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Urolithiasis in Small Animals
2
1
Alice Defarges, DVM, DACVIM (SAIM) , Michelle Evason, DVM, DACVIM (SAIM) ,
3
Marilyn Dunn, DVM, DACVIM (SAIM) , and Allyson Berent, DVM, DACVIM (SAIM) 4
1 Ontario Veterinary College, University of Guelph, Guelph, ON, Canada
2 Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PE, Canada
3 Département des sciences cliniques, Centre Hospitalier Universitaire Vétérinaire (CHUV), Université de Montréal, Saint‐Hyacinthe, QC, Canada
4 The Animal Medical Center, New York, USA

Overview numerous patient‐related factors, urine crystals may
begin to aggregate into a nucleus. The crystal nucleus,
Definition commonly referred to as the nidus, can then develop into
a urolith, aka the “stone.”
Urine is a complex solution that contains multiple crystal Additional theories regarding urolith development
components. The ionic elements of crystals in urine are have been postulated. One of these, homogenous nuclea­
referred to as crystalloids, which can be organic or inor­ tion, refers to the spontaneous precipitation of crystals
ganic. Uroliths are organized assemblages of crystalloids, from a pure solution. This process is considered uncom­
combined with a small amount of organic matrix, located mon in urine, and urine crystal formation is believed to
within the urinary tract. occur through heterogenous nucleation. This theory is
similar to the matrix‐nucleation theory, whereby another
Urolith Formation substance (e.g., bacteria, inflammatory cells or debris)
acts as a nidus for crystal precipitation. Once again, after
Formation of urinary calculi (uroliths) has been hypoth­ the crystal nidus has formed, it may develop into a uro­
esized to occur through multiple mechanisms or pro­ lith dependent on the following: degree and duration of
cesses. Three of the more common of these theories are: urine supersaturation, and sufficient time for the nidus
the precipitation‐crystallization theory: supersaturation to remain in the urinary tract without being flushed out.
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of urine with crystalloids is followed by growth of calculi Factors that predispose to urine stasis also play an
the matrix‐nucleation theory: an abnormal substance important role in urolithiasis formation. Consequently,
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in the urine is responsible for calculi development factors that induce frequent and regular voiding of urine
the crystallization‐inhibition theory: the absence of a crit­ may be protective and aid in prevention of urolith devel­
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ical inhibitor (or the presence of a promoter) of crystal opment and progression.
formation is the primary factor in calculi development.
All three of the above theories (individually or in combi­ History and Physical Examination
nation) likely contribute to development of individual
uroliths.
Urine is commonly supersaturated with crystalloids, History
and this is a prerequisite for urolith formation (Figure Dogs and cats with uroliths may present for a variety of
123.1). Supersaturation of urine with crystalloids depends animal owner concerns. Most commonly, dogs and cats
on urine crystal volume, renal crystal excretion, urine present with lower urinary tract signs (i.e., pollakiuria,
concentration, urine pH, and the presence of urolith hematuria, stranguria) related to the irritating presence of
promoters or lack of inhibitors. Following establish­ uroliths in the bladder or urethra. In patients with upper
ment of these supersaturation criteria, and dependent on urinary tract uroliths (i.e., nephroliths, ureteroliths), signs

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical