Ureteral obstruction is a common cause of acute azotaemia in cats. The most frequent cause of ureteral obstruction is a calcium oxalate urolith but the small size (often only 1-2mm) means that these cannot always be detected radiographically. Other possible causes of obstruction include strictures, blood clots, anatomical variations as well as other types of urolith. FUO is usually diagnosed when pelvic and/or ureteric dilation is recognised with abdominal ultrasound. Sometimes the ureter can be traced to an obstructing stone with acoustic shadowing, but this is not always possible even when the study is being performed by a diagnostic imaging specialist. Even if no urolith is observed when dilation is marked, obstruction is almost always the cause. When dilation is mild other aetiologies (ureteritis, pyelonephritis, marked diuresis) can also be considered. Sometimes when obstruction is very acute, or if the kidney is non-distensible for example due to extensive fibrosis, then pelvic dilation may not be evident at all and diagnosis of FUO in these patients is extremely challenging.

Unilateral ureteral obstruction is not expected to result in azotaemia unless the contra-lateral kidney is compromised. This means that sometimes FUO is discovered as an incidental finding when imaging a patient for a different problem, or due to investigation of palpable renal asymmetry or pain in a non-azotaemic cat. Unfortunately, many cats are chronic or repeated stone formers, resulting in ongoing insidious renal injury prior to a more fulminant presentation with newly diagnosed azotaemia and overt obstruction. In cases of severe FUO, life-threatening hyperkalaemia may develop in addition to severe azotaemia.

FUO is challenging to manage, and surgical intervention can be costly. The normal internal diameter of the feline ureter has been estimated to be only 0.4 mm which means it becomes obstructed easily and is not amenable to surgical management in general practice.  Nephrectomy is not advisable as these cats will often develop uroliths or strictures in the contra-lateral kidney and the majority of cats have concurrent kidney disease. If a cat is referred for treatment by a surgical specialist, it is most likely that a SUBTM device will be placed. A SUB is essentially a tube system that by-passes the ureter providing an alternative conduit for urine to flow through. One end of the tube is curled up within the renal pelvis, the other in the urinary bladder and these are joined at a subcutaneous access port attached to the abdominal wall allowing periodic sampling/flushing of the device with a Huber needle. A limited number of specialist surgeons, mainly those with extensive experience of micro-surgical techniques may perform ureteral surgery instead of placing SUBs.  Periodic follow-ups to monitor for recurrence of obstruction and progression of their kidney disease and flushing of the SUB (when present) is generally recommended when possible.

What can be done for FUO when referral for surgery is not possible? Medical management can be an option for these patients if they are stable (not anuric or severely hyperkalemic). Sometimes (in less than 25% of cases) stones will pass either spontaneously or while supportive care is being provided. This is most likely to happen if the stone is very distal and small (<1.5mm). Occasionally uroliths in the proximal ureter will move in a retrograde direction back into the renal pelvis relieving the obstruction, if only temporarily. To try to facilitate urolith passage it is reasonable to provide judicious intravenous fluids being very careful not to fluid overload the patient. Occasionally diuretics may be used. Opioid or other forms of analgesia should be provided. Many drugs have been given to relax the ureter and facilitate urolith passage (e.g. amitriptyline, prazosin, amlodipine, tamsulosin). Anti-inflammatory doses of steroids are sometimes given to decrease ureteritis.  Antibiotics are indicated in patients where pyonephrosis (dilation of the renal pelvis due to infection) is suspected, pending results of urine culture. While short-term non-surgical management is reasonable in select cases, it is not recommended in patients where referral for more definitive treatment is possible, or in cats with very severe azotaemia and/or hyperkalaemia where unfortunately euthanasia may be more appropriate if referral for surgery is not an option.

*Reflecting the international nature of IRIS the integrity of the author’s British English has been preserved.