Ultrasonography of bladder disorders (Proceedings)

The bladder should be thoroughly scanned from left to right in the longitudinal plane and cranial to caudal in the transverse plane. The bladder should be moderately distended for complete evaluation. This fluid-filled structure can be a useful acoustic window for evaluating adjacent structures such as ureters, lymph nodes, vessels, uterus/prostate and colon. 

The normal distended bladder is a pear-shaped structure with a thin wall and anechoic content (urine). It is located on midline in the caudal abdomen ventral to the descending colon, aorta and caudal vena cava. Normal urinary bladder wall thickness can range between 1-3 mm in dogs and 1-2 mm in cats depending on the degree of filling. With increasing filling the wall thickness decreases. The intra-abdominal segment of the urethra should be also evaluated as far caudally as possible, in both male and female pets.  The urinary bladder is a common site to encounter side-lobe and pseudo-sludge artifacts, mimicking echogenic sediment in urine.

Bladder disorders will be divided into 2 categories to ease the presentation: wall abnormalities and content abnormalities.

Wall abnormalities can appear as focal or diffuse thickening, mural nodules or masses, or rupture.  It is important that the bladder be sufficiently distended to evaluate its thickness.  Cranio-ventral wall thickening is most commonly seen in cystitis, although the inflammation/infection can affect more extensively the bladder. Nodules/masses are often seen with neoplasia. Transitional cell carcinoma (TCC) is the most common neoplasm of the urinary bladder. It is commonly an irregular bladder wall mass with a broad-based attachment. The echogenicity is often mixed and has an overall appearance that can be hyperechoic (mineralized foci), isoechoic or hypoechoic compared to the bladder wall.  The masses are most commonly located at the bladder trigone region and dorsal bladder wall, but it is not uncommon to see ventral spread.  If the lesion affect the trigone, unilateral or bilateral hydroureter can occur. It is then possible to see the mass   extending into the proximal urethra. A wide variety of other bladder tumor types are possible including epithelial (squamous cell carcinoma) and mesenchymal tumors (botryoid rhabdomyosarcoma, chemodectoma, leiomyosarcoma, leiomyoma, fibroma, fibrosarcoma, hemangioma, hemangiosarcoma, lymphoma, mast cell tumor). Ultrasonographic differentiation of tumor type and differentiating from non-neoplastic disease is often impossible without a biopsy. However, a mass with a smooth luminal surface is more likely to have a mesenchymal origin. Uncommonly, bladder tumors can diffusely invade the bladder wall. 

The most common urethral neoplasms in dogs include transitional cell carcinoma, squamous cell carcinoma or adenocarcinoma. Proximal urethral neoplasms are often the result of local spread of bladder or prostatic neoplasms. Urethral transitional cell carcinoma has a markedly hyperechoic non-shadowing mucosal line and may be associated with hypoechoic thickening of the urethral wall. Enlargement of the medial iliac, hypogastric, sacral or superficial inguinal lymph nodes is not specific to bladder or urethral disease however they should be located and measured to aid the staging of a suspected neoplastic disease.

Polypoid cystitis may appear as one or several nodules/pedunculated masses projecting within the lumen. A suction biopsy is required for definitive diagnosis.

Gas can be seen within the bladder wall in cases of emphysematous cystitis.

Urinary bladder rupture is a challenging diagnosis to make via ultrasonography. In some cases of known uroabdomen, ultrasonographic findings of a small bladder with a mural defect will lead to the diagnosis; however, it is difficult to find the defect in most cases. Not finding the defect does not rule out bladder rupture. Also, edge shadowing artifact may give the impression of a bladder wall defect that is not actually present. A positive contrast cystourethrogram is a less technically challenging method for evaluating cases of suspected rupture.

Content abnormalities include: gas, calculi, blood clots, sediment, or foreign bodies.  Calculi and mineralized sediment are found in the dependent portion of the bladder and are most commonly associated with strong far shadowing, whereas gas is found in the nondependent portion of the bladder and is associated with "dirty" (gray) shadowing/reverberation.  Gas in the bladder is often iatrogenic (cystocentesis, catheter placement).  Blood clots are often echogenic without acoustic shadowing. They may be adherent to the bladder wall, therefore it may be difficult to distinguish them from neoplasia or polyps. Color flow Doppler examination may assist to distinguish blood clot (no flow) from a mass.

Interventional procedures

Percutaneous cystocentesis can be readily performed with ultrasound guidance. A 22 gauge needle is readily visible as a hyperechoic linear interface in the anechoic lumen of the bladder.

Suction biopsies of bladder and urethral tissue can be obtained using a red-rubber urinary catheter. This technique is used preferentially over trans-abdominal fine needle-aspiration or biopsies, when neoplasia is suspected as it avoids the risk of tumor seeding along a percutaneous needle track.

References

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Leveille R, Biller DS, Paugh Partington B, Miyabayashi T. Sonographic Investigation of Transitional Cell Carcinoma of the Urinary Bladder in small animals. Vet Radiol Ultrasound. 1992;33:103-107.

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Martinez I, Mattoon JS, Eaton KA, Chew DJ, DiBartola SP. Polypoid cystitis in 17 dogs (1978-2001). J Vet Intern Med. 2003;17:499-509.

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Hanson JA, Tidwell AS. Ultrasonographic appearance of urethral transitional cell carcinoma in ten dogs. Vet Radiol Ultrasound. 1996;37:293-299.