Page 744 - Small Animal Internal Medicine, 6th Edition

716 PART V Urinary Tract Disorders

plain radiography, ultrasonography can help delineate which The 12-month survival rate after medical treatment of 52
ureter is obstructed and the severity of hydronephrosis and cats with ureteral obstruction was 66%. However, 32% of
VetBooks.ir hydroureter that may be present. A combination of survey these cats did not respond to medical therapy and were
euthanized or died within 1 month of diagnosis. Large case
radiography and ultrasonography has a sensitivity of 90% for
the diagnosis of ureterolithiasis, so it is the preferred
published, likely because this disease is more common in the
approach. In subacute ureteral obstructions, ureteral and series evaluating dogs with ureteral obstructions are not
pelvic dilation may have not yet developed, so it is critical to cat. If azotemia or pyelonephritis is present, surgery or mini-
consider ureteral obstruction as a differential diagnosis in mally invasive techniques (e.g., ureteral stents) for the resto-
appropriate cases, even when dilation is not present. Addi- ration of urine flow should be considered. Cats with marked
tional imaging modalities, such as antegrade pyelography or hyperkalemia or fluid overload at the time of initial examina-
computed tomography (CT), are usually not necessary, and tion may benefit from aggressive medical management, such
interventional procedures are recommended if obstruction as hemodialysis.
is present.
SURGICAL INTERVENTION FOR
MEDICAL TREATMENT TREATMENT OF URETERAL CALCULI
As mentioned previously, dissolution of suspected struvite, In cats and dogs, the optimal time for medical management
urate, and cystine calculi can be attempted in stable animals before making a decision to pursue surgery has not been
(as mentioned previously, these mineral types can occur in determined, and the improvement in renal function after
the upper tract of dogs) without complete ureteral obstruc- stone removal is variable. However, early intervention with
tion. There is no dissolution protocol for CaOx calculi, which surgical or minimally invasive procedures is likely indicated
is the most common calculi identified in the upper urinary in an effort to maintain functional kidneys. Surgical removal
tract of cats and can certainly occur in dogs. Conservative of ureteral calculi can be considered when there is evidence
medical management for cats with minimal or no renal com- of partial or complete ureteral obstruction. The number of
promise can be attempted. Although there are no studies to stones, degree of obstruction, operator experience, and avail-
evaluate the efficacy of any of the treatments mentioned in ability of appropriate equipment should all be considered
cats and dogs, most experienced clinicians agree that expul- when deciding whether to proceed with ureterotomy, stent,
sive therapy may play a role in the management of this or subcutaneous ureteral bypass (SUB). Ureterotomy may be
disease in stable patients. Suggested therapies can include preferred when only a single calculus is identified, but the
intravenous fluid diuresis with the administration of the latter two minimally invasive procedures are more often per-
diuretic mannitol, with or without other drug therapies. formed for a variety of reasons. Major factors determining
In humans with ureterolithiasis, the α-adrenergic antago- the recovery of renal function after reestablishing ureteral
nist tamsulosin is often used, with favorable outcome, par- patency include the degree of renal dysfunction before the
ticularly when calculi are in the distal third of the ureter. This development of the obstruction and the duration and extent
and other α-antagonists, such as phenoxybenzamine and of the obstruction. Ureteral strictures secondary to scar
prazosin, have been anecdotally used in cats with variable tissue formation can also form at the ureterotomy site,
responses. In one report, amitriptyline, a tricyclic antide- leading to reobstruction; these complications are much less
pressant, was shown to facilitate the passage of urethral plugs likely to occur when ureteral stenting or SUBs is performed.
in cats. Further studies evaluating ureteral tissue from rats, When a subset of cats with ureteral obstruction was followed
pigs, and humans found that amitriptyline inhibits smooth after surgical intervention, 14 of 35 (40%) had recurrent
muscle contractions, suggesting it might be a useful therapy episodes of ureterolithiasis.
for ureteral obstruction in cats. Analgesic therapy such as Ureteral stents and subcutaneous
buprenorphine should also be used to prevent ureteral ureteral bypass
“spasm,” which could prevent ureterolith movement. Placement of ureteral stents or SUB is being performed
During conservative management, it is crucial to critically as the treatment of choice for ureteral obstruction in cats,
evaluate the cat’s stability and fluid status. Animals should particularly if there are multiple ureteroliths. Evidence sug-
be monitored by serial measurements of serum creatinine gests that these less invasive procedures have lower morbid-
(and possibly SDMA) because this is often the best clinico- ity and mortality rate for ureteral obstruction than do
pathologic indicator currently available that the obstruction traditional surgical approaches in cats. A lower re-obstruc-
has improved or progressed. Body weight should be assessed tion rate and decreased severity of azotemia have been asso-
at least twice daily to be certain the animal is not becoming ciated with stent placement. A skilled operator is necessary
overhydrated. It is important to remember that, if significant for these referral procedures. Attempts to place these stents
intrinsic renal damage has occurred, passage of ureteral using a retrograde cystoscopic approach have been made but
obstruction does not always lead to immediate improvement have been largely unsuccessful in cats; however, this tech-
in azotemia. In cats with severe kidney disease before nique can be used for many dogs. In cats, a relatively mini-
obstruction, azotemia may persist. Serial radiography and mally invasive surgical placement is used (Video 43.4),
ultrasonography can be useful for monitoring the success of whereby the surgeon performs a laparotomy and places the
medical management for ureterolithiasis. stent from the kidney and into the bladder using a coaxial