Chronic pyelonephritis

Etiology and pathogenesis

Chronic pyelonephritis develops against the background of a continuous or recurrent urinary tract infection. The most important predispositions are obstruction and vesicoureteral reflux (VUR).

In clinical practice, the term "reflux nephropathy" is frequently used—it describes scarring and irreversible kidney damage resulting from VUR suffered in childhood. However, it is important to remember that the formation of fibrous scars in the kidney can also occur in the complete absence of reflux. The inevitable outcome of massive fibrosis is end-stage renal disease (ESRD).

Basic imaging semiotics

The diagnostician's key task is to identify signs of fibrosis, atrophy, and compensatory renal reactions. The overall picture often demonstrates pronounced renal asymmetry.

• Scarring and atrophy: the cortical layer becomes thinned. The underlying scar pulls on the renal papilla, causing its retraction. As a result, calyceal clubbing forms.
• "Pseudotumors": against the background of thinned scar tissue, areas of normal parenchyma undergo compensatory hypertrophy. Visually, this can strongly resemble a neoplastic process.
• Calyceal system: in addition to deformation, its thickening and dilation are noted.

A rare but severe form of inflammation is alkalobacterian (crusted) pyelitis and cystitis. It is caused by Gram-positive urease-producing bacteria. On unenhanced CT, this process appears as linear hyperdense calcifications located along the thickened urothelium.

Comparison of diagnostic imaging modalities

Various modalities are used to evaluate chronic pyelonephritis, each having its own strengths:

1. Ultrasound (US): reveals reduced kidney size with focal contour irregularities (due to areas of fibrosis). Hydronephrotic changes against the background of generalized atrophy can be visualized.
2. Computed tomography (CT): contrast-enhanced CT is ideal for differential diagnosis. It allows for confidently distinguishing a true tumor from hypertrophied parenchyma ("pseudotumor"), and a post-infectious scar from an infarction zone.
3. Magnetic resonance imaging (MRI): provides information comparable to unenhanced CT data.
4. 99mTc-DMSA scintigraphy: a highly sensitive method demonstrating photopenia (areas of reduced radiopharmaceutical uptake) strictly in the areas of scarring.
5. Excretory urography (IVU): is used less frequently in modern practice. It is less sensitive than CT and ultrasound because preserved renal function is required to visualize contours and calyces. In addition, the method poorly differentiates post-infectious scars, infarctions, papillary necrosis, and fetal lobulation.

Renal tuberculosis: key features of involvement

Although this topic is dedicated to nonspecific pyelonephritis, renal tuberculosis (TB)—the most frequent localization of extrapulmonary TB—must always be included in the differential diagnosis.

The infection reaches the periglomerular and peritubular zones hematogenously from a pulmonary focus (while only 50% of patients with renal TB have pulmonary manifestations). Clinical presentation is insidious: dysuria, flank pain, low-grade fever. Sterile pyuria (culture-negative pyuria) and hematuria are characteristic.

The process spreads from the cortex to the medulla and pelvicalyceal system, causing destruction, papillary necrosis, and calcification up to the complete replacement of the kidney with calcifications—autonephrectomy (autonephrectomy).

Imaging findings of renal tuberculosis

The picture of TB relies on detecting parenchymal destruction and papillary necrosis.

• Specific symptoms: fibrous strictures of the infundibula (infundibular strictures) cause obstruction of the segments. The "excluded" or "phantom" calyx sign appears (phantom calyx).
• CT findings: papillary necrosis ("moth-eaten appearance" / moth-eaten appearance), scarring, caseous masses, and calcifications. Upon reactivation of the process, vasoconstriction and inflammation produce the "striated nephrogram" (striated nephrogram) sign on contrast-enhanced CT.
• MRI signs: excellently visualizes fistulas and sinus tracts. A macronodular tuberculoma on T1-WI appears hypointense, has a thick irregular wall, and exhibits a fluid-debris level.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Pathogenesis of the formation of calyceal clubbing.
• Differential diagnosis of "pseudotumors" and renal cell carcinoma on CT.
• Features of the development of encrusted pyelitis.
• The striated nephrogram sign: causes of vasoconstriction.
• The role of scintigraphy (DMSA) in pediatric nephrology for assessing scarring.

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

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