Hydronephrosis is not an independent disease, but is caused by various causes of urinary tract obstruction, resulting in a lower rate of urine discharge than urine production, and excessive urine accumulation in the renal pelvis, resulting in what is clinically known as “hydronephrosis”. The urinary system is a continuous system of tubes starting from the kidneys and ending at the urethra, which consists of the calyces, pelvis, ureter, bladder and urethra from top to bottom. The smooth flow of all components of the ductal system is a prerequisite for the proper discharge of urine. Narrowing or obstruction of the ducts in any part and abnormal peristalsis can lead to impaired urine drainage and consequently hydronephrosis. What can cause hydronephrosis to occur? In general, hydronephrosis can be divided into two types: primary and secondary. The main cause of primary hydronephrosis is obstruction at the junction of the renal pelvis and ureter, which is often due to the inability of the ureter in this area to peristaltically move properly, resulting in poor urine drainage. Other causes include ureteral valves, ectopic vessels compressing the ureter, horseshoe-shaped kidneys, abnormal kidney rotation during embryonic development, high ureteral openings, and congenital ureteral ectopia, cysts, and double ureter. Secondary hydronephrosis is mostly due to ureteral stenosis or obstruction caused by diseases of other systems that compress the ureter. The cause may be from lesions inside the ureter or from lesions outside the ureter. Internal ureteral lesions can be: ureteral tumors, ureteral stones, ureteral tuberculosis, ureteral injury, etc.; causes from outside the ureter include: compression by various tumors in the abdominal cavity or pelvis, retroperitoneal fibrosis, compression of the ureter by an enlarged uterus during pregnancy, etc. In addition, some diseases of the lower urinary tract can also cause bilateral hydronephrosis, such as prostatic hyperplasia, prostate cancer, urethral stricture, vesicoureteral reflux, etc. What are the symptoms of hydronephrosis? The symptoms of hydronephrosis vary greatly from patient to patient because the clinical manifestations are related to the site and duration of obstruction, the speed of obstruction, the presence or absence of secondary infection, and the nature of the primary disease causing the hydronephrosis. Low back pain is a common symptom of hydronephrosis, and the degree of pain is related to the speed and degree of hydronephrosis. Generally, moderate or severe hydronephrosis that occurs in a short period of time is often accompanied by severe low back pain, such as renal colic that often occurs with ureteral stones; while slowly progressing hydronephrosis often has an insidious onset and is rarely accompanied by pain even when it develops into severe hydronephrosis, such as hydronephrosis resulting from compression by extra-ureteral malignant tumors, but this type of hydronephrosis is much more destructive to However, this type of fluid is much more destructive to the kidneys than the former. The decrease of urine volume is a serious complication of bilateral hydronephrosis. Generally, the 24-hour urine volume of healthy adults is about 1500~2000ml; under normal diet, less than 400ml urine volume in 24 hours is called oliguria, and less than 50ml urine volume is called anuria. Patients with acute hydronephrosis suffer from gastrointestinal symptoms due to nerve reflexes in the internal organs caused by renal colic, while patients with chronic hydronephrosis suffer from metabolites and toxins accumulation in the body due to renal impairment, which affects the function of the digestive tract. In addition, hematuria, lump in the lower back and abdomen, hypertension and fever are also common accompanying symptoms of hydronephrosis. What are the consequences of hydronephrosis? The most common and serious consequence is kidney atrophy of different degrees. Due to the obstruction of urination, the renal pelvis is enlarged, the pressure in the pelvis increases, and the blood vessels in the kidney are compressed, resulting in kidney ischemia, and then kidney atrophy and impaired kidney function. In severe cases, the kidney will become a huge non-functional sac. Mild hydronephrosis can be recovered, while in severe hydronephrosis the atrophied kidney tissue is difficult to recover. Infection often occurs in the hydronephrosis pelvis, as urine is retained in the pelvis and ureter, resulting in bacterial growth and reproduction, which can lead to pyelonephritis, urinary extravasation and perinephritis, and in severe cases, pus in the kidney. The obstruction of the ureter by stones leads to obstruction of urine flow and hydronephrosis, and electrolytes in the obstructed urine are easily precipitated to form urine salt crystals and develop into stones, so hydronephrosis and stones are mutually beneficial. The bacteria and necrotic tissues in the hydronephrosis pelvis become the core of kidney stone formation, especially in the infected urine where urine salt crystals are more likely to precipitate and form stones. Complete bilateral ureteral obstruction leads to anuria within a short period of time, and the accumulation of large amounts of water in the body after anuria increases the load on the heart, which can lead to acute heart failure, a fatal complication, in severe cases. Bilateral hydronephrosis causes chronic renal impairment, and with the accumulation of metabolic waste and toxins, chronic renal insufficiency until uremia occurs, and patients will face the risk of lifelong dialysis at this time. This shows that patients should be more alert to hydronephrosis and should not take it lightly. The causes of hydronephrosis are diverse and should be treated for different causes, and the key to treating hydronephrosis is to remove the obstruction and restore kidney function as soon as possible. The efficacy of timely treatment is more satisfactory. Acute complete ureteral obstruction should be removed by surgery in time, and renal damage will occur after 24h of obstruction; renal function will decrease by 30% after 10 days of obstruction; renal function will be difficult to recover after 30-40 days of complete ureteral obstruction. The sooner the chronic ureteral obstruction is released, the better, and the kidney function may be partially restored after a period of time.