A child with macroscopic crystalluria: Answers

The most likely diagnosis is crystalluria secondary to amoxicillin therapy. Amoxicillin was prescribed within the range of the recommended dosage for pneumonia (100 mg/kg/day), but a concomitant acid urine pH and dehydration (the child admitted low fluid intake with, consequently, elevated urinary density) may have played a role in the precipitation of crystals. There were no other predisposing causes, such as urinary tract infection nor structural anomalies of the urinary tract. A differential diagnosis of drug-induced acute tubulointerstitial nephritis (TIN) was very unlikely, considering the preservation of renal function and the absence of rash, arthralgia, and peripheral eosinophilia.

Comparison of the morphology of the crystals with images of crystals appearing in the literature [1, 2] revealed that the crystals found in the urine of our patient differed remarkably from those more commonly seen in urine, such as uric acid and calcium oxalate crystals. These atypical crystals took on the form of ‘needles,’ and when aggregated looked like ‘sheaves of wheat’, ‘broom bush,’ or ‘sea urchin’ [2]. In a compatible setting, this appearance is diagnostic of amoxicillin crystalluria and does not require further investigations [1]. Subsequent spontaneous recovery after removal of the drug confirmed the diagnosis.

The correct management of amoxicillin crystalluria is drug withdrawal and high fluid intake to improve drug solubility in the urine. Once amoxicillin therapy has been stopped, spontaneous and complete recovery occurs rapidly, even in case of renal impairment. Intravenous re-hydration and urine alkalinization are indicated only in more severe and symptomatic cases [1, 2]. In our patient, urinary crystals rapidly disappeared within 24 h after drug discontinuation, without sequelae.