Friday, June 26, 2009

Urine Crystals: Pattern Recognition

One of the easier aspects to taking an examination like the boards is pattern recognition: there are certain images or associations that should be immediate triggers for a particular diagnosis. One good example of this is a knowledge of what different types of urine crystals (causing nephrolithiasis) look like under the microscope. A quick review with examples I swiped from the Internet:

1) Ca-oxalate stones. Crystals of calcium oxalate can take two basic forms. The dihydrate form looks like little square envelopes:

The monohydrate form in contrast looks like elongated rods or sometimes dumbells. Monohydrate crystals are the predominant form of oxalate crystal seen with ethylene glycol poisoning.
Uric acid crystals in the urine are more tricky because they are pleimorphic--they can have many shapes. Some look almost football-shaped; other look more like crystal aggregates. They generally only form in an acidic urine.

Struvite stones are easy--they look like "coffin lids" and are usually found alkaline urine often with evidence of a UTI.
Though uncommon, cystine stones (seen in the genetic condition cystinosis) are hexagonal-shaped crystals. This is pathognomonic.
Finally, different medications can form urine crystals which may have a characteristic shape. One website with a lot of good images documenting many of these drug crystals can be found here.

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