Matrix calculi in systemic lupus erythematosus complicated by distal renal tubular acidosis in a solitary kidney

Nephrolithiasis is characterized by a composite pathology involving low urine volume, excess solute secretion, urinary pH changes, and lack of crystal formation inhibitors.1 Distal renal tubular acidosis predisposes patients to stone formation.1 Matrix calculi are notable for their predominant organic composition of protein matrix and their predisposition for females and urinary tract infection.2 Glomerular diseases with proteinuria are rare causes of matrix calculi, which are dependent on the quantity of proteinuria.3 Here, we present a rare case of matrix calculi in a patient with SLE endowed with a solitary kidney and the factors predisposing to the formation of matrix calculi in a patient with lupus. A 39-year-old female patient with systemic lupus erythematosus (SLE), treated with prednisolone for 7 years, presented to the emergency department with acute-onset right flank pain and hematuria. On examination, she had tenderness in the right flank, without any other systemic abnormalities. Laboratory tests revealed serum potassium 2.6 mmol/L, serum creatinine 1.3 mg/dL, serum sodium 130 mmol/L, serum chloride 112 mmol/L, serum bicarbonate 10 mmol/L, serum protein 6.9 g/dL, and serum albumin 3.5 g/dL. Arterial blood gas analysis revealed normal anion-gap acidosis, with an anion gap of 10.6. She had low C3 levels (64 mg/dL, normal range: 80–160 mg/dL) and normal C4 levels (16 mg/dL, normal range: 15–40 mg/dL) with a positive IgG anti-dsDNA level (25 U/mL). The patient's urinary pH was 7, which is alkaline, and the urine protein level was 2+, with plenty of red blood cells and 2–3 pus cells/high power field. The urinary electrolytes revealed urinary spot sodium 54 mmol/L, urinary spot potassium 22 mmol/L, and urinary spot chloride 48 mmol/L with a positive urinary anion gap of 28, indicative of distal renal tubular acidosis. The urine culture was sterile. She had a Systemic Lupus Erythematosus Disease Activity Index 2000 score of 12 points. Ultrasonography and abdominal computed tomography revealed a solitary right kidney with multiple pelvicalyceal calculi, and abdominal computed tomography also revealed obstructive calculi at the vesicoureteral junction (Figure 1A). She had no features of secondary Sjogren's syndrome and had no antibodies against SSA and SSB. She underwent emergency right ureteroscopic lithotripsy with double-J stenting and percutaneous nephrolithotomy with the retrieval of multiple pliable soft calculi suggestive of matrix calculi (Figure 1B). Twenty-four-hour urinary electrolyte analysis revealed urine calcium of 180 mg/day, urinary oxalate of 34 mg/day, urinary uric acid of 540 mg/day, and urinary citrate levels of 260 mg/day, suggestive of hypocitraturia. Twenty-four-hour urine protein estimation was 2170 mg/day a week after surgery. Global stone composition analysis revealed an 83% protein matrix and 17% calcium oxalate monohydrate composition. The patient was treated with sodium bicarbonate, potassium citrate, telmisartan at the maximum tolerated doses, and mycophenolate mofetil. The patient did not consent for renal biopsy of the solitary kidney. On follow-up at 3 months, she had a creatinine of 0.7 mg/dL, with normal serum potassium and bicarbonate, urine spot protein creatinine ratio of 0.4, and normal complement levels without recurrence of new stones. Distal renal tubular acidosis (dRTA) is clinically characterized by a defect in normal urinary acidification in the background of normal renal function, with SLE contributing approximately 12% of dRTA.4 Renal tubular acidosis is predominantly observed in tubulointerstitial involvement in SLE, and antibodies to carbonic anhydrase, high disease activity score, and tubulointerstitial inflammation have been implicated in the pathogenesis of dRTA.5 SLE-associated dRTA is a proton secretion defect that leads to persistent normal anion gap acidosis, thereby contributing to nephrolithiasis by inducing hypocitraturia as a compensatory phenomenon.4 Hypocitraturia, alkaline urine, and structural abnormality of the urinary tract leading to stagnation of urine would have contributed to nephrolithiasis in our case.1, 4, 5 Matrix calculi are soft, pliable, and amorphous calculi which conform to the structure of pelvicalyceal system and present as acute flank pain and recurrent urinary tract infection.2, 6 They are observed in urinary tract infections caused by Escherichia coli and Proteus mirabilis, and their visibility on ultrasound and abdominal radiographs is influenced by their mineral content.6 Matrix calculi associated with glomerular diseases are rare and are characterized by their laminated texture, structural anisotropy under polarized light, and peculiar microfibrils under transmission electron microscopy.3 Lupus nephritis, an inflammatory glomerular disorder, is postulated to predispose to matrix calculi since the medical literature provides evidence of inflammatory proteins S100-A8 and S100-A9 and neutrophil defensin 1 as major components of these matrix variants of renal calculi.7 Percutaneous nephrolithotomy (PCNL) or retrograde treatment of matrix calculi is influenced by stone location, stone burden, and PCNL scores over the retrograde approach for near-complete clearance of matrix stones.6 Our case was managed by PCNL, immunosuppressants for active SLE, angiotensin receptor blockers for proteinuria reduction, and correction of distal renal tubular acidosis with potassium citrate supplementation. This case is unique because matrix calculi occurring in the clinical context of sterile urine and SLE are rarely reported in the medical literature. This highlights the pathogenesis of matrix calculi formation in secondary glomerular diseases with subnephrotic proteinuria in the background of distal renal tubular acidosis and structurally abnormal kidneys. All authors have contributed to the conceptualization, visualization, drafting, investigation, clinical management, revision, and approval of the submitted manuscript. The authors acknowledge the wholehearted encouragement of Dr. S.Illango, Consultant nephrologist for this case report. The authors declare no conflict of interest. This case report was drafted after written informed consent of the patient in conformity with CARE clinical case reporting guidelines.