Secondary
hyperoxaluria is due either to increased intestinal
oxalate absorption or to excessive dietary
oxalate intake. Certain
intestinal diseases like
short bowel syndrome, chronic
inflammatory bowel disease or
cystic fibrosis and other
malabsorption syndromes are known to increase the risk of secondary
hyperoxaluria. Although the urinary
oxalate excretion is usually lower than in
primary hyperoxaluria, it may still lead to significant morbidity by recurrent
urolithiasis or progressive
nephrocalcinosis. A clear distinction between primary and secondary hyperoxalurias is important. As correct classification may be difficult, appropriate diagnostic tools are needed to delineate the metabolic background as a basis for optimal treatment. We developed an individual approach for the evaluation of patients with suspected secondary
hyperoxaluria. First, 24 h urines are examined repeatedly for lithogenic (e.g.
calcium, oxalate,
uric acid) and stone-inhibitory (e.g.
citrate,
magnesium) substances, and the patients are asked to fill in a dietary survey form. Urinary saturation is calculated using the computer based program EQUIL2, and the BONN-Risk-index is determined. The measurement of plasma
oxalate and of urinary
glycolate helps to distinguish between primary and secondary hyperoxalurias. If secondary
hyperoxaluria is suspected, the stool is examined for Oxalobacter formigenes, an intestinal
oxalate degrading bacterium, as lack or absence may lead to increased intestinal
oxalate absorption. The last diagnostic step is to study the intestinal
oxalate absorption using [13C2]
oxalate. Depending on the results, various therapeutic options are available: 1) a diet low in
oxalate, but normal or high in
calcium, 2) a high fluid intake (>1.5 L/m2/d), 3) medications to increase the urinary solubility, 4) specific therapeutic measures in patients with
malabsorption syndromes, depending on the underlying pathology, and 5) intestinal recolonization of Oxalobacter formigenes or the treatment with other
oxalate degrading bacteria.