Minimal change disease and foot process effacement
Minimal change disease injures podocytes (the cells forming the outer layer of the kidney's filter), flattening their finger-like foot processes so that protein leaks into the urine.
Remember
In a young child with nephrotic syndrome (heavy protein loss in the urine with swelling), think of minimal change disease: the podocyte foot processes flatten, the filter leaks protein, and there are no immune complex deposits (clumps of antibody and antigen).
In minimal change disease, a circulating factor (a substance in the blood whose exact identity is still unclear) is thought to injure the podocytes. These are the cells that wrap around the capillaries of the glomerulus and help to form the filtration barrier. Their foot processes flatten and fuse, a change called effacement, so the barrier becomes leaky to protein, mainly albumin (the blood protein that helps to hold water in the circulation).
Light microscopy, the ordinary microscope used on stained biopsy sections, shows glomeruli that are normal or nearly normal, which gives the disease its name. Only an electron microscope, which magnifies far more, shows the flattened foot processes. Immunofluorescence, a stain that uses fluorescent antibodies to show immune deposits, is negative, because immune complexes do not cause the disease.
As albumin is lost, the blood albumin falls and water moves out into the tissues. Together with heavy protein loss in the urine (proteinuria), this makes up nephrotic syndrome, with oedema (fluid swelling) usually appearing first around the eyes and then at the ankles. In the typical picture there is no haematuria (blood in the urine), because the glomerulus is not inflamed, and kidney function is usually normal.
Minimal change disease is the most common cause of nephrotic syndrome in children over 1 year of age. A child with this typical picture is usually started on corticosteroids (steroids) without a biopsy, and a good response supports the diagnosis. Most children respond, but relapses are common, and the disease can recur if the steroids are stopped.
Key facts
- Cause
- a circulating factor is thought to injure the podocytes, with no immune complex deposition
- Light microscopy
- the glomeruli look normal or nearly normal
- Electron microscopy
- the podocyte foot processes are effaced (flattened and fused)
- Immunofluorescence
- no immune deposits are seen
- Typical patient
- a child over 1 year of age, in whom minimal change disease is the most common cause of nephrotic syndrome
- Treatment
- in a child with the typical picture, corticosteroids are usually started first, with biopsy only if treatment fails or the picture is atypical
Exam traps
- Clumps of antibody and antigen building up along the filter cause membranous nephropathy, which mostly affects adults. Minimal change disease shows no immune deposits, so immune complex deposition is not its mechanism.
- IgA nephropathy (IgA deposits in the mesangium, the supporting tissue between the capillary loops) usually presents with haematuria, so it does not explain isolated heavy proteinuria with oedema.
- Crescents (proliferating cells in the space around the glomerulus) mark severe inflammation and give a nephritic picture, with blood in the urine and falling kidney function, whereas minimal change disease shows no inflammation.
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