Cystic kidney diseases include medullary sponge kidney, polycystic kidney dysplasia, autosomal recessive polycystic kidney, autosomal dominant polycystic kidney, simple renal cyst, and acquired renal cyst. According to the causes, renal cystic diseases can be divided into three categories: congenital developmental, hereditary and acquired.
I. Congenital developmental
1. Medullary Sponge Kidney (MSK)
MSK is a congenital developmental abnormality of the kidney, characterized by spongy porous manifestations of the dilated collecting ducts of the renal medulla and the papillary ducts of the conus and the formation of small stones in the renal papillae or medulla. Patients are mostly asymptomatic and may present with hematuria, urinary tract infection or renal calculus.
(1) Pathological basis
Medullary spongy kidney, also known as medullary tubular dilation, was named medullary spongy kidney by Cacchi and Ricci in 1949 based on the porous or spongy structure of the lesion in the cone of the renal section. The incidence is about 1/5000 to 1/20000, with a male to female ratio of about 2.5:1, mostly in middle-aged patients and rarely in children. The papillary duct enters the calyces with a sphincter-like effect, and this structure is hypertrophic and overly tight, causing obstruction of the papillary duct and collecting duct with small cystic dilatation. The anatomic abnormality causes local urinary retention and deposition of urine salts in the cystic dilatation of the collecting ducts or papillary ducts, which promotes stone formation when combined with infection and bleeding. The lesions usually involve both kidneys, but can also be seen on one side or as segmental lesions. Approximately 50% of patients with MSK may have renal hypercalcemia. The disease is present at birth and is usually asymptomatic. It is usually detected incidentally during an adult physical examination or at the age of 40 to 50 years due to complications such as stones or infections. The main clinical manifestations are recurrent urinary tract infections and kidney stones.
(2) Ultrasound findings
Although the diagnosis of medullary spongy kidney is currently confirmed mainly by intravenous pyelography, routine ultrasound screening is still important for the early detection and diagnosis of this disease [1].
The ultrasound presentation of the spongy kidney is a consistent hyperechoic area of the kidney cones arranged in a radial pattern with no obvious cystic echogenicity. When the lesion is confined to a few cones, it needs to be differentiated from renal malformations, renal tuberculosis, renal calculus and renal stones and intrarenal calcified foci. Renal tuberculosis is usually associated with symptoms of tuberculosis intoxication and is characterized by an increase in kidney volume, uneven envelope bulges, and poorly defined echogenic areas with irregular margins in the renal parenchyma, or irregular masses or plaques with significant posterior acoustic shadowing in the kidney. The renal calcification foci are mostly located in the renal cortex or under the renal envelope.
2. Multicystic Dysplastic Kidney (MCDK)
MCDK is a clinically rare congenital developmental anomaly of the kidney and is the most serious consequence of early fetal ureteral obstruction [2], with no apparent familial inheritance. Its polycystic presentation is often misdiagnosed as polycystic kidney on ultrasound. The prenatal routine ultrasound is often positive around 28 weeks of gestation [3], with an incidence of 1 in 5000 to 10,000. 76% of patients have unilateral renal polycystic kidney dysplasia and 24% of patients have bilateral. The incidence is higher in males than females when the lesion involves unilateral kidneys and twice as high in females than males when the lesion involves bilateral kidneys. The prognosis is better for unilateral lesions and worse for bilateral lesions, and patients with bilateral lesions tend to die of renal failure or respiratory insufficiency within a few days after birth.
(1) Pathological basis
The renal collecting ducts are abnormally wide and cystic in appearance. The kidney loses its normal shape and is replaced by irregular lobulated cysts. The cysts vary in size and number and are often associated with ureteral malformations, defects, solid cords, and mid-segment atresia, and may involve part of the duplicated kidney or part of the horseshoe kidney. The intercapsular tissue contains normal glomeruli, proglomerular tubules or their primary forms, connective tissue, and foci of cartilage. The contralateral kidney may have compensatory hypertrophy and hydronephrosis due to narrowing of the ureteropelvic junction.
(2) Ultrasound findings
MCDK can be divided into three forms [4]: the classic form, in which the kidney is filled with cysts of different sizes and shapes, interspersed with dysplastic, thinning renal parenchyma. In addition to the classic form, the hydronephrosis type is characterized by a cystic dilatation of the collecting system with a clear border and surrounded by cystic structures. Solid cysts are rare and are characterized by ultrasonographically indistinguishable microscopic cysts that fill the entire kidney and appear only as mildly enlarged, slightly echogenic kidneys on sonogram.
The most common type is the classic lesion [5]: in the early stages, the affected kidney increases in size. As the disease progresses, a number of non-echoic cystic structures of varying size and disparity are seen in the kidney. As the disease progresses, the morphology of the kidney becomes distorted and the sinus structures disappear; eventually, the kidney shrinks or even disappears and function is lost.
When the lesions are bilateral, prenatal ultrasound shows: large masses in both kidney areas; empty bladder; low amniotic fluid; often combined with dilated intestines, sometimes with cleft lip and palate.
II. Heredity
1. Autosomal Recessive Polycystic Kidney Disease (ARPKD)
ARPKD is an autosomal recessive cystic kidney disease with an unknown etiology [6]. ARPKD is not limited to infants, but can also occur in children and adults, and is usually associated with congenital hepatic fibrosis and intrahepatic bile duct dilatation. The incidence is 1/5500 to 1/6000, with no significant gender differences. Most of the children are born with low birth weight (1000-2500 g) and about 40% of them die at birth, while others die from respiratory distress 24-48 h after birth.
(1) Pathological basis
ARPKD is caused by a gene defect on autosome 6, and the pathogenesis is unclear [8]. The kidney pelvis is compressed and deformed by the swollen renal parenchyma, and the bile ducts in the hilar region may be dilated and increased, and the surrounding connective tissue is hyperplastic.
(2) Ultrasound performance
The main manifestations of the diseased kidneys are as follows: Bilateral enlargement of the kidneys, usually two to three times the size of normal kidneys, sometimes filling the entire abdominal cavity, while the outline of the kidneys is still present. Diffuse echogenic enhancement, occasionally showing discrete microscopic or slightly large cysts (1-2 mm). Echo-enhanced renal cone structures, similar to spongy kidneys, may be seen in the perinatal period.
Potter’s syndrome: A fetus severely affected by amniotic fluid deficiency may have multiple malformations such as pulmonary hypoplasia, facial deformities, and abdominal insufficiency, with a high mortality rate. The main signs include: Potter’s face (widening of the distance between the eyes, an abnormal fold in the upper corner of the eyes, flattened and short nose, backward sloping chin, large and soft auricles without cartilage, etc.); skeletal deformities such as bell-shaped chest, foot entropion, etc.; ophthalmologic deformities, pulmonary hypoplasia, ventral fissure syndrome, cardiovascular deformities, etc.
Prenatal liver lesions are not obvious, but occasionally the liver and bile duct wall echogenicity is enhanced, which is an early ultrasound sign of liver and bile duct fibrosis.
2. Autosomal Dominant Polycystic Kidney Disease (ADPKD)
ADPKD is an autosomal dominant cystic kidney disease and is the most common type of cystic kidney disease [9], with approximately 1 in 1000 people carrying the defective gene for ADPKD. Because symptoms of ADPKD usually do not appear until the age of 30-40 years, it used to be called Adult Polycystic Renal Disease (APCD). Both kidneys show diffuse, progressive cystic changes, and some cases may be combined with cystic changes in the liver, spleen, pancreas, ovaries and other organs. The clinical manifestations of ADPKD vary greatly among individuals, with low back pain, abdominal masses and renal insufficiency being the most common. 50-70% of patients have hypertension and 20%-46% of cases have liver cysts, which are the most common extra-renal manifestations of ADPKD, and the incidence of ADPKD combined with intracranial aneurysms is also high. There are also many cases that carry the gene for ADPKD for life without symptoms. Renal failure and secondary infection are the main causes of death. Clinical diagnostic criteria include echogenic cysts of varying sizes in the renal cortex, a clear family history of ADPKD, and positive results of gene linkage analysis. Ancillary diagnostic bases include polycystic liver, renal insufficiency, abdominal hernia, abnormal heart valves, pancreatic cysts, intracranial aneurysms, and seminal cysts.
(1) Pathological basis
ADPKD is one of the most common genetic diseases, caused by mutations in PKD1 and PKD2 on autosome 16 [10], with an incidence of 1/500 to 1/1000, mostly in adults. The pathogenesis is still unclear, but it is believed that it is caused by the obstruction of the collecting duct connection and fluid retention during embryonic development.
(2) Ultrasound manifestations
The main manifestations of the diseased kidney are as follows [11]: the kidney is mildly enlarged and the parenchymal echogenicity is enhanced, and the dermal medulla is poorly defined. The enlarged kidney sonogram is similar to that of ARPKD, but due to the relatively mild kidney lesion, renal function is mostly normal and amniotic fluid deficiency is rare. The diagnosis of ADPKD can be made if there is an enlarged subcortical cyst on ultrasound and one of the fetal parents has this disease. The diagnosis of ADPKD can be helped if there is a combined liver, pancreas or spleen cyst.
III. Acquired
1. Simple renal cysts
Simple renal cysts are the most common renal cystic disease, patients are usually asymptomatic, most are detected on physical examination, and the incidence increases with age [11]. Autopsy studies have shown the presence of simple renal cysts in 50% of those aged >50 years.
(1) Pathological basis
It is currently believed that simple renal cysts originate from tubular diverticula, with some degree of urological obstruction and tubular basement membrane degeneration as predisposing factors.
(2) Ultrasound findings
The ultrasound manifestation is thin-walled anechoic area within the renal parenchyma, with smooth wall, clear boundary and posterior echogenic enhancement of the lesion; in case of secondary infection, the cyst wall is thickened with sparse dotted echogenicity inside; with intracapsular hemorrhage, there may be local echogenic enhancement and other complex cyst sonograms; the cyst wall is occasionally calcified.
2. Acquired Renal Cystic Disease (ARCD)
ARCD refers to renal cysts that occur as a result of chronic renal insufficiency. The lesions occur in patients with uremia who have been on dialysis for a long time, and cystic lesions appear in the kidneys where there were no cysts.
Although it has no obvious clinical symptoms, it can be complicated by retroperitoneal hemorrhage and has a high tendency to become cancerous, with a cancer rate of 5.8% to 20% reported in foreign literature, and even kidney transplantation cannot eliminate the potential malignancy of ARCD. Therefore, patients with ARCD belong to the high-risk group of tumors and deserve attention and research.
(1) Pathological basis
The sonographic features of ARCD depend on the pathological basis, with a thick and gross cyst wall and irregular morphology, which is consistent with ARCD consisting of tubular basement membrane changes, epithelial hyperplasia, interstitial fibrosis and tubular dilatation. ARCD is a prominent local manifestation of diffuse lesions.
(2) Ultrasound findings
The sonogram of ARCD has the following characteristics: (1) the cyst wall is thicker and grosser than that of simple cysts, and some of the cyst wall is accompanied by dotted strong echogenicity on one side; (2) the morphology is irregular; (3) small cysts are common; (4) multiple cysts are predominant, or single cysts may occur; (5) cysts are mostly located in the lower pole of the kidney, mostly accumulating in both kidneys, with the right kidney slightly more than the left kidney; (6) the size of cysts does not change on follow-up.
In conclusion, ultrasound can help to screen for cystic kidney disease and is the imaging method of choice.