Page 684 - Small Animal Internal Medicine, 6th Edition

656 PART V Urinary Tract Disorders

have extremely hyposthenuric urine (i.e., USG = 1.001–1.003). recognized as an incidental finding. The kidneys of cats with
Although not consistently present, mild hyponatremia in a ADPKD become progressively enlarged and irregular over
VetBooks.ir dog with marked hyposthenuria is suggestive of PPD. Dogs time as the cysts increase in number and size. Renal failure
usually does not develop until 7 or 8 years of age. Currently,
with PPD of recent onset often have a normal response to
abrupt water deprivation testing, but those with long-stand-
ADPKD in affected cats. In one study, renal ultrasonography
ing PPD develop renal medullary washout of solute because ultrasonography is the clinical test of choice for identifying
the release of vasopressin from the pituitary gland is sup- had a sensitivity of 75% when performed at 4 months of age
pressed by plasma hypo-osmolality. Vasopressin normally and a sensitivity of 91% when performed at 9 months of age
facilitates urea reabsorption in the inner medulla of the for the diagnosis of ADPKD. Primary and metastatic renal
kidney and helps maintain medullary hypertonicity. Gradual tumors can cause renomegaly, and lymphosarcoma often is
water deprivation testing allows time for restoration of the responsible, especially in cats. Renal lymphoma is usually
renal medullary solute gradient and is the preferred diagnos- bilateral in cats and often is associated with the alimentary
tic test in dogs with PPD. Treatment of PPD is accomplished form of the disease. Occasionally renal involvement may
by gradual water restriction into the normal range over be extensive enough to cause renal failure. The diagnosis
several days. of renal lymphosarcoma often can be made by cytologic
evaluation of a fine-needle aspirate of the kidney that identi-
RENOMEGALY fies a monomorphic population of immature lymphocytes.
Renomegaly refers to enlargement of the kidney. It may be Renal obstruction by a nephrolith or ureterolith, or inad-
unilateral or bilateral, and bilateral enlargement may be sym- vertent ligation of a ureter during ovariohysterectomy, can
metric or asymmetric. Renomegaly may be acute or chronic result in hydronephrosis and renomegaly. Uncommonly,
in onset. In most animals with renomegaly, onset is insidi- ureteral fibrosis, secondary to previous ureterolith removal
ous. Acute renomegaly is uncommon and when it occurs or idiopathic in nature, can result in hydronephrosis. Blunt
(e.g., acute obstruction of a kidney by a nephrolith), the abdominal trauma can cause subcapsular hemorrhage and
presentation is that of acute abdomen (i.e., abdominal pain, renomegaly, but the renal capsule usually limits the extent of
reluctance to move, vomiting). Chronic renomegaly typically enlargement. Bacterial infection can result in a renal abscess
is moderate or severe but occasionally can be mild. For or pyonephrosis. The noneffusive form of feline infectious
example, mild enlargement may occur in some dogs with peritonitis often affects the kidneys, liver, mesenteric lymph
renal amyloidosis and in some cases of acute renal failure nodes, central nervous system, and eyes. Renal involvement
(e.g., leptospirosis) because of renal swelling. The renal occurs in many affected cats, and they may have enlarged,
capsule, however, limits the extent of acute swelling that can irregular kidneys on abdominal palpation.
occur. Unilateral renal enlargement can occur because of
compensatory hypertrophy in animals with a solitary kidney Perinephric Pseudocysts
or with severe end-stage disease in the contralateral kidney. Although not a renal disease, perinephric pseudocysts can
Normally, feline kidneys are 3.5 to 4.5 cm in length and easily be confused with renomegaly on routine abdomi-
can be readily palpated in cooperative cats. The size of canine nal palpation. Perinephric pseudocysts are fluid-filled
kidneys varies according to body size, and generally kidneys fibrous sacs that surround the kidney but are not lined by
are more difficult to palpate in dogs than in cats. Occasion- epithelium—hence the term pseudocyst. They are idiopathic
ally the left kidney can be palpated in a cooperative dog. in origin but often occur in association with chronic kidney
Renal length and volume in dogs is correlated with body disease (CKD) in older (>10 years) cats of both sexes and any
weight. For example, dogs up to about 15 kg in weight are breed. Most cats with perinephric pseudocysts have at least
expected to have kidneys 3 to 5.5 cm in length, whereas dogs mild CKD. Occasionally small kidneys and CKD are diag-
in the 30- to 45-kg range are expected to have kidneys 7 to nosed before the development of perinephric pseudocysts.
8 cm in length. A rule of thumb that has been used for many The presenting complaints may be related to underlying
years is based on the ratio of the length of the kidney as CKD (e.g., PU-PD, anorexia, and weight loss), but usually
observed on plain abdominal radiographs to the length of abdominal distention is the only abnormality detected by
the second lumbar vertebra (L2). On plain abdominal radio- the owner. Perinephric pseudocysts may be unilateral or
graphs, this ratio is 2.5-3.0-to-1 in cats and 2.5-3.5-to-1 in bilateral. Renal ultrasonography is the diagnostic test of
dogs. choice and demonstrates an accumulation of anechoic fluid
Disorders that cause renomegaly include polycystic between the capsule and renal parenchyma of one or both
kidney disease, neoplasia, and obstruction. Renomegaly can kidneys (Fig. 38.4). Definitive treatment involves surgical
occur in dogs and cats, but it is more common in cats. Poly- resection of the pseudocyst capsule. The kidney should not
cystic kidney disease is inherited as an autosomal dominant be removed in cats with unilateral perinephric pseudo-
trait in Bull Terriers and Persian cats (autosomal dominant cysts because progression of renal disease in the remnant
polycystic kidney disease [ADPKD]). It is caused by a muta- kidney can be accelerated dramatically, and renal failure may
tion in exon 29 of the polycystin-1 gene in Persian cats, and worsen rapidly (Fig. 38.5). Recently, a laparoscopic approach
prevalence in the breed approaches 30%. Many young Persian has been used for fenestration or subtotal resection of peri-
cats with ADPKD are asymptomatic, and renomegaly is nephric pseudocyst capsules. Ultimately, the prognosis of