Clinical presentation of feline urolithiasis and associated diagnostic testing

Urolithiasis refers to the identification of urinary calculi which can be located in the upper (nephroliths and ureteroliths) or the lower (cystoliths and urethroliths) urinary tract. The presence of nephroliths must be differentiated from mineralisation of renal parenchyma (nephrocalcinosis). It is currently unknown whether medullary mineralisation in cats is on a continuum with the future development of nephrolithiasis. Urolithiasis is a common clinical presentation, but differences exist in terms of calculi composition not only by location of the stones (upper versus lower) but also based on breed, sex, age and underlying disease conditions. A full evaluation of potential risk factors for the formation of uroliths together with diagnostic investigation to try and establish the likely stone type and burden is important, in order to optimise not only the initial management but also to facilitate prevention strategies to reduce the risk of recurrence.

Upper urinary tract uroliths can be considered either obstructive or non-obstructive, and the degree of obstruction may be partial or complete; a differentiation that can be challenging to determine, and can occur at any level (for example, both in the renal pelvis and the ureter). Ureterolithiasis is the most common cause of upper urinary tract obstruction (82%; Berent et al, 2018) with the most common urolith type in the upper urinary tract of cats being calcium oxalate (>98%; Kyles et al, 2005). Other types of urolith are possible, but much less common (for example xanthine, urate, struvite, apatite, blood concretions) (Kyles et al, 2005). One of the difficulties in assessing the composition of uroliths from the upper urinary tract is that it is rare for them to be submitted for analysis, at least partly because management strategies (such as theplacement of subcutaneous ureteral bypass devices) do not require removal. Whilst all upper urinary tract uroliths will originate in the kidney, not all will migrate into the ureter. One study suggests that it is more common to localise ureteroliths to the proximal ureter than either the mid or distal ureter or ureterovesicular junction, and logically larger uroliths are more likely to cause obstruction proximally (Nesser et al, 2018).