Medicine – Renal Calculi

Renal calculi, or kidney stones, are crystalline concretions that form within the urinary tract when substances in the urine become supersaturated and precipitate. They may remain within the kidney, pass into the ureter, or cause obstruction, infection, haematuria, and severe colicky pain.

The overall prevalence varies by population and has increased in many countries over time, so the older figure of 3% in the UK should be regarded as historical rather than a fixed modern prevalence estimate.

1. Types of Renal Calculi

The major stone types are:

Calcium-containing stones.

Uric acid stones.

Struvite stones.

Cystine stones.

Calcium-containing stones are the most common.

2. Calcium Stones

Most renal stones contain calcium, usually as:

Calcium oxalate

and less commonly:

Calcium phosphate.

They account for the majority of urinary calculi.

3. Hypercalciuria

Hypercalciuria is one of the most important metabolic risk factors for calcium stones.

It may occur even when the:

Serum calcium is normal.

This is called:

Idiopathic hypercalciuria

and is a common cause of recurrent calcium stone formation.

4. Hypercalcaemia

Conditions causing hypercalcaemia can increase filtered urinary calcium and predispose to stones.

Important examples include:

Primary hyperparathyroidism.

Malignancy-associated hypercalcaemia.

Excess vitamin D states.

Sarcoidosis in selected patients.

A classic exam association is:

Primary hyperparathyroidism → hypercalcaemia + hypercalciuria → calcium stones.

5. Renal Tubular Acidosis

Distal, or type 1, renal tubular acidosis is strongly associated with:

Calcium phosphate stones.

It causes:

Inappropriately alkaline urine.

Hypocitraturia.

Nephrocalcinosis.

These changes favour calcium phosphate precipitation.

6. Uric Aciduria

Increased urinary uric acid can contribute to stone formation.

This may produce:

Uric acid stones

or may promote calcium oxalate crystallisation.

Conditions associated with high uric acid include:

Gout.

High cell turnover.

Tumour lysis.

Myeloproliferative disorders.

High purine intake.

7. Uric Acid Stones

Uric acid stones are favoured particularly by:

Persistently acidic urine.

They are associated with:

Gout.

Metabolic syndrome.

Diabetes.

Obesity.

High purine intake.

High cell turnover.

A key feature is that uric acid stones are usually:

Radiolucent on plain X-ray

although they are visible on CT.

8. Hyperoxaluria

Hyperoxaluria increases the risk of:

Calcium oxalate stones.

This may result from:

Excess intestinal oxalate absorption.

Enteric disease.

Malabsorption.

Short bowel disease.

Certain bariatric procedures.

Rare inherited primary hyperoxaluria.

9. Enteric Hyperoxaluria

In fat malabsorption, free fatty acids bind intestinal calcium.

This leaves more oxalate unbound and available for absorption.

Therefore:

Fat malabsorption → less calcium available to bind oxalate → increased oxalate absorption → hyperoxaluria → calcium oxalate stones.

10. Low Urinary Citrate

An important additional risk factor is:

Hypocitraturia.

Citrate normally binds calcium and reduces crystal formation.

Low urinary citrate therefore predisposes to:

Calcium stone formation.

It may occur in:

Distal renal tubular acidosis.

Chronic diarrhoea.

Metabolic acidosis.

11. Dehydration

Low urine volume is one of the most important general risk factors for all major stone types.

Dehydration causes urine to become more concentrated.

Therefore:

Low fluid intake or high fluid loss → concentrated urine → increased supersaturation → stone formation.

12. Structural Predisposing Factors

Structural abnormalities of the urinary tract may promote:

Urinary stasis.

Recurrent infection.

Crystal retention.

These factors increase the likelihood of stone formation.

13. Polycystic Kidney Disease

Polycystic kidney disease may be associated with renal stones.

Distorted renal anatomy and altered urinary composition can contribute.

Patients may also develop:

Haematuria.

UTI.

Flank pain.

which can overlap with stone symptoms.

14. Reflux Nephropathy

Reflux nephropathy may predispose indirectly through:

Urinary tract distortion.

Scarring.

Recurrent infection.

However, it is not one of the strongest primary metabolic causes of stone disease.

15. Nephrocalcinosis

Nephrocalcinosis means diffuse calcium deposition within renal tissue.

It is not itself the same as a urinary stone, but conditions causing nephrocalcinosis often also predispose to calculi.

Examples include:

Hyperparathyroidism.

Distal renal tubular acidosis.

Medullary sponge kidney.

16. Medullary Sponge Kidney

Medullary sponge kidney is a congenital disorder involving cystic dilatation of collecting ducts.

It is associated with:

Nephrocalcinosis.

Calcium stones.

Haematuria.

Recurrent urinary tract infection.

17. Struvite Stones

Struvite stones are composed mainly of:

Magnesium ammonium phosphate.

They form in alkaline urine due to infection with:

Urease-producing organisms.

The classic organism is:

Proteus.

Other urease-producing organisms may also contribute.

18. Mechanism of Struvite Stone Formation

Urease breaks down urea into ammonia.

This raises urinary pH.

The sequence is:

Urease-producing bacteria → alkaline urine → magnesium ammonium phosphate precipitation → struvite stone.

19. Staghorn Calculi

Large struvite stones can grow to fill the renal pelvis and calyces.

These are called:

Staghorn calculi.

They are strongly associated with:

Chronic urinary infection.

Recurrent pyelonephritis.

Renal damage.

20. Cystine Stones

Cystine stones occur in:

Cystinuria.

Cystinuria is an inherited defect in renal tubular reabsorption of certain dibasic amino acids.

A useful mnemonic is:

COLA

for:

Cystine.

Ornithine.

Lysine.

Arginine.

21. Cystine Crystal Appearance

Cystine crystals classically appear:

Hexagonal

on urine microscopy.

This is a very high-yield examination finding.

22. Clinical Presentation

Renal calculi may present in several ways.

Possible presentations include:

No symptoms.

Renal colic.

Haematuria.

Urinary infection.

Obstruction.

Hydronephrosis.

Pyelonephritis.

23. Asymptomatic Stones

Some renal stones are discovered:

Incidentally on imaging.

They may remain asymptomatic if they do not obstruct urine flow or irritate the urothelium.

24. Renal Colic

The classic symptom of a ureteric stone is:

Renal colic.

This is severe intermittent pain caused by:

Ureteric obstruction and muscular spasm.

25. Character of Renal Colic

Renal colic typically begins in the:

Flank or loin

and may radiate toward the:

Groin.

Lower abdomen.

Testicle in men.

Labia in women.

The pain often comes in waves and may be extremely severe.

26. Restlessness

Unlike patients with peritonitis, who often lie still, patients with renal colic are typically:

Restless and unable to find a comfortable position.

This is a useful clinical distinction.

27. Haematuria

Stones can traumatise the urothelium and cause:

Microscopic haematuria

or

Visible haematuria.

However, absence of haematuria does not completely exclude a stone.

28. Proteinuria

The original list includes:

Proteinuria.

Mild proteinuria can occur, but it is not one of the most characteristic features of uncomplicated renal calculi.

Prominent proteinuria should prompt consideration of:

Additional renal pathology.

29. Cystitis and Lower UTI

Stones can predispose to:

Recurrent urinary tract infection

through urinary stasis and persistent bacterial colonisation.

Lower infection may cause:

Dysuria.

Frequency.

Urgency.

Suprapubic discomfort.

30. Pyelonephritis

Stones can predispose to:

Upper urinary tract infection.

A patient may develop:

Fever.

Flank pain.

Systemic illness.

Pyuria.

The combination of infection and obstruction is particularly dangerous.

31. Infected Obstructed Kidney

An:

Obstructed infected urinary system

is a urological emergency.

It can rapidly lead to:

Sepsis.

Renal damage.

Pyonephrosis.

32. Pyonephrosis

Pyonephrosis is infection of an obstructed renal collecting system with accumulation of pus.

This requires:

Urgent drainage

in addition to:

Antibiotic therapy.

33. Urinary Obstruction

A stone lodged in the ureter can obstruct urine flow.

This may produce:

Hydroureter.

Hydronephrosis.

Reduced renal function.

If obstruction is bilateral or occurs in a solitary functioning kidney, it may cause:

Acute kidney injury.

34. Common Sites of Ureteric Obstruction

Stones tend to lodge at naturally narrower points.

Traditional teaching includes:

Pelvi-ureteric junction.

Crossing of the iliac vessels.

Vesicoureteric junction.

The vesicoureteric junction is a particularly common site.

35. Investigation – Urinalysis

Urinalysis may show:

Blood.

Leukocytes.

Nitrites if infection is present.

Crystals.

Urine pH can also give clues to stone type.

36. Urine pH

Acidic urine favours:

Uric acid stones.

Cystine stones.

Alkaline urine favours:

Calcium phosphate stones.

Struvite stones.

37. Imaging

For suspected acute renal colic, the most useful imaging test in many adults is:

Non-contrast CT of the kidneys, ureters and bladder.

This is often called:

CT KUB.

It is highly sensitive for most urinary calculi.

38. Ultrasound

Renal ultrasound can identify:

Hydronephrosis.

Some renal stones.

It avoids radiation and is particularly useful in:

Pregnancy.

Children.

Certain follow-up situations.

39. Plain Abdominal X-Ray

Some stones are radiopaque and may be visible on:

Plain KUB X-ray.

Calcium stones are usually radiopaque.

Uric acid stones are usually:

Radiolucent.

CT, however, detects both.

40. Treatment – Acute Renal Colic

Initial treatment focuses on:

Pain relief.

Assessment for obstruction.

Assessment for infection.

Assessment of renal function.

Hydration appropriate to clinical status.

NSAIDs are often effective analgesics if not contraindicated.

41. Increased Fluid Intake

Long-term prevention often includes:

Increased fluid intake.

The aim is to produce a high urine volume and reduce urinary supersaturation.

A general preventive principle is:

More dilute urine → lower risk of crystal precipitation.

42. Important Correction – Do Not Routinely Restrict Calcium

The original note states:

“Reduced Ca²⁺ intake.”

This is generally not recommended for most patients with calcium stones.

A normal dietary calcium intake is usually preferred.

Too little dietary calcium may actually increase intestinal oxalate absorption and increase calcium oxalate stone risk.

43. Dietary Calcium

The better principle is:

Maintain normal dietary calcium intake.

At the same time, reduce excessive:

Sodium intake.

High sodium intake increases urinary calcium excretion.

Therefore:

High salt → increased calciuria → increased calcium stone risk.

44. Oxalate Reduction

In selected patients with hyperoxaluria or calcium oxalate stones, reducing excessive high-oxalate foods may help.

Examples include:

Spinach.

Rhubarb.

Nuts.

Certain teas.

However, dietary changes should be targeted rather than excessively restrictive.

45. Sodium Restriction

Reducing:

Excess dietary sodium

can lower urinary calcium excretion.

This is particularly useful in patients with:

Hypercalciuria.

46. Animal Protein

Excessive animal protein intake may increase stone risk by:

Increasing uric acid excretion.

Lowering urinary citrate.

Increasing acid load.

Moderation may therefore help recurrent stone formers.

47. Thiazide Diuretics

Thiazide diuretics reduce urinary calcium excretion.

Therefore, they may be useful in selected patients with:

Recurrent calcium stones + persistent hypercalciuria.

Their preventive benefit is greatest when combined with appropriate dietary measures, especially lower sodium intake.

48. Potassium Citrate

An important modern preventive treatment is:

Potassium citrate.

It increases urinary citrate and can also alkalinise urine.

It is useful in selected patients with:

Hypocitraturia.

Recurrent calcium stones.

Uric acid stones.

Cystine stones in some cases.

49. Uric Acid Stones – Treatment

Uric acid stones may often be treated or prevented by:

Urinary alkalinisation.

This is commonly achieved with:

Potassium citrate.

The aim is to raise urinary pH so that uric acid becomes more soluble.

50. Allopurinol

Allopurinol may be useful in selected patients with:

Hyperuricosuria.

Recurrent uric acid stones.

Certain recurrent calcium oxalate stones with high urinary uric acid.

It is not required for every patient with a uric acid stone.

51. Struvite Stones – Treatment

Struvite stones require treatment of:

The underlying urinary infection

and often:

Complete stone removal.

Residual infected stone material can lead to recurrence.

52. Cystine Stones – Treatment

Management may include:

Very high fluid intake.

Urinary alkalinisation.

In refractory cases, additional drugs that bind cystine may be considered.

These patients often require specialist metabolic follow-up.

53. Stone Removal

Intervention is considered when stones:

Do not pass spontaneously.

Cause persistent obstruction.

Cause uncontrolled pain.

Cause infection.

Threaten renal function.

Are large or anatomically unsuitable for conservative management.

54. Extracorporeal Shock-Wave Lithotripsy

Extracorporeal shock-wave lithotripsy, or ESWL, uses external shock waves to fragment stones.

The fragments can then pass through the urinary tract.

Suitability depends on:

Stone size.

Stone location.

Stone composition.

Patient anatomy.

55. Ureteroscopy

Ureteroscopy allows direct endoscopic access to ureteric stones.

The stone can be:

Extracted

or

Fragmented with laser lithotripsy.

56. Percutaneous Nephrolithotomy

Large renal stones, especially:

Staghorn calculi,

may require:

Percutaneous nephrolithotomy, or PCNL.

This involves accessing the kidney through the skin and removing or fragmenting the stone.

57. Emergency Decompression

If there is:

Infection + urinary obstruction,

the priority is not definitive stone fragmentation.

The priority is:

Urgent drainage of the infected obstructed system.

This may be achieved using:

Ureteric stent

or

Percutaneous nephrostomy.

Definitive stone treatment is usually performed after infection has been controlled.

58. Metabolic Evaluation

Patients with recurrent stones may need metabolic investigation.

This can include:

Serum calcium.

Renal function.

Uric acid.

Bicarbonate.

Parathyroid hormone if hypercalcaemia.

Urinary stone analysis.

24-hour urine studies in selected recurrent/high-risk patients.

59. Stone Analysis

Whenever possible, a passed or removed stone should be sent for:

Stone composition analysis.

This can identify:

Calcium oxalate.

Calcium phosphate.

Uric acid.

Struvite.

Cystine.

The result helps guide prevention.

60. Renal Calculi – Note Form

Most common type:

Calcium-containing stones.

Especially calcium oxalate.

Metabolic risk factors:

Hypercalciuria.

Hypercalcaemia, especially primary hyperparathyroidism.

Distal renal tubular acidosis.

Hyperuricosuria.

Hyperoxaluria.

Hypocitraturia.

Structural associations:

Polycystic kidney disease.

Medullary sponge kidney.

Nephrocalcinosis-associated disorders.

Urinary tract abnormalities causing stasis.

General risk factor:

Dehydration / low urine volume.

61. Clinical Features – Note Form

Asymptomatic:

Some stones are incidentally detected.

Renal colic:

Severe loin-to-groin colicky pain.

Patient often restless.

Haematuria:

Microscopic or visible.

UTI:

Stones may predispose to cystitis or pyelonephritis.

Pyonephrosis:

Pus in an obstructed collecting system.

Urological emergency.

Obstruction:

May cause hydronephrosis and renal impairment.

62. Treatment – Note Form

Fluids:

Increase fluid intake for prevention.

Diet:

Maintain normal dietary calcium.

Reduce excessive sodium.

Reduce excessive oxalate in selected patients.

Moderate excessive animal protein.

Underlying cause:

Treat hyperparathyroidism, infection, RTA, metabolic abnormalities, or structural disease where possible.

Hypercalciuria:

Thiazide may be used in selected recurrent calcium stone formers.

Hypocitraturia / uric acid stones:

Potassium citrate may be useful.

Stone removal:

ESWL.

Ureteroscopy.

PCNL.

Choice depends on stone size and location.

Infected obstruction:

Urgent decompression with ureteric stent or nephrostomy.

63. Important Corrections to the Original Notes

The statement:

“Reduce calcium intake”

should generally be corrected.

For most calcium stone formers:

NORMAL DIETARY CALCIUM IS PREFERRED.

Excessive calcium restriction can increase oxalate absorption and may worsen calcium oxalate stone risk.

The original list does not include two very important stone types:

Struvite stones

and

Cystine stones.

Another important metabolic risk factor is:

Hypocitraturia.

The original symptom:

Proteinuria

is possible but not a classic dominant feature of uncomplicated nephrolithiasis.

Marked proteinuria should suggest additional renal disease.

Key Clinical Pattern

Think:

CALCIUM OXALATE = MOST COMMON RENAL STONE.

Important associations:

HYPERCALCIURIA → CALCIUM STONES.

PRIMARY HYPERPARATHYROIDISM → HYPERCALCAEMIA/HYPERCALCIURIA → CALCIUM STONES.

DISTAL RTA → ALKALINE URINE + CALCIUM PHOSPHATE STONES.

PROTEUS → UREASE → ALKALINE URINE → STRUVITE / STAGHORN STONE.

GOUT / ACIDIC URINE → URIC ACID STONES.

CYSTINURIA → HEXAGONAL CYSTINE CRYSTALS.

For acute presentation:

SEVERE LOIN-TO-GROIN COLIC + HAEMATURIA → THINK URETERIC STONE.

And the major emergency is:

STONE + FEVER/SEPSIS + OBSTRUCTION = INFECTED OBSTRUCTED KIDNEY → URGENT DRAINAGE.