Medullary sponge kidney is a congenital disorder characterized by malformation of the terminal collecting ducts in the pericalyceal region of the renal pyramids [1-3]. This collecting duct dilatation, or ectasia, is associated with the formation of both small (microscopic) and large medullary cysts that are often diffuse but do not involve the cortex.
Medullary sponge kidney is usually asymptomatic. The diagnosis is commonly made as an incidental finding during an imaging test performed for a different indication. Although considered benign, the associated complications of medullary sponge kidney (eg, nephrolithiasis and urinary tract infections) may rarely lead to chronic kidney disease and even renal failure .
ETIOLOGY AND PATHOPHYSIOLOGY
The changes seen in medullary sponge kidney are thought to reflect a developmental abnormality, though the underlying defect is not fully understood. The association of medullary sponge kidney with malformations affecting the kidney such as hemihypertrophy and Beckwith-Wiedemann syndrome has led to speculation that the disorder results from a disruption of the interface between the ureteral-bud and the metanephric blastema during embryonic development .
Evidence for genetic transmission of medullary sponge kidney is limited. Some patients with medullary sponge kidney have mutations in the gene for glial cell-derived neurotrophic factor (GDNF), a protein that is pivotal in renal development . Although occasional families with medullary sponge kidney appear to show autosomal dominant inheritance [1,3], a positive family history is often absent. Despite this, family clustering of medullary sponge kidney may be common. In a study of 50 patients with documented medullary sponge kidney, 27 had affected family members identified with imaging (either contrast-enhanced computerized tomography or ultrasound) or, among deceased family members, a suggestive clinical history [7,8]. The responsible genotypes for this clustering are uncertain and were not due to mutations in GDNF.
The infrequent positive family history and a benign clinical course help distinguish medullary sponge kidney from two other diseases that are genetically transmitted and typically progressive: autosomal dominant polycystic kidney disease and autosomal dominant interstitial kidney disease. Whether medullary sponge kidney disease is related to these disorders with respect to pathogenesis is currently not known. (See "Course and treatment of autosomal dominant polycystic kidney disease" and "Autosomal dominant interstitial kidney disease (medullary cystic kidney disease)".)