Urogenital Imaging

Medullary sponge kidney

a disease characterized by the presence of cystic dilatations of distal collecting tubules and collecting ducts within medullary pyramids. The aetiology of medullary sponge kidney is not known. The incidence has been estimated at 0.5% of the population and is generally diagnosed in adults, although cases have been reported in children. Medullary sponge kidney is generally felt to be a sporadic disease with a few reported hereditary cases. Other entities that have been associated with medullary sponge kidney include Carolis disease, hemihypertrophy and Ehlers Danlos syndrome.

The disease is often progressive, and may begin as tubular ectasia with linear collections of contrast identified within medullary pyramids. With time, these will progressively dilate and form cystic cavities. These cavities result in urinary stasis, leading to stone formation. Advanced cases of medullary sponge kidney will demonstrate extensive medullary nephrocalcinosis.

Medullary sponge kidney may be an incidental finding on ultrasound or intravenous urography (IVU) performed for other indications, particularly early in its course. Later, patients often present with symptoms related to the passage of renal calculi, including flank pain and haematuria. Renal function is usually normal, except in very advanced cases. The principle method for diagnosis of medullary sponge kidney is IVU. The scout film will frequently demonstrate the presence of medullary nephrocalcinosis. Although a specific diagnosis of medullary sponge kidney cannot be made on plain film alone, a typical pattern of linear and rounded calcifications is frequently identified in patients with medullary sponge kidney (Fig.1). Following administration of intravenous contrast, the calcifications become largely obscured by contrast filling in the cystic, dilated renal tubules (Fig.2) (Fig.3). In addition, other linear and cystic areas of contrast are identified in the medullary pyramids in areas where calcifications were not present on the scout film. These findings enable a specific diagnosis of medullary sponge kidney as opposed to other causes of medullary nephrocalcinosis such as renal tubular acidosis and hyperparathyroidism.

Medullary sponge kidney must be distinguished from benign tubular ectasia. These entities have frequently been treated as interchangeable and synonymously. Tubular ectasia is a relatively common finding and produces linear "paint brush" striations arising from the medullary pyramids which fill with contrast on IVU. However, unlike medullary sponge kidney, there is no evidence of cystic dilatation within the tubules or formation of calcifications. There is no good evidence that benign tubular ectasia progresses to medullary sponge kidney. The presence of medullary nephrocalcinosis is not necessary for a diagnosis of medullary sponge kidney, if cystic dilatation of the tubules is identified on IVU.

Ultrasound demonstrates echogenic medullary pyramids in patients with medullary sponge kidney. In most cases, however, ultrasound cannot distinguish medullary sponge kidney from other types of medullary nephrocalcinosis. However, medullary sponge kidney may demonstrate echogenic medullary pyramids, even in the absence of medullary nephrocalcinosis.

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Fig.1

Plain abdominal radiograph demonstrates bilateral nephrocalcinosis, due to medullary sponge kidney.
Medullary sponge kidney, Fig.1
Medullary sponge kidney, Fig.2
Medullary sponge kidney, Fig.3