maintain a closed hydraulic system to contain the extraordinary pressures generated as the ischiocavernosus muscles rhythmically contract against the blood‐filled crura of the penis [2, 4]. If the integrity of the fibrous tunica albuginea is compromised, anastomoses between the CCP and the surrounding peripenile vasculature may form and provide an escape route for blood contained in the CCP. Should this occur, pressure sufficient to achieve or maintain erection cannot develop [28, 37]. Communication of the CCP with the corpus spongiosum penis will produce a similar result because venous drainage of the CSP is not occluded during erection [29, 38]. Formation of vascular shunts may follow traumatic disruption of the tunica albuginea or be associated with a congenital weakness of the tissues of the tunica albuginea.
The presence of vascular shunts as a cause of erection failure can be suspected based on findings of an observed test mating, and confirmation depends on demonstration of the vascular communication of the CCP and surrounding vasculature or CSP using radiographic contrast studies. Cavernosography (Figure 15.12) utilizes water‐soluble radiographic contrast media (Renografin 76, Squibb Diagnostics, New Brunswick, NJ). Best results are obtained with the bull restrained in lateral recumbency on a tilt table. Extend the penis and place towel clamps under the dorsal apical ligament and apply sufficient traction on the towel clamps to maintain extension of the penis. A length of suture or umbilical tape should be attached to the towel clamps to keep the hands of the assistant out of the radiograph beam. At the same time, place a 30‐cm loop of suture through the skin under the retractor penis muscles and apply traction to pull the more proximal portion of the penis away from the abdomen for better radiographic visualization. Make an initial scout film without contrast media to establish appropriate radiographic settings. While utilizing the towel clamps to extend the penis and the loop of suture under the retractor penis muscles to pull the penis from the abdomen, insert a 16‐gauge needle into the CCP on the dorsum of the free portion of the penis and attach an infusion set of sufficient length to keep the operator's hands out of the radiographic field (Figure 15.21). Inject saline into the CCP to ensure proper placement, and once proper needle placement is ascertained, inject 15 ml of the radiographic contrast media and begin the radiographic series. Have the assistant slowly inject additional contrast media over the next 60 seconds as serial radiographs are taken of as much of the penis as possible. Take enough exposures to allow visualization of the free portion of the penis and penile shaft to the level of the distal sigmoid flexure [39].
Figure 15.21 Bull prepared for cavernosography. Note suture placed through the abdominal skin and under retractor penis muscles to pull the penis away from the abdomen (a). As the bull is positioned, an extension set is attached to a hypodermic needle placed into the CCP of the free portion of the penis, the penis is held in extension with a towel clamp placed under the dorsal apical ligament, and traction is applied to the preplaced sutures under the retractor penis muscles (b).
Formation of vascular shunts is recognized as a potential complication following rupture of the tunica albuginea as described under the section on penile hematoma. Anastomoses which develop following disruption of the integrity of the tunica albuginea and subsequent penile hematoma formation are located at the site of injury, the dorsum of the penis at the level of the distal bend of the sigmoid flexure (Figure 15.22). Penetrating injury to the tunica albuginea at sites other than the location of a penile hematoma may also be followed by shunt formation. Surgical correction of vascular shunts that occur following either type of trauma may be successful.
Figure 15.22 Cavernosogram of the penis of a bull demonstrating escape of contrast media into the peripenile circulation at the level of the distal sigmoid flexure.
Source: Courtesy of Dwight Wolfe.
Cases of erection failure due to multiple vascular shunts involving defects in the tunica albuginea of the free portion of the penis are thought to be the result of a congenital weakness in the structural integrity of the tunica. In such cases there is no history of penile trauma or injury. Affected bulls fail to achieve erection following sexual stimulation and may have bluish discoloration of the penile or preputial skin when attempting to breed or when attempts are made to induce erection with an electroejaculator. Discoloration is the result of blood exiting the CCP through the peripenile vasculature. Multiple distal vascular shunts are most often diagnosed in bulls during the first breeding season. Vascular shunts in the distal penis are readily demonstrable with cavernosography (see Figure 15.13). Unlike vascular shunts that form secondary to traumatic disruption of the tunica albuginea, surgical correction of multiple congenital shunts is unsuccessful.
Filling Defects of the CCP
Erection and maintenance of the normal penile form is dependent on complete distension of the unobstructed cavernous spaces with blood under pressure. If the cavernous spaces are blocked sufficiently to prevent blood from completely filling the CCP, engorgement of the penis can only progress from the proximal portion of the penis to the area of the filling defect [40]. The clinical presentation depends on the location of the obstruction. Complete obstruction of portions of the CCP may result in extension of the penis without erection of the portions distal to the blockage (Figure 15.23). Partial blockade of the cavernous spaces in the distal free portion of the penis may sometimes cause the erect penis to deviate ventrally or laterally.
Figure 15.23 Failure of the distal portion of the penis to become erect due to occlusion of the cavernous spaces of the corpus cavernosum penis.
Cavernosography can demonstrate filling defects in portions of the penis located distal to the sigmoid flexure. Filling defects may be congenital, secondary to fibrosis following trauma, or subsequent to cavernositis.
Denervation Injury
When mounting an estrus female the bull must position himself to make searching motions and use the penis to locate the vulva in order to make intromission. Sensory innervation to the glans and free portion of the penis is necessary for the bull to align the penis, achieve intromission, and successfully complete the breeding act. Without sufficient sensory innervation of the distal penis the bull is unable to locate the vagina and the coital act cannot be completed [41].
Sensory input from the distal penis is transmitted by branches of the paired dorsal nerves of the penis through the pudendal nerves to reach the spinal cord and brain. Although disruption of any portion of the neurologic pathway could result in loss of sensation, damage to the dorsal nerves of the penis is the most likely etiology of penile desensitization. Denervation injury does not interfere with erection, but affected bulls are unable to breed by natural service and usually cannot be successfully collected with an AV. Semen collection with an electroejaculator for artificial insemination is possible.
An observed test mating utilizing an estrus female in a confined area with adequate footing remains the best method to evaluate