Nephrolithiasis and nephrocalcinosis — formative SAQs

SAQ 1 (10)

A 9-year-old boy presents to the emergency department with acute left-sided colicky loin pain radiating to the groin, nausea and visible haematuria. He is afebrile and haemodynamically stable. A renal ultrasound shows a 6 mm stone at the left vesicoureteric junction with mild hydronephrosis but no perinephric collection. [1][10]

1. Define nephrolithiasis and explain how it differs from nephrocalcinosis, using the imaging finding to justify your classification. (2) [5][1]
2. Outline the immediate and definitive management of this uncomplicated distal ureteric stone, naming the drugs, doses and the role of surgery, and state the specific red flag that would change the tempo. (5) [10][9]
3. Describe the metabolic workup you would arrange after the stone is passed or removed, and explain why a first-episode evaluation is justified in a child. (3) [3][11]

Model answer

Definition and distinction. Nephrolithiasis is a stone in the collecting-system lumen — the renal pelvis, calyces, ureter or bladder — whereas nephrocalcinosis is calcium deposited in the renal parenchyma itself, usually the medulla. This child has nephrolithiasis: the ultrasound shows a discrete 6 mm calculus at the vesicoureteric junction (a luminal stone) with hydronephrosis from obstruction, not echogenic parenchymal deposits. The lumen-versus-tissue distinction matters because a stone is managed by passage or removal while nephrocalcinosis directs the search toward inherited tubulopathies and metabolic disease. [5][1]

Immediate and definitive management. Immediate management is analgesia with a non-steroidal anti-inflammatory drug (diclofenac or ibuprofen) first-line, adding intravenous paracetamol or an opioid for breakthrough pain, an antiemetic, and hydration to correct dehydration rather than to flush the stone. Because the stone is a 6 mm distal ureteric stone in an afebrile, stable child with controlled pain, the definitive step is medical expulsive therapy with tamsulosin 0.4 mg once daily (off-label, for up to four weeks with planned review), supported by the Aldaqadossi trial showing improved passage of distal ureteric stones under 1 cm in children. If the stone fails to pass within four weeks, or if obstruction worsens, ureteroscopy with holmium laser lithotripsy is the surgical fallback; ESWL suits more proximal stones and PCNL is for large renal stones. The specific red flag that changes tempo is fever with obstruction — an infected obstructed system — which mandates urgent urological decompression, cultures and intravenous antibiotics. [10][9]

Metabolic workup and rationale. After stone passage or removal, arrange the metabolic workup: serum calcium, phosphate, magnesium, bicarbonate (venous gas), creatinine, urate and parathyroid hormone; a urine culture and urinalysis for pH and crystals; a spot urine calcium-to-creatinine ratio with an age-specific threshold (the upper limit falls from about 0.8 mg/mg in infants to below 0.3 mg/mg in older children); a 24-hour urine confirming hypercalciuria above 4 mg per kilogram per day plus oxalate, citrate, uric acid and cystine; and stone analysis. A first-episode evaluation is justified because a metabolic or genetic cause underlies the majority of paediatric stones, monogenic disease explains about a fifth, and without prevention roughly half of children recur within three to five years. [3][11]

SAQ 2 (10)

A 10-month-old infant born at 27 weeks gestation is found to have bilateral medullary nephrocalcinosis on a renal ultrasound performed for a urinary tract infection. She has failure to thrive, and a venous gas shows a metabolic acidosis with a urine pH of 6.8. She is no longer on furosemide. [5][6]

1. What is the most likely diagnosis, and how do you distinguish distal renal tubular acidosis from the other principal causes of medullary nephrocalcinosis in an infant? (4) [6][5]
2. Give the immediate and long-term management of her condition, naming the drug class, its purpose and its metabolic target. (4) [6][5]
3. Outline the surveillance plan and explain why the diagnosis is important for her growth and kidney function. (2) [6]

Model answer

Diagnosis and distinction. The most likely diagnosis is distal renal tubular acidosis. The combination of medullary nephrocalcinosis, failure to thrive, a metabolic acidosis on the venous gas, and a paradoxically alkaline urine (pH of 6.8, when the kidney should be acidifying to below 5.5 in the face of systemic acidosis) is the signature of a distal acidification defect. The principal alternatives are prematurity and furosemide-related hypercalciuria — but furosemide has been stopped, making iatrogenic disease less likely — and the inherited tubulopathies such as Dent disease (hypercalciuria with low-molecular-weight proteinuria, X-linked in boys) and primary hyperoxaluria (early-onset, progressive nephrocalcinosis with progressive kidney disease). Dent disease is far less likely in a girl, and primary hyperoxaluria would show marked hyperoxaluria on the metabolic workup. The urine pH greater than 5.5 despite acidosis is the tie-breaker for distal renal tubular acidosis. [6][5]

Management. The immediate and long-term management is alkali replacement — potassium citrate or sodium-potassium citrate — titrated to correct the metabolic acidosis and to suppress the hypercalciuria. The metabolic target is a normal serum bicarbonate, which halts bone calcium release, and a urine citrate high enough to inhibit calcium crystallisation; the dose is weight-based and adjusted to the serum bicarbonate and urinary citrate. Adequate alkali corrects the acidosis, stops the hypercalciuria and hypocitraturia that drive the nephrocalcinosis, and protects growth. Potassium citrate is preferred over sodium because sodium can worsen hypercalciuria. [6][5]

Surveillance and importance. Surveillance includes serial growth (the acidosis causes failure to thrive and rickets), serum bicarbonate and electrolytes, urinary calcium-to-creatinine ratio and citrate, renal ultrasound to track the nephrocalcinosis, and screening for rickets and nephrocalcinosis complications. The diagnosis matters because untreated distal renal tubular acidosis causes progressive growth failure, rickets, worsening nephrocalcinosis and, over time, chronic kidney disease — all preventable with sustained alkali. Genetic testing to define the inherited form guides counselling and prognosis. [6]