Complications
Complications associated with medullary sponge kidney include the following:

Kidney stones
Urinary tract infection (UTI)
Blood in the urine
Distal renal tubular acidosis (Type 1 RTA)
Chronic kidney disease (rarely)
Marked chronic pain

Cause
In recent studies, insight has been obtained on the genetic basis of this disease, supporting the hypothesis that MSK is due to a disruption at the ureteric bud-metanephric mesenchyme interface. This explains why so many tubular defects coexist in this disease, and particularly a distal tubular acidification defect of which the highly prevalent metabolic bone disease is one very important consequence. In addition to the typical clinical phenotype of recurrent stone disease, other clinical profiles have now been recognized, that is, an indolent, almost asymptomatic MSK, and a rare form characterized by intractable, excruciating pain. It was previously believed that most cases of medullary sponge kidney were sporadic; however, recent studies show familial clustering of MSK is common and has an autosomal dominant inheritance, a reduced penetrance, and variable expressivity. Other theories suggest that dilatation of a collecting duct may occur, caused by occlusion by uric acid during fetal life or resulting from tubular obstruction due to calcium oxalate calculi secondary to infantile hypercalciuria.A rare, autosomal recessive form is associated with Caroli disease. Diagnosis
Classically, MSK is seen as hyperechoic papillae with clusters of small stones on ultrasound examination of the kidney or with an abdominal x-ray. The irregular (ectatic) collecting ducts are often seen in MSK, which are sometimes described as having a "paintbrush-like" appearance, are best seen on intravenous urography.
Through statistics, it is suspected that one-half of cases are inherited, and one-half are the result of new,  de novo mutations. Cause loss of hearing
The hearing loss caused by NF2 is gradual and results from the presences of bilateral cochleovestibular schwannomas, also known as acoustic neuromas, which damage to cochlear nerve causing hearing loss. Hearing loss may also result from benign tumors that grow on the vestibular and auditory nerves, which lead to the inner ear. Pathogenesis
NF2 is caused by a defect in the gene that normally gives rise to a product called Merlin or Schwannomin, located on chromosome 22 band q11-13.1. Merlin was first discovered as a structural protein functioning as an actin cytoskeleton regulator. Later merlins tumour suppressant role was described. Merlin regulates multiple proliferative signalling cascades such as receptor tyrosine kinase signalling, p21-activated kinase signalling, Ras signalling, MEK-ERK cascade, MST-YAP cascade. In a normal cell, the concentrations of active (dephosphorylated) merlin are controlled by processes such as cell adhesion (which would indicate the need to restrain cell division). It has been shown that Merlin inhibits Rac1 which is crucial for cell motility and tumour invasion. Also, merlins interaction with cyclin D was described. It is known that Merlins deficiency can result in unmediated progression through the cell cycle due to the lack of contact-mediated tumour suppression, mainly because of the cell:cell junction disruption, sufficient to result in the tumors characteristic of Neurofibromatosis type II.