What is perinephric stranding?

Perinephric Stranding: Radiological Finding and Clinical Significance

Perinephric stranding refers to the increased density or haziness in the fat surrounding the kidney seen on imaging studies, particularly CT scans, which indicates inflammation or fluid in the perinephric space.

Definition and Appearance

Perinephric stranding appears as linear or curvilinear areas of increased density within the perinephric fat on CT imaging. It is characterized by:

• Linear or hazy densities in the fat surrounding the kidney
• Typically bilateral and symmetrical in normal variations
• May be asymmetric or more pronounced in pathological conditions
• Often described as a "dirty" appearance of the normally homogeneous perinephric fat 1

On MRI, perinephric stranding typically shows:

• Low signal intensity on T1-weighted images
• High signal intensity on fat-suppressed T2-weighted images
• Enhancement on contrast-enhanced fat-suppressed T1-weighted images 2

Clinical Significance

Perinephric stranding is associated with several clinical conditions:

1. Acute Pyelonephritis

• Commonly seen in acute kidney infections
• When severe, may be referred to as a "hairy kidney" appearance 1
• Associated with higher rates of bacteremia (55.2% vs. 23.1% in patients without stranding) 3
• Indicates more severe inflammation but has limited diagnostic value on its own
• Sensitivity of 72% and specificity of only 58% for diagnosing acute pyelonephritis 4

2. Urinary Tract Obstruction

• Frequently seen with obstructing ureterolithiasis
• Moderate-severe perinephric stranding correlates with elevated serum creatinine levels 5
• May represent pyelovenous/pyelolymphatic backflow due to increased pressure in the collecting system

3. Other Conditions

• May be seen in:
  • Renal trauma
  • Post-surgical changes
  • Renal vein thrombosis
  • Renal infarction
  • Erdheim-Chester disease (a rare form of histiocytosis) 1

Diagnostic Value

The diagnostic value of perinephric stranding varies:

• Limited specificity: Perinephric stranding alone is not a powerful diagnostic tool for acute pyelonephritis (positive likelihood ratio of only 1.7) 4
• Better when asymmetric: Remarkable asymmetry in the degree of stranding is more likely to represent pathology 2
• Clinical correlation required: Should be interpreted in conjunction with clinical presentation and laboratory findings
• Prognostic value: In acute pyelonephritis, presence of stranding is associated with higher peak body temperatures and increased likelihood of bacteremia 3

Imaging Modalities

Perinephric stranding can be visualized on different imaging modalities:

1. CT scan (non-contrast or with contrast)

   • Most sensitive for detection
   • Appears as increased density in the normally low-density perinephric fat
   • In severe cases with urinary tract obstruction, may be accompanied by hydronephrosis 1

2. MRI

   • Can detect stranding as curvilinear areas in the perinephric fat
   • More sensitive than CT for detecting mild cases 1

3. Ultrasound

   • Limited sensitivity for detecting perinephric stranding
   • May show increased echogenicity around the kidney

Clinical Implications

When perinephric stranding is identified:

• Consider obtaining blood cultures, especially in patients with suspected pyelonephritis, as it predicts higher rates of bacteremia 3
• Evaluate renal function, as moderate-severe stranding is associated with elevated creatinine in obstructive uropathy 5
• In patients with sepsis and perinephric stranding, consider urgent decompression of the collecting system if obstruction is present 1, 6
• In patients with acute flank pain, fever, and perinephric stranding, consider both infectious and obstructive etiologies 1

Pitfalls and Caveats

• Normal variant: Mild, symmetric perinephric stranding can be a normal finding, especially in older patients or those with renal dysfunction 4
• Age-related changes: The frequency of perinephric stranding increases with age and renal dysfunction 4
• Limited specificity: Do not rely solely on the presence of perinephric stranding for diagnosis of pyelonephritis 4
• Post-procedural changes: Recent renal biopsy or intervention can cause perinephric stranding 2

In summary, while perinephric stranding is a common radiological finding associated with various renal pathologies, it should be interpreted within the clinical context and in conjunction with other imaging findings to determine its significance and guide appropriate management.