Medullary sponge kidney (ORPHA:1309)
A rare renal tract malformation characterized by dilated malformation of the medullary collecting ducts (typically bilateral), and associated with stone formation, renal colic, hematuria, urinary tract infection, nephrocalcinosis, calcium nephrolithiasis, pyelonephritis, hypercalciuria and hypocitraturia. The disease is associated with abnormal distal tubular functions.
Epidemiology
Prevalence of medullary sponge kidney (MSK) in the general population is unknown; rough estimations suggest a prevalence of 1/2,000-20,000. However, it is much higher, up to 20%, in recurrent renal calcium stone formers. MSK affects males and females equally.
Clinical description
The majority of patients with MSK are asymptomatic; diagnosis of symptomatic patients typically occurs between 30 to 50 years of age, though the range is wider. The major clinical manifestations are kidney stones, hematuria, and urinary tract infections. Clinical manifestations including hypercalciuria, impaired urine acidification, and hyperparathyroidism may contribute to decreased bone density, leading to osteopenia and osteoporosis that is observed in some MSK patients. Medullary ectasias vary between 1 to 7 mm in size and are frequently without stones. Stones are primarily composed of calcium phosphate and calcium oxalate, and typically recur every two years. Flank pain can occur in the presence or absence of stones, can be acute resembling renal colic. Chronic pain is rare, but can lead to significant disability. Hematuria can occur in the presence or absence of stones, can be microscopic or gross, singular or recurrent, and is typically painless unless clots lead to ureteral obstruction or it is associated with nephrolithiasis. Abnormal tubular functions observed include defective urinary concentrations, incomplete and overt distal tubular acidosis, and hypocitraturia. Occurrence in children is rare but typically manifests with severe bone-related consequences of distal renal tubular acidosis (failure to thrive, short stature and rickets-like symptoms). MSK may be associated with developmental anomalies and tumors, such as Wilms tumor, horseshoe kidney, contralateral congenital small kidney, Beckwith-Wiedemann syndrome, and congenital hemihypertrophy.
Etiology
Whilst no disease-causing gene mutations have been identified, MSK may be associated with GDNF variants. In addition, HNF1B variants are associated with a spectrum of kidney disorders and two family members with an MSK phenotype have been identified with HNF1B mutations.
Diagnostic methods
Urography was the cornerstone of MSK diagnosis, and reveals dilated papillary collecting ducts with collection of contrast medium appearing as striations or bouquets of flowers. These features can also be disclosed by multidetector computed tomography (CT) urography. MRI and Ultrasound do not have the required specificity to diagnose MSK.
Differential diagnosis
The differential diagnosis is essentially that of medullary nephrocalcinosis and other causes of multiple medullary cysts such as autosomal dominant interstitial kidney disease and autosomal dominant polycystic disease.
Genetic counseling
Typically, the disease is sporadic; however, family clustering of MSK has been observed and, in certain cases, is suggestive of autosomal dominant inheritance.
Management and treatment
Treatment with potassium citrate is effective in reducing calciuria and stone recurrence rate. A diet with a high intake of water, vegetables and fruit, and low in sodium and proteins, is recommended. Thiazides are suggested where calciuria does not improve. Whilst small stones can pass spontaneously, in recurring stone formers and selected symptomatic patients, pain can be controlled with laser papillotomy. Renal autotransplantation with pyelovesicostomy, a last resort in intractable, chronic painful stone disease, has also been reported in one MSK patient.
Prognosis
Long term prognosis is excellent.

                            Last update: 
                            March 2021


                            - Expert reviewer(s): 
                            Pr Giovanni GAMBARO