Medullary Sponge Kidney - Pathogenesis, Etiology, Symptoms, and Treatment

Introduction

Medullary sponge kidney (MSK) was first described in 1939 by Landerduzzi. It is a birth defect in which the dilatation of the kidney tubules occurs as a result of the formation of cysts in one or both kidneys. These cysts, in turn, reduce the urine flow from the kidney causing further complications. Patients with this disorder are more prone to getting urinary tract infections and kidney stones.

What Is Medullary Sponge Kidney?

Medullary sponge kidney, also known as Cacchi - Ricci disease, is a benign congenital disorder affecting one or both the kidneys as a result of cystic dilatation of the collecting tubules. The small cysts may measure 1 to 8 millimeters in diameter. The definitive etiology of this condition is unknown, and in most cases, it is asymptomatic.

What Is the Reason Behind the Terminology Medullary Sponge Kidney?

On crossing the kidney tissues, the small cysts formed in the collecting tubules would give a spongy appearance to the kidney; thus, the name medullary sponge kidney came into existence.

What Is the Pathogenesis of Medullary Sponge Kidney?

The possible pathogenesis of MSK could be due to the disruption in the ureteric bud (a protrusion from the mesonephric duct) metanephros interface during embryogenesis resulting in dilatation of the collecting tubules and cyst formation. The formation of small cysts in the tubules or the collecting ducts of the kidney will result in obstruction of the outward flow of urine from the kidney. This obstruction would result in the deposition of calcium, leading to the development of renal calculi (kidney stones) and urinary tract infections.

What Is the Epidemiology of Medullary Sponge Kidney?

It is reported to be a rare disorder with an overall incidence of 0.02 to 0.005 % in the general population. Studies report that it is seen in 12 to 21 % of patients with renal stones, and in 70 % of cases, it affects both the kidneys. Frequently reported in women than men, the symptoms are usually manifested around the age of 20 to 30 years.

What Is the Etiology of Medullary Sponge Kidney?

The definitive etiology of MSK is unknown, though studies report that about five percent of MSK shows an autosomal dominant pattern of inheritance and hereditary link. Most cases are sporadic and are generally not considered inheritable. Few studies have shown gene mutations in the glial cell-derived neurotrophic factor (a protein that promotes the development of many neurons in the human body) and receptor tyrosine kinase (membrane-bound receptors that mediate cell-cell communication) in medullary sponge kidney patients.

What Are the Anomalies Associated With Medullary Sponge Kidney?

It includes,

• Beckwith-Wiedemann Syndrome- A genetic disorder that affects many parts of the body).

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• Hyperparathyroidism - The increased parathyroid hormone in the blood.

• Wilms Tumor - Type of kidney cancer.

• CAKUT Syndrome- Congenital anomaly of kidney and urinary tract.

• Ehlers Danlos Syndrome - A genetic disorder that affects the connective tissues.

• Marfan Syndrome - A genetic disorder that affects the connective tissues.

• Horseshoe Kidney - This is an anatomical variation of the kidney.

• Polycystic Renal Disease - Development of multiple cysts in the kidney.

• Caroli’s Disease - A genetic condition with widened bile ducts.

What Are the Symptoms of Medullary Sponge Kidney?

Symptoms include,

• Flank pain in the back, abdomen, or groin.

• Dysuria (pain or burning sensation during urination).

• Fever.

• Chills.

• Nausea.

• Renal colic.

• Hematuria (blood in the urine).

What Are the Complications of Medullary Sponge Kidney?

• Renal stones.

• Urinary tract infections.

• Medullary nephrocalcinosis (deposition of calcium salts in the medulla of the kidney).

• Distal tubular acidosis (failure of hydrogen in secretion in the distal nephrons).

• Hypocitraturia (decreased urinary citrate excretion of less than 320 mg).

• Hypercalciuria (increased excretion of calcium in the urine).

How Is Medullary Sponge Kidney Diagnosed?

The steps in diagnosis include,

• History: Proper medical and family history to know the symptoms and if anyone in the family is affected by the same phenomenon would help in further investigation and diagnosis of the disease. It should be suspected when the patient has a history of repeated urinary tract infections and renal stones.

• Renal Ultrasound: Ultrasound examination would reveal the hyperechoic papillae with clusters of small stones in the kidney.

• Intravenous Pyelogram: In this method, a special dye is injected into the kidney to visualize the cysts or any blockage in the tubules of the kidney. The paintbrush-like appearance of the irregular collecting ducts may be visualized in the intravenous pyelogram.

• Computerized Tomography (CT) Scan: CT scans are the most preferred method in diagnosing the medullary sponge kidney. Multidetector contrast CT urography is a method in which a special dye is injected into the kidney, and the collecting tubules and the urinary tract are examined using a computerized tomography scanning machine. This would reveal the distinctive papillary blush and the parallel striations that extend from the papilla to the medulla.

• Histopathology: Microscopic examination of the affected tissue would reveal the dilated papillary collecting ducts lined with flattened or cuboidal epithelium. The cyst may contain calculi, or it may show inflammatory cells. The cortex remains unaffected.

How Is Medullary Sponge Kidney Treated?

The goal of management is to treat the complications of the medullary sponge kidney.

• Urinary Tract Infections - It is managed with appropriate antibiotics and diligent personal hygiene habits.

• Renal Stones- The stones in the medullary sponge kidney are usually small, and they tend to pass spontaneously in urine. The patients are counseled to take a good amount of water sufficient to generate 2000ml of urine per day. A diet that is rich in potassium, low sodium, normal calcium, and protein levels would help. In case of stones that are not eliminated in the urine, other procedures to eliminate stones such as surgery, lithotripsy, or ureteroscopy may be needed.

• Hypercalciuria And Hypocitraturia - 24-hour urine test is recommended in patients to check for hypercalciuria (increased excretion of calcium in the urine) and hypocitraturia (decreased citrate in the urine). If the presence of both is confirmed, then the treatment should be started at the earliest. Hypercalcemia can be treated with Thiazide diuretics. Potassium citrate supplements help treat hypocitraturia; the dosage should be titrated to make the 24-hour urinary citrate level greater than 500 mg with a urinary pH of around 6.5. Potassium citrate supplements also help in reducing the incidence of bone loss seen in medullary sponge kidney patients.

What Are the Differential Diagnosis of Medullary Sponge Kidney?

The conditions that show similar presentations include,

1. Hyperparathyroidism - The increased parathyroid hormone in the blood.

2. Hypervitaminosis D - Increased level of vitamin D in the blood.

3. Milk Alkali Syndrome - High level of calcium in the body.

What Is the Prognosis of Medullary Sponge Kidney?

Medullary sponge kidney is a benign condition; the majority of patients have normal kidney function throughout their lifetime. But studies report that about ten percent of patients with MSK develop renal failure due to repeated kidney stones and severe urinary tract infections.

Conclusion

Medullary sponge kidney is a rare benign congenital disorder of unknown etiology and pathogenesis. It is asymptomatic in the majority of patients but with an increased risk of kidney stones and urinary tract infections. Ten percent of patients with MSK have an increased risk of morbidity due to repeated renal stones and urinary tract infections. The overall prognosis of MSK is good; the majority of patients have normal kidney function throughout their lives.