Matrix Stone

Matrix stones are soft, friable calculi made mainly of organic protein matrix with little mineral. They are rare, and are found far more often in patients with chronic urinary infection.

They may cause obstruction, recurrent infection and loss of renal function, and are frequently mistaken for tumour or clot.

Pathophysiology. The matrix is a mucoprotein with carbohydrate and a minor mineral component, usually under 10% of dry weight. Urease producing bacteria, especially Proteus mirabilis, create alkaline, protein rich urine with inflammatory debris that aggregates.

Risk factors: recurrent infection, chronic kidney disease, haemodialysis and peritoneal dialysis, kidney transplantation, urinary diversion, indwelling catheters, previous urological surgery and urinary stasis.

Matrix may also form the core of other infection related stones.

Flank pain, recurrent or persistent urinary infection with Proteus, haematuria, passage of soft material, and lower urinary tract symptoms with bladder involvement. Renal failure or anuria can occur when the ureter or a solitary kidney is obstructed. Fever with obstruction is an emergency.

• Urinalysis, culture and sensitivity. Urine pH is often 7 to 8.
• Ultrasound: a poorly shadowing, echogenic filling defect.
• Non-contrast CT: low attenuation, usually under about 100 HU, which is much lower than calcium stones. Contrast CT may show a rim of enhancement.
• Plain film: normal or only faint opacity.
• Retrograde or antegrade pyelography: filling defect.
• Ureteroscopy or percutaneous endoscopy confirms soft grey white material. Send for stone analysis and histology (matrix stains with periodic acid-Schiff), and culture.
• Creatinine and eGFR, and split function by scintigraphy before surgery.

Differential diagnosis.

• Urothelial tumour
• Blood clot
• Fungal ball
• Sloughed papilla
• Uric acid or xanthine stone
• Struvite stone
• Pyonephrosis

• Obstruction with infection: urgent decompression by stent or nephrostomy, blood and urine cultures, and ceftriaxone 2 g intravenously once daily, adjusted to culture.
• Pre-operative antibiotics directed by culture for 1 to 2 weeks, a single prophylactic dose at induction (cefazolin 2 g intravenously, 3 g if over 120 kg), and continued treatment for 2 to 4 weeks after surgery.
• Removal: percutaneous nephrolithotomy for renal matrix stones, and ureteroscopy with basket or forceps for ureteric stones. The stone is soft, so laser and shock waves are ineffective, and removal needs forceps, irrigation and suction. Complete clearance is needed.
• Irrigation of residual matrix through a nephrostomy with 0.25% acetic acid has been reported in small series, with limited evidence.
• Treat the cause: remove or change catheters, correct stasis and reflux, and optimise dialysis care.
• Long term suppression for patients with repeated infection after clearance: nitrofurantoin 50 to 100 mg orally at night when eGFR is over 45, reviewed every 3 to 6 months.
• Nephrectomy for a non-functioning infected kidney.

Monitoring. Urine culture monthly for 3 months, then every 3 months for 2 years. Ultrasound or low dose CT at 3 months, then every 6 to 12 months, since recurrence is common.

Referral and escalation. Refer to urology and nephrology. Urgent referral for infected obstruction, renal impairment or a solitary kidney.