Clinical Exposures: A perinephric pseudocyst in a cat

An 11-year-old 14.5 lb (6.6 kg) castrated male domestic shorthaired cat was presented for evaluation of a progressively distended abdomen. The cat lived exclusively indoors and was receiving insulin (Humulin L; 4 U b.i.d.) and undergoing routine monitoring for diabetes mellitus that had been diagnosed when the cat was 8 years old. Except for the abdominal distention, the owners had not noticed lethargy, changes in appetite, or any other clinical signs.

Physical examination and differential diagnoses

The cat was fractious and had to be sedated with a combination of butorphanol tartrate, medetomidine, and ketamine hydrochloride given intramuscularly. The physical examination revealed a distended abdomen with a large palpable mass. An abdominal radiographic examination revealed a large mass with soft tissue radiopacity in the area normally occupied by the kidneys (Figure 1). The differential diagnoses included renal neoplasia, hydronephrosis, polycystic kidney disease, pyonephrosis, and a perinephric pseudocyst.

Figure 1. A lateral abdominal radiograph demonstrating the large mass.

Diagnostic testing

The results of a complete blood count were normal. The only abnormality on a serum chemistry profile was mild hyperglycemia (glucose 191 mg/dl; reference range = 63 to 139 mg/dl). All other values were normal, including creatinine (0.9 mg/dl; reference range = 0.7 to 2.4 mg/dl), blood urea nitrogen (23.4 mg/dl; reference range = 17.2 to 31.1 mg/dl), and phosphorus (7.3 mg/dl; reference range = 2.7 to 8.1 mg/dl) concentrations. Additional diagnostic tests were not performed because of financial constraints. The cat's owners opted for exploratory surgery and treatment. Although the owners did not pursue additional diagnostics, a urinalysis to further evaluate kidney function and abdominal ultrasonography and intravenous pyelography to evaluate the renal morphology would have been valuable to help narrow the differential diagnoses before exploratory surgery.

Exploratory surgery and treatment

The cat was sent to a referral center for exploratory surgery. The cat was sedated with the same protocol as before followed by mask induction with isoflurane for intubation and maintenance during surgery. A ventral midline laparotomy was performed. A fluid-filled structure was readily apparent (Figure 2), and about 500 ml of clear, colorless fluid was removed to allow better exposure. The structure was determined to be a perinephric pseudocyst of the left kidney. The kidney within the fluid-filled capsule was grossly abnormal with an irregularly depressed surface. Because there was no clinical evidence of renal disease, a nephrectomy was thought to be in the patient's best interest. The nephrectomy was uneventful, and the cat recovered well. Postoperative pain was controlled with buprenorphine (0.008 mg/kg subcutaneously b.i.d.). The cat was discharged from the referral facility to the owners the next day. The owners administered buprenorphine for three additional days.

Figure 2. An intraoperative view of the perinephric pseudocyst before fluid removal. The pseudocyst contained a total of 800 ml of fluid (transudate).

Pathology

The pseudocyst contained a total of 800 ml of clear, colorless fluid. Fluid analysis revealed a specific gravity of 1.010 and low cellularity and was consistent with a transudate. On gross examination, the kidney had an irregular shape and several areas of infarction and hemorrhage (Figures 3 & 4). Histologic examination of the kidney revealed regions with infiltrates of lymphocytes, plasma cells, and macrophages (moderate multifocal perivascular and interstitial nephritis), as well as mild interstitial and periglomerular fibrosis. Sections of the cyst wall contained large amounts of collagen and fibroblasts.

Figure 3. The opened perinephric pseudocyst with attachment at the renal hilus.

Follow-up

Because of the cat's fractious nature, the cat was not brought back to the clinic for follow up. But when I did see the cat again two months later, it was doing well with no signs of renal disease or abdominal distention. No pseudocyst development in the contralateral kidney was evident. The blood urea nitrogen and creatinine concentrations also remained normal.

Figure 4. The left kidney has been opened longitudinally, showing several areas of infarction and hemorrhage. The surface of the kidney is irregularly depressed.

Discussion

Perinephric pseudocyst formation, characterized by the accumulation of a variable amount of serous fluid in fibrous sacs surrounding one or both kidneys, is a relatively uncommon disease in cats.1-3 Because the cyst is not lined with epithelium, the term pseudocyst is used. The pathogenesis of the perinephric fluid accumulation is not completely understood, but underlying renal parenchymal disease may be a factor.2-5 Chronic interstitial renal disease is often present in association with pseudocysts, and progressive renal parenchyma contraction that impairs venous or lymphatic drainage could result in transudation. The fluid may accumulate in a subcapsular or an extracapsular location and is usually characterized as a transudate, having a low protein content, a low specific gravity, and a low cell count.6,7 It is also possible to find urine (uriniferous pseudocyst) or blood within the pseudocyst. Uriniferous pseudocysts occur from extravasation of urine between the kidney and renal capsule usually because of urinary tract trauma or an obstruction. Blood that accumulates in pseudocysts may be secondary to blood dyscrasias, blood vessel damage from neoplasia, external trauma, aneurysm rupture, or surgery.

Perinephric pseudocysts occur primarily in male cats with a mean age of 11 years.1-7 Cats usually appear outwardly healthy with an increasingly distended, nonpainful abdomen, but signs of concomitant renal failure may be present.3,4 While abdominal palpation reveals what may seem like dramatic renomegaly, radiography demonstrates a large, fluid-filled mass (or masses) in the area occupied by the kidneys, and ultrasonography shows the anechoic fluid surrounding the renal parenchyma.

Treatments for perinephric pseudocysts are ultrasound-guided aspiration of the fluid, combined nephrectomy and cyst resection, and resection of the cyst wall with or without adjunctive omentalization.1,8,9 Because of continual fluid production, needle drainage provides only temporary relief to the patient, lasting from days to months, and should be repeated as needed. Instilling tetracycline into the pseudocysts can prolong the period between recurrences but is associated with extremely high fever in some patients.10 Nephrectomy should be avoided, if possible, when underlying renal disease is present or suspected because of the potential for rapid progression of underlying chronic renal failure in the remaining kidney.1,3 Renal biopsy of the contralateral kidney is useful to identify underlying parenchymal disease, but complications such as hemorrhage and deterioration of renal function must be considered.

Currently, resecting the cyst wall is the most common treatment. If the cyst wall is insufficiently removed, a new pseudocyst may form, necessitating dissection to the renal hilus.9 Because of the continued fluid production from the cyst remnants after capsulectomy and the inability of the peritoneum to remove the excess fluid, ascites can occur.11 Using the omentum to enhance physiologic draining of the fluid has been reported and may be useful in minimizing abdominal distention.8,9 Surgically resecting the pseudocyst wall is often effective in relieving clinical signs, but renal disease will continue to progress. The patient's prognosis is related to the severity of renal dysfunction at the time the perinephric pseudocyst is diagnosed.

The photographs and information for this case were provided by Becky L. Morrow, DVM, Biology Department, College of NaturalSciences and Mathematics, Indiana University of Pennsylvania, Indiana, PA 15705.

REFERENCES

1. Ochoa VB, DiBartola SP, Chew DJ, et al. Perinephric pseudocysts in the cat: A retrospective study and review of the literature. J Vet Intern Med 1999;13:47-55.

2. DiBartola SP, Westropp J. Perinephric pseudocysts. In: August JR, ed. Consultations in feline internal medicine. Vol. 3. Philadelphia, Pa: WB Saunders Co, 1997;341-344.

3. Osborne CA, Finco DR. Diseases of the kidney. In: Canine and feline nephrology and urology. Philadelphia, Pa: Lippincott Williams & Wilkins, 1995;466-470.

4. Lulich JP, Osborne CA, et al. Perirenal pseudocysts. In: Tilley LP, Smith FWK, eds. The 5-minute veterinary consult: canine and feline. 2nd ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2000;1004.

5. Beck JA, Bellenger CR, Lamb WA, et al. Perirenal pseudocysts in 26 cats. Aust Vet J 2000;78:166-171.

6. DiBartola SP. Perinephric pseudocyst. In: Selected diseases of the feline kidney. Lakewood, Colo: American Animal Hospital Association, 1992;11-12.

7. Brace JJ. Perirenal cysts (pseudocysts) in the cat. In: Kirk RW, ed. Current veterinary therapy VIII small animal practice. Philadelphia, Pa: WB Saunders Co, 1983;980-981.

8. Hill TP, Odesnik BJ. Omentalisation of perinephric pseudocysts in a cat. J Small Anim Pract 2000;41:115-118.

9. Inns JH. Treatment of perinephric pseudocysts by omental drainage. Aust Vet Pract 1997;27:174-176.

10. Mattoon J. Renal ultrasound. Available at: www.vet.osu.edu/docs/ClinSci/radiology/urinary/renalus.html. Accessed March 3, 2005.

11. Rishniw M, Weidman J, Hornof WJ. Hydrothorax secondary to a perinephric pseudocyst in a cat. Vet Radiol Ultrasound 1998;39:193-196.