Flank pain
The flank is the anatomical region located lateral to the vertebral column between the pelvis and ribs. The innervation of the kidneys and ureters is mediated by visceral fibers from spinal segments T10 to L1. Pathology of these organs causes referred pain transmitted via somatic nerves to the corresponding lumbar dermatomes.
The most common cause of acute pain is urinary tract obstruction by calculi. Urolithiasis affects up to 5% of the population, with a higher prevalence in men, and the peak incidence occurs before the age of 60.
There are three typical anatomical sites of stone impaction:
- Ureteropelvic junction (ureteropelvic junction).
- The site where the ureter crosses the iliac blood vessels.
- Vesicoureteric junction (vesicoureteric junction).
Patient management largely depends on the size of the calculus: while stones $\le$ 5 mm pass spontaneously, this is highly unlikely for formations larger than 10 mm.
Differential diagnosis and imaging methods
Patient complaints are often non-specific, necessitating a thorough differential diagnosis. Acute pain can be triggered by urological issues (blood clots, obstruction, acute pyelonephritis) as well as non-urological pathologies: biliary colic, appendicitis, pneumonia, ovarian/adnexal torsion, or leaking abdominal aortic aneurysm. Chronic pain frequently accompanies neoplasms, tuberculosis, or spinal disorders.
According to the American College of Radiology guidelines, non-contrast low-dose CT of the pelvis and abdomen is the first-line imaging modality for suspected urolithiasis, with a sensitivity reaching 96%. MRI urography is used less frequently due to poor visualization of calcifications and is generally reserved for pregnant women.
For tumor screening (e.g., transitional cell carcinoma, which appears on ultrasound as a mass erasing the hyperechoic sinus fat), MRI has advantages over CT. It provides superior contrast resolution and is safe for patients with renal impairment. In cases of trauma or to detect iatrogenic hematuria (e.g., following nephrostomy tube removal), non-contrast CT or angiography is used.
Key nephrological syndromes
The clinical and laboratory presentation of urinary system disorders is grouped into several characteristic syndromes:
- Urinary syndrome. Isolated proteinuria and hematuria without overt clinical manifestations.
- Nephrotic syndrome. The end stage of many nephropathies, manifesting as massive proteinuria, hypoproteinemia, hyperlipidemia, and pronounced edema (urine may appear "foamy"). Protein loss triggers a cascade of reactions leading to lipid accumulation within nephrocytes and the renal interstitium.
- Nephritic (acute nephritic) syndrome. Typical of acute glomerular injury. It is characterized by the classic triad: hematuria ("smoky" or cola-colored urine), arterial hypertension, and edema, accompanied by a drop in the glomerular filtration rate (GFR).
- Hypertensive syndrome. Secondary arterial hypertension resulting from renal tissue ischemia and activation of the renin-angiotensin-aldosterone system. It is important to differentiate it from essential hypertension by the presence of urinary abnormalities (cylindruria, proteinuria) prior to blood pressure spikes.
- Tubulointerstitial disorders. Manifest as tubular dysfunction: polyuria, hyposthenuria, and nocturia (predominance of nocturnal diuresis). Over time, they can lead to acidosis and nephrocalcinosis.
- Urinary tract obstruction syndrome. Impaired urine outflow elevates pelvicalyceal pressure. This leads to tubular epithelial atrophy, decreased blood flow, and consequently, secondary infection (pyelonephritis) or hydronephrosis development.
To memorize the three typical sites of ureteral stone localization, use the mnemonic UPV (or equivalent): Ureteropelvic junction, Pelvic vessels crossing, Vesicoureteric junction.
Class and exam questions
What imaging method is optimal for acute flank pain when urolithiasis is suspected?
Non-contrast low-dose computed tomography (stone protocol CT) is the gold standard and first-line imaging modality, offering exceptional specificity and sensitivity.
What is the main difference between nephrotic and nephritic syndrome?
Nephrotic syndrome is dominated by massive urinary protein loss, severe edema, and lipid metabolism disorders. Nephritic syndrome is characterized by prominent hematuria, elevated blood pressure, and decreased renal filtration function.
Why does fatty degeneration of the kidneys develop in nephrotic syndrome?
Massive proteinuria induces compensatory hyperlipidemia. Excess lipids are filtered through damaged glomeruli (lipiduria) and actively reabsorbed by tubular epithelium, accumulating in the cytoplasm of nephrocytes.
What else to study on this topic
- Pathogenesis and mechanisms of hyperlipidemia in nephrotic syndrome
- Differential diagnosis of renovascular and essential arterial hypertension
- Ultrasound and CT findings of transitional cell renal carcinoma
- Iatrogenic hematuria: angiographic monitoring during nephrostomy removal
- Morphological changes in membranoproliferative glomerulonephritis
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