Acute pyelonephritis

Acute bacterial pyelonephritis remains common and continues to have significant morbidity in certain patients groups.

Epidemiology

The incidence of acute pyelonephritis parallels that of lower urinary tract infections: approximately five times more common in females with a sharp increase following puberty ⁶.

Clinical presentation

Clinical presentation is fairly specific and classical in most cases, consisting of rapid onset of high fevers and flank pain and tenderness. In many instances less specific or non-urinary symptoms and signs may also be present which may lead to clinical confusion ¹.

White cells and bacteria are usually present in the urine, and blood tests reveal the expected changes: increased WCC, CRP and/or ESR. In severe cases, systemic sepsis may be present.

In many instances patients respond promptly to antibiotics and no imaging is required.

Pathology

The most commonly implicated organisms are from the gastrointestinal tract ⁵:

• E. coli (most common)
• Klebsiella sp.
• Proteus sp.
• Enterobacter sp.
• Pseudomonas sp.
• Haemophilus influenzae

Infection gains access to the upper urinary tract by passing retrogradely up the ureter from the bladder, facilitated by virulence factors which allow bacteria to adhere to the urothelium (e.g. adhesin P) and inhibit ureteric peristalsis (endotoxins) ^1,5. Infection then passes into the collecting tubules and results in an interstitial nephritis, with resulting alterations in renal filtration and blood flow in the affected region. Localised ischaemia secondary to inflammatory changes results in altered imaging and potentially eventually results in necrosis and scarring ².

Rarely, the kidney may be seeded haematogeneously, in which case usually peripherally located renal abscesses develop rather than pyelonephritis. 

Radiographic features

In many instances imaging is not required. Situations in which imaging is indicated include:

• exclude obstructed kidney
• high risk patients: diabetics, elderly, immunocompromised
• those with mixed clinical picture
• previous renal pathology

Plain film

Plain films have a limited role to play, especially if patients are likely to go onto CT. They may demonstrate obstructing urinary tract calculi and occasionally demonstrate gas within the collecting system (emphysematous pyelonephritis).

Ultrasound

Ultrasound is insensitive to the changes of acute pyelonephritis, with most patients having 'normal' scan, and abnormalities only identified in ~25% of cases ¹. Possible features include:

• particulate matter in the collecting system
• reduced areas of of cortical vascularity by using power doppler 
• gas bubbles (emphysematous pyelonephritis)
• abnormal echogenicity of the renal parenchyma ¹
  • focal/segmental hypoechoic regions
  • mass like change

Ultrasound is however useful in assessing for local complications such as hydronephrosis, renal abscess formation, renal infarction, perinephric collections, and thus guiding management.

CTTCT

CT is the most sensitive modality for the renal tract, able to assess for renal calculi, gas, perfusion defects, collections and obstruction. Unfortunately it does have a significant radiation burden and should be used sparingly, especially in young patients.

There is usually no need for a three or four phase CT IPV (CT urography). A single 45-90 second post contrast scan usually suffices, although clinical accumen is required to optimise the scan time and limit radiation ^1,3. For example, if renal colic is suspected then a non contrast scan is often required to assess for renal calculi. If renal ischaemia is suspected than an arterial scan (15-25 seconds) is ideal to assess perfusion ³.

Non-contrast CT

Often the kidneys appear normal. Affected parts of the kidney typically may appear swollen and of lower attenuation. Renal calculi or gas within the collecting system may be evident.

Post-contrast CT

Following administration of contrast, one or more focal wedge like regions will appear swollen and demonstrate reduced enhancement compared to the normal portions of the kidney. Of note, the periphery of the cortex is also affected, helpful in distinguishing so called lobar nephronia from a renal infarct (which tends to spare the periphery; so-called rim sign).

If imaged during the excretory phase, a striated nephrogram may also be visible ^3-4.

If for some reason the kidney is imaged again within 3-6 hours, persistent enhancement of the affected regions may be evident due to slow flow of contrast through involved tubules ^1,3.

MRI

MRI is usually reserved for patients who are pregnant, and findings mirror those seen on CT. The kidney demonstrates wedge shaped regions of altered signal:

• T1: affected region(s) appear hypointense compared to normal kidney parenchyma
• T2: hyperintense compared to normal kidney parenchyma
• T1 C+: reduced enhancement

A fast inversion recovery sequence obtained after contrast administration has been shown to be particularly effective in outlining affected regions which appear hyperintense compared to the low signal parenchyma. The contrast is thought to represent a combination of local oedema, and decreased T2 signal due to Gadolinium in the perfused 'normal' portions ².

Nuclear medicine

Technetium-99m dimercaptosuccinic acid (DMSA) demonstrates a similar reduction in renal perfusion and function, which one or more wedge like defects in the outline of the kidneys ².

Treatment and prognosis

Most patients respond rapidly to appropriate antibiotic therapy, and often no imaging whatsoever is required. Even when imaging has been obtained, unless patients do not respond clinically no follow-up imaging is usually required, and it is important to note that imaging changes can take up to five months to resolve ¹.

Presence of an upper urinary tract infection in the presence of obstruction can threaten the viability of the kidney and a percutaneous nephrostomy is usually required on an emergency basis.

Complications

Complications include ²:

• renal abscess
• renal infarction, necrosis and scarring
• chronic renal impairment
• hypertension

Differential diagnosis

General imaging differential considerations include

• renal infarction
  • typically spares the peripheral aspect of the cortex, e.g. cortical rim sign
• other causes of interstitial nephritis
  • sarcoidosis
  • drug induced