1.Why do kidney stones cause hydronephrosis? Hydronephrosis refers to the presence of obstruction in the urinary tract, the discharge of urine secreted by the kidneys is blocked, or the passage is not smooth, urine accumulates in the kidneys, and the pressure in the renal pelvis gradually rises, after a certain period of time, resulting in dilatation of the renal pelvis and/or renal calyces, called hydronephrosis. Patients with kidney stones often have hydronephrosis in combination with hydronephrosis. The cause of hydronephrosis is urinary tract obstruction. The cause of urinary tract obstruction can be congenital (e.g., pelvic-ureteral junction stenosis) or acquired (e.g., stones). Obstruction and stones are causal, as obstruction can cause stone formation; in turn, stones can cause or worsen urinary tract obstruction. When a kidney stone lodges at the exit site, such as the pelvic-ureteral junction or the calyx neck, it can cause dilatation and fluid accumulation in the renal pelvis or the corresponding calyces. If the stone stagnates when it is discharged to the ureter, it is more likely to lead to hydronephrosis because of the narrow lumen of the ureter. 2.What are the symptoms of patients after hydronephrosis caused by kidney stones? After the formation of hydronephrosis, due to the rise of pressure in the renal pelvis and calyces, patients will have symptoms such as pain in the affected side of the back. In case of acute obstruction, patients may have colic, which is manifested as severe pain in the lumbar region, unbearable, even restlessness, profuse sweating, also accompanied by nausea and vomiting, and obvious percussion pain in the affected lumbar region during physical examination. In chronic obstruction, the symptoms are mostly discomfort and distension in the lumbar region. If secondary urinary tract infection occurs, systemic symptoms such as increased pain, chills and fever may appear. As the function of urine secretion by the glomerulus is reduced after hydronephrosis, toxins cannot be discharged from the body. Patients with bilateral hydronephrosis or congenital isolated kidney with only one kidney may show symptoms of renal insufficiency such as anorexia, depression, anemia and swelling. A small number of patients with bilateral kidney stones or isolated kidney stones develop acute urinary tract obstruction and clinical manifestations of acute anuria. 3.What tests should be done if kidney stones are suspected to be combined with hydronephrosis? Ultrasound is a simple, safe and non-invasive test that can not only understand the location, size and number of kidney stones, but also determine the extent of hydronephrosis and the condition of the kidney parenchyma. However, the resolution of ultrasound image is poor, and it is affected by the intestinal contents, so it is usually only used as a preliminary examination. More accurate examination methods are CT and intravenous urography (IVU). there are two types of CT, plain and enhanced, and the latter requires intravenous injection of contrast agent, which has a certain impact on kidney function. If the patient has renal insufficiency, enhanced CT is not suitable, and likewise IVU is not suitable.