Medullary sponge kidney is a congenital multicystic deformity of the collecting renal tubules of the Malpighian pyramids, giving the renal tissue the appearance of a finely porous sponge. With the development of complications of the medullary sponge kidney (nephrocalcinosis and pyelonephritis), renal colic, hematuria, and pyuria are observed. Pathology is detected with excretory urography, retrograde pyelography. Treatment in the presence of clinical manifestations is aimed at eliminating complications; if conservative therapy is ineffective, kidney stones are removed, nephrostomy, kidney resection, nephrectomy.
Meaning
Medullary sponge kidney is a congenital pathology characterized by a change in the structure of the organ. It usually has a two-sided character. It can be asymptomatic for a long time and become an accidental finding when conducting an examination of the genitourinary system for another reason. The function of the kidney remains normal for a long time; its distortion often provokes an infectious process, progression of stone formation and violation of the patency of the upper urinary tract.
Causes
Most studies in the field of modern urology confirm that the medullary sponge kidney as an anomaly of intrauterine development is close to the polycystic kidney in etiology and pathogenesis. It is believed that the occurrence of the anomaly is associated with late disorders of embryogenesis, and changes in the collecting renal tubules can be observed in the postnatal period. The hereditary nature of the medullary sponge kidney is confirmed, but the type of inheritance in most cases is sporadic. Pathology is diagnosed mainly in middle-aged and older people, mainly in men. Due to the long latent course of the disease in childhood, the medullary sponge kidney is found relatively rarely.
Pathanatomy
In most cases, there is a bilateral anomaly of the medullary substance of the kidneys, while cystic changes of varying severity may affect some or all of the renal papillae. Unlike polycystic kidney, medullary sponge kidney has a regular shape, smooth contours, smooth surface and slightly increased in size compared to the age norm. On the incision, the extensions of the terminal renal tubules with multiple small cysts and cavities in the area of the pyramids are determined.
The sizes of cysts range from 1 to 5 mm, increasing towards the center. There are two types of cysts – lateral spindle- or diverticular protrusions into the lumen of the renal tubules lined with cylindrical epithelium, or closed cystic cavities formed by the fusion of small cysts isolated from the renal tubules and lined with flat epithelium. Cystic cavities may contain a clear yellowish liquid (in the absence of inflammation), exfoliated cells, calcified concretions (from grains of sand to small stones). As a result of washing out, small stones from the cystic tubules may appear in the cups or renal pelvis.
Renal tissue in the area of the pyramids, as a rule, is dense and fibrous-altered, with concomitant pyelonephritis – with inflammatory transformation. Calcification of the renal parenchyma (nephrocalcinosis) is a secondary disorder, since urine stasis in the dilated renal tubules and cystic cavities contributes to the deposition of calcium salts.
Symptoms
As a rule, the anomaly does not manifest clinically for a long time. Symptoms usually occur in the age range from 20 to 40 years with the addition of various complications: stone formation in the cystic cavities, infections of the kidneys and urinary tract. The main clinical manifestations of complicated medullary sponge kidney are dull or acute paroxysmal pain in the lower back, macro– and microhematuria, pyuria.
Papillary nephrocalcinosis occurs in medullary sponge kidney in more than 60% of cases. Renal colic occurs due to the migration of small stones from the cystic cavities into the calyx and pelvis. The development of inflammation due to the ingress of small stones into the calyx-pelvic system and impaired urine outflow is manifested by a periodic increase in body temperature, violation of urination. Rarely, with severe urolithiasis and recurrent secondary infection, purulent melting and death of the renal parenchyma occurs, which is manifested by signs of renal failure.
Diagnostics
The diagnosis of a medullary sponge kidney is established by a nephrologist based on the results of an extended urological examination, in which the main method is excretory urography. The urogram shows intensely contrasting, mosaic-like and fan-shaped cluster-shaped cystic cavities and extensions of the collecting tubules. Morphological changes usually affect the distal medullary zone of the kidney, while the cortical-medullary zone and the cortical substance remain unchanged. In the cystic cavities of the papillary zone, the X-ray contrast substance lingers longer than in the cups, which indicates stasis in the collecting tubules. Papillary concretions obscured by radiopaque means indicate nephrocalcinosis.
Less often, retrograde pyelography is used in the diagnosis of a medullary sponge kidney, because changes in the enlarged collecting ducts of the kidney are not always detected on pyelograms. It is advisable to conduct an X-ray examination of the kidneys when a medullary sponge kidney is combined with nephrocalcinosis or nephrolithiasis to detect calcinates and microliths located in the distal sections of the pyramids. In this case, the overview image shows the shadows of small stones in the papillary cysts, which partially or completely coincide with the shadows of the cavities on the excretory urogram.
Ultrasound of the kidneys does not always allow you to detect the smallest cysts in the deep layers of renal tissue. Laboratory tests help to confirm the presence of hematuria, pyuria, minor proteinemia, hypercalciuria. Differential diagnosis of medullary sponge kidney is carried out with diseases in which there are polycystic lesions of medullary renal tissue (polycystic kidney disease, cystic pyelitis, papillary necrosis, chronic pyelonephritis), as well as nephrocalcinosis, tuberculosis, kidney stone disease.
Treatment
With uncomplicated abnormality and its asymptomatic course, no treatment is carried out; preventive prescriptions are indicated to reduce the risk of complications. In clinical manifestations of medullary sponge kidney, treatment is focused on eliminating secondary urinary tract infection and metabolic disorders (further deposition of calcium salts in cystic-altered renal tubules). When pyelonephritis is attached, copious drinking, a low-Ca diet, prolonged antibacterial therapy are prescribed. To prevent the development of iatrogenic infection, urological instrumental manipulations of patients with medullary sponge kidney are indicated in exceptional cases.
Nephrostomy is required when the pathology is complicated by urolithiasis or pyelonephritis and there is no effectiveness of conservative treatment. In case of focal cystic deformity affecting a separate segment of the kidney, its resection is performed. Kidney removal (nephrectomy) is performed very rarely and strictly with unilateral kidney damage. Migration of small concretions that disrupt the outflow of urine may be an indication for surgical removal of kidney stones – pyelolithotomy, nephrolithotomy, percutaneous nephrolithotripsy, remote nephrolithotripsy.
Prognosis and prevention
In uncomplicated cases, the outcome of medullary sponge kidney is favorable, kidney function is preserved. With the development and progression of nephrocalcinosis, the infection may worsen over time. Preventive measures have not been developed.