Description
Urolithiasis refers to the formation of calculi anywhere in the urinary tract, including the kidneys, bladder, and ureters. It’s a highly prevalent condition that can cause significant pain and discomfort.
Pathogenesis
The development of urolithiasis is multifactorial and often involves a combination of genetic, dietary, and environmental factors. Stone formation usually begins when there is supersaturation of urinary solutes such as calcium, oxalate, phosphate, and uric acid, causing them to precipitate and aggregate into stones. In addition, decreased urinary volume, abnormally acidic or alkaline urine, and urinary stasis can also contribute to stone formation.
Epidemiology, Risk Factors & Associations
Urolithiasis is a common condition worldwide, with a lifetime prevalence estimated at 10-12% in men and 5-6% in women. The recurrence rate within five years of the first stone is up to 50%.
Risk factors include:
- Dietary factors: High protein, sodium, or sugar intake can increase the risk of stone formation. For example, diets high in protein and low in fibre are associated with a 250% increased risk of urolithiasis.
- Insufficient fluid intake: Low fluid intake, leading to concentrated urine, is a well-known risk factor. Increasing water intake to achieve a urine volume of 2 litres per day can reduce the risk of stone recurrence by nearly 50%.
- Certain medications: Drugs like loop diuretics and calcium-based antacids can increase calcium in the urine, increasing the risk of stone formation.
- Medical conditions: Hyperparathyroidism increases the risk due to raised calcium levels. Gout can promote the formation of uric acid stones. Inflammatory bowel disease can increase the risk due to changes in the gut’s absorption and excretion of certain ions.
- Genetic predisposition: Certain genetic disorders, such as cystinuria and primary hyperoxaluria, significantly increase the risk of urolithiasis.
Clinical Features
The classic symptom of urolithiasis is severe, intermittent flank pain that can radiate to the groin, known as renal colic. Other symptoms may include:
- Haematuria (visible or microscopic)
- Dysuria
- Urgency and frequency of urination
- Nausea and vomiting
Complications
Complications of urolithiasis include urinary tract obstruction, recurrent urinary tract infections, renal failure, sepsis, and staghorn calculi (large stones that occupy a significant portion of the renal pelvis).
Subtypes
The composition of urinary stones can provide clues to the underlying metabolic disorder.
- Calcium stones: The most common type (70%), made up of calcium oxalate or calcium phosphate.
- Struvite stones: Associated with urinary tract infections caused by urease-producing bacteria.
- Uric acid stones: Common in gout and conditions with high cell turnover such as malignancy.
- Cystine stones: Rare and related to a genetic disorder causing cystinuria.
Pathological Features
Urolithiasis encompasses urinary tract stones, forming due to a blend of genetic, environmental, anatomical, and metabolic factors.
Histopathology
- Stones demonstrate layered structures microscopically, mirroring the episodic growth of stones.
- Organic and inorganic materials can alternate within layers.
- Certain bacteria such as Proteus mirabilis, and other urease-producers, often instigate struvite stone formation by raising urine pH, leading to precipitation of struvite (magnesium ammonium phosphate) and calcium phosphate.
Biochemistry
- Over 75% of stones are calcium-based, either as calcium oxalate or calcium phosphate.
- Calcium stones emerge when urinary concentrations of calcium and either oxalate or phosphate supersede their solubility.
- Uric acid stones, around 10% of urinary stones, form in acidic urine, often linked with conditions like gout or chemotherapy.
- Less common stones include struvite stones (10-15%) often linked to urinary tract infections, and cystine stones (<1%) tied to the genetic disorder cystinuria (defects in renal amino acid resportion).
Genetics
- Mutations in the SLC34A1 gene may lead to idiopathic infantile hypercalcaemia, a condition associated with calcium stone formation.
- Primary hyperoxaluria, a rare condition causing recurrent calcium oxalate stones, is caused by mutations in AGXT, GRHPR, and HOGA1 genes.
Serology
- Elevated blood calcium or uric acid levels may suggest the formation of calcium or uric acid stones, respectively.
- Persistent urinary tract infection may indicate struvite stones.
Radiological Features
General Features
- Location:
- Renal calyces and renal pelvis: Initial sites of stone formation.
- Ureters: Points of physiological narrowing such as the pelviureteric junction (PUJ), mid-ureter over the iliac vessels, and vesicoureteric junction (VUJ) are frequent locations.
- Bladder: Stones can reside here, especially in patients with urinary stasis conditions (e.g., neurogenic bladder, bladder outlet obstruction).
- Size and shape:
- Small calculi: Less than 5mm in size, often pass spontaneously.
- Medium-sized calculi: Between 5-10mm, may require intervention due to lower rates of spontaneous passage.
- Large calculi: Greater than 10mm or “staghorn calculi” (large stones that occupy the entire renal pelvis and calyces), often require surgical intervention. Staghorn calculi are particularly significant due to their association with infection, sepsis, and renal damage.
- Complications:
- Hydronephrosis: Dilatation of the renal pelvis and calyces, commonly occurs due to obstruction from a stone. Moderate hydronephrosis is typically indicated by a renal pelvis measuring over 10mm in AP diameter, severe hydronephrosis is often when the renal pelvis measures over 20mm.
- Hydroureter: Dilatation of the ureter, also due to obstruction. Typically, an ureteral diameter greater than 7mm is considered abnormal and suggestive of obstruction.
- Signs of infection: Perinephric stranding, abscess formation, or gas within the renal tract on CT should prompt urgent urology referral, as these can indicate urinary tract infection or pyelonephritis, especially in the context of obstruction.
KUB XR
- KUB is often the initial imaging modality due to its availability and low cost.
- Approximately 85-90% of urinary tract stones, mainly calcium-based, are visible on plain radiography due to their radiopaque nature.
- Limitations include the inability to detect radiolucent stones (e.g., uric acid and some medication-related stones), and potential masking by bowel gas or bony structures.
US
- May show hydronephrosis and can occasionally detect stones directly, especially if they are large or in the renal pelvis.
- Less sensitive (45-60%) and specific (70-80%) for stone detection compared to CT.
CT
- Non-contrast CT is the gold-standard modality with a sensitivity and specificity approaching 100%.
- Dual-energy CT is an advanced imaging modality that improves stone characterisation by differentiating materials based on their attenuation profiles at two different X-ray energy levels.
IVU
- Intravenous urography less sensitive (77-80%) than NCCT
- IVU can depict radiolucent stones missed on KUB and provides functional information.
MRU
- Magnetic resonance urography, a less commonly used modality, provides excellent visualisation of the urinary tract anatomy without radiation exposure.
- While its sensitivity for stone detection is lower than NCCT, MRU can be a valuable alternative in patients who cannot receive radiation or iodinated contrast.
Differential Diagnosis
- Phleboliths: Venous calcifications often located in the pelvis. They typically have a central lucency on CT and do not cause obstructive symptoms.
Management
Management of urolithiasis depends on the size and location of the stone, as well as the severity of symptoms. For smaller stones (<5mm), conservative management with analgesics and increased fluid intake may be sufficient. Larger stones may require intervention such as extracorporeal shock wave lithotripsy (ESWL), ureteroscopy, or in severe cases, percutaneous nephrolithotomy. Long-term management involves dietary modifications and addressing underlying metabolic abnormalities.