Page 548 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.2 The male reproductive tr act 523

VetBooks.ir within the scrotum with the left hand. The calipers Examination of the internal genitalia
The internal genitalia are examined by rectal pal-
are held in the right hand and the measurement is
taken across the widest part of the scrotum, while
the left hand pushes both testicles firmly downwards pation and ultrasonography. Examination is easi-
est after semen collection and aided by the use of
into the scrotum. stocks. To reduce the risk of rectal tearing or other
Testicular size varies among stallions depending injury, appropriate patient selection and restraint are
on breed, season, age and reproductive status. Each critical. On transrectal palpation, only the seminal
testis of a post-pubertal stallion weighs between 150 vesicles and ampullae are palpable, although all four
and 300 g and measures approximately 50–80 mm accessary glands are visualised readily using ultra-
wide, 60–70 mm high and 80–140 mm long. Testis sonography. The prostate gland is located at the
parenchymal weight correlates highly with daily neck of the bladder, about 10 cm cranial to the anal
sperm production and, therefore, is a useful predictor sphincter, and consists of a central isthmus and two
of breeding potential; however, because parenchymal lobes. The prostatic isthmus measures approximately
weight cannot be measured in the live stallion, esti- 3 × 4 cm in size. Ultrasonographically, its appear-
mates must be used. Ultrasonographic measurements ance varies between homogeneously grey in sexually
may be more accurate than calipers, although proper rested stallions to a more mixed anechoic appearance
placement of the probe across the testes to ensure due to increase fluid production in a teased or sexu-
that a cross-sectional image is obtained is critical. ally active stallion. The vesicular glands give rise
Deviation towards an oblique image dramatically to the gel portion of the ejaculate. They are thin-
affects measurements obtained with this method. walled, paired structures located craniolateral to the
The ultrasound probe is placed longitudinally across prostate, on either side of the ventral midline, and
one testicle, while the examiner’s left hand pushes the may extend over the pelvic brim. In normal stallions
opposite testicle dorsally out of the way. Once the they may be difficult to appreciate except during
length has been measured, the probe can be turned to sexual arousal or following teasing, when they are
a cross-sectional axis in order to measure the width. filled with fluid, or if inflamed. They are symmet-
Turning the probe once again at the pole of the testicle rical and approximately 2 cm wide and 5 cm long,
obtains the height measurement. Testicular volume although their size and ultrasonographic appearance
can then be calculated from the individual measure- can vary greatly, both between stallions and within
ments and used to predict daily sperm production. the same stallion at different examinations. They
As a testis approximates the shape of an ellipsoid, the are identified ultrasonographically by their loca-
following formula is used to convert length, width tion, echogenic wall and a central anechoic lumen
and height measurements into testicular volume: filled with varying amounts of fluid. The paired
ampullae are tubular structures located cranial to
Testis volume = 4/3 π (length/2) (width/2) the prostate and medial to the seminal vesicles, on
(height/2) the pelvic floor. They are palpable at the midline
or = 0.5236 × H × L × W in cm over the bladder neck as approximately 1 cm wide
by 12–25 cm long paired tubular structures. In nor-
Predicted daily sperm production = mal stallions, they are easily palpable, but should
[0.024 × (vol L + vol R)] – 0.76 not be firm or distended (like a banana) as is seen in
stallions with ampullary blockage. On ultrasonog-
Predicted daily sperm production can be com- raphy, the ampullae are identified by a hypoechoic
pared with actual daily sperm production to deter- central lumen, surrounded by a uniformly echogenic
mine spermatogenic efficiency. A stallion whose wall and a hyperechoic outer muscular layer. One or
actual daily sperm production is below that predicted more cystic structures, between the ampullae, may
for his testicular size should be further evaluated for be identified on ultrasonography of stallions. This is
disease conditions of the testes, epididymides and termed the uterus masculinis and represents cystic
accessory glands. remnants of the müllerian duct system. The internal