The symptoms of medullary sponge kidney are similar to those of other hereditary diseases, making its onset and prevalence difficult to predict. Most patients with medullary sponge kidney are asymptomatic, and many people with MSK likely go undiagnosed. An estimated 1 in 5,000 people in the U.S. have medullary sponge kidney, and about 12-20% of people who have experienced calcium-based kidney stones are diagnosed with MSK. This rare disease affects people of different races and geographical locations equally, yet women are more likely to be born with MSK than men. A person diagnosed with other renal (kidney) malformations, such as Beckwith-Wiedemann Syndrome or Caroli’s Disease (CD), may also exhibit medullary sponge kidney. 

There is currently no known genetic cause of medullary sponge kidney. Research shows that those with a family history of MSK likely inherit it from one parent as an autosomal dominant trait, but this inheritance pattern has not been proven. Autosomal dominance refers to a pattern of inheriting two copies of each chromosome, one from each parent, in an offspring (see RareShare Guide on Genetic Inheritance). If a gene is autosomal dominant, it expresses its trait in an individual even if it is present on only one of the two chromosomes. Medullary sponge kidney may be related to mutations in the glial cell-derived neurotrophic factor receptor tyrosine kinase(GDNF-RET) gene, associated with nephron development in the kidney. This gene, along with its associated RET repressor, is essential to early growth of the kidneys during embryonic development. The GDNF-RET gene and its pathway initiates a process known as ureteric budding of the kidneys from the nephric duct in an embryo, one of the key steps in the development of the urinary tract. Studies are currently underway to determine if mutations to this gene and its subsequent pathway cause medullary sponge kidney.

Medullary sponge kidney is often diagnosed during adulthood, yet is likely inherited and present from birth. Though the genetic cause remains unknown, the physical cause of medullary sponge kidney is a malformation of the kidneys in the human embryo. This means that the process of kidney development in the embryo is interrupted or not carried out properly. During embryonic development, two tissue types arise that later differentiate into the functional areas of the kidney: the nephrons of the kidneys, the internal mechanics that conduct the kidneys’ function, and the ureteric-bud, which serves as the renal collecting system and is located just after the nephrons to filter and collect waste in the urine. During embryonic development, malformations in the formation of either of these systems lead to the development of cysts and the dilation of kidney tubules that cause medullary sponge kidney.

In most cases of medullary sponge kidney, the patients do not exhibit any symptoms and the cysts on the kidneys do not cause any discomfort. Problems can arise due to the presence of cysts in the medulla that reduce the kidney’s ability to filter waste and produce urine. The outward signs and symptoms that result from cysts can include pain originating in the side, back, abdomen, and/or groin, painful or burning urination, cloudy urine, blood present in the urine (hematuria), fever, chills, nausea, and vomiting. 

Two common symptoms that lead to testing and diagnosis of medullary sponge kidney are frequent urinary tract infections and generation and passage of kidney stones. Urinary tract infections (UTIs) can occur in any part of the passage urine takes to leave the body, and they cause pain and irritation during urination. Kidney stones are deposits of minerals and salts that form hard “stones” that must be excreted from the body in the urine. Both symptoms are treatable but can cause pain and discomfort.

While unlikely, medullary sponge kidney can lead to more damaging effects to the kidney such as kidney failure, chronic kidney disease, and kidney damage. Kidney failure (end-stage renal disease) as a result of MSK reportedly occurs in 10% of patients. Chronic kidney disease and kidney damage (renal tubular acidosis) can occur in patients with MSK but is very rare and often not seen until adulthood.

Neonatal Form

Medullary sponge kidney originates from complications in embryonic development of the kidney(s). It is present from birth but often does not cause symptoms. 

Infantile Form

Medullary sponge kidney in young children is often benign with no noticeable symptoms.

Juvenile Form

While medullary sponge kidney is usually benign throughout a patient’s life, symptoms can occur starting as early as teenage years. 

Adult Form

Symptoms of MSK, if any, are most likely to appear in adults between the ages of 30 and 50. These symptoms can include pain in the side(s), back, and stomach, urinary tract infections, and kidney stones.

Patients with medullary sponge kidney are thought to often live their entire lives without a diagnosis since the disease is usually benign. However, patients that experience kidney stones and/or urinary tract infections with some frequency are often tested for MSK.

A patient who has experienced urinary tract infections and/or kidney stones may be recommended a few diagnostic tests for medullary sponge kidney by their doctor:

1. Intravenous Pyelogram (IVP) - This is the most common and accurate test of medullary sponge kidney to date. The patient is injected with a dye called contrast medium through an IV, which then travels to the kidneys for removal in the urine. The patient’s kidneys are examined by x-ray to visualize the contrast medium in the urine and check for the presence of blockage or cysts in the urinary tract. 

2. Ultrasound - During an ultrasound, a transducer device held over the abdomen releases sound waves that bounce off major organs in the body. The doctor examines the kidneys for calcium deposits or abnormalities.

3. Computed Tomography (CT) Scan - Similarly to IVP, a CT scan involves the injection of contrast medium and then exposure to x-rays in a tunnel-like device. This will show whether the tubules of the kidney medulla are expanded or blocked.

4. Blood and urine tests can also be used by doctors to screen for the health and infection status of the kidneys.

Medullary sponge kidney is not always associated with the development of symptoms, and thus does not always require specific treatment. Patients are treated for any symptoms that arise, and those that are diagnosed with MSK should receive regular examinations of their kidneys to monitor the size and location of cysts. This will aid in the diagnosis and prevention of frequent kidney stones and urinary tract infections. Though there is no cure for MSK, treatments do exist for the two major symptoms associated with the disorder: urinary tract infections and kidney stones. 

Urinary tract infections (UTIs) are treated with antibiotics. A medical professional will often advise drinking clear fluids to maintain the health of the urinary tract.

Kidney stones often pass on their own out of the urinary tract, however in extremely painful and large cases they may require surgery or therapeutic procedures to remove. These treatments to remove kidney stones include lithotripsy (the use of ultrasound shock waves to break up the obstruction), ureteroscopy (the insertion of a scope into the urinary tract to observe and break up stones using a laser), and percutaneous nephrolithotomy (the insertion of a scope and instruments into the kidney through an incision in the back to remove the obstruction). In an effort to prevent kidney stones from developing, the patient may be prescribed medications that lower calcium levels in the urine: potassium citrate and thiazide.

Individuals with medullary sponge kidney often do not develop symptoms related to this disorder. Symptomatic individuals  often receive treatment for pain, urinary tract infections, and/or kidney stones. While some can develop more serious kidney problems and/or kidney failure, many people with MSK do not experience these complications.

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