History: A 75-year-old man with a chronic history of microhematuria was referred for a CT urogram by his urologist.
Scout, coronal precontrast, axial nephrographic/excretory phase images, and volume-rendered coronal reconstructions of the left and right kidneys are shown below.
Additional history: The patient has a prior history of microhematuria and kidney stones. Review of the PACS for prior imaging reveals an intravenous pyelogram (IVP) from 2005. Scout and postcontrast exposures are shown below.
CT urogram (2017): Noncontrast images demonstrate multiple bilateral nonobstructing renal calculi, the largest measuring 6 mm in the right kidney. Some of the calcifications could represent nephrocalcinosis — for example, those in the upper pole of the right kidney. The kidneys show symmetric enhancement without suspicious renal mass. Multiple bilateral exophytic simple renal cysts are noted, the largest in the midpole of the right kidney measuring 3.6 x 4.3 cm. There are additional subcentimeter low-attenuation lesions that are too small to characterize. There is a “paintbrush sign” appearance to the renal medullae in keeping with a history of medullary sponge kidney. There is mild bladder wall thickening and trabeculation that may be related to chronic outlet obstruction. Further evaluation is deferred to cystoscopy.
CTU：CT平扫可见双肾多发非梗阻性肾结石，右肾最大者直径约6mm，其中一些钙化可能代表肾结石，例如，右肾上极的那些。肾脏对称性强化，未见可疑肾肿物。双肾可见多发单纯性肾囊肿，大者位于右肾中部，大小约3.6 x 4.3 cm；另可见不足1cm的低密度灶，其太小而不能显示。肾髓质表现为“毛刷征”，符合髓质海绵肾。膀胱壁轻度增厚并小梁形成，与慢性流出道梗阻有关。进一步评估需膀胱镜检查。
IVP (2005): Preliminary scout images demonstrate a cluster of at least three radiopaque stones in the upper pole of the right kidney; the largest two stones in this cluster each measure 6 mm in diameter. In addition, there are probable clusters of smaller stones in the interpolar right kidney and right lower pole. Following the uneventful intravenous administration of 150 mL of nonionic contrast material, prompt nephrograms develop, which show the kidneys to be normal in size, shape, appearance, and internal architecture. There is diffuse bilateral tubular ectasia. Contrast is excreted promptly into otherwise normal-appearing pyelocalyceal systems. The calculi are no longer seen and likey reside in dilated tubules. The ureters are normal in course, caliber, and appearance. The urinary bladder has a mildly trabeculated appearance. There are two small urinary bladder diverticula: one of the bladder dome and one arising from the left lateral bladder wall. No filling defects are seen.
- Medullary sponge kidney
- Renal cortical necrosis
- Renal papillary necrosis
- Renal tuberculosis
- Renal pyramid “blush”: Normal homogeneous enhancement of pyramids with no tubular dilation
Diagnosis: Medullary sponge kidney
Medullary sponge kidney (MSK)
- Medullary sponge kidney refers to a sporadic condition in which the medullary and papillary portions of the collecting ducts are dysplastic and dilated and, in most cases, develop medullary nephrocalcinosis. Occasionally, it can be hereditary.
- Patients are predisposed for urinary stasis and resulting urolithiasis.
- As many as 33% to 50% of patients have hypercalcemia (i.e., hyperparathyroidism).
- The etiology and pathogenesis are unknown.
- On cross section, the kidney appears as a sponge and there are multiple cystic cavities in the renal pyramids.
- Cysts contain yellow-brown fluid and desquamated cells or calcified material.
- Calculi within cysts and their walls consist of calcium oxalate ± calcium phosphate.
- The size and number of renal calcifications typically increase with time.
- The condition may be bilateral (75%) or unilateral (25%), and it may involve a single pyramid or segment.
- The prognosis with MSK is generally good and patients will typically lead a normal life unless complications ensue. Complications include infection, extensive calculus formation, renal insufficiency, and renal failure.
- The condition is more common in women.
- The prevalence is estimated at 1 in 5,000 to 20,000.
- Incidence at urography: 0.5%.
- Incidence in patients with nephrolithiasis: 2% to 21%.
- Most patients remain asymptomatic throughout their life, and the diagnosis is only made incidentally when the renal tract is imaged for other reasons.
- However, approximately 10% of patients may present with a complication such as urinary tract infection, hematuria, or urolithiasis.
Imaging features 影像表现
Radiography: May see medullary nephrocalcinosis (calcifications within medulla) or urolithiasis.
- Gross deformity of papillae with beaded or striated cavities
- Distortion broad, shallow, or widely cupped calyces
- Numerous large calcifications
- May or may not have nephrolithiasis
- More prominent linear densities
- Clusters of small, rounded opacities in papillae (cystic dilatation of collecting ducts)
- With or without enlarged papillae and splayed calyceal cups
- “Bouquet of flowers” appearance: Ectatic collecting ducts filled with calcification
- Large and dense medullary calcifications
- “Paintbrush” appearance: More than three discrete linear densities in more than one papilla.
- Collecting ducts wider than the normal 200-300 μm in diameter.
- Mild/Moderate/Advanced ductal ectasia:
- Evaluate the adjacent calyx for tumor (e.g., transitional cell carcinoma) or for obstructing calculi.
- 集合管增宽，超过正常直径200-300 μm
- No filling or poor filling of dilated collecting ducts is seen; contrast injection should not be forceful.
- Tips of papillary ducts are not prone to reflux, regardless of disease severity.
- Can help differentiate cysts of medullary sponge kidney from cysts of other medullary cavities (e.g., papillary necrosis or tuberculosis).
- Has a “paintbrush” appearance: Retention of contrast within dilated tubules in pyramids.
- May or may not have medullary nephrocalcinosis and urolithiasis.
- In severe disease, will see extracalyceal contrast accumulation within papillae or abscess.
- May see hydronephrosis and hydroureter secondary to obstruction.
- MRI is an insensitive modality for detecting calcium.
- Has decreased sensitivity in detecting tubular ectasia.
There is no treatment for medullary sponge kidney. Treatment is based on managing complications when they arise, such as the following:
- Antibiotics for infection
- Thiazide and alkali therapy to decrease stone formation
- Extracorporeal lithotripsy and percutaneous nephrolithotomy for stone removal