Medullary Sponge Kidney

What happens in medullary sponge kidney

Each kidney contains about a million filtering units that drain urine through collecting ducts in the medulla, the inner part of the kidney. In MSK, these ducts are widened and form small cysts, usually 1 to 8 mm. One or both kidneys may be affected, sometimes only part of a kidney.

Urine moves slowly through the widened ducts. This stagnation, combined with changes in urine chemistry (often high urine calcium and low citrate), encourages calcium deposits and stones, and allows bacteria to grow.

Causes

MSK is present from birth and is thought to result from abnormal kidney development. Most cases are sporadic, but some families show autosomal dominant inheritance. It has been linked with genes involved in kidney development (such as GDNF) and with some other congenital conditions, including Beckwith-Wiedemann syndrome and hemihypertrophy.

Symptoms

Many people with MSK never know they have it. Symptoms usually begin in the teens to 40s and include:

• Kidney stones: severe pain in the side or back that may spread to the groin, often recurrent
• Blood in the urine: visible or found on a urine test
• Urinary tract infections: burning, frequency, cloudy urine, and kidney infections with fever
• Chronic flank pain in some people, even without obvious stones
• Nephrocalcinosis: calcium deposits in the kidney tissue, seen on scans

Associated metabolic problems

• Hypercalciuria: too much calcium in the urine
• Hypocitraturia: too little citrate, a natural stone inhibitor
• Incomplete distal renal tubular acidosis: the kidneys do not acidify urine properly, promoting stones and sometimes affecting bone health

Diagnosis

1. Imaging: - CT urography or the older intravenous urogram shows contrast filling the dilated ducts in streaks ("paintbrush") or clusters ("bouquet of flowers"), the classic sign - Ultrasound may show bright, calcified areas in the medulla - Plain CT shows stones and calcifications
2. Urine tests: urinalysis, culture for infection, and a 24-hour urine collection to measure calcium, citrate, oxalate, uric acid, volume and pH
3. Blood tests: kidney function, calcium, bicarbonate and potassium
4. Stone analysis if a stone is passed or removed

Other causes of nephrocalcinosis and stones, such as hyperparathyroidism, are ruled out.

Treatment

There is no treatment to reverse the structural changes, so care focuses on preventing and treating complications.

Preventing stones

• Drink plenty of fluids to produce at least 2.5 liters (about 85 oz) of urine a day. Pale urine is the goal
• Limit salt, which increases urine calcium
• Keep normal dietary calcium, from food rather than supplements. Low-calcium diets increase stone risk
• Moderate animal protein
• Medicines based on 24-hour urine results: - potassium citrate to raise urine citrate and correct acidosis - thiazide diuretics to lower urine calcium - occasionally other medicines for specific stone types

Treating stones

• Pain relief and fluids for small stones that may pass
• Shock wave lithotripsy, ureteroscopy with laser, or percutaneous nephrolithotomy for larger or blocking stones
• Urgent drainage with a stent or tube if a stone blocks an infected kidney

Treating infections

UTIs are treated with antibiotics guided by urine culture. People with frequent infections may need longer courses or preventive strategies.

Monitoring

Periodic imaging, urine studies and kidney function tests help track stone formation and kidney health.

Outlook

Most people with MSK have normal kidney function throughout life. The main burden is recurrent stones and infections, which can usually be reduced with prevention measures. Kidney failure is uncommon and usually linked to repeated obstruction or infection.

Frequently asked questions

What is medullary sponge kidney?

Medullary sponge kidney is a condition present from birth in which small tubes inside the kidney are widened into tiny cysts, giving a sponge-like appearance. It can cause kidney stones, blood in the urine and urinary tract infections, though many people have no symptoms.

What are the symptoms of medullary sponge kidney?

Common symptoms are recurrent kidney stones causing flank pain, blood in the urine, and urinary tract infections. Some people have chronic flank pain. Many have no symptoms and are diagnosed incidentally on scans.

Is medullary sponge kidney serious?

Usually not. Most people keep normal kidney function for life. The main problems are recurrent stones and infections, which can be painful but are manageable. Kidney failure is rare.

How is medullary sponge kidney treated?

Treatment focuses on preventing stones with high fluid intake, reduced salt, normal dietary calcium, and medicines such as potassium citrate or thiazides based on urine tests. Stones and infections are treated when they occur.

Is medullary sponge kidney hereditary?

Most cases occur without a family history, but some families show an inherited pattern. Research has linked it to genes involved in kidney development. Relatives of affected people may sometimes have it too.