Medullary sponge kidney

Synonym: Cacchi-Ricci syndrome

Medullary sponge kidney (MSK) is a congenital disorder that can affect one or both kidneys, or only part of one kidney. There are ectatic and cystic changes of the medullary and papillary collecting ducts. This cystic dilatation of tubules may have an effect on urinary flow. Cysts can be 1-7 mm in diameter. Cyst formation is commonly associated with the development of small calculi within the cyst. The rest of the kidney is usually normal unless affected by complications such as pyelonephritis or obstruction secondary to renal calculi.

Eziologia

This is uncertain, but thought to be a developmental abnormality. There is increasing evidence for the involvement of genetic factors, as there is familial clustering with apparent autosomal dominant inheritance. It is thought to be due to disruption at the "ureteric bud-metanephric blastema" interface during embryonic development, caused by developmental genes functioning abnormally.¹ It has been demonstrated that mutations in glial cell-derived neurotrophic factor (GDNF) account for a small proportion, around 12%, of MSK cases.² Many cases, however, are sporadic, and may have no genetic component, so much remains to be understood.

There are no recent figures for prevalence, but it is estimated to have a prevalence in the general population of between 5 in 10,000 and 5 in 100,000.³

As many as 12% of people with recurrent renal stones may have MSK.⁴

• Although it is a congenital disorder, diagnosis is often not made until the second or third decades.

• It may be asymptomatic.

• It can be an incidental finding on radiological investigations performed for other reasons.

• Haematuria is common and usually microscopic. Macroscopic haematuria can occur with associated infection or calculi.

• Recurrent urinary tract infection (UTI) is a common presentation and affects more women than men. There may also be sterile pyuria.

• Flank pain, which in a few cases can be severe and intractable.¹

• It may present with renal calculi and their complications including renal colic, haematuria, infection and urinary obstruction. As many as 70% of patients with known MSK have evidence of renal calculi.⁵ Stones may be composed of calcium oxalate monohydrate, calcium oxalate dihydrate, calcium phosphate apatite, and uric acid.

MSK is associated with a number of other congenital disorders. Some of the more common associated conditions include:

• Congenital hemihypertrophy: this is the most frequent association. There is enlargement of a limb or one side of the body compared with the contralateral side beyond the limits of normal variation. 5-10% of people with congenital hemihypertrophy have MSK.⁶

• Sindrome di Beckwith-Wiedemann: a growth disorder, which is characterised by macroglossia, macrosomia, omphalocele, visceromegaly, adrenal cortex cysts, neonatal hypoglycaemia, embryonal tumours (Tumore di Wilms, hepatoblastoma, neuroblastoma, rhabdomyosarcoma) and renal abnormalities including MSK.⁷

• Congenital hepatic fibrosis.

• Congenital pyloric stenosis.

• Anodontia.

• Malattia policistica renale.

• Horseshoe kidney.

• Distal renal tubular acidosis.³

• Sindrome di Ehlers-Danlos.

• Sindrome di Marfan.

• Caroli's syndrome.

• Multivessel fibromuscular dysplasia (a rare non-atherosclerotic, non-inflammatory vascular disease that particularly affects the renal arteries and other vessels).⁸

Except for the possible presence of other congenital abnormalities, including hemihypertrophy, there is usually no abnormality to be found on examination.

• Renal calculi: stones are often small and can pass spontaneously.

• Infezione del tratto urinario e pielonefrite.⁶

• Renal failure: due to recurrent infection or obstruction due to calculi.

• Reduced bone density. Defective bone mineralisation associated with MSK.⁹

• Tumore di Wilms: children with MSK have a greater risk of developing Wilms' tumour. This is because of the association of Wilms' tumour with congenital hemihypertrophy and Beckwith-Wiedemann syndrome.

• Distal renal tubular acidosis.

Investigation may be initiated because of recurrent UTI or renal calculi.

Radiology

• Plain abdominal X-ray: may reveal nephrocalcinosis.

• Ultrasound scan: not very sensitive for MSK or stones, but either may be picked up coincidentally.

• Intravenous pyelogram (IVP): the usual standard investigation. There are radial linear striations in the papillae ('brushlike' patterns). Contrast medium collects in the ectatic collecting ducts and is seen as a 'bouquet of flowers'. MSK was traditionally often picked up coincidentally on an IVP done for renal calculi. However, this test is used decreasingly due to the nephrotoxic and allergenic effects of the contrast medium.

• Recent articles suggest CT urography should be the investigation of choice for MSK.¹⁰¹¹

Altri

• Urine:
  

  • Dipstick for leukocytes, blood cells, nitrates.

  • Culture if infection is suspected.

  • Urinary calcium: may be hypercalciuria.

  • Urinary citrate tends to be low.

  • Urinary pH: if distal renal tubular acidosis is present, there is inability to form an acid urine in the distal tubule. Urine pH is above 6 and there is associated hypokalaemia with a hyperchloraemic metabolic acidosis.

• U&Es, eGFR for renal function.

• Stone analysis where appropriate.

• The presence of MSK does not exclude other pathology and it can co-exist with other diseases such as polycystic kidney disease.

• Sterile pyuria can be due to inadequately treated UTI or tuberculosis of the kidney.

• Haematuria requires the exclusion of malignancy of the urinary tract, including clear cell carcinoma of kidney, bladder tumours or Wilms' tumour in children.

No treatment can eliminate the cysts in MSK; therefore, management centres around treating and preventing UTIs and renal stones:

• Patient education about the usually benign nature of the condition.

• Complicating UTI and pyelonephritis should be treated with antibiotics.

• Complicating renal calculi should be managed appropriately. This may include extracorporeal shock-wave therapy, percutaneous surgery or ureteroscopy. In severe cases of recurrent renal stones, partial nephrectomy may be required. Rarely, total nephrectomy may be necessary in cases of severe recurrent sepsis.

• Investigate for other co-existing pathology, including the exclusion of malignancy if there is haematuria.

• Regular urinalysis and plain abdominal X-ray can help to detect infection and renal tract calcification but there is no general agreement about optimum frequency. Renal function can be monitored using U&Es, electrolytes, creatinine and eGFR. Affected children should have surveillance for Wilms' and other abdominal tumours.

• Potassium citrate administration has been shown to improve bone density.¹² It also reduces the frequency of stone formation.¹³

• Lifestyle advice as for those with renal calculi, ie:¹⁴
  

  • Increase fluid intake to maintain urine output at 2-3 litres per day to reduce the risk of both stones and UTI.

  • Riduci l'assunzione di sale.

  • Reduce the amount of meat and animal protein eaten.

  • Reduce oxalate intake (foods rich in oxalate include chocolate, rhubarb, nuts) and urate-rich foods (eg, offal and certain fish).

  • Drink regular cranberry juice: increases citrate excretion and reduces oxalate and phosphate excretion.

  • Maintain calcium intake at normal levels (lowering intake increases excretion of calcium oxalate).

• Depending on the composition of the stone, medication to prevent further stone formation is sometimes given - eg, thiazide diuretics (for calcium stones), allopurinol (for uric acid stones) and calcium citrate (for oxalate stones).

MSK is normally a benign condition that does not affect life expectancy. Renal impairment and failure are uncommon. Recurrent infections/stones and chronic pain cause most of the ongoing morbidity.