Management of cats with urethral obstruction (Proceedings)
It is helpful to divide cats with FLUTD into obstructive and non-obstructive uropathy for treatment purposes. Obstructive uropathy is most commonly seen in male cats due to the small diameter of the male urethra. Typical clinical signs include dysuria, hematuria, frequent attempts to urinate, and licking at the penis or prepuce. Some cats will also have signs of systemic illness, such as lethargy and anorexia. Cats suffering from prolonged obstruction may present moribund.
Urethral plugs and urethroliths have been identified as the most common causes of obstruction in male cats. Other potential causes include urethrospasm, trauma, congenital defects, stricture and neoplasia. Since 1980, the frequency of urethral obstructions in cats has been declining, paralleling a similar decline in the frequency of urethral plugs and urethroliths. It is currently not known what causes urethral plugs to form. Many urethral plugs are composed of struvite crystals in a proteinaceous matrix. One theory suggests that urethral plugs form in cats with underlying idiopathic cystitis. Plasma proteins enter urine from suburothelial vascular leakage and may trap crystals in the lumen of the urethra, resulting in obstruction. Oozing of plasma proteins into urine combined with active inflammation may increase the urine pH, thus contributing to the precipitation of struvite crystals. Although calcium oxalate uroliths are now at least as common as struvite uroliths, the mineral composition of urethral plugs continues to be predominantly struvite. Other urethral plugs are composed almost totally of matrix (mucoproteins, albumin, globulins, cells, debris) or sloughed tissue and blood. Uroliths are organized concretions containing primarily crystalloids with a small amount of organic matrix. The most common components of uroliths are struvite and calcium oxalate, but recently uroliths composed of dried solidified blood have been reported. Urine is commonly supersaturated with crystalloids, so crystalluria itself is not a disease and does not need to be treated unless it is associated with clinical signs of FLUTD.
Cat with urethral obstruction should be treated as emergencies when presented. In particular, cats that have been obstructed 48 hours or more may be severely ill and require crisis management. A thorough assessment of the cat's condition should be made before attempting to relieve the obstruction. Placement of an IV catheter should be the first procedure in order to administer fluids and medication. Analgesia should be provided at the earliest opportunity. Appropriate agents include butorphanol, buprenorphine, hydromorphone, and fentanyl transdermal patch. Blood samples are collected for a complete blood cell count and serum chemistries/electrolytes. Treatment of the patient can begin before all the results are received. For critically ill cats, an emergency database would be PCV, TP, electrolytes, ionized calcium, blood glucose and BUN. Venous blood gases are also useful if available. Obstructed cats may have moderate to severe dehydration. A balanced electrolyte solution (lactated Ringer's or Plasmalyte) or 0.9% saline is adequate for rehydration and stabilization. In severely dehydrated or moribund cats, 20-30 ml/kg may be administered as an initial intravenous bolus.Common abnormalities in cats with urethral obstruction that may require correction include azotemia, hyperkalemia, metabolic acidosis and hypocalcemia. In one study, about 24% of 199 obstructed cats had mild to severe hyperkalemia (≥ 6.0 mEq/L).9 About 12% of the cats in the same study had multiple, life-threatening metabolic derangements (particularly hyperkalemia with concurrent hypocalcemia). With severely ill patients, part of the assessment plan should include an ECG (lead II). In most cases, azotemia, acidosis and hyperkalemia resolve with administration of fluids and relief of the obstruction. Severe acidosis has profound effects on the cardiac, respiratory and central nervous systems. Sodium bicarbonate may be required to resolve severe metabolic acidosis (pH <7.1). If blood gases are not available and metabolic acidosis is suspected, administer 1-2 mEq/kg of sodium bicarbonate slowly. It is important to monitor serum calcium as sodium bicarbonate lowers the ionized portion of plasma calcium and some patients are already hypocalcemic at presentation.