Your kidneys serve a vital role in removing waste and excess fluid from your body through urination. Sometimes one of the kidneys becomes swollen. A swollen kidney is generally due to a serious disorder that requires medical attention. Causes may range from kidney infection to kidney cysts, a kidney tumor, blockage of blood flow to or from the kidney or urine accumulation in the kidney, called hydronephrosis.
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An infection in the kidney, called pyelonephritis, usually originates as an infection in the bladder, called cystitis. The kidney becomes infected when bacteria in the bladder move up through the ureter -- the tube connecting the kidney to the bladder. Less commonly, bacteria enter the kidney directly from the bloodstream. Pyelonephritis causes inflammation with swelling of the kidney. It often produces pain in the flank or back, fever, nausea and vomiting. Blood may be seen in the urine, especially in women. Kidney infections require treatment with antibiotics.
Accumulation of urine in the kidney is usually caused by partial or complete blockage of the ureter, preventing normal flow of urine to the bladder. Blockage is often caused by a kidney stone or blood clot within the ureter. Other causes include narrowing of the ureter by an injury, surgery or infection in or around the ureter. If the blockage is left untreated, hydronephrosis can impair kidney function and eventually lead to kidney failure.
Blocked Blood Flow
The kidneys receive blood through renal arteries. If blood flow into the kidney is partially or totally blocked, kidney cells will die because of lack of oxygen and other nutrients. This causes kidney infarction, characterized by swelling of the dying cells and surrounding tissue. Blockage of blood flow into the kidney is often caused by cholesterol-containing plaques -- atherosclerosis -- of the renal artery or the main artery of the abdomen, called the aorta.
Blood leaves the kidneys through renal veins. If blood flow from a kidney is partially or totally blocked, blood will accumulate inside the kidney, causing swelling. Renal vein blockage may be caused by a blood clot. This is often due to a coagulation disorder that causes a person to develop blood clots in various parts of the body. Abdominal tumors that compress the renal vein or grow into the vein are other possible causes.
Kidney Cysts and Tumors
Fluid-filled sacs in the kidney, called renal cysts, can cause kidney swelling. Renal cysts are usually benign but are occasionally cancerous. They may occur as single or many cysts. Unilateral renal cystic disease is a rare disorder characterized by multiple cysts in just one kidney. It generally causes no symptoms and does not interfere with normal kidney function.
Kidney tumors may also cause swelling of one kidney. These tumors may be benign or cancerous. Kidney cancer is the eighth most common cancer in the United States, according to the National Cancer Institute. In the early stages of kidney cancer, there are usually no symptoms. Flank pain, blood in the urine and fever often appear with more advanced disease. Sometimes kidney cancer enters the renal vein or causes a blood clot to form in the renal vein, producing further swelling of the kidney.
When one kidney is surgically removed, often as treatment for cancer, the remaining kidney may swell as it takes over the function of both kidneys. Disorders that cause swelling of both kidneys may primarily affect one kidney, especially early in the course of the disease. For example, early-stage kidney disease caused by diabetes typically produces kidney swelling, which may sometimes be apparent in just one kidney.
Reviewed and revised by: Mary D. Daley, MD