Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Three The Abd omen 359
The retrograde urethrogram will evaluate the entire urethra. Water-soluble radiopaque
contrast medium is used at a dosage of 0.5 ml/kg (0.25 ml/lb) with a maximal volume per
injection of 20 ml for the largest male dogs. The study should be performed carefully to
avoid air bubbles in the contrast medium, because these may be mistaken for radiolucent
calculi. A catheter with a balloon near the tip (e.g., Foley) is recommended. 438,451 The tip
of the catheter should be positioned just beyond the urethral papilla in the female and just
caudal to the os penis in the male. The injection should be made manually with as much
force as is easily obtained, and the radiograph should be exposed as the last portion of contrast
is injected. Both lateral and ventrodorsal views should be made. The ventrodorsal in
the male should be slightly oblique to prevent superimposition of the distal and proximal
urethra. Distention of the urinary bladder prior to retrograde urethrography is not always
necessary. 428,433,451-460
Sedation or anesthesia are not required but are recommended to reduce the chance that
the patient will move during the contrast injection. In addition, sedation or anesthesia
helps reduce the possibility of infection by limiting movement-induced catheter contamination
during insertion into the urethra.
Trauma to the urethra and postcatheterization infection are the only complications
resulting from contrast urethrography. 450,461,462 Contrast material may gain access to the
systemic circulation if rupture or ulceration of the urethral mucosa is present. This is not
a problem if positive-contrast material is used. However, when air has been used, deaths
from air embolism have occurred. 443,444
Vaginourethrography. In female dogs and cats, especially when ectopic ureters are suspected
or the animal is too small for urethral catheterization, vaginourethrography may be
performed. This study is performed by placing a Foley or other balloon-tipped catheter
into the vagina distal to the urethral papilla and injecting iodinated contrast into the
vagina. The urethra usually will fill with contrast as well. An atraumatic forceps usually is
required to close the vagina, keep the catheter in place, and prevent contrast leaking to the
outside. 463,464 This technique also can be used for the evaluation of urethrorectal or rectovaginal
fistulas. Anesthesia may be required to keep the patient from struggling during
catheter and forceps placement or contrast injection.
Normal Retrograde Urethrogram. In the male dog, there are three portions of the urethra.
These include the prostatic portion, which is surrounded by the prostate gland; the
membranous portion between the prostatic portion and the ischial arch; and the penile
portion extending from the ischial arch to the external urethral orifice. During imaging, the
urethral width varies depending on the degree of bladder distention at the time of urethrography
and the volume and pressure of the retrograde injection during radiographic
exposure. Depending on the technique used, the male canine urethra may be either of (1)
relatively uniform width except for a slight narrowing at the pelvic brim, bladder neck, and
at the ischial arch or (2) nonuniform width wherein the prostatic portion is wider than
either the membranous or penile portions and there are no focal narrowings that correspond
with anatomical location, although some may be seen due to spasm, particularly in
the proximal membranous portion.* An indentation of the prostatic portion of the urethra
may be present dorsally because of the colliculus seminalis. 465 Reflux of a small amount of
contrast into the prostate is normal. 449,450 The urethra of the male cat is narrowest in the
penile portion, with minimal diameter variation from there through the vesicourethral
junction when reasonable retrograde pressure is applied. 454-456 Although opinions vary,
there are apparently no significant effects of prepuberal gonadectomy on urethral diameter.
466 The urethra of the female dog and female cat is short and wide and tapers to the
external sphincter, but it is like that of the male in that there is variability depending on
bladder fullness and the volume and pressure of the retrograde injection during radiographic
exposure. 434,435,454,455
*References 435, 449-452, 456, 457, 460.