Prussian blue

Organ: Lung

Description:

The slide is filled with brown-blue-blackish pigments. The pigments are found especially in the alveoli.

Diagnosis: Brown induration of the lung

Causes:

• Severe congestive left ventricle failure due to mitral stenosis due to rheumatic fever

Theory:

Prussian blue is a staining that stains iron blue and is used for detection of iron.

The word “induration” means hardening. In brown induration of the lung, the lung becomes firmer while also becoming brown due to the haemosiderin pigment.

Haemosiderin (blue pigment on the slide) can especially be seen in the alveoli. This is because in this disease, macrophages in the alveoli contain the haemosiderin pigment.

This figure shows some macrophages filled with haemosiderin (the roundish dark blue spots) inside alveoli. Note how there’s also small amounts of hemosiderin everywhere.

The pathomechanism is complicated. In congestive left ventricular failure, the pressure inside the pulmonary circulation, and therefore in the pulmonary capillaries, will increase significantly. The pressure is so high that blood is pressed out of the capillaries into the alveoli. Inside the alveoli will the RBCs be broken down and eaten by macrophages. The iron, coming from the haemoglobin, will stay inside the macrophages, causing a local haemosiderosis. These macrophages filled with haemosiderin are called heart failure cells as they’re commonly found in heart failures.

The only disease that can cause such a severe congestive left ventricular failure that lung induration forms is mitral stenosis, where the mitral valve is so narrowed that blood struggles to flow through it, creating backward congestion into the lung. Mitral stenosis was historically most frequently caused by rheumatic fever, an infection disease. However, nowadays rheumatic fever can be treated with antibiotics, meaning that the disease never progresses enough to cause mitral stenosis and therefore brown induration of the lung. Brown induration of the lung is therefore a very rare disease nowadays.