Medullary Sponge Kidney is a rare congenital birth defect where the tiny, tube-like structures deep within the kidneys’ inner part (the medulla) are malformed. In MSK, the collecting ducts, the final tiny pipelines that transport urine into the central collecting system of the kidney are dilated, widened, and cystic. This gives the inner part of the kidney a “spongy” appearance on certain types of X-rays, which is where the condition gets its name.

To understand MSK, it is helpful to visualize the kidney’s intricate plumbing system. Each kidney is a complex filter that produces urine. This urine flows through millions of tiny nephrons and ultimately drains into a series of small tubes called collecting ducts, which are located in the kidney’s inner medulla.

A helpful analogy is to think of this drainage system as the branches of a tree. The nephrons are the leaves producing water, which then flows into the smallest twigs, the collecting ducts. These twigs merge into larger branches and finally into the main trunk (the ureter), which carries the fluid away.

• In a normal kidney, these “twigs” are smooth, narrow, and efficient, allowing urine to flow freely.
• In Medullary Sponge Kidney, it is as if these tiny twigs were formed with small, sac-like bulges or pockets along their length.

This structural change itself is not harmful, but it creates two main problems that can lead to symptoms:

1. Stagnation of Urine: Urine can pool in these dilated ducts.
2. Formation of Crystals and Stones: The stagnant, slow-moving urine in these ducts creates an ideal environment for calcium and other minerals to crystallize and form kidney stones. The buildup of tiny calcium deposits in the kidney tissue is known as nephrocalcinosis, a hallmark feature of MSK.

In my practice, MSK often shows up during evaluations for frequent kidney stones. Most patients are surprised to learn they’ve had it since birth but only now feel symptoms.

The cause of MSK is a malformation during embryonic development of the kidneys. The precise reason why this developmental error occurs is unknown. It is not caused by the mother’s actions during pregnancy. While a few specific gene mutations have been found in some rare familial cases, for the vast majority of people with MSK, there is no known genetic cause.

I tell patients it’s like a blueprint error during kidney development, nothing they did caused it, and it often runs quietly in the background until it causes trouble later in life.

You get MSK by being born with it. It develops during fetal kidney formation. It is not contagious, not caused by diet or behavior, and doesn’t result from any lifestyle factors.

Sporadic Occurrence

In over 95% of cases, MSK is sporadic. This means it occurs by chance in an individual, with no other affected family members and no clear pattern of inheritance. It is considered a random developmental event.

Rare Familial Cases

In a very small number of cases, MSK has been observed to run in families. In these rare instances, it may be inherited, possibly in an autosomal dominant pattern, which means an affected person would have a 50% chance of passing the condition to each of their children. It can also be associated with other rare genetic syndromes, such as Beckwith-Wiedemann syndrome or hemihypertrophy. However, for the average person diagnosed with MSK, it is not considered a heritable disease they need to worry about passing on (NORD, 2023).

In many cases, people only discover MSK after a CT scan or IVP done for stone pain. It’s not something they developed over time, it’s something they were born with.

Many people with MSK are asymptomatic and never require treatment. When symptoms do occur, they are not caused by the “spongy” kidney itself, but by the complications that can arise from it, namely, kidney stones and urinary tract infections (UTIs). The signs and symptoms are therefore the classic symptoms of these two conditions.

Symptoms related to kidney stones (renal colic) are often the reason MSK is discovered. These include:

• Severe, sharp pain in the side and back (flank pain).
• Pain that radiates to the lower abdomen and groin.
• Pain that comes in waves.
• Nausea and vomiting.
• Hematuria, which is blood in the urine, making it appear pink, red, or brown.

Symptoms related to a urinary tract infection (UTI) can include:

• A burning sensation during urination (dysuria).
• Frequent, urgent need to urinate.
• Cloudy or foul-smelling urine (sign of a UTI).
• Fever and chills (if the infection is severe).

If someone keeps getting kidney stones despite good hydration and diet, I consider MSK. Repeated UTIs or visible blood in urine are also strong clues, especially in younger adults.

Because MSK is asymptomatic, it is almost always diagnosed as an incidental finding on an imaging test that is being performed for another reason, usually for recurrent kidney stones.

The diagnosis is made with imaging that can visualize the inner structures of the kidney.

• Computed Tomography (CT) Scan: This is now the most common and effective way to diagnose MSK. A CT scan, especially with contrast dye (CT urogram), can clearly show the dilated ducts, any calcifications within them (nephrocalcinosis), and any kidney stones.
• Intravenous Pyelogram (IVP): This was historically considered the “gold standard” for diagnosis. It involves injecting an iodine-based dye into a vein and taking a series of X-rays as the dye is filtered by the kidneys. In MSK, the dye fills the dilated ducts, creating a characteristic “bunch of grapes” or “bouquet of flowers” appearance. This test is used much less frequently today.
• Ultrasound: A standard kidney ultrasound may suggest the presence of MSK by showing bright, echogenic spots (nephrocalcinosis) in the renal pyramids, but it is not as definitive as a CT scan or IVP.

Once a diagnosis is made, a doctor may recommend further tests, such as a 24-hour urine collection, to analyze the composition of the urine and identify specific risk factors for kidney stone formation. Blood tests to check overall kidney function are also standard, and in almost all cases of MSK, the kidney function is normal (NIH, 2020).

I still rely on CT imaging for most MSK diagnoses today, but IVP remains the gold standard when we need to see the classic “brush-like” pattern in the renal papillae.

There is no cure for MSK, but symptom management and prevention of complications are the main goals. The goal is to maintain good kidney health and minimize the painful episodes of renal colic.

1. Preventing Kidney Stones (The Cornerstone of Management)

• High Fluid Intake: This is the single most important intervention for anyone with MSK. Drinking a large amount of fluid, ideally 2.5 to 3 liters per day, is crucial. This keeps the urine dilute, which helps to prevent minerals from crystallizing and forming stones.
• Dietary Modifications: A doctor may recommend specific dietary changes based on the type of stones a person forms. This can include:
  • Reducing sodium intake.
  • Limiting animal protein.
  • A diet low in oxalate (for calcium oxalate stones).
• Medications: A doctor may prescribe medications like thiazide diuretics or potassium citrate to help prevent stone formation.

2. Treating Active Kidney Stones

• Small Stones: Most small stones can be passed with increased fluid intake and pain medication.
• Large Stones: If a stone is too large to pass, is causing unbearable pain, or is completely blocking the ureter, a urologist will need to intervene. Common procedures include shock wave lithotripsy (ESWL), ureteroscopy, or percutaneous nephrolithotomy (PCNL).

3. Treating UTIs

Prompt treatment with antibiotics is necessary for any urinary tract infection to prevent the infection from spreading to the swollen and vulnerable kidneys.

I emphasize hydration as the single most effective preventive tool. Most of my patients with MSK avoid surgery by simply drinking enough water and managing diet and calcium levels.

Medullary Sponge Kidney is a benign congenital condition that, for most people, will never cause any health problems. It does not lead to kidney failure, cancer, or a shortened lifespan. Its significance lies entirely in its potential to create an environment where painful kidney stones and urinary tract infections can form more easily. For individuals who are diagnosed with MSK, typically during an investigation for recurrent stones, the key to living well with the condition is prevention. What I always tell patients is this: you can’t fix the spongy kidneys, but you can control what flows through them. Stay hydrated, stay aware, and MSK won’t control your life.

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). (2020). Medullary Sponge Kidney. Retrieved from https://www.niddk.nih.gov/health-information/kidney-disease/medullary-sponge-kidney

National Organization for Rare Disorders (NORD). (2023). Medullary Sponge Kidney. Retrieved from https://rarediseases.org/rare-diseases/medullary-sponge-kidney/

The Cleveland Clinic. (2022). Medullary Sponge Kidney. Retrieved from https://my.clevelandclinic.org/health/diseases/15494-medullary-sponge-kidney