Improving management of urolithiasis: therapeutic caveats
As discussed in last month's Diagnote, the underlying causes of different types of uroliths vary. It follows that medical and/or surgical procedures designed to safely and effectively treat different types of uroliths also vary. The objective of the second part of this series is to provide an overview of risks and benefits associated with surgical and medical therapy of urolithiasis. Therapeutic caveats associated with treatment of specific types of uroliths will be the subject of next month's Diagnote.
Surgical removal of uroliths 1. Surgery plays an important role in the management of active uroliths in many patients. However, detection of uroliths is not, in itself, an indication for surgery. Many cases may be managed by medical therapy alone. Candidates for surgery include: (a) patients with urolith-induced obstruction to urine outflow that cannot be corrected by nonsurgical techniques, especially when obstruction is associated with concomitant urinary tract infection; (b) patients with active uroliths that are refractory to current methods of medical dissolution (e.g. silica, calcium oxalate and calcium phosphate uroliths); (c) patients with uroliths that are increasing in size and/or number despite medical therapy designed to inhibit their growth or cause their dissolution (especially if they are causing obstruction to urine outflow and/or progressive deterioration in renal function); (d) patients with nephroliths and renal dysfunction of such nature that the time required to induce medical dissolution is likely to be associated with more renal dysfunction than that associated with surgical procedures; (e) patients with anatomical defects of the urogenital tract that predispose to recurrent urinary tract infection (UTI) and urolithiasis and that are amenable to surgical correction at the time uroliths are removed; and (f) patients that do not respond to medical management because clients are unwilling or unable to comply with therapeutic recommendations.
2. Although surgery facilitates immediate elimination of uroliths, it is associated with several limitations, including: ( a) persistence of underlying causes associated with a high rate of recurrence of uroliths following surgery; (b) patient factors that enhance adverse consequences of general anesthesia or surgery; and (c) inability to remove all uroliths or fragments of uroliths during surgery. In addition, situations occasionally arise in which owners of companion animals will not consent to surgical therapy but will consider medical therapy. For these and other reasons (that is, the urolith is asymptomatic), medical dissolution of uroliths is often considered.3. Bacterial UTI's associated with uroliths that completely obstruct urine outflow through one or both kidneys and ureters, or through the urethra should be regarded as an emergency. In this situation, rapid spread of infection and associated damage to the urinary tract, especially the kidneys, may induce septicemia and peracute renal failure caused by a combination of obstruction and pyelonephritis.
4. Surgery or lithotripsy should be considered for renoliths and/or ureteroliths associated with (1) outflow obstruction and substantial impairment of function of the associated kidney; (2) symptomatic abdominal pain; or (3) recurrent bacterial UTI's. However, before performing these procedures, the risk of iatrogenic damage to the kidneys causing further decline in renal function should be considered. Sterile nonobstructive nephroliths may persist for years without substantial change in renal function.
5. Dissolution requires sustained contact of uroliths with urine that has been modified so that it is undersaturated with calculogenic minerals. Therefore, struvite, urate, or cystine uroliths located in the ureters or urethra cannot be dissolved by medical protocols because they are only intermittently in contact with urine. If urethroliths are returned to the urinary bladder by retrograde urohydropulsion, they may be subsequently managed by surgery, medical dissolution or lithotripsy.
6. Combined use of surgical removal of uroliths followed by medical calculolytic protocols may be of value in some patients. Examples include patients in which struvite, urate, or cystine uroliths or fragments of these uroliths remain following surgery or lithotripsy. If protein restricted calculolytic diets are used, meticulous procedure should be used in repairing surgical incisions.
7. Most cases that require surgical intervention also require medical management designed to prevent urolith recurrence. Persistent urinary tract disease may develop and urolithiasis may recur if surgical removal of uroliths is the only form of therapy used.
8. The most common reason for detection of uroliths within days to weeks following surgery is incomplete removal of uroliths during the previous surgery. We have observed this type of "pseudorecurrence" in approximately 20 percent of patients following cystotomies. Therefore, radiographs should be obtained immediately following surgery to evaluate completeness of urolith removal.
Medical dissolution of uroliths 1. Protocols have been developed to promote dissolution of canine and feline struvite uroliths, the dissolution of canine ammonium urate and cystine uroliths, and the prevention of all major types of canine and feline uroliths. Because therapy designed specifically for one type of urolith (for example, struvite) may be detrimental to patients with a different type of urolith (for example. calcium oxalate), medical therapy should not be initiated before appropriate samples have been collected and evaluated. The goal is to base treatment recommendations on an accurate diagnosis.
2. The overall objectives of medical management of uroliths are to arrest further growth and/or to promote urolith dissolution by correcting or controlling underlying abnormalities. For therapy to be effective, it must induce undersaturation of urine with calculogenic crystalloids by: (a) increasing the solubility of crystalloids in urine, (b) increasing the volume of urine in which crystalloids are dissolved or suspended, and (c) reducing the quantity of calculogenic crystalloids in urine. For example, attempts to increase the solubility of crystalloids in urine often include administration of medications designed to change urine pH in order to create a less favorable environment for crystallization. Likewise, induction of diuresis is a method commonly used to increase the volume of urine in which crystalloids are dissolved or suspended. Change in diet is an example of a method to reduce the quantity of calculogenic crystalloids in urine.
3. In general, medical treatment should be formulated in stepwise fashion, with the initial goal of reducing the urine concentration of calculogenic substances. Medications that have the potential to induce a sustained alteration in body composition of metabolites, in addition to urine concentration of metabolites, should be reserved for patients with active or frequently recurrent uroliths. Caution must be used so that the side effects of treatment are not more detrimental than the effects of the uroliths.
4. The size and number of uroliths per se do not dictate the likelihood of response to therapy. We have had success in dissolving uroliths that are small and large, single and multiple. However the rate of dissolution is related to size and surface area of the urolith exposed to urine. Just as one large ice cube dissolves in water more slowly than an equal volume of crushed ice, one large urolith will dissolve more slowly in urine than an equal volume of many smaller uroliths. The point is that the rate of dissolution is influenced by surface area of the urolith exposed to undersaturated urine.
5. In general, increasing water intake with the goal of decreasing urine concentration and increasing urine volume should be considered as a key component of medical management for all types of uroliths. However, it is unlikely that water hardness plays a significant role in the formation of uroliths. The quantity of water consumed is much more important. Therefore, high-moisture (canned) diets are usually preferable to low-moisture (dry) diets. Use of distilled water is of questionable value, unless its flavor enhances water consumption.
6. Difficulty in inducing complete dissolution of uroliths by creating urine that is undersaturated with the suspected calculogenic crystalloid should prompt consideration that: (a) the wrong mineral component was identified; (b) the mineral composition of the nucleus of the urolith is different than outer portions of the urolith; and/or, (c) the owner or the patient is not complying with therapeutic recommendations.