This case was selected to demonstrate the appearance of sevelamer crystals in the lower gastrointestinal tract. Sevelamer is an anion-exchange resin used in the treatment of hyperphosphatemia in patients with chronic renal failure. It is known to cause mucosal abnormalities at various levels of the GI tract. Patients may be asymptomatic or may present with dysphagia, dyspepsia, nausea/vomiting, diarrhea, and/or rectal bleeding. The endoscopic appearance may be normal or may show inflammatory changes (erythema, ulceration). Histologic examination often demonstrates chronic mucosal injury with architectural distortion and Paneth cell metaplasia, active inflammation, inflammatory polyp formation, ischaemic changes, or necrosis, as well as deposition of characteristic angular crystals with a "fish scale" appearance. The crystals may be variable in colour on H&E, but they are commonly rusty yellow with pink accentuations. This is in contrast to the deep purple/magenta of Kayexalate crystals or the orange crystals seen with cholestyramine use (the latter also lacks the fish scale appearance).

In this case, the biopsies show crystals consistent with sevelamer, and review of patient records indicated that the patient was taking this medication. Histologically, the mucosa shows near-normal architecture and mildly increased lamina propria cellularity, with mild focal active inflammation. There is some attenuation and sloughing of the superficial epithelium but no withered crypts or increase in intraepithelial lymphocytes; however, the subepithelial collagen band does appear to be focally thickened. Given that the biopsies show sevelamer deposition as well as some, but not all, the diagnostic features of collagenous colitis, the main considerations on the differential diagnosis remain medication effects versus collagenous colitis. However, other etiologies such as a resolving infectious colitis also need to be considered and excluded clinically.

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1) Swanson BJ., et al. Am J Surg Pathol. 2013 Nov;37(11):1686-93