Under light microscopy (using a PAS stain), the glomeruli are normal.
Immunofluorescence is also negative, as immunoglobulin or complement deposition is not a feature of this disease.
Under electron microscopy, there is a diffuse effacement of the foot processes. Often incorrectly described as "fusion of the foot processes", the actual architectural derangement is flattening, retraction and swelling of the foot processes. The podocytes also exhibit other changes such as cytoplasmic vacuoles, cellular swelling and formation of microvilli. The latter is not well-demonstrated in this image.
Minimal change disease (MCD) is the most common cause of nephrotic syndrome in children (~75%) and a less frequent cause in adults (~15%). Lipid droplets can be found in the urine (oval fat bodies) as implied by the historical name "lipoid nephrosis". Under light microscopy, the cells composing the convoluted tubules accumulate lipid and protein droplets in their cytoplasm. However, the glomeruli look virtually normal and in glomerulopathies, the diagnosis depends on the morphological changes of the glomeruli, thus, the name of minimal change disease disease was given to this entity.
Under electron microscopy, the actual pathology of the glomeruli was demonstrated to be effacement of the foot processes of the visceral epithelial cells (aka podocytes). These changes are reversible. Note that foot process effacement "podopathy" can be found in other conditions besides MCD such as membranous glomerulopathy or diabetes, but in MCD, it is the only morphological change in the glomeruli.
Peak incidence occurs between age 2-6 with a male predominance (M:F ratio of 2.5:1). In adults, there is no sex difference. Commonly presents with heavy selective proteinuria (mostly albumin rather than larger molecules such as immunoglobulins). Nephritic syndrome features such as hematuria, hypertension and azotemia are usually not present. Although immune-mediated, deposition of immunoglobulin and complement is not part of this disease, thus serum complement levels are normal.
In children, most cases (80-90%) are idiopathic, however, secondary causes such as infections, immunizations, drugs, ingestion of heavy metals causes a minority of cases. In adult, elderly patients, the use of NSAIDS as well as Hodgkin's lymphoma have been documented to cause minimal change disease.
Children respond very well to steroids, achieving complete remission rapidly. In adults, the improvements are much more gradual, and it make take up to 12 weeks to achieve even partial remission. Approximately 1/2 of patients relapse, however, these relapses are still responsive to steroid therapy. Patients who achieve only partial remission may benefit from immunosuppressants.1 . The prognosis is generally excellent in all cases.
1 Rosai, J. Rosai and Ackerman's Surgical Pathology. 9th Ed. Philadelphia, PA: Elsevier; 2004: 1169-1170.
2 Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th Ed. Philadelphia, PA: Elsevier; 2005: 981-2.
3 Zhou M, Magi-Galluzzi, C. Genitourinary Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elvesier; 2006: 351-4.
EM photo courtesy of Dr. Alexis Harris, Nephropath.