Calcium oxalate monohydrate
Calcium oxalate monohydrate crystals vary in size and may have a spindle, oval, or dumbbell shape (for examples, see the two unlabeled crystals in the lower left corner of the image to the right). These forms of calcium oxalate monohydrate indicate supersaturation of the urine with calcium and oxalate and, along with calcium oxalate dihydrate crystals ("square envelopes"), can be seen in the urine of animals that have no urologic problems or those suffering from oxalate urolithiasis, hypercalciuric or hyperoxaluric disorders, or rarely ethylene glycol toxicosis. They are infrequent in the urine of normal dogs and cats but can be seen commonly in the urine from healthy horses.
A particular form of calcium oxalate monohydrate are flat, elongated, six-sided crystals ("picket fences") which are the larger crystals in the image above (which represents urine from a dog with ethylene glycol toxicosis). The arrow in the photo indicates a "daughter" crystal forming on the face of a larger underlying crystal. These "picket fence" forms of calcium oxalate monohydrate are frequently associated with ethylene glycol intoxication in dogs and cats, but are not always observed in the urine of affected animals (i.e. not 100% sensitive). They can also be seen in the urine of animals with hypercalciuria from other causes, e.g. paraneoplastic hypercalcemia with lymphoma.
Another rare form of calcium oxalate monohydrate are the "hempseed" variant or "orzo" (as in the pasta shape). The image on the right is from the urine of a dog with many of these crystals. The dog did not have ethylene glycol poisoning and the crystals are assumed to be secondary to supersaturation of the urine with calcium and oxalates, which precipitated in the acidic urine.
Ammonium urate (or biurate) crystals generally appear as brown or yellow-brown spherical bodies with irregular protrusions ("thorn-apples"). In some urine samples, they do not have irregular protrusions but have smooth borders and can resemble calcium carbonate (although these do not occur in the urine from dogs and cats). Though possible in urine of any pH, their formation is favored in neutral to alkaline urine. They are frequently seen with amorphous urates.
These crystals are fairly common in dogs and cats with congenital or acquired portal vascular anomalies, with or without concomitant ammonium urate uroliths. They can be seen in urine from normal Dalmatians and Bulldogs, both of which .are predisposed to urate urolithiasis. They are rarely, if ever, seen in urine from normal cats or dogs of other breeds and have not been reported in large animals.
Cystine
Cystine crystals are flat colorless plates and have a characteristic hexagonal shape with equal or unequal sides. They often aggregate in layers. Their formation is favored in acidic urine.
Cystine crystalluria or urolithiasis is an indication of cystinuria, which is an inborn error of metabolism involving defective renal tubular reabsorption of certain amino acids including cystine. Sex-linked inheritance is suspected since male dogs are almost exclusively affected. Many breeds, as well as mongrels, have been reported affected . Renal function otherwise appears to be normal and, aside from a tendency to form uroliths, the defect is without serious consequence.
Drug crystals
Many drugs excreted in the urine have the potential to form crystals. Hence, a review of the patients drug history is prudent when faced with unidentified urine crystals.
Most common among these are the sulfa drugs. Both panels on the right are from patients receiving trimethoprim-sulfadiazine. The differing appearance may relate to variation in drug concentration, urine pH, and other factors. The upper panel is from a feline case, the lower from a horse. The inset in the lower panel shows the crystals as they appeared when polarized.
Other examples include radiopaque contrast agents (Hypaque, Renografin) and ampicillin which may precipitate in acid urine as fine needle-like crystals (not shown).
Other crystals
We frequently see several different types of crystals that are of uncertain origin (see urine sediment atlas for examples). Whenever an "unknown" crystal is encountered, we usually perform solubility studies (chemical, i.e. hydrochloric acid, glacial acetic acid and sodium hydroxide, or heat) to help identify the crystals. Some crystals, such as uric acid, calcium phosphate and sulfa drug-related crystals have specific solubility characteristics which, with their shape and pH of the urine (and of course, clinical history), can aid in their identification. Sometimes, despite doing these solubility studies, the identity of the crystal remains a mystery. The clinical relevance of such crystals is questionable, but drug or chemical toxicity should always be considered in an animal presenting with clinical signs of renal failure and unidentified crystals in the urine. The crystals shown in the image above were seen in the urine of a cat with acute renal failure due to melamine toxicosis. These crystals were highly characteristic of melamine and facilitated diagnosis in affected animals.
Source : © Cornell University
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