Crystalluria can occur in animals with normal urinary tracts without any clinical signs of lower urinary tract disease. Why?
Because the crystals are usually eliminated in the urine before they reach a large enough size to cause clinical problems.
Even in animals that do have evidence of lower urinary tract disease, crystalluria itself does not contribute to the development
of clinical signs. Although identification of crystals in otherwise healthy animals does not in itself necessitate therapy,
it may indicate the need to perform further diagnostic tests to detect any underlying diseases or to monitor the patient for
evidence of larger urinary stones (uroliths).
Figure 1: Cystine in the urine sediment of a dog (25X original magnification; unstained).
Causes of urine crystals and their relation to urolithiasis
Just because crystals alone do not cause clinical signs does not mean that they are a normal finding. Mineral crystals often
form in the urine because there is a favorable environment. Many common crystals require that the urine be more acidic (e.g.,
calcium oxalate, cystine, xanthine), and others form in neutral or alkaline urine (e.g., struvite, calcium phosphate). Diet
can influence urine pH, and some breeds can be predisposed to certain urine crystals. Additionally, crystalluria may be indicative
of underlying subclinical disease in other body systems. For example, ammonium acid urate crystals are found in animals with
liver shunts. Cystine crystalluria (Figures 1-4) results from a disorder characterized by an impaired ability to reabsorb
the amino acid cystine. So identifying the type of crystals found in urine is important as it points to the likely cause of
the crystalluria and perhaps to an underlying problem.
Figure 2: Six-sided cystine crystals (center) and calcium oxalate dihydrate (dipyramidal) crystals in the urine sediment of
a dog (25X original magnification; unstained).
Urinary stone formation, or urolithiasis, is a concern in both dogs and cats and can result in serious illness. Crystals found
in the urinary tract do not necessarily lead to the formation of uroliths, which are large stones that can cause urinary tract
irritation and obstruction. However, crystalluria does represent a risk factor for urolith formation. In addition to crystalluria,
other abnormalities must be present before uroliths develop. Struvite crystals are a common finding in both dog and cat urine
but generally do not form uroliths unless there is also a bacterial infection.
Figure 3: Scanning electron micrograph of cystine crystals in the urine sediment of a dog.
Once crystals of one mineral type form in urine, the conditions can be improved for the formation of other types. In other
words, a crystal of one mineral type serves as a risk factor for formation of crystals of other types and provides one explanation
of why large, or macroscopic, uroliths may contain more than one mineral.
Figure 4: Aggregate of cystine crystals and red blood cells in the urine sediment of a 2-year-old domestic shorthaired cat
with cystine urocystoliths (40X original magnification; unstained).
Combinations of minerals may be unevenly mixed throughout the urolith, or they may be deposited in layers (laminations). The
core of some uroliths is composed of one mineral type (e.g., calcium oxalate), whereas outer layers are composed of different
mineral types (e.g., struvite; Figures 5 & 6).
Figure 5: Radiograph of a compound urolith in an adult neutered male Yorkshire terrier. The center contains calcium oxalate
(white arrow), and the shell contains struvite (green arrow).
In addition, some medications may also precipitate as crystals within the urinary tract and become incorporated within uroliths.
Sulfadiazine is a notable example. Xanthine, resulting from excessive allopurinol dosing, is another (Figure 7).
Figure 6: Compound urolith removed from the dog described in Figure 5. The center contains calcium oxalate (white arrow),
and the shell contains struvite (green arrow).
The key to interpreting a finding of crystalluria is to remember that the clinical significance needs to be evaluated along
with all relevant clinical findings. Only then will the importance of the crystalluria be crystal clear.
Figure 7: Xanthine crystals in the urine sediment of a dog being treated with allopurinol. These crystals cannot be distinguished
from ammonium urate crystals by light microscopy.