PERIRENAL EFFUSION IN DOGS AND CATS WITH ACUTE RENAL

FAILURE

ANDREW HOLLOWAY, ROBERT O’BRIEN

Perirenal fluid accumulation has been described as an ultrasonographic feature of urine leakage, hemorrhage,

abscessation, or neoplasia. The purpose of this retrospective study was to report perirenal effusion as an

additional ultrasonographic finding in canine and feline patients with acute renal failure. The causes of acute

renal failure in 18 patients included nephrotoxicity (4), leptospirosis (3), ureteral obstruction (2), renal lym-

phoma (2), ureteronephrolithiasis (2), prostatic urethral obstruction (1) and interstitial nephritis and ureteritis

(1). An underlying cause was not identified in three patients. The sonographic finding of perirenal fluid was

bilateral in 15 patients. Unilateral perirenal fluid was identified ipsilateral to the site of ureteric obstruction in

two patients. Large effusions extended into the caudal retroperitoneal space. Additional sonographic findings

suggestive of renal parenchymal disease included mild (5), moderate (5) or severe (2) pyelectasia, increased renal

echogenicity (11), increased (9) or decreased renal size (2) and ureteral and/or renal calculi (3). There did not

appear to be an association between the volume of perirenal fluid and the severity of renal dysfunction. All

patients with large effusions underwent euthanasia. Perirenal fluid developing in acute renal failure is thought

to be an ultrafiltrate associated with tubular back-leak into the renal interstitium that overwhelms lymphatic

drainage within the perirenal and retroperitoneal connective tissues although obstruction to urine flow may also

play a role. Localized perirenal retroperitoneal free fluid may be a useful ultrasonographic feature to assist with

the characterization of, and determination of prognosis in, patients with suspected renal disease. Veterinary

Radiology & Ultrasound, Vol. 48, No. 6, 2007, pp 574–579.

Key words: cat, dog, ultrasound, perirenal.

Introduction

PERIRENAL FLUID IN the dog and cat can result from

urine leakage,1,2 hemorrhage,3 abscessation,4 perirenal

pseudocysts,5 and neoplasia.6 Large fluid volumes are seen

with urine leakage and small to moderate collections of

subcapsular or perirenal fluid occur in dogs due to le-

ptospirosis,7 ethylene glycol toxicity8 and, anecdotally, sec-

ondary to acute urinary tract obstruction6 and in cats due

to lymphoma9 and feline infectious peritonitis.

In humans bilateral perirenal fluid has been recognized

as an additional sonographic feature of renal failure caused

by systemic hypertension, glomerulonephritis, acute tubu-

lar necrosis, and sepsis.10 This sonographic finding was

identified in 14% of patients with renal dysfunction over a

1 year period. A similar sonographic finding of perirenal

fluid in renal failure has been recognized in the goat in

nephrotoxicity caused by Narthecium ossifragum,11 a pe-

rennial herb of the lily family, and in ethylene glycol

toxicity and leptospirosis in the dog but it has not been

reported in more recently described nephrotoxicoses

caused by several lily species in the cat12 or raisins and

grapes in the dog13 or with other causes of acute renal

failure. Herein we describe the sonographic features and

underlying causes of perirenal effusion in dogs and cats

with acute renal failure and discusses the potential patho-

physiologic mechanisms responsible for the accumulation

of such effusions.

Materials and Methods

Six cats and 12 dogs with acute renal failure and an

ultrasonographic diagnosis of unilateral or bilateral peri-

renal fluid accumulation were identified in this multicenter

study. Patients with acute renal failure secondary to rup-

ture of the collecting system or lower urinary tract were not

evaluated. The histories were evaluated for species, breed,

age and gender, clinical laboratory results, sonographic

findings, final diagnosis, and clinical outcome.

The degree of renal dysfunction was categorized as

moderate in patients with a measured blood creatinine

between 2.50–5mg/dl (normal range 0.6–1.2mg/dl), and as

severe renal dysfunction in patients with a measured blood

creatinine of 45mg/dl.14

The volume of perirenal effusion in these patients was

subjectively classified as small where a thin layer of an-

Address correspondence and reprint requests to Andrew Holloway, atthe above address. E-mail: andrew.holloway@aht.org.uk

Received November 5, 2006; accepted for publication April 17, 2007.doi: 10.1111/j.1740-8261.2007.00300.x

From the Department of Veterinary Medicine, University of Cam-bridge, Madingley Road, Cambridge, CB30ES, UK (Holloway), Depart-ment of Clinical Sciences, 0221 Mosier Hall, Kansas State University,Manhattan, KS 66506 (O’Brien).

574

echoic fluid surrounded the kidney alone, and moderate

where, in addition to the small volume of perirenal fluid, a

triangular pocket of fluid surrounded the caudal pole of the

kidney. Large effusions were classified as perirenal fluid

greater in size than the thickness of the adjacent renal cor-

tex and which extended markedly into the caudal retro-

peritoneal space. A large effusion was also termed perirenal

retroperitoneal to denote the extent of the effusion.

Results

Of the six cats there were four domestic Shorthair, one

Burmese and one Siamese. Of the 12 dogs there was one

crossbreed and 11 purebreds. Of the purebred dogs there

were two German Shepherds, two miniature Schnauzers,

and two Labrador Retrievers and one Golden Retriever,

Cavalier King Charles Spaniel, Bull Mastiff, Great Dane

and Yorkshire Terrier. The age of affected cats ranged

from 2 to 9 years old with a mean of 6 years. The age of

affected dogs ranged from 8 months to 10 years with a

mean of 6 years. Nine patients were male (of which eight

were neutered) and nine were female (of which eight were

neutered).

The severity of acute renal failure was classified as mod-

erate in three patients and severe in 13 patients. In two

patients, a dog with moderate hydronephrosis due to a

ureteral calculus and in a cat with severe hydronephrosis

due to a ureteral stricture, creatinine was normal and

slightly raised, respectively. Despite supportive treatment

by the referring veterinarian, four patients were anuric and

two oliguric at the time of presentation.

Perirenal fluid was recognized either as an extracapsular

accumulation of fluid conforming to the renal margin or as

a fusiform or triangular fluid accumulation surrounding

the caudal pole of the affected kidney on dorsal or sagittal

plane images (Fig. 1). On transverse plane images perirenal

fluid was recognized dorsomedial and dorsolateral to the

kidney (Figs. 2 and 3) but did not extend along the ventral

border of the kidney. In several patients with large effu-

sions the caudal pole but not the cranial pole of the affect-

ed kidney was surrounded by fluid. Hyperechoic fusiform

to angular structures consistent with retroperitoneal fat

were identified within (Fig. 4) or at the periphery of the

retroperitoneal fluid accumulations. Large retroperitoneal

fluid accumulations could be followed caudally beneath the

hypaxial muscles (Fig. 5) and recognized dorsal to the

bladder at the pelvic inlet.

Fig. 1. Dorsal plane ultrasonographic image of the caudal pole of the leftkidney of a lactating bitch in acute renal failure. Anechoic retroperitonealfluid (arrow) is present surrounding the caudal pole and has a triangular tofusiform appearance. The cause of acute renal failure in this dog was notdetermined.

Fig. 2. Transverse plane ultrasonographic image of the left kidney of acat. Dorsal is to the right of the image and ventral is to the left. Notemoderate pyelectasia (arrowhead) and perirenal fluid dorsolateral (arrow) tothe kidney. Ureteric obstruction was suspected but not proven to be thecause of hydronephrosis and hydroureter in this patient. The right kidney(not shown) had an uneven margin and an overall, reduced corticomedullarydifferentiation and overall increase in renal echogenicity suggestive of chron-ic nephritis.

Fig. 3. Transverse plane ultrasonographic image caudal to the hilus of theright kidney of a dog with acute renal failure due to Leptospira Bratislava.Anechoic perirenal retroperitoneal fluid is recognized dorsomedial (arrow)and dorsolateral (arrowhead) to the kidney but does not extend along theventral margin of the kidney.

575PERIRENAL RETROPERITONEUM EFFUSIONVol. 48, No. 6

In 15 of the 18 patients the perirenal fluid accumulations

were bilateral. In two of the three patients with a unilateral

fluid accumulation there was hydronephrosis and hydro-

ureter due to ureteral obstruction from a calculus or stric-

ture (Fig. 6), respectively. The perirenal effusion in these

patients was ipsilateral to the ureteric obstruction. In the

third patient with unilateral effusion both kidneys were

abnormal due to lymphoma.

The volume of perirenal fluid was considered small in 10

patients, including six where the renal failure was catego-

rized as severe. In one patient with a moderate volume of

perirenal fluid renal dysfunction was classified as moderate.

In the other seven patients a large volume of perirenal

retroperitoneal fluid was present and the severity of renal

failure in these patients was considered severe.

Additional sonographic findings suggestive of renal

parenchymal disease included mild (5), moderate (5) or

severe (2) pyelectasia, increased renal echogenicity (11),

increased (9), or decreased renal size (2) and ureteral

and/or renal calculi (3).

Fig. 4. Transverse plane ultrasonographic image (A) of the left retro-peritoneal space caudal to the kidney of a dog with renal, ureteric, andbladder lymphoma. The marked accumulation of fluid with the retroperi-toneal space has a ‘‘marble-like’’ appearance (arrow) due to alternatingbands of retroperitoneal fat and fluid. Photograph of the right kidney andright retroperitoneal space (B) of the same dog at postmortem. There ismassive enlargement of the right caudal retroperitoneal space (arrowhead)due to a fluid accumulation caudal to the right kidney. The appearance of theleft retroperitoneal space was similar. Note the absence of retroperitonealfluid around the ventral border and cranial pole of the right kidney (arrow).

Fig. 5. Transverse plane ultrasonographic image of the dorsal abdomencaudal to the left kidney of a dog in acute renal failure due to LeptospiraBratislava. Note the fluid (white arrow) within left caudal retroperitonealspace fluid. Aorta (AO, black arrow) and caudal vena cava (CVC, whitearrow head).

Fig. 6. Dorsal plane ultrasonographic image of the right kidney of a catwith a ureteral stricture. There is extensive hydronephrosis including dilationof the renal diverticuli. Caudal to the kidney an angular accumulation ofperirenal fluid is evident. The retroperitoneal fat has a mottled appearance.

576 HOLLOWAYAND O’BRIEN 2007

An underlying cause of the renal failure was identified in

15/18 patients based on histopathologic examination of

renal tissue (4), known historical exposure to toxins (3),

serology (3), surgical findings (2), and imaging findings (3).

Nephrotoxicity was confirmed in four patients and includ-

ed lily, raisin, and ethylene glycol (n¼ 2) toxicities. Ureteral

obstruction was the cause of renal failure in three patients

(calculus, stricture, and distal obstruction presumed due to

prostatic neoplasia). Infection was the cause of renal failure

in four patients (leptospirosis (n¼ 3), interstitial nephritis

and pyogenic ureteritis (1)). Acute decompensation of pre-

existing chronic renal failure was identified in one patient

with bilateral renal and ureteric calculi and in another with

bilateral renal calculi. Lymphoma was identified in two

patients. In three patients a cause of the renal failure was

not determined.

All patients with a large volume of perirenal retroperi-

toneal fluid were euthanased due to a poor response to

medical management. This included all patients with an-

uric or oligouric renal failure. Two patients with a small

perirenal effusion underwent euthanasia. The remaining

eight patients with a mild effusion, and one with a mod-

erate effusion were discharged.

Discussion

Renal changes recognized sonographically in acute renal

failure are often nonspecific, including increased renal size,

increased cortical echogenicity (in acute tubular necrosis,

acute interstitial nephritis, hypercalcemic nephropathy)

and occasionally abnormalities of the renal pelvis.6–8 In

our patients small perirenal to large perirenal retroperito-

neal fluid accumulations were found in animals with acute

renal failure in the absence of rupture of the urinary tract.

Although excretory urography was not performed in any

patient here, rupture was excluded on the basis of histor-

ical, clinicopathologic, postmortem, or ultrasonographic

findings. Perirenal fluid has been noted in acute renal

failure due to ethylene glycol toxicity,8 leptospirosis,7 and

anecdotally secondary to obstructive uropathy but an

association with acute renal failure itself has not been

made.

Sonographically, perirenal fluid may be difficult to dis-

tinguish from subcapsular fluid. Perirenal fluid is not clear-

ly defined by a capsule but conforms, as previously

reported, to the margin of the kidney and may have a

pointed shape on the nonrenal border confluent with the

retroperitoneal space.7,8 In several patients reported here

perirenal fluid was extensive within the retroperitoneal

space caudal to the kidney.

The sonographic recognition of perirenal fluid may be

influenced by the anatomy of the retroperitoneal space.15,16

The volume of the retroperitoneal space is small except in

obese individuals due to the accumulation of adipose

tissue. Based on dye infusion studies in the dog, partial

compartmentalization within the retroperitoneal space has

been identified and prerenal, perirenal, and caudal retro-

peritoneal spaces reported.16 Communication between

these compartments (spaces) may occur with large vol-

umes of fluid.16 The sonographic appearance of these

compartments has not been described but their presence

likely accounts for the dorsomedial and dorsolateral accu-

mulation of fluid around the caudal pole of the kidney and

the absence of fluid around the ventral margin of the kid-

ney in several patients with large effusions documented

here.

Excluding rupture within the upper urinary tract, the

mechanism by which fluid accumulates in the perirenal

retroperitoneal space in acute renal failure is uncertain.17,18

Proposed mechanisms include pyelolymphatic drainage in-

to the perirenal space in obstructive renal disease or due to

the tubular back-leak following increased permeability of

the highly metabolically active proximal tubular epithelium

in nephrotoxic or ischemic acute renal failure.19,20 Drain-

age of ultrafiltrate from within the interstitium via renal

capsule lymphatics into the retroperitoneal space is sug-

gested to occur. In one patient with ethylene glycol toxicity

reported here, dilated lymphatics extending from the renal

pelvis to an edematous renal capsule were identified histo-

pathologically. Retroperitoneal, and particularly perirenal,

edema is reported in pigs and cattle due to acute tubular

necrosis caused by ochratoxin or A. retroflexus (redroot

pigweed), and in oak poisoning (Quercus sp.)17,18 in cattle.

Perirenal edema in cattle has also been a feature of ne-

phrotoxicity caused by the shrub Nolletia Gariepina21 and

the herbaceous perennial, Narthecium asiaticum, a member

of the Lily family.22 Severe interstitial renal edema is also

reported in ethylene glycol,8 lily poisoning,12 and in other

causes of acute tubular necrosis in the dog and cat.17,18 In

the rat,19 nephrotoxin-induced acute renal failure led to

perinephric retroperitoneal effusion with a distribution and

severity (dose dependent) similar to those observed in our

patients. In that study19 the kidney was the source of per-

inephric fluid, probably from glomerular filtrate, as fluid

did not develop following ligation of the renal hilum or

unilateral nephrectomy. This proposed renal origin of fluid

may explain the predominance of bilateral perirenal fluid in

15 of the 18 patients reported here.

In two of our patients with confirmed ureteral obstruc-

tion, fluid accumulated ipsilaterally suggesting that

obstruction may play some role in perirenal fluid accumu-

lation. Furthermore, partial compartmentalization of the

retroperitoneal space may have been partially responsible

for the unilateral distribution of fluid. In obstructive

hydronephrosis in humans23 it is suggested that glomerular

filtration continues, even in sudden complete obstruction,

resulting in pyelolymphatic drainage of filtrate into the

perirenal space. This potential for perirenal fluid accumu-

577PERIRENAL RETROPERITONEUM EFFUSIONVol. 48, No. 6

lation in ureteral obstruction, particularly acute obstruc-

tion has also been documented using MR imaging in hu-

mans with sequences very sensitive to fluid.24 Further

investigation is required in the dog and the cat to determine

whether such effusions are recognized sonographically.

Experimentally induced perirenal effusion19 has a very

low protein content, suggesting that it is an ultrafiltrate and

that some residual renal function is required for its for-

mation. An increase in vascular hydrostatic pressure or

reduced vascular oncotic pressure could also result in the

accumulation of such a low protein fluid in the patients

described here. It is possible that aggressive intravenous

fluid loading in the face of minimal urine production may

in itself, by these mechanisms, have lead to the accumu-

lation of the perirenal fluid in our patients. However no

patient had pulmonary edema, pleural effusion, ascites, or

clinical changes suggestive of fluid overload. Hence al-

though it is recognized that continued fluid administration

is necessary for the formation of perinephric fluid19 the

mechanism(s) by which this fluid forms, and continues to

accumulate, as noted in four patients in anuric renal fail-

ure, is unclear. It would be useful to examine the perirenal

space in patients with acute renal failure before and after

fluid therapy to determine the influence of fluid loading on

the appearance of such fluid accumulations.

In experimental, nephrotoxin-induced perirenal effu-

sion19 the severity of the effusion was dose related. In our

patients, as in one human study,10 the severity of renal

failure did not seem to be associated with the volume of the

perirenal fluid. In several patients classified with severe

renal failure only small fluid accumulations were noted

sonographically. However all patients with large perirenal

retroperitoneal effusions in this study failed to respond to

aggressive medical therapy and underwent euthanasia sug-

gesting that large volumes of fluid together with severe

renal dysfunction may be a poor prognostic finding.

Aspiration of perirenal retroperitoneal fluid was at-

tempted in several of our patients, but even with large

effusions this was not successful. At postmortem examin-

ation, the affected retroperitoneal tissue invariably had a

gelatinous or edematous texture accounting for nondiag-

nostic aspirates.

A further factor to consider in two of the patients de-

scribed here is the role that widespread loss of ureteral

transitional epithelium, identified in lymphoma and nec-

rotizing ureteritis, may have had in the development of

fluid accumulation. The normal urothelium is a very dense

structure and usually responds to inflammation by prolif-

eration and metaplasia hence in its absence in the above

patients it is speculated that direct leakage of urine into the

perirenal space may have occurred.

In three patients presented here an underlying cause of

the renal failure was not determined but these patients are

included as they had severe acute renal failure, perirenal

fluid and no history or evidence of trauma. In one lactating

bitch, acute renal failure developed shortly after parturi-

tion. In humans bilateral renal cortical necrosis is linked

with pregnancy-associated hypertension25 but no such as-

sociation has been made in the dog nor could this be

demonstrated in our patient. In two cats in which an un-

derlying cause of the renal failure was not determined acute

decompensation of preexisting renal failure was suspected

as a diffuse, heterogeneous increase in renal echogenicity

and loss of corticomedullary differentiation was present

that was considered suggestive of chronic nephritis.

Conclusion

The group of patients described here suggest an associ-

ation between acute renal failure and perirenal retroperi-

toneal fluid accumulations. The fluid accumulation may be

unilateral or bilateral, depending on the symmetry of the

underlying lesion or whether the patient has had a systemic

insult. Although the mechanism by which perirenal fluid

develops is uncertain, its presence, location and extent may

be important. Severe perirenal retroperitoneal effusion may

be a poor prognostic factor in patients with severe renal

dysfunction.

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