A renal cyst is the most common lesion of the kidney. Renal cysts are so ubiquitous that they are present in approximately 40% of the patients undergoing imaging. The cystic renal disease can be focal, multifocal, one-sided, or bilateral. Renal cysts can be a result of a congenital disease or acquired. The acquired form is the most common. Renal cysts can range from benign to malignant. A categorization for renal cysts was introduced in the late 1980s known as the Bosniak classification. It was introduced in attempts to standardize characterization and management of renal cysts.
The terminology of congenital renal cysts has changed throughout the years, with the current characterization known as the Potter classification. The Potter classifications have four categories: type I is infantile polycystic kidney disease, type II is multicystic dysplastic kidney disease, type III is adult polycystic kidney disease, and type IV is obstructive renal dysplasia. Infantile polycystic kidney disease is also known as autosomal recessive polycystic kidney disease. ARPKD demonstrates no gender predilection with a ratio of 1:1. The range of diagnosis is neonate to childhood, depending on the severity of the disease. Multicystic dysplastic kidney disease is a non-inherited kidney disease that develops in utero. It is most commonly unilateral with a higher incidence on the left-side. The diagnosis is often made while still in utero or very early in a neonate. Adult polycystic kidney disease is also known autosomal dominant polycystic kidney disease. A patient with ADPKD has a normal appearance of the kidneys at birth and begins to develop multiple cysts bilaterally in their 20s to 30s. It is the most common inherited cause of end-stage renal failure with more than 50% of patients requiring dialysis by the age of 60 years. Obstructive cystic renal dysplasia results from an obstruction during development that causes scattered cysts throughout the affected kidney.
Autosomal recessive kidney disease is caused by a mutation in the PKHD1 gene located on chromosome 6 resulting in cyst formation in the collecting ducts. Multicystic dysplastic kidney disease is most commonly idiopathic. Autosomal dominant kidney disease is caused by a mutation in either the PKD1 gene on chromosome 16 or PKD2 gene on chromosome 4. Obstructive cystic renal dysplasia results from an obstruction of the ureter, bladder or urethra. Acquired renal cysts are most commonly idiopathic. Renal cysts can develop in the infectious setting, can be a result of multisystem diseases such as Von-Hippel-Lindau and tuberous sclerosis, or can form during end-stage renal disease.
The presence of acquired renal cysts increases with increasing age.
Multicystic dysplastic kidney disease is caused by nonfunctioning kidney tissue in the affected area which develops into cysts. It can ultimately lead to renal agenesis of the effected side. Obstructive cystic renal dysplasia results from an obstruction of the ureter, bladder, or urethra that increases pressure to the kidney. This process creates cysts.
The presentation of a patient with cystic kidney disease varies with the underlying disease etiology. Physical examination is usually only pertinent to inherited renal disease, presenting as a palpable mass in a neonate or child. Patients with ADPKD commonly present with flank pain, renal failure, hypertension or palpable masses. Acquired cystic kidney disease rarely becomes large enough to become palpable on physical examination.
Renal cysts can be diagnosed by ultrasound, CT, and MRI.
Infantile polycystic kidney disease is usually first evaluated with ultrasound in utero. The mother usually has is usually present with oligohydramnios from the poor renal function. After birth, an ultrasound is performed that will show enlarged kidneys with numerous small renal cysts bilaterally. The kidneys can be further evaluated with MRI which normally shows a diffuse increase in T2 signal. Multicystic dysplastic kidney disease is usually evaluated in utero versus early in neonates and presents as multiple non-communicating renal cysts. Adult polycystic kidney disease is often evaluated with ultrasound or by CT. The classification of more than two cysts per side by the age of 30 helps in the diagnosis. Obstructive cystic renal dysplasia is diagnosed in utero demonstrating a normal to small kidney size of the affected kidney with multiple scattered cysts.
Ultrasound evaluation of a renal cyst can be described as simple, complex. A simple renal cyst has the following characteristics: a well-marginated anechoic lesion with possible posterior acoustic enhancement. A minimally complex cyst may demonstrate a few septations. A complex cyst will demonstrate thick septations or wall and can have a hypoechoic appearance or signs of internal debris.
Acquired single or multifocal renal cysts are classified using the Bosniak classification on a contrast-enhanced CT and can be adjusted for MRI use. A Bosniak I lesion is a simple cyst with imperceptible walls and round. No follow-up is needed, and there is zero percent chance of malignancy. A Bosniak II lesion demonstrates minimal complexity: a few thin septations less than 1 mm in diameter, thin calcification, or a high attenuated lesion less than 3 cm in diameter without enhancement. Bosniak II lesions require no follow-up and have a zero percent chance of malignancy. A Bosniak IIF lesion demonstrates minimal complexity — an increased number of thin septations, with possible "perceived" enhancement with no true measurable enhancement — or a high attenuated lesion larger than 3 cm. These lesions require follow-up with CT or ultrasound. No definitive interval timeline is given, but most references say approximately 6 months. A Bosniak IIF lesion has a 5% chance of malignancy. A Bosniak III lesion is indeterminant in appearance with thick septations with possible nodularity or measurable wall enhancement greater than ten Hounsfield units. The recommended treatment is the biopsy and partial nephrectomy versus ablation. A Bosniak III lesion has a 55% chance of being malignant. A Bosniak IV lesion describes a solid mass with cystic versus necrotic components. These are described as being unquestionably malignant. The recommended treatment is partial or total nephrectomy. The Bosniak lesion has a 100% chance of being malignant.
A simple cyst on MRI will demonstrate low T1 signal, high T2 signal, with no evidence of enhancement post contrast.
Infantile polycystic kidney disease treatment is mostly supportive, with dialysis and transplant, but overall patients tend to have a poor prognosis. Multicystic dysplastic kidney disease has a controversial treatment of a nephrectomy of the affected side. If a nephrectomy is not performed follow-up is recommended because there is a low risk of malignant transformation. The patients will have a normal life expectancy as long as the contralateral kidney has normal function. Adult polycystic kidney disease is the leading inherited cause of end-stage kidney disease and results in the requirement of dialysis and transplant is the vast majority of cases by middle age through the early 60s. Removal of the obstruction treats obstructive cystic renal dysplasia. Acquired lesions classified as Bosniak I and II requires no further evaluation. A Bosniak IIF recommends follow-up with ultrasound or CT. A Bosniak III recommends surgical intervention with biopsy and partial nephrectomy or ablation. A Bosniak IV recommends partial or total nephrectomy.
The most common associated disease with infantile polycystic kidney disease is Caroli disease of the liver and hepatic fibrosis. There are many associated findings of adult polycystic kidney disease including berry aneurysms, hypertension, liver cysts, and cysts within different organs. There is an association of VACTERAL and congenital heart disease with obstructive cystic renal dysplasia.