Benign ureteral obstruction in cats: Outcome with medical management

Abstract

Background

Limited information is available regarding the outcome of medical management (MM) of benign ureteral obstruction in cats (BUO).

Hypothesis

Describe clinical characteristics and outcome of MM of BUO.

Animals

Seventy‐two client‐owned cats with 103 obstructed kidneys.

Methods

Medical records of cats diagnosed with BUO between 2010 and 2021 that received >72 hours of MM were retrospectively reviewed. Clinical data, treatment, and outcome were reviewed. Outcome was classified as success, partial success, or failure based on ultrasound findings. Factors associated with outcome were assessed.

Results

Seventy‐two cats with 103 obstructed kidneys were enrolled. The causes of obstruction were uroliths in 73% (75/103), strictures in 13.5% (14/103), and pyonephrosis in 13.5% (14/103) of affected kidneys. Median serum creatinine concentration at presentation was 4.01 mg/dL (range, 1.30‐21.3 mg/dL). Outcome after MM was considered a success in 30% (31/103), partial success in 13% (13/103), and failure in 57% (59/103) of kidneys. Success was reported in 23% (17/75) of kidneys with uroliths, 50% (7/14) with pyonephrosis, and 50% (7/14) with strictures. Median time to a successful outcome was 16 days (range, 3‐115 days). Distal and smaller uroliths (median length, 1.85 mm) were significantly associated with success (P = .05 and P = .01, respectively). Median survival times were 1188 days (range, 60‐1700 days), 518 days (range, 7‐1812 days), and 234 days (range, 4‐3494 days) for success, partial success, and failure, respectively.

Conclusions and Clinical Importance

We found a higher success rate for MM of BUO than previously reported. Smaller distal uroliths (<1‐2 mm) were more likely to pass.

Abbreviations

bpH

blood pH

BUO

benign ureteral obstruction

GLM

generalized linear model

IR

interventional radiology

MM

medical management

MST

median survival time

MSu

median time to success

RPS

renal pelvis size

SCr

serum creatinine

SUB

subcutaneous ureteral bypass

UD

ureteral diameter

1. INTRODUCTION

Ureters in cats have a small internal diameter (0.3‐0.4 mm), thus predisposing to intraluminal obstruction (benign ureteral obstruction [BUO]). Benign intraluminal obstruction occurs mostly secondary to uroliths (of which 98% are composed of calcium oxalate), acquired strictures, and purulent exudate (pyonephrosis). ¹ , ² , ³ Treatment options include medical management (MM), surgery, and interventional treatment. No current official guidelines are available in veterinary medicine to assist in decision‐making. Generally, MM is instituted 24‐72 hours before a definitive procedure, to relieve obstruction, stabilize the patient and allow for proper treatment planning. ¹ , ²

In people, the internal diameter of the ureter is 6‐8 mm, thus allowing more likelihood of urolith passage. In people, uroliths <5 mm and between 5 and 10 mm have 75%‐98% and 25%‐47% passage rates, respectively. ⁴ , ⁵ , ⁶ Uroliths >10 mm generally require intervention. Passage rates of 25%, 45%, and 70% are reported for uroliths located in the proximal, mid, or distal ureter, respectively. ⁵ , ⁷ In veterinary medicine, standard surgical options (e.g., ureterotomy, neoureterocystostomy) for ureterolithiasis have high rates of morbidity (13%‐30%) and mortality (8%‐20%) along with high recurrence rates (22%‐40%). ⁸ , ⁹ , ¹⁰ , ¹¹ Placement of a subcutaneous ureteral bypass (SUB) device is the standard of care considering hospital discharge rates of 94% and median survival times of 821‐827 days. Complications after SUB device placement include kinking of the device (3%‐10%), luminal mineralization (17%‐25%), and chronic infection (24%‐25%). ¹² , ¹³ However, the cost of the procedure and perceived necessity of evaluations to assess patency of the implant discourage some clients. Given these limitations, MM sometimes is chosen by owners. Medical management consists of fluid therapy, ureteral muscle relaxants (e.g., alpha‐1 adrenergic antagonists such as prazosin), analgesia, diuretics (e.g., mannitol, furosemide), and corticosteroids. ¹ , ² , ³ , ¹² Current veterinary literature is limited to a single retrospective study with a low success rate (8%‐13%) in 52 cats. ¹⁰ No information regarding outcome of obstruction (other than urolith), impact of size and location of uroliths or the efficacy of treatments is available. Our impression is that a higher rate of urolith passage is observed clinically than reported.

Our primary objective was to describe the clinicopathologic findings and outcome in cats with BUO that underwent MM for at least 72 hours and compare these findings to those in cats that underwent SUB placement or were euthanized within the first 72 hours after presentation. Our secondary aim was to determine if any pretreatment features predicted success or failure of MM.

2. MATERIALS AND METHODS

Retrospective review of medical records of cats diagnosed with BUO from August 2010 to September 2021 was performed at the Veterinary Teaching Hospital of the University of Montreal (CHUV). A computerized search using keywords was done. Keywords used were ureteral obstruction, ureteral blockage, pyonephrosis, ureteral stone, ureteral calculi, urolith, ureterolith, ureteral stricture, SUB, ureteral stent, ureteritis, ureteral dilatation, hydroureter, mannitol, prazosin, pelvic dilatation, pyelectasia, and hydronephrosis.

2.1. Inclusion criteria

Cats were included if a diagnosis of BUO was established on the basis of ultrasound examination performed by a board‐certified radiologist and if a complete medical record was available including serum biochemistry at presentation (5 cats were excluded because of incomplete data). Diagnosis was based on a combination of several criteria, such as renal pelvis dilatation, diverticula dilatation, ureteral dilatation or some combination of these, and visualization of an obstructive intraluminal lesion. ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ Causes were divided into urolith, pyonephrosis, and suspected stricture. Urolith was defined as an intensely hyperechoic structure within the lumen of the ureter associated with distal acoustic shadowing. ¹⁵ , ¹⁹ Pyonephrosis was defined as a conglomerate of nonvascularized hyperechoic material, compatible with purulent exudate, in the renal pelvis or ureter or both. ¹⁵ , ²⁰ , ²¹ , ²² A stricture was suspected when an abrupt decrease in ureteral luminal diameter was observed without evidence of a cause for the obstruction. ¹⁹ , ²³ , ²⁴ Cats with a malignant or extraluminal cause of obstruction were excluded.

Cats were divided into 3 groups. Group “MED” if they underwent MM for >72 hours after presentation and received at least 1 of the following treatments: IV or SC fluid therapy, ureteral muscle relaxant, corticosteroids, diuretics, or some combination of these. Antibiotic treatment was necessary for cats to be included in the pyonephrosis group. The treatment plan was established at the discretion of the attending clinician, and the patient could be hospitalized or treated on an outpatient basis. Placement of a SUB device or ureteral stent had been declined by the owners. Cats were included in group “IR” if they underwent an interventional procedure (placement of a SUB device or ureteral stent) within the first 72 hours after presentation. Group “EUTH” consisted of cats that were euthanized (or died) within the first 72 hours after presentation. Reason for euthanasia was recorded and retrospectively classified as financial, clinical deterioration, or because of the presence of other comorbidities. For this group only, the diagnosis of BUO could be based on an ultrasound examination by a board‐certified radiologist, internist, or criticalist (13 cats).

Cats in Group “MED” were excluded if they did not have at least 1 reevaluation, a serum biochemistry (i.e., serum blood urea nitrogen [BUN], creatinine, and electrolyte concentrations), and a repeated urinary tract ultrasound examination by a board‐certified radiologist or internist (exclusion of 6 cats).

2.2. Data collection

At presentation, the following data were collected for all 3 groups: signalment, weight, hematocrit, as well as blood pH (bpH) and concentrations of serum creatinine (SCr), BUN, electrolytes, phosphorus, and bicarbonate. Cause of BUO also was recorded (e.g., urolith, pyonephrosis, suspected stricture). If the cause of the obstruction was not identified on ultrasound examination (for group EUTH), the cause was classified as “unknown.” It was noted if the obstruction was unilateral or bilateral.

Additional data were collected for group MED. Ultrasound examination details of the obstructed kidney recorded at time of diagnosis included kidney length, transverse renal pelvic dilatation (RPD), largest ureteral diameter (UD), number of uroliths (1‐5 or >5), size (length and width), and location (proximal, middle, or distal) of the longest urolith and presence of nephroliths. Medications administered and urine culture results also were recorded. Duration of hospitalization, bpH as well as SCr, potassium, and bicarbonate concentrations (24 hours after hospitalization, 48 hours after hospitalization, and at the time of discharge) were noted. ²⁵ Development of oligo‐anuria and fluid overload during hospitalization were recorded if present. Follow‐up time points were divided into 3 periods (short‐term: 0‐1 week, mid‐term: 1 week‐3 months, and long‐term subdivided as 3‐6 months, 6‐12 months, and >12 months). Placement of a SUB was recommended if obstruction was still present at each follow‐up. Data collected during follow‐ups included medications administered, hematocrit, SCr and BUN concentrations, and ultrasound findings (e.g., RPD). If a patient underwent an interventional radiology (IR) procedure or recurrence of obstruction was recorded, it was noted and additional data from this patient were excluded.

For group MED, outcome was retrospectively classified for each obstructed ureter as success, partial success, or failure based on ultrasound findings. Success was defined as resolution of obstruction or resolution of pelvic and ureteral dilatation or both. Time to success was the time to the first ultrasound examination that reported success. Partial success was defined as a decrease of at least 50% in RPD with persistence of the obstructive cause. Failure was defined as persistence of the obstructive cause with worsening RPD, or <50% improvement in RPD, if the patient was euthanized or died because of BUO, or if a decision was made to proceed with an IR procedure after 72 hours. After exclusion of patients with failure of MM, time to recurrence of BUO was recorded as well as decision to proceed with an IR procedure, when available. After exclusion of all patients that underwent an IR procedure, time to euthanasia or death was recorded.

2.3. Statistical analysis

For population description and clinicopathologic features, effects of treatment groups (MED, IR, and EUTH) on signalment (age, sex, breed, and weight), laboratory findings (hematocrit, bpH as well as serum BUN, creatinine, potassium, bicarbonate, and phosphorus concentrations at presentation) and final diagnosis as dependent variables were evaluated. Because these dependent variables were of different types, different analyses were used to test the effect of group. First, to study differences among groups (MED, IR, and EUTH) on continuous dependent variables, normality and homoscedasticity assumptions were evaluated using Shapiro‐Wilks and Levene tests, respectively. If the data distribution were not normal or if the variance was not homogeneous among groups, a Kruskal‐Wallis (KW) test was used to compare groups. Almost all of the variables violated at least 1 of these conditions. For sex (dichotomous variable), a generalized linear model (GLM) was used. Finally, for the qualitative variable (cause of obstruction), a multinomial logistic regression was used. When independent variables consisted of more than 2 modalities (e.g., the effect of the group was composed of 3 modalities) and had a statistically significant effect (P < .05) on the dependent variable, post hoc tests were carried out in order to know which terms differed between them. Thus, following the KW tests, Dunn's post hoc tests were performed. In the case of GLM and multimodal models, Tukey's post hoc tests were used. In all cases, because of multiple comparisons during post hoc tests, corrections to the P values were made, using the Benjamini‐Hochberg method.

To address our main objective, further statistical analysis focused on group MED. For statistical purposes, outcome was classified per cat (individual) as success, partial success, or failure, based on the previously established kidney outcome. However, in cases of bilateral obstruction with a different outcome per kidney, reclassifications were made. Among 31 bilaterally obstructed cats, 7 cats had different outcomes in each kidney. Patient's outcome was reclassified as partial success if the combination of kidney outcomes was partial success/success or success/failure. It was reclassified as failure if the combination was partial success/failure. Overall survival times were obtained using Kaplan‐Meier curves for all 3 outcome categories, and we analyzed survival time using log‐rank tests. Cats still alive at the end of the study were censored. Additional statistics were performed between success and failure groups. Partial success kidneys and cats were excluded from statistical analysis to simplify analysis. Because the dependent variable (outcome) was dichotomous (0 for fail and 1 for success), GLM (with a logic link) was used to test the effect of laboratory results at admission (hematocrit, bpH and serum BUN, creatinine, phosphorus, potassium, and bicarbonate concentrations), ultrasonographic characteristics at admission (length of kidney, RPD and UD, presence of circumcaval ureter, cause of obstruction, and unilateral or bilateral obstruction), urolith characteristics at admission (number, location, and length; width of longest urolith), laboratory results at 24 and 48 hours after admission (hematocrit, bpH, serum potassium, creatinine, and BUN concentrations), treatment received during MM (IV fluids, analgesia, mannitol, furosemide, corticosteroids, prazosin, and amitriptyline), development of fluid overload or anuria during hospitalization, and positive bacterial culture at admission.

P values <.05 were considered significant. All statistics were determined using R software version 4.0.3. ²⁶

3. RESULTS

3.1. Population description

A total of 217 cats were diagnosed with BUO from August 1, 2010, to September 1, 2021, representing 0.83% (217/26094) of the CHUV's cat caseload. Seventeen cats were excluded: 4 did not receive any treatment, 6 did not meet follow‐up criteria, 2 had concomitant urethral obstruction and 5 had incomplete medical records. Two‐hundred cats were included and further divided into 3 groups: 72 in group MED, 78 in group IR, and 50 in group EUTH. Among group EUTH, 48/50 (96%) cats were euthanized within 24 hours. Financial limitations were noted for every cat. Additional reasons for euthanasia included deterioration of overall condition despite MM since admission (19/50 cats, 40%) and concurrent comorbidities (9/50, 18%).

Of the 72 cats that underwent MM for >72 hours (group MED), 41 were spayed females (57%), 30 were neutered males (42%), and 1 was an intact female (1%). Median age was 8 years (range, 1‐17 years). Fifty‐five were domestic shorthairs (76%). Other breeds included Siamese (5/72, 7%), Himalayan (3/72, 4%), Persian (2/72, 3%), and 1 of each of the following breeds (Singapura, Sokoke, Abyssinian, Scottish Fold, and Lynx). Median weight was 3.92 kg (range, 2.35‐9.55 kg). Signalment of group IR and group EUTH are presented in Table 1. No statistical difference was found in signalment among the 3 groups (P > .05).

TABLE 1.

Signalment and obstructive cause for patients with interventional procedures performed within 72 hours (Group IR) and patients euthanized within 72 hours (Group EUTH).

Group	IR	EUTH
	n = 78	n = 50
Age	8 y (range, 2‐17 y)	8 y (range, 2.5‐16 y)
Sex	39/78 (50%) spayed females 39/78 (50%) neutered males	24/50 (48%) spayed females, 25/50 (50%) neutered males, and 1/50 (2%) intact female
Breed	61/78 DSH (78%), 7/78 Siamese (9%), 2/78 (3%) Abyssinian/Persian/Burmese, and 1/78 (1%) Maine Coon/Tonkinese/Ragdoll/Bengal	41/50 (82%) DSH, 2/50 (4%) Siamese, and 1/50 (2%) Himalayan/Abyssinian/Persian/Egyptian Mau/Jungle Curl/Highland fold/Highland lynx
Total obstructed kidneys	111 obstructed kidneys: 33/78 (42%) bilateral and 45/78 (58%) unilateral	76 obstructed kidneys: 26/50 (52%) bilateral and 24/50 (48%) unilateral
Cause of obstruction	86/111 (77%) uroliths, 24/111 (21%) strictures, and 1/111 (1%) unknown cause	31/76 (41%) uroliths, 7/76 (10%) strictures, 1/76 (1%) pyonephrosis, and 37/76 (49%) unknown cause

Note: Group IR: patients that underwent an interventional procedure within the first 72 hours (1 cat had a ureteral stent placed and 77 cats had SUB placement) and Group EUTH: patients that were euthanized within first 72 hours.

Abbreviation: DSH, domestic short hair.

3.2. Clinicopathologic features

Among the 72 cats in group MED, 103 ureters were obstructed. There were 41 unilateral (57%) and 31 bilateral obstructions (43%). Ureteral obstruction was secondary to uroliths in 73% (75/103), suspected stricture in 13.5% (14/103), and pyonephrosis in 13.5% (14/103) of ureters. Causes of obstruction in groups IR and EUTH are presented in Table 1. Significantly more unknown causes were recorded in Group EUTH compared to the 2 other groups (P < .001). Significantly more obstructions were caused by pyonephrosis (vs by suspected strictures or uroliths) in group MED compared to group IR (P = .01 and P = .02, respectively). Other causes were not significantly different among groups.

In group MED, at the time of diagnosis, median kidney length, median RPD, and median UD were 39.75 mm (range, 17‐67 mm), 4.5 mm (range, <1‐25.7 mm), and 2.8 mm (range, 1.1‐7.5 mm), respectively. Nephroliths were reported in 74% (77/103) of obstructed kidneys.

On presentation, group MED had a median hematocrit, SCr, and BUN concentrations of 32% (range, 19‐56%), 4.0 mg/dL (range, 1.3‐21.3 mg/dL), and 62 mg/dL (range, 19‐206 mg/dL), respectively. Median potassium concentration was 4.3 mmol/L (range, 1.8‐8.3 mmol/L). Urine culture obtained by cystocentesis was positive in 12/60 (10/12 E. coli and 2/12 Staphylococcus spp.). In the 11 cats (14 kidneys) diagnosed with pyonephrosis, urine culture grew E. coli (3/11) and was negative in 8/11 cats (4 cats were receiving antibiotics at the time of culture). Table 2 presents clinical data from all 3 groups on admission. Serum creatinine (P = .001) and phosphorus (P < .001) concentrations were significantly different among all 3 groups (highest in group EUTH for both variables). Hematocrit was significantly different among groups (P < .001). It was lower in group IR compared to group EUTH (P < .001) and group MED (P = .03). Bicarbonate concentration and bpH were significantly lower in group EUTH compared to groups IR and MED (P < .001). No significant difference was found in serum potassium concentration and fluid overload among the 3 groups (P = .08, P = .1).

TABLE 2.

Comparison of clinicopathologic data on admission of the 3 groups (MED, EUTH, and IR).

Median value group	Hematocrit (%)	Serum creatinine concentration (mg/dL)	Blood urea concentration (mg/dL)	Serum phosphorus concentration (mg/dL)	Serum potassium concentration (mmol/L)	Blood pH	Blood bicarbonate concentration (mEq/L)
MED	32 (19‐56)	4.0 (1.3‐32.3)*	61.6 (18.8‐205.3)	5.7 (2.2‐19.8)*	4.3 (1.8‐8.3)	7.32 (6.887‐7.45)	17.5 (7.3‐32.3)
EUTH	33 (21‐55)	10.7 (0.8‐29.6)*	140.0 (24.5‐389.3)	12.8 (3.3‐33.3)*	4.5 (2.7‐9.5)	7.17 (6.94‐7.63)*	11.95 (5.8‐39.4*
IR	30 (15‐47)*	5.9 (1.3‐31.3)*	100.3 (22.1‐317.8)	9 (2.8‐21.6)*	4.28 (2.3‐7.5)	7.31 (7.04‐7.52)	15.9 (7.8‐28.4)

Abbreviations: EUTH, euthanized; IR, underwent SUB or ureteral stent placement; MED, medically managed.

^*A significant difference (P < .05) of the value between the 2 other groups.

3.3. Description of medical management (group MED)

Eighty‐two percent (59/72) of cats were hospitalized and received IV fluids for a median of 96 hours (range, 24‐216 hours). After discharge, 23/72 (32%) cats received SC fluids for a median of 14 days (range, 2‐205 days). Other treatments included prazosin in 62/72 (86%) cats (0.125 mg/cat PO q12h to 0.5 mg/cat PO q8h) for a median of 11 days (range, 3 days to >1 year). Amitriptyline was administered in 2 cats (3%, 5 mg/cat PO q12‐24h for 3 or 11 days). Thirty‐three percent of cats (24/72) received corticosteroids (always in combination with prazosin). Fifteen hospitalized cats received dexamethasone (0.05‐0.1 mg/kg IV q24h followed by 0.05 mg/kg IV q24h). Twenty‐two cats received prednisolone PO at a median starting dosage of 0.49 mg/kg/day (range, 0.3‐1.3 mg/kg/day) for a median of 33 days (range, 3‐106 days). Among them, 13 cats had received dexamethasone during hospitalization. Diuretics (furosemide or mannitol) were only administered to hospitalized cats. Furosemide (29%, 21/72) was administered by bolus in 16/21 cats (range, 0.25‐2 mg/kg IV q12h) or by constant rate infusion in 5/21 cats (0.2 mg/kg/h IV). Median duration of treatment was 1 day (range, 1‐7 days). Among 11/16 cats that received furosemide, 8 cats experienced fluid overload, 1 was oliguric, and 2 had both (fluid overload and were oliguric). Mannitol was administered to 9/72 cats (12.5%). All received a bolus (range, 0.25‐0.5 g/kg over 20 minutes) either as a single dose (3/9 cats), followed by 2 boluses over 24 hours (3/9 cats), or followed by a constant rate infusion of mannitol (1 mg/kg/min over 8 hours; 3/9 cats). None of these cats was reported to develop fluid overload after mannitol (2/9 were obstructed bilaterally).

Analgesics were prescribed in 67% (48/72) cats: 8% (4/48) received remifentanil (range, 3‐9 μg/kg/h), 83% (40/48) buprenorphine (range, 0.010‐0.015 mg/kg IV or sublingual q8‐12h), and 14% (7/48) gabapentin. Remifentanil was changed to buprenorphine in 3 cats. Buprenorphine was used at home in 21 cats. Gabapentin was prescribed as sole analgesia in 6 cats, and was started in hospital for 3 and prescribed at home in 7 cats.

Antibiotic treatment for cats with pyonephrosis consisted of chloramphenicol (1 cat, 50 mg total PO q12h), enrofloxacin (10 cats; range, 3.4‐5.1 mg/kg PO q24h) with a median duration of 35 days (range, 35‐180 days).

3.4. Outcome of medical management (group MED)

Success was reported in 30% of ureters (31/103). Overall median time to success (MSu) was 16 days (range, 3‐115 days). Success was documented at discharge from hospitalization in 8% of obstructed ureters (7/84). Success was reported in 23% (17/75) of ureters obstructed by uroliths with MSu of 14 days (range, 3‐53 days), 50% (7/14) of pyonephrosis with MSu of 47 days (range, 3‐115 days), and 50% (7/14) of suspected ureteral strictures with a MSu of 22 days (range, 4‐79 days). Partial success was reported in 13% (13/103) of ureters with median time to partial success of 27 days (range, 9‐55 days). Finally, failure was reported in 57% (59/103) of cats. By disease category, partial success and failure were reported in 15% (11/75) and 63% (47/75) of uroliths, 7% (1/14) and 43% (6/14) of suspected strictures, and 7% (1/14) and 43% of (6/14) of pyonephrosis. Among failures, 19 owners accepted a procedure (SUB device or stent placement) after median duration of MM of 7 days (range, 3‐221 days).

3.5. Risk factors associated with outcome (group MED)

The only factor associated with outcome by cat was age (P = .01; Table 3). Cats with successful outcome (median age, 4 years) were significantly younger than cats with failure outcome (median age, 8 years). Blood test results at admission and 24 and 48 hours after admission were not associated with outcome. Bilateral obstruction in cats in the failure group was more prevalent (54%) compared to the success group (25%). This difference was not significant (P = .12). A recorded occurrence of oligo‐anuria or volume overload during hospitalization was not significantly associated with outcome (P = .11 and P = .23, respectively). Although corticosteroids were administered more often in the success group (40%) compared to the failure group (23%), this difference was not significant (P = .26). Furthermore, there was no association of any treatment category with outcome.

TABLE 3.

Selected clinicopathologic variables compared between success and failure groups in cats.

Factor studied		Outcome (n = number of patients with available data)	Median value of studied factor (range: min‐max)	Number of patients presenting studied factor (%)	P value
Clinicopathologic variables at admission	Age (years old)	Success (n = 20)	4 (1‐15)	NA	.01
		Failure (n = 39)	8.5 (3‐17)
	Weight (kg)	Success (n = 20)	4.17 (2.9‐7.3)	NA	.94
		Failure (n = 39)	3.7 (2.35‐9.55)
	Sex	Success (n = 20)	NA	9 spayed females, 1 intact female, and 11 neutered males	.4
		Failure (n = 39)		24 spayed females and 15 neutered males
	Hct (%)	Success (n = 18)	32.5 (25‐56)	NA	.14
		Failure (n = 35)	32 (19‐47)
	Serum creatinine (mg/dL)	Success (n = 20)	4.7 (1.4‐21.3)	NA	.34
		Failure (n = 39)	3.8 (1.7‐17.5)
	Urea (mg/dL)	Success (n = 16)	62.7 (18.8‐206.5)	NA	.72
		Failure (n = 32)	66.1 (26.9‐180.1)
	Bicarbonates	Success (n = 19)	18 (9.2‐24.4)	NA	.32
		Failure (n = 37)	17.8 (8.70‐32.3)
	pH	Success (n = 11)	7.32 (7.196‐7.416)	NA	.9
		Failure (n = 16)	7.335 (7.13‐7.45)
	Phosphorus (mg/dL)	Success (n = 16)	6.0 (3.4‐19.8)	NA	.31
		Failure (n = 33)	5.2 (2.2‐16)
	Potassium (mmol/L)	Success (n = 20)	4.39 (2.5‐6.12)	NA	.32
		Failure (n = 39)	4.2 (1.8‐5.9)
	Positive culture	Success (n = 16)	NA	1 (6%)	.17
		Failure (n = 34)		8 (23%)
Clinical variables at 24 h after admission	Serum creatinine concentration (mg/dL)	Success (n = 16)	4.9 (1.4‐15)	NA	.51
		Failure (n = 27)	5 (1.5‐15)
	Serum potassium concentration (mmol/L)	Success (n = 16)	3.95 (2.9‐7.9)	NA	.96
		Failure (n = 26)	3.95 (2.9‐6.2)
	Blood pH	Success (n = 8)	7.3825 (7.264‐7.46)	NA	.36
		Failure (n = 18)	7.37 (6.997‐7.449)
	Blood bicarbonate concentration	Success (n = 12)	16.95 (11.8‐30.8)	NA	.71
		Failure (n = 19)	17.8 (8.2‐23.7)
Clinical variables at 48 h after admission	Serum creatinine (mg/dL)	Success (n = 14)	4.1 (1.4‐15)	NA	.72
		Failure (n = 29)	4 (1.6‐15)
	Potassium (mmol/L)	Success (n = 13)	4 (3.2‐5)	NA	.11
		Failure (n = 27)	4.24 (3.03‐7.5)
	pH	Success (n = 8)	7.4155 (7.238‐7.48)	NA	.17
		Failure (n = 16)	7.35 (6.97‐7.443)
	Bicarbonates —HCO3 −	Success (n = 10)	19.3 (13.5‐28)	NA	.27
		Failure (n = 21)	19.4 (7.7‐22.6)
Treatment	Prazosin	Success (n = 20)	NA	18 (90%)	.57
		Failure (n = 39)		33 (84%)
	Amitryptiline	Success (n = 20)	NA	0	.99
		Failure (n = 39)		2 (5%)
	Corticosteroids	Success (n = 20)	NA	8 (40%)	.26
		Failure (n = 39)		9 (23%)
	Furosemide	Success (n = 20)	NA	4 (20%)	.32
		Failure (n = 39)		12 (31%)
	Mannitol	Success (n = 20)	NA	2 (10%)	.57
		Failure (n = 39)		6 (15%)
	Analgesia	Success (n = 20)	NA	13 (65%)	.95
		Failure (n = 39)		25 (64%)
	Hospitalization and IV fluids	Success (n = 20)	NA	17 (85%)	.97
		Failure (n = 39)		33 (85%)
Complication during hospitalization	Oligo‐anuria	Success (n = 20)	NA	3 (15%)	.11
		Failure (n = 39)		1 (3%)
	Volume overload	Success (n = 20)	NA	2 (10%)	.24
		Failure (n = 39)		9 (23%)
Characteristics of obstruction	Bilateral vs unilateral	Success (n = 20)	NA	15 unilateral (75%) 5 bilateral (25%)	.12
		Failure (n = 39)	NA	21 unilateral (54%) 18 bilateral (46%)

Note: P < .05 is considered significant. Bold values represent statistically significant results.

Abbreviation: NA, not applicable.

When analyzing factors associated with outcome per kidney (Table 4), obstructed kidneys were significantly larger in cases with success (median length, 43.2 mm) than failure (median length, 36.6 mm; P = .01). Renal pelvic dilatation, UD, presence of a circumcaval ureter, and cause of obstruction were not significantly associated with outcome. The length of the longest urolith from the success group was significantly less than that of the failure group (median, 1.85 mm vs 3 mm, respectively; P = .01). The longest urolith to pass was 3.3 mm. Uroliths <1.44 and <1.05 mm had a 50% and 60% probability of passage, respectively. Urolith width was not significantly different between outcome groups (median, 1.2 mm vs 1.8 mm for success and failure outcomes, respectively) but width was not measured in every patient (available for 58% of success vs 30% of failure cases, P = .34). Proximal uroliths were significantly more prevalent in the failure group (51%) compared to the success group (25%; P = .05). Distal uroliths were significantly more prevalent in the success group (62.5%) compared to the failure group (31%; P = .05). Number of uroliths was higher in the failure group compared to the success group, but this difference was not significant (P = .06). Only 1 urolith was identified by ultrasound examination in 12/16 (75%) of ureters in the success group compared to 20/45 (44%) in the failure group.

TABLE 4.

Selected ultrasound variables compared between success and failure outcomes by kidney/ureter.

	Factor studied	Outcome (n = number of available data)	Median value of factor studied (range: min‐max)	Number presenting factor studied (%)	P value
Admission, ultrasound measurements from obstructed ureter	Length of kidney (mm)	Success (n = 29)	43.25 (26‐67)	NA	.01
		Failure (n = 54)	36.6 (17‐57)
	Presence of circumcaval ureter	Success (n = 31)	NA	3 (10%)	.99
		Failure (n = 59)		0 (0%)
	Transverse pelvis size (mm)	Success (n = 30)	3.9 (<1‐15)	NA	.56
		Failure (n = 56)	4.3 (<1‐25.7)
	Ureteral diameter	Success (n = 31)	2.95 (1.4‐7.3)	NA	.91
Characteristics of urolith(s)		Failure (n = 47)	2.7 (1.1‐7.5)
	Number of uroliths	Success (n = 16)	1 (1‐>5)	NA	.06
		Failure (n = 47)	2 (1‐>5)
	Length of urolith	Success (n = 16)	1.85 (1‐3.3)	NA	.01
		Failure (n = 46)	3 (1‐7.7)
	Width of urolith	Success (n = 10)	1.25 (0.9‐2.5)	NA	.34
		Failure (n = 14)	1.8 (0.6‐3.2)
	Location (proximal) a	Success (n = 16)	NA	4 (25%)	.05
		Failure (n = 45)		23 (51%)
	Location (middle) a	Success (n = 16)	NA	2 (12.5%)	.21
		Failure (n = 45)		8 (18%)
	Location (distal)	Success (n = 16)	NA	10 (62.5%)	NA
		Failure (n = 45)		14 (31%)
Cause	Stricture b	Success (n = 31)	NA	7 (23%)	1
		Failure (n = 59)		6 (10%)
	Urolith b	Success (n = 31)	NA	17 (55%)	.06
		Failure (n = 59)		47 (80%)
	Pyonephrosis b	Success (n = 31)	NA	7 (23%)	NA
		Failure (n = 59)		6 (10%)

Note: P < .05 is considered significant. Bold values represent statistically significant results.

Abbreviation: NA, not applicable.

^a Baseline location used for comparison was distal.

^b Baseline cause used for comparison was pyonephrosis.

3.6. Follow‐up and recurrence (group MED)

Follow‐up data were divided into 4 categories: at hospital discharge, short‐term, mid‐term, and long‐term. Data at discharge, in the short‐term and in the mid‐term were available for 55/59 (93%), 25/63 (40%), and 44/54 (81%) of cats, respectively. Median SCr concentration was 2.4 mg/dL (range, 1.2‐15 mg/dL) at discharge, 2.6 mg/dL (range, 1.3‐14.4 mg/dL) at short‐term follow‐up, and 2.5 mg/dL (range, 1.2‐9.9 mg/dL) at mid‐term follow‐up. At discharge, >20% improvement of SCr was documented in all 16 cats from the success group, in 90% (9/10) of cats from the partial success group, and in 18/29 (62%) of cats from the failure group. Table 5 summarizes the data in accordance with outcome.

TABLE 5.

Clinicopathologic data during short‐term and mid‐term follow‐ups classified by outcome category.

Follow‐up time‐point	Median (n = number of cats with available data)	Hematocrit (%)	Serum creatinine concentration (mg/dL)	Blood urea concentration (mg/dL)
Short‐term (<1 wk)	All outcomes (n = 25)	28 (20‐38)	2.6 (1.3‐14.4)	47.8 (17.6‐120)
	Failure (n = 15)	25 (20‐37)	3.4 (1.7‐14.4)	50.7 (39.5‐120)
	Partial success (n = 3)	26.5 (23‐30)	2.0 (1.5‐3.6)	27.5 (27.5‐80.5)
	Success (n = 7)	34 (29‐38)	1.6 (1.3‐5.3)	25.4 (6.2‐11.2)
Mid‐term (>1 wk‐<3 mo)	All outcomes (n = 44)	31.5 (15‐42)	2.5 (1.3‐9.9)	42.5 (20.2‐44.9)
	Failure (n = 16)	27 (15‐41)	3.8 (1.4‐9.9)	51.1 (20.2‐125)
	Partial success (n = 12)	32 (28‐42)	2.1 (1.5‐3.2)	39.5 (26.0‐63.6)
	Success (n = 16)	34 (18‐42)	1.8 (1.2‐9.1)	10.2 (21.4‐98.3)

Long‐term follow‐up was divided into 3 timespans: 3‐6 months, 6‐12 months, and 12‐24 months. Twenty‐eight cats were excluded from long‐term follow‐up (9 dead or euthanized and 19 had IR procedure in <3 months). Data were available for 34/44 (78%) cats after 3 months (15 successes, 8 partial successes, and 11 failures). Between 3 and 6 months median SCr concentrations, for all cats, and for cats with successful (n = 11), partially successful (n = 7), and failure outcomes (n = 9) were 2.4 mg/dL (range, 1.3‐7.9 mg/dL), 1.9 mg/dL (range, 1.3‐3.4 mg/dL), 2.4 mg/dL (range, 1.4‐6.9 mg/dL), and 3.4 mg/dL (range, 1.6‐7.9 mg/dL) respectively. Additional results are presented in Table 6.

TABLE 6.

Clinicopathologic data during long‐term follow‐up classified by outcome.

	3‐6 mo				6‐12 mo				12‐24 mo
Median outcome (individual)	Number of patients with data available	Hematocrit (%)	Serum creatinine concentration (mg/dL)	Blood urea concentration (mg/dL)	Number of patients with data available	Hematocrit (%)	Serum creatinine concentration (mg/dL)	Blood urea concentration (mg/dL)	Number of patients with data available	Hematocrit (%)	Serum creatinine concentration (mg/dL)	Blood urea concentration (mg/dL)
Success (n = 15)	n = 11	36 (21‐48)	1.9 (1.3‐3.4)	29.7 (12.5‐65.0)	n = 4	29 (24‐41)	2 (1.7‐219)	29.7 (12.6‐65)	n = 6	34 (25‐51)	1.9 (1.7‐2.7)	27.2 (22.7‐53.8)
Partial success (n = 8)	n = 7	31.5 (28‐41)	2.4 (1.4‐6.9)	44.5 (29.7‐55.4)	n = 4	34 (24‐39)	2.9 (3.6‐5.6)	44.5 (29.7‐55.4)	n = 3	40 (35‐45)	2 (1.5‐2.7)	37.8 (25.5‐38.6)
Failure (n = 11)	n = 9	25 (17‐34)	3.4 (1.6‐7.9)	54.6 (25.5‐78.7)	n = 3	N/A	2.8 (2.5‐4.1)	38.4 (37.8‐73.7)	n = 5	29 (27‐39)	3.9 (1.4‐13.0)	46.2 (52.9‐109.8)
All outcome categories	n = 27	31 (17‐48)	2.4 (1.3‐7.9)	46.5 (12.6‐78.7)	n = 11	30 (24‐41)	2.6 (1.7‐5.6)	44.5 (37.8‐73.7)	n = 14	34.5 (25‐51)	2.0 (1.43‐13.0)	13.8 (22.7‐109.8)

Note: 38 cats were excluded from long‐term follow‐up (9 were dead or euthanized; 19 had SUB device placement within 3 months from their admission; 6 were alive with no follow‐up blood work; and 4 were lost to follow‐up).

Information regarding recurrence was available for 81% of cats (27/33) after exclusion of 39 failures. Recurrence was reported in 22% (6/27 cats) with a median time to recurrence of 83.5 days (range, 7‐200 days). Median SCr was 3.2 mg/dL (range, 1.6‐7.7 mg/dL). Causes of recurrence were uroliths for 3 cats (1 with nephroliths, 2 without nephroliths), pyonephrosis for 1, and stricture for 2 cats. Placement of a SUB for 2 cats was accepted by the owners at recurrence (1 with a stricture and 1 with a urolith). One cat was euthanized at recurrence of pyonephrosis. Specific patient details are available in Table S1.

Survival data were available for 45/51 (88%) cats, after exclusion of 21 cats that underwent an IR procedure. Median survival times with successful, partially successful, and failure outcomes were 1188 days (range, 60‐1700 days; 16 cats), 518 days (range, 7‐1812 days; 8 cats), 234 days (range, 4‐3494 days; 20 cats), respectively. Seventy‐three percent (17/23) had severe azotemia at the time of euthanasia. Survival estimates calculated by Kaplan‐Meier curve did not show significant difference in survival time based on outcome (Χ ² = 1.700, df = 2, P = .43; Figure 1).

FIGURE 1.

Kaplan‐Meier survival curves of 45 cats that underwent medical management per outcome.

4. DISCUSSION

Little information is available describing the clinical course of MM of BUO in cats when decompressive procedures (e.g., surgery, SUB placement) are declined. Our findings show an overall higher success rate of 30% after at least 72 hours of MM compared to a previous report. ¹⁰ Specifically, successful outcomes were reported for uroliths, pyonephrosis, and suspected stricture in 23%, 50%, and 50% of kidneys, respectively. The only other study of MM of ureteroliths in 52 cats reported an improvement in SCr concentration in 13% of cats, among which only 57% had documented ureterolith passage. ¹⁰ Median survival times for success, partial success, and failure outcomes were 1188, 518, and 234 days, respectively, with 14% of cats (6/44) dying within the first month of diagnosis. No significant difference in survival time was found using Kaplan‐Meier curves. Small population size per outcome group could account for failure to find a difference. These results are more encouraging than the previous study reporting 30% of deaths occurring within the first month of diagnosis after MM. ¹⁰

Recurrence rate in our study was 22%. Factors associated with recurrence were not studied because only 6 cats presented with recurrence, of which 2 had nephroliths. The prevalence of nephroliths was 74%. Nephroliths were associated with recurrence in a previous study (12/14 cats with recurrence had nephroliths) ¹⁰ but were not associated with reobstruction after neoureterocystostomy in another study. ⁹ Additional studies are necessary to identify risk factors for reobstruction.

Resolution of ureteral strictures in 50% of our cats (7/14 cats) was unexpected because the histopathologic definition suggests an irreversible process (narrowing of the ureteral lumen caused by fibrosis). ²³ There was no history of ureteral surgery. Among them, 3 had evidence of bladder uroliths at time of diagnosis, suggesting that inflammation or spasm after passage of the urolith was possible. A presumptive diagnosis of stricture was based on the high specificity (98%) of ultrasound for strictures in cats that underwent subsequent exploratory laparotomy in a previous study. ¹⁹ Resolution of strictures in our study suggests that, in some cases, a reversible inflammatory process or spasm is possible. Histopathology would be required to verify the etiology of what is referred to here as a stricture. Given these results, MM could be attempted in cats with ultrasound findings suggestive of stricture.

A secondary objective of our study was to identify risk factors associated with outcome. Young cats had a higher chance of recovery. In people, a similar association with age was described (lower rate of spontaneous urolith passage in older individuals). ²⁷ A distal urolith was more likely to pass in our cats compared to a proximal urolith as is reported in people. ⁴ , ⁵ Urolith length was associated with outcome, and smaller uroliths were more likely to pass. Particularly, if a urolith was <1.44 mm, it had a 50% chance of passing. Urolith width was not associated with outcome, but width was not recorded for most uroliths because of the retrospective nature of the study. In people, urolith width has a high correlation with length. ⁵ A study with independent multiple measurements by different radiologists (to limit measurement bias) is needed to further validate these results. Ultrasonography tends to overestimate urolith size and it, therefore, is important to note that our measurements are based on ultrasound examinations. Other imaging methods such as nonenhanced computed tomography could allow better detection and measurement of uroliths. ²⁸ Our preliminary results are useful to help guide selection of cats for MM.

Median time to success varied depending on the cause. Median times to success of 14, 47, and 22 days were found for uroliths, pyonephrosis, and strictures, respectively. These data could guide clinicians with recommendations regarding duration of treatment and timing of evaluations. Median time to success may have been overestimated because it corresponds to the time when resolution was first documented on ultrasound examination. Prospective studies with larger cohorts and standardized time points for ultrasound examination are needed to validate these results.

In our study, there was no association of treatment with outcome. However, our study was not designed to evaluate singular treatment efficacy given large variability in treatment protocols with no control group. Alpha‐adrenergic blockers decrease pressure and ease spasms by acting on ureteral smooth muscle and this treatment was commonly used in our cats (86%). A study in dogs suggested that tamsulosin was more effective than prazosin and warrants further investigation. ²⁹ In people, tamsulosin is associated with shorter treatment time, higher expulsion rate for distal uroliths, and decreased analgesic requirements. ³⁰ No studies in cats evaluating the efficacy of alpha blockers in ureters are available. Corticosteroids at anti‐inflammatory dosage were used in our study to decrease ureteral inflammation and facilitate passage of uroliths. A study showed increased inflammation (significantly higher CD+ B‐cells) in ureters obstructed by uroliths compared to healthy ureters. ³¹ In humans, corticosteroid use remains controversial. Evidence remains insufficient to support its use as monotherapy, but in association with other treatments (e.g., alpha‐blockers, calcium channel blockers, phosphodiesterase‐5 inhibitors), it may improve passage rates and shorten time to expulsion. ⁷ , ³² Additional prospective studies are needed in cats to better target treatment and improve outcome.

Cats included in group MED underwent at least 72 hours of MM. The time criterion was established to evaluate the effect of MM, but also to limit inclusion of cases referred for SUB device placement and those euthanized upon diagnosis. Significantly higher sCr concentration was identified with increasing severity from group EUTH to group IR to group MED. This finding could indicate a selection bias of the MED population with inclusion of cats potentially less severely affected. Pursuing MM may not be indicated in every patient, especially those with urosepsis, fluid overload, anuria, worsening azotemia, life‐threatening hyperkalemia, increasing RPD, or some combination of these. ¹ Additionally, depending on the onset and severity of obstruction, variable degrees of irreversible renal damage may have occurred in an already potentially compromised kidney because 80% of geriatric cats suffer from chronic kidney disease. ³³ Thus, if obstruction remains after 24‐72 hours of MM, the standard of care remains renal decompression by SUB device placement as supported by the 95% success rates in 2 large retrospective studies. ¹² , ¹³ In group MED, SUB device placement initially was declined by owners. Conversely, selection bias might be caused in part by the clinician's therapeutic decision (e.g., elected for euthanasia or renal decompressive procedure based on owner preference or clinicopathologic severity). In group MED, no significant association of outcome with hematocrit, azotemia, serum potassium concentration, or acidosis at presentation or at different timepoints (24 and 48 hours after presentation) or characteristics of obstruction (unilateral vs bilateral, presence of circumcaval ureter) was identified.

Limitations of our study include its retrospective nature, lack of standardization of diagnostic testing (e.g., bacterial culture, urinalysis), treatment protocols, missing data, and lack of follow‐up. Another limitation was the diagnosis of pyonephrosis on ultrasonography, and not all cats had positive bladder culture, pyuria, or neutrophilia on CBC. ²¹ , ²² Although ultrasonography is highly specific in people and dogs, additional studies are needed in cats. Other limitations include small sample size of the success group, which underpowered our study and affected our ability to detect significant differences. Intraoperator variability when performing ultrasonography could have affected the detection of ureteroliths and other anomalies such as circumcaval ureters and nephroliths. Moreover, measurements of UD were not standardized in follow‐ups and therefore could not be used as a criterion for partial success. Finally, outcome was based on ultrasound criteria. Although clinically important, azotemia is not a reliable marker of resolution of obstruction. Improvement may be noted despite persistent obstruction (i.e., correction of dehydration) or azotemia may persist depending on the presence of underlying kidney damage.

In conclusion, we report a higher success rate of 30% in cats that underwent MM for BUO. Younger age and small distal uroliths (1‐2 mm) were significantly associated with success. Cats with ureteral strictures and pyonephrosis were more likely to have successful outcomes (50%) than cats with uroliths (23%). Placement of a SUB device remains standard care and is associated with the highest success rates.

CONFLICT OF INTEREST DECLARATION

Authors declare no conflict of interest.

OFF‐LABEL ANTIMICROBIAL DECLARATION

Authors declare no off‐label use of antimicrobials.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) OR OTHER APPROVAL DECLARATION

Authors declare no IACUC or other approval was needed.

HUMAN ETHICS APPROVAL DECLARATION

Authors declare human ethics approval was not needed for this study.

Supporting information

Table S1. Initial diagnosis and outcome of patients with recurrent ureteral obstruction.

Merindol I, Vachon C, Juette T, Dunn M. Benign ureteral obstruction in cats: Outcome with medical management. J Vet Intern Med. 2023;37(3):1047‐1058. doi: 10.1111/jvim.16709