Acute pyelonephritis - Urinary tract infections

Acute pyelonephritis

Definition

Acute upper urinary tract infection, which causes inflammation of the interstitium of the kidney.

Aetiology

Bacterial infection, either ascending from the lower urinary tract or, less commonly due to haematogenous spread in bacteraemia or septicaemia. The most common organism is E. coli, as for other UTIs.

Pathophysiology

Predisposing factors to ascending infection include pregnancy (progesterone dilates the ureters), diabetes mellitus (does not increase incidence of UTI’s but does seem to make them more likely to be severe), urinary stasis due to obstruction, dilatation or neurological causes and reflux.

Clinical features

Fever >38â—¦ C, rigors, loin pain and tenderness with or without lower urinary tract symptoms. Nausea and vomiting are common. Features may be less specific in the elderly.

Macroscopy/microscopy

The kidneys appear hyperaemic, and tiny yellow-white spherical abscesses may be seen in the cortex. There is neutrophilic infiltration, and bacteria may be seen.

Complications

Gram negative septicaemia causing shock is uncommon in young, otherwise healthy patients, but may lead to multiorgan failure. Necrotic renal papillae due to inflammatory thrombosis of the vasa recta, can be shed, causing obstruction and acute renal failure.

Recurrent infections cause renal scarring and impaired renal function, which may cause hypertension.

See also perinephric abscess (see below).

Investigations

Urine microscopy and culture. Urinalysis is usually abnormal, but may not be grossly so.

FBC and differential. U&Es and creatinine (assess hydration and renal function).

Blood culture.

Renal USS and plain KUB X-ray may be performed if response to treatment is slow, or in suspected complicated cases, to exclude any underlying renal tract abnormality, and the presence of stones. If there is any evidence of obstruction this requires rapid drainage. Some stones are not radio-opaque, and will be missed with these tests, in which case an IVU or CT scan is more definitive.

Management

Mild cases may respond to oral antibiotics as for urinary tract infection, but many require intravenous therapy such as gentamicin and ciprofloxacin. In hospitalised patients, once clinically improving and able to tolerate oral medications, i.v. antibiotics and fluids can be converted to oral. Antibiotics should be tailored to the sensitivity and specificity, and continued for 10–14 days (longer courses in patients who were more unwell, complex, immunosuppressed or responded slowly).