Uroliths account for approximately 20 percent of feline lower urinary tract disease (FLUTD) (Gerber et al., 2005), with calcium oxalate uroliths accounting for just over 40 percent of uroliths analysed (Osborne et al., 2009). Calcium oxalate stones are also the most common type of stone in the upper urinary tract, comprising over 90 percent of nephroliths and ureteroliths in cats (Lulich et al., 2016). They tend to affect cats over the age of seven years, and certain breeds including British Shorthair, Himalayan and Persian cats are over-represented. Nutritional factors can both influence the likelihood of formation of calcium oxalate stones and help reduce the risk of recurrence of uroliths.
|• Increased dietary intake of oxalates (eg in cereals) that are available for uptake by the gastrointestinal tract|
• Increased dietary intake of the amino acid hydroxyproline (high in collagen tissue), which is a substrate for oxalate
• Dietary calcium levels: calcium restriction was previously recommended in order to decrease the amount of urinary calcium. More recent research has demonstrated increased calcium may complex with oxalate in the intestine and decrease absorption and urinary excretion
• Increased dietary levels of vitamin C, glycine and glyoxylate may all increase production and excretion of oxalate
• Deficiencies in vitamin B6, a co-factor in oxalate metabolism, have been shown to increase urine oxalate in kittens
Factors involved in the pathogenesis of calcium oxalate urolithiasis in cats are not fully understood but involve urine supersaturation with calcium and oxalate. Some dietary factors may contribute to the formation of these uroliths (Box 1). Additional factors include decreased water intake and hypercalcaemia (often idiopathic in nature in cats).
Calcium oxalate uroliths are not amenable to dietary dissolution. When identified, calcium oxalate uroliths should be removed by the most minimally invasive procedure possible. This may include voiding urohydropropulsion, basket retrieval by cystoscopy (in female cats), intracorporeal laser lithotripsy, percutaneous cystolithotomy or other extraction procedures that do not involve surgical intervention. Such procedures are associated with shorter hospitalisation times, fewer adverse effects, fewer residual stones because of improved visualisation and possibly lower stone recurrence rates compared to surgical cystotomy (Lulich et al., 2016). Unfortunately, even with experienced surgeons there is a failure to remove all uroliths in 20 percent of cats (Lulich et al., 1993), so diligence during surgery and imaging following surgery to confirm complete urolith removal are recommended.
Decreasing the risk of recurrence
Decreasing the risk of recurrence of calcium oxalate uroliths can be challenging as the exact mechanisms underlying their formation are still not understood. However, the key recommendations are to decrease urine concentration, avoid urine acidification (pH less than 6.5) and avoid diets with excessive animal protein content (more than 10g/100kcal) (Lulich et al., 2016), particularly those high in collagen.
Decreasing the risk of recurrence of calcium oxalate uroliths can be challenging as the exact mechanisms underlying their formation are still not understood
Monitoring calcium levels
Evaluation of total and ionised serum calcium levels should be performed in all cats to rule out hypercalcaemia, an intrinsic risk factor. In one study, when serum ionised calcium was measured in 194 cats with normal total calcium, 25 percent were found to be hypercalcaemic (Schenck and Chew, 2010), emphasising the importance of measuring ionised calcium. Where hypercalcaemia is identified it should be managed appropriately. This can be challenging in cats with idiopathic hypercalcaemia since no single treatment has been demonstrated to be effective. Management may include glucocorticoids, bisphosphonate administration or dietary modification using a high-fibre diet with or without potassium citrate administration (Lulich et al., 2016).
Dietary risk factors such as those mentioned above should be minimised to help decrease hypercalciuria. Adapting a maintenance diet to decrease the recurrence of calcium oxalate uroliths is difficult, so the use of therapeutic commercial diets is recommended, with several specifically formulated commercial diets available.
Adapting a maintenance diet to decrease the recurrence of calcium oxalate uroliths is difficult, so the use of therapeutic commercial diets is recommended
They commonly incorporate increased levels of vitamin B6 and increased citrate, which forms soluble complexes with calcium to inhibit calcium oxalate formation. They also typically contain restricted levels of vitamin C, a precursor for calcium oxalate, and controlled levels of calcium. In one study of 10 cats with calcium oxalate urolithiasis, feeding a urolith prevention diet also increased the urine concentration of glycosaminoglycans, which are glycoprotein inhibitors of growth and aggregation of calcium oxalate crystals (Lulich et al., 2012).
A target urinary pH of greater than 6.5 should be aimed for (Lulich et al., 2016). Where diet alone does not achieve this, potassium citrate is recommended as an alkalinising salt; it also helps to chelate calcium ions.
Urinary pH should be measured with a pH meter to maximise accuracy and should be evaluated at least two hours after a meal to avoid interference due to the post-prandial alkaline tide. Where possible, the diet should be fed in multiple small meals daily to minimise urinary pH fluctuations.
Maximising fluid intake is paramount for successful calcium oxalate management, with a target USG less than 1.030 (Lulich et al., 2016). The most effective means for this is a high moisture diet, for example a canned diet or pouch. Increased dietary sodium content within a dry diet may increase the total amount of calcium excreted but overall it decreases urinary calcium saturation due to the production of more dilute urine, so it remains a valid option for cats refusing a wet diet. Encouraging intake of water via methods including flavouring the water with broths or use of a water fountain may be successful in some cats.
Maximising fluid intake is paramount for successful calcium oxalate management, with a target USG less than 1.030 (Lulich et al., 2016). The most effective means for this is a high moisture diet
In patients with recurrent calcium oxalate uroliths, thiazide diuretics may be recommended in order to enhance renal tubular reabsorption of calcium and help to reduce USG. A 65 percent decrease in urinary calcium oxalate relative supersaturation was reported in clinically normal cats receiving hydrochlorothiazide at a dosage of 1mg/kg q12h (Hezel et al., 2007).
Probiotics may play a future role in management of calcium oxalate uroliths. In vitro studies have shown that manipulation of the gut lactic acid bacteria (eg with the probiotic Lactobacillus acidophilus) may decrease intestinal oxalate and correspondingly decrease intestinal oxalate absorption and renal excretion (Weese et al., 2004; Cho et al., 2013). In vivo studies are needed to determine whether probiotics containing L. acidophilus decrease urine oxalate concentrations and reduce the risk of calcium oxalate urolith recurrence.
As discussed in this two-part series, different risk factors influence the likelihood of formation of struvite and calcium oxalate uroliths. Management differs significantly both during the initial treatment of any uroliths identified and when considering the optimum approach to minimise the risk of recurrence. Nutritional factors can influence the likelihood of formation of calcium oxalate uroliths, and nutritional management plays a key role in reducing the risk of recurrence of calcium oxalate uroliths in individuals that develop them. Regular urinalysis and other monitoring strategies including imaging should be conducted to ensure management strategies employed are effective, and owners should be educated to ensure they are aware of the importance of dietary management. This can help to maximise the likelihood of long-term success.