Urology- Nephrolithiasis

Renal Anatomy and Physiological Compartments

• The kidneys are specifically described as bean-shaped structures located within the retroperitoneal compartment.

• Clinical Significance of Retroperitoneal vs. Intraperitoneal Location:

  • Organs in the retroperitoneum tend to manifest pain that radiates to the back when pathology is present.
  • Examples of back-radiating pain associated with specific retroperitoneal conditions include:
    • Pancreatic cancer (pain radiating to the back).
    • Aortic rupture (especially in elderly men presenting with a palpable mass and pain radiating to the back).
    • Renal pain (presents as back and flank pain).
  • Practitioners must integrate this anatomical knowledge into clinical examinations and medical histories when assessing patients for back pain.

• The Psoas Muscle Association:

  • The psoas muscle wraps down to the pelvis where it fuses with the iliacus muscle to form the iliopsoas.
  • Clinical Relevance: Due to its extreme proximity to the kidney, pathology in the psoas muscle—such as abscesses or hematomas—can infiltrate the adjacent kidney. Conversely, renal pathology can become psoas-focused.

• Protective Layers:

  • The kidney is surrounded by layers of fat and fascia designed for protection against mechanical injury.
  • The peritoneal line sits in front of the kidney, while major vessels like the aorta sit behind the peritoneal lining as retroperitoneal structures.

Internal Kidney Structure and the Nephron

• Macro-Anatomy Divisions:

  • The kidney is divided into three sections: the upper pole, mid pole, and lower pole.
  • The Outer Layer: Known as the cortex.
  • The Inner Layer: Known as the medulla, which contains the filtration system.
  • The Renal Sinus: Located in the middle of the kidney, the sinus houses the collecting system.

• The Collecting System Hierarchy:

  • Minor calyces come together to form major calyces.
  • Major calyces converge to form the renal pelvis.
  • The renal pelvis transitions into the ureter to transport urine.

• Neurological Components:

  • Adjacent to the cortex are nerve endings that function as stretch detectors (stretch receptors).
  • When the kidney is obstructed and begins to dilate (hydronephrosis), these receptors are stimulated, causing the sensation of renal pain.

• Vascular Supply:

  • The kidney is supplied by the renal artery and drained by the renal vein.

• The Nephron:

  • The nephron is the functional unit of the kidney.
  • There are approximately $1 \times 10^6$ nephrons in each kidney.
  • Regeneration Warning: Nephrons do not regenerate when damaged. Loss of nephrons due to pathology (like stones or chronic disease) is irreversible and leads eventually to renal failure.
  • Functions of the Nephron:
    • Filtration.
    • Reabsorption of essential substances: Glucose, amino acids, and water.
    • Secretion.
    • Excretion of urinary waste into the urine.

Anatomy of the Ureter and Bladder

• Classification of the Ureter:
  • The ureter connects the kidney to the bladder.
  • It is divided into the upper ureter, mid ureter, and distal (sub)ureter.
  • These divisions are crucial for documenting the exact location of kidney stones.
• Ureter Histology and Function:
  • The ureter is a muscular pocket containing both longitudinal and circular muscle layers.
  • These muscle layers facilitate a "squeezing" or contraction function to propel urine downward to the bladder.
• Common Sites of Ureteral Obstruction/Impaction:
  1. UPJ (Ureteropelvic Junction): The junction between the renal pelvis and the kidney.
  2. Iliac Vessel Crossing: Where the ureter crosses over the iliac vessels.
  3. UVJ (Ureterovesicular Junction): Where the ureter joins the bladder.

Pathophysiology of Kidney Stones (Nephrolithiasis)

• Epidemiology:
  • Nephrolithiasis is the most common condition affecting the urinary system.
  • Yearly incidence: Approximately $600,000$ cases.
  • Consequence: Damage to nephrons leading to Chronic Kidney Disease ($CKD$), increased morbidity, and increased mortality.
• Etiology: Supersaturation and Crystallization:
  • Urine contains various crystals; saturation levels determine if they become pathological stones.
  • Primary Factor: Hydration levels. Decreased water intake leads to higher saturation and crystal congregation.
  • Citrate: A natural compound in the body that helps dissolve crystals. A lack of citrate increases stone risk.
• Chemical Types and Radiographic Appearance:
  • Radio-opaque (Visible on X-ray):
    • Calcium stones (Calcium Oxalate or Calcium Phosphate).
    • Struvite stones.
  • Radiolucent (Invisible on X-ray):
    • Uric Acid stones.
  • Clinical Application: If a patient is in pain but a KUB ($Kidney, Ureter, Bladder$) X-ray shows no stones, you cannot conclude the patient is stone-free, as they may have radiolucent uric acid stones. Further imaging (Sonogram or CT) is required.

Risk Factors and Congenital Variations

• Personal and Family History:
  • Personal history of a stone: $50\%$ chance of forming another within $10$ years.
  • Family history of stones: $2.5 \times$ higher risk of stone formation.
• Other Risk Factors:
  • Malabsorption symptoms.
  • Urinary Tract Infections ($UTIs$).
• Horseshoe Kidney (Fusion Anomaly):
  • The lower poles of the kidneys are fused in $90\%$ of cases ($10\%$ involve upper poles).
• Malrotation:
  • Normal kidney orientation has the renal pelvis positioned medially.
  • In malrotated kidneys, the pelvis might be lateral, meaning the kidney is twisted into a different orientation (cross-axis instead of transverse).
  • While often asymptomatic, malrotated kidneys are at a significantly increased risk for stone formation.
• Situs and Orientation:
  • The right kidney is normally slightly lower than the left due to the position of the liver.
  • When evaluating imaging, one must assess location, gross appearance, size, presence/absence of intrarenal and extrarenal pathology, and cortical thickness.

Clinical Presentation and Pathophysiology of Pain

• Symptoms: Acute pain in the abdomen, back, and flank, typically radiating to the groin or testes, often accompanied by nausea and vomiting.
• Testicular Referred Pain:
  • Caused by the distribution of the ilioinguinal and genitofemoral nerves.
  • As a stone obstructs and stretches the ureter, signals are transmitted to these pelvic nerves, referring pain to the scrotum/testicles.
  • Clinical Warning: Don't be misled into thinking scrotal pain is exclusively a testicular problem.
• Nausea and Vomiting Pathophysiology:
  • Result of shared nerve pathways between the Gastrointestinal ($GI$) and Genitourinary ($GU$) systems.
  • Activation of GU nerves sends signals to the vomiting center in the Medulla Oblongata, causing the body to respond as if the etiology is gastrointestinal.
• Fever and Chills: Indicate systemic infection or renal inflammation, which may result from an ascending bladder infection.
• CVA Tenderness: Indicates renal involvement or potential failure.

Evaluation and Diagnostic Testing

• Urinalysis ($UA$):
  • Hematuria: Blood in the urine is a classic sign of stones. If imaging is negative, hematuria might indicate a stone has already passed or suggest other pathology (e.g., cancer).
  • Infection Signs: Look for White Blood Cells ($WBCs$), Nitrites, and Leukocyte Esterase. These indicate infection requiring antibiotics even before the culture results return.
  • Prophylaxis: If imaging shows hydronephrosis without immediate signs of infection, antibiotics are often given preventatively.
• Serum Creatinine:
  • Used to assess kidney function. Always compare to the patient's baseline (e.g., $3$ or $6$ months ago) to determine if it is "trending up."
• The Diagnostic Dilemma: Medical vs. Surgical:
  • Elevated creatinine requires a distinction between chronic medical disease and acute surgical obstruction.
  • Sonogram Protocol: If hydronephrosis is present on sonogram, it is a surgical/urology referral. If no hydronephrosis is present, it is a medical/nephrology referral.
• Imaging Modalities:
  • CT Scan (Computed Tomography): The test of choice. Specifically, a CT without contrast ($C-$) is used because contrast in the kidney can mimic the appearance of stones.
  • KUB (Kidney, Ureter, Bladder): A plain X-ray. Good for calcium stones but misses radiolucent stones.
  • Sonogram: Used to visualize hydronephrosis (black areas representing dilated calyces) and can detect "bladder jets."

Diagnostic Interpretation Scenarios

• Displaced/Malrotated Structures:
  • If an organ is displaced on a CT scan, do not automatically assume it is a congenital variation. You must rule out a tumor displacing or pushing the structure.
• Stone Location Significance:
  • Calyceal Stones: Stones in the calyx are generally less of a threat as they don't typically cause full obstruction.
  • Pelvic/Ureteral Stones: These are much more significant as they cause obstruction and potential renal damage.
• Hydronephrosis and Cortical Thickness:
  • Acute: Dilated calyces/pelvis with preserved cortical thickness.
  • Chronic: Minimal cortical tissue/thickness indicates long-standing obstruction. This suggests the kidney may have minimal to no remaining function (potential for a high creatinine).
• Bladder Jets:
  • Seen on color Doppler ultrasound. They represent the turbulence created as urine enters the bladder from the ureter.
  • If bladder jets are visualized/patent, the ureter is not fully obstructed, even if hydronephrosis is present. If they are absent, it confirms obstruction.

Specialized Conditions and Emergencies

• Staghorn Calculus: A large stone that occupies the renal pelvis and extends into multiple calyces, resembling the horns of a stag.
• XGP (Xanthogranulomatous Pyelonephritis):
  • A severe infectious process associated with staghorn calculi.
  • Surgical Emergency: Requires an immediate nephrectomy.
  • High Risk Group: Diabetic patients presenting with flank pain, fever, and a kidney stone should be screened for XGP.
  • Imaging Findings: CT shows lobulations and significant deformity of the kidney.
• Nephrocalcinosis: High levels of calcium in the kidney that can progress to chronic renal failure; requires metabolic management and fluids.

Clinical Management and Treatments

• Emergency Room Stabilization:
  • Pain Management: It is advised to perform a physical exam before giving heavy pain medication to avoid masking signs like CVA tenderness.
  • Hydration: IV fluids help flush fragments through the system. Do NOT restrict fluids.
• Decision to Admit versus Discharge: Dependent on stone location, stone size (stones less than $5\,mm$ have a high chance of passing), patient symptoms, and comorbidities (Diabetes, age, cancer).
• Pharmacological Treatment:
  • Flomax ($Tamsulosin$): Used to relax the muscular ureter to help push stones out.
• Surgical/Interventional Options:
  • Ureteral Stents: Curl-ended tubes placed to relieve hydronephrosis temporarily. Warning: Stents must be removed (usually within a few weeks); if left for months/years, they can become encrusted with new stones.
  • Shock Waves (ESWL - Extracorporeal Shock Wave Lithotripsy): Suitable for renal and renal pelvic stones; breaks stones into fragments to be passed. Potential long-term risks include hypertension or diabetes.
  • Ureteroscopy: A scope is passed into the ureter/kidney, and stones are broken with a laser.
  • PCNL (Percutaneous Nephrolithotomy): For very large stones (like staghorn calculi). Requires an opening through the skin into the kidney; usually requires inpatient admission for a couple of weeks.
• Differential Diagnosis: Always consider chronic nephritis, ovarian cysts, ectopic pregnancies, cholecystitis, or herpes as alternatives for flank/abdominal pain.

Questions & Discussion

Q: Why don't you give pain meds immediately in the ER?A: To avoid masking physical exam findings such as CVA (costovertebral angle) tenderness, which helps verify the location and severity of the problem.

Q: How do you tell the difference between a medical kidney issue and a surgical one?A: Use a sonogram. Hydronephrosis indicates a surgical/obstructive issue (Urology). No hydronephrosis with high creatinine indicates a medical filtration issue (Nephrology).

Q: Is the malrotated kidney usually symptomatic?A: No, it is usually found incidentally on a scan. It is not malrotated because of the liver (the right kidney is just lower because of the liver); it is malrotated because of a congenital variation.

Q: If a patient has a horseshoe kidney, does that cause symptoms?A: In most cases, it's fine, but like malrotation, it increases stone risk.

Q: Can you describe the croissant-shaped kidney on the scan?A: This refers to a kidney that is rotated upward (long axis) instead of being transverse. When you see this displacement, you must consider if a tumor is pushing it.

Q: What do you do if a patient has blood in the urine but a scan shows no stones?A: Repeat the urinalysis in $1$ to $2$ weeks. If the stone passed, the blood should be gone. If it's persistent, refer for further workup like a cystoscopy to rule out cancer.

Q: How does a patient urinate with a stent in?A: The stent does not affect the flow of urine to the bladder, though it can cause some physical discomfort.

Q: What is the significance of "Bladder Jets"?A: Their presence indicates that the ureters are patent and not obstructed. If you see hydronephrosis but see bladder jets, the patient is not fully obstructed.

Q: Is XGP a surgical emergency for everyone?A: Yes, especially in diabetics. The longer the infection stays, the more endangered the patient is. It requires nephrectomy.