Section 11. Kidney and Urinary Tract Disorders

Chapter 149. Urinary Tract Infections

Topics: Introduction | Urethritis | Cystitis | Ureteritis | Pyelonephritis

Pyelonephritis

Pyelonephritis is a bacterial infection of one or both kidneys.

Pyelonephritis is more common in women than in men. Escherichia coli, a type of bacteria that is normally found in the large intestine, causes about 90% of cases of pyelonephritis among people who live in the community. Infections usually ascend from the genital area through the urethra to the bladder, up the ureters, into the kidneys. In a person with a healthy urinary tract, an infection is usually prevented from moving up the ureters into the kidneys by the flow of urine washing organisms out and by closure of the ureters at their entrance to the bladder. However, any physical obstruction to the flow of urine, such as a structural abnormality, kidney stone, or an enlarged prostate, or the backflow (reflux) of urine from the bladder into the ureters increases the likelihood of pyelonephritis.

Infections can also be carried to the kidneys from another part of the body through the bloodstream. For instance, a staphylococcal skin infection can spread to the kidneys through the bloodstream.

The risk of pyelonephritis is increased in people with obstruction of the ureters, diabetes, in people with a weakened immune system (which reduces the body's ability to fight infection), and in pregnant women. During pregnancy, the enlarging uterus puts pressure on the ureters, which partially obstructs the normal downward flow of urine. Pregnancy also increases the risk of reflux of urine up the ureters by causing the ureters to dilate and reducing the muscle contractions that propel urine down the ureters into the bladder.

Symptoms and Complications

Symptoms of pyelonephritis often begin suddenly with chills, fever, pain in the lower part of the back on either side, nausea, and vomiting.

About one third of people with pyelonephritis also have symptoms of cystitis, including frequent, painful urination. One or both kidneys may be enlarged and tender, with tenderness felt in the small of the back on the affected side. Sometimes the muscles of the abdomen are tightly contracted. Irritation from the infection or the passing of a kidney stone can cause spasms of the ureters. If the ureters go into spasms, the person may experience episodes of intense pain (renal colic). In children, symptoms of a kidney infection often are slight and more difficult to recognize (see Section 23, Chapter 272).

In a long-standing infection (chronic pyelonephritis), the pain may be vague, and fever may come and go or not occur at all. Chronic pyelonephritis occurs only in people who have major underlying abnormalities, such as a urinary tract obstruction, large kidney stones that persist or, most commonly, reflux of urine from the bladder into the ureters (which occurs mostly in young children). Tuberculosis and fungal infections rarely cause pyelonephritis. Rarely, chronic pyelonephritis can eventually severely damage the kidneys.

Diagnosis

The typical symptoms of pyelonephritis lead a doctor to perform two common laboratory tests to determine whether the kidneys are infected: examining a urine specimen under a microscope and culturing bacteria in a urine specimen to determine which bacteria are present. Blood tests are also performed to check for elevated white blood cells or bacteria in the blood.

Additional tests are performed in people who have intense back pain from renal colic, in those who do not respond to antibiotic treatment within 48 hours, in those whose symptoms return shortly after antibiotic treatment is finished, and in men (because they so rarely develop pyelonephritis). Ultrasound or x-ray studies performed in these situations may reveal kidney stones, structural abnormalities, or other causes of urinary obstruction.

Treatment

Antibiotics are started as soon as the doctor suspects pyelonephritis and urine and blood samples have been taken for laboratory tests. The choice of drug or its dosage may be modified based on the laboratory test results, how sick the person is, and whether the infection started in the hospital, where bacteria tend to be resistant to antibiotics.

Treatment with antibiotics given by mouth for 14 days is sometimes successful if the person has no nausea or vomiting, no signs of dehydration, pain that is controlled with drugs taken by mouth, and no high fever or chills. Otherwise, the person is usually treated initially in the hospital. If hospitalization is needed, after 1 or 2 days of injected or intravenous antibiotics, treatment can usually be switched to antibiotics given by mouth.

Antibiotic treatment to prevent recurrence of the infection usually continues for 2 weeks but may last as long as 6 weeks for men, in whom the infection is commonly more difficult to eradicate. A final urine sample is usually taken 4 to 6 weeks after the antibiotic treatment is finished to make sure the infection has been eradicated.

Surgery may be needed if tests reveal a predisposing condition, such as an obstruction, a structural abnormality, or a stone.

People who have frequent episodes of pyelonephritis or whose infection returns after antibiotic treatment is finished may be advised to take a small dose of antibiotic every day as preventive therapy. The ideal duration of such therapy is unknown, but it is often discontinued after a year. If the infection returns, therapy may be continued indefinitely.