Use of Ultrasonography in the Normal and sub-fertile Mare
Introduction Ultrasonography provides the opportunity to :
In addition ultrasonographic evaluation of the uterus and ovaries assists in:
Recently the usefulness of ultrasound as a diagnostic tool for subfertile mares has become apparent. Ultrasonography can be helpful in the following situations:
Nowadays, diagnostic ultrasound plays a pivotal role in the reproductive management of the mare and should be ROUTINE at every gynaecological examination of a mare. A thorough understanding of normal ultrasonographic anatomy is vital for veterinarians involved in broodmare work. Ultrasonography is non-invasive and can be confidently used for repeated examinations of the reproductive tract without impairment of breeding potential or adversely affecting the conceptus. . For routine mare gynaecological work a 5 MHz linear-array transducer is the most satisfactory, offering a compromise between a reasonable depth of penetration combined with adequate tissue resolution. It is essential to understand that the ultrasound machine is only as good as the person using it. Experience and accurate interpretation of the image obtained are VITAL.
Patient Preparation and Imaging Technique The scanner should be as close to eye level as practicable and the control panel of the machine within easy reach of the operator. The scanner can be placed either side of the mare. Where the operator's left hand holds the transducer, the scanner is placed obliquely to the right side of the mare's hind-quarters allowing the right hand to make any notes or adjustments to the controls. To facilitate correct orientation of the transducer, a groove for the finger of the operator is usually located on the transducer, on the opposite side to the working face. The fingers should always be in front of the transducer as it is being introduced and later manipulated rather than pushing the transducer on ahead. For reasons of hygiene, it may be desirable to have the transducer in a plastic sleeve. Coupling gel should be used to exclude air from between the transducer and its protective cover. Using copious amounts of lubricant, which also acts as a coupling medium to ensure good contact and prevent air interference, the transducer and hand are gently inserted into the rectum. Should the mare strain, the examination should be stopped and one should wait for the rectum to relax. However, straining is usually not a significant problem. It is best to examine the reproductive tract systematically and to scan the entire uterus and both ovaries at least twice. The transducer is held within the rectum in the longitudinal plane and since the uterus of the mare is 'T-shaped', the uterine body appears as a rectangular image in the longitudinal plane. When scanning the uterine body, it is important to move the transducer forwards and backwards and from side to side so that no feature is missed. The transducer should be moved slowly at all times. To image the uterine horns and ovaries the transducer should be rotated slowly to the right and then the left side. Therefore, the uterine horns appear as circular images in cross-section. If difficulties are encountered with finding a structure, the transducer can be withdrawn a short distance and the structure located by palpation. Ultrasound examination can then be resumed. On an ultrasound image fluid does not reflect sound waves and appears black on the screen the image being termed anechoic. Such an image would be given by follicular fluid etc. Dense tissues such as fetal bone strongly reflect sound waves and the image appears white on the screen, being correctly termed hyperechoic. Soft tissues, fluid in the uterus etc reflect sound waves to produce an image of varying grey shades termed hypoechoic.
Use of Ultrasonography in the Normal Mare
Breeding Management : Timing of Breeding/Insemination The mare has a long and variable oestrous period, mating should take place within the time frame of 24 hours before to 4 hours after ovulation for maximum pregnancy rates. This mean we should:
Breeding Management : Rectal/Ultrasound Examination Findings
Breeding Management : Control after Breeding Normal mare:
Not ovulated?
Abnormal mare:
Endometrial Ultrasonographic Appearance Dioestrus
Homogenous echotexture
Oestrus
Heterogenous echotexture The grading of endometrial oedema is a useful additional aid to predicting ovulation
time: endometrial oedema scores generally decline 24 to 36 hours before ovulation Use of Ultrasonography in the Sub-fertile Mare Sub-fertility in the Mare There are many causes of subfertility which can act either alone or in combination with
each other. The causes can be categorised : Infectious subfertility will be dealt with in the next article and the remainder of
this article will focus on use of ultrasonography to diagnose structural and functional
causes of subfertility. Mare Subfertility Non-infectious : Structural Mare Subfertility Non-Infectious: Functional Anestrus a). Anestrus caused by ovarian quiescence i) Winter anestrus b). Anestrus caused by prolonged luteal function c). Anestrus caused by behaviour i) Prolonged diestrus Shortened Luteal Phase a). endometritis Irregular or Prolonged Estrus a).Transitional (`spring') estrus The specific conditions where ultrasound has an important application will now be
considered in more detail and for the purposes of this talk will be considered under
either ovarian or uterine abnormalities. However, it must be remembered that several of
the conditions involve both the ovaries and the uterus. OVARIAN ABNORMALITIES 1. Lack of Ovulatory Follicles/ Silent Ovulation During the transitional period before the first ovulation of the year mares demonstrate
erratic oestrous behaviour of varying intensity. The presence of multiple large follicles,
possibly as large as 30 mm, makes detection of ovulation difficult by palpation alone.
Even outside this transitional period, misinterpretation of ovulation, even by experienced
palpators has been shown to be as high as 50% in some studies. It is much easier to
visualise the corpus haemorrhagicum/early corpus luteum with ultrasound when the anechoic
follicle is replaced by an intensely echoic area representing the early corpus luteum. Because of the considerable variation in the duration of oestrus during the
transitional period, efficient breeding of the mare can be difficult. It is recommended
that the interval between matings should not exceed 2 or 3 days, although there have been
no critical studies on the survival time of sperm in the mares' genital tract. It is
important not to begin mating too early or this will result in the mare being mated many
times. The appearance of uterine oedema is an indication that the follicle should ovulate
within a few days. A key factor in the emergence out of vernal transition is the
development of steroidogenic competence by the follicle leading to an increase in
circulating oestrogen concentrations which cause release of LH from the pituitary due to a
positive feedback mechanism. Oestrogen is responsible for the appearance of uterine oedema
(in the absence of progesterone) and so this may be why the detection of uterine oedema is
important in signalling the emergence of the mare from the transitional period. Double ovulation can be more accurately diagnosed than by rectal examination alone,
particularly when the follicles are adjacent to each other in the same ovary. 2. Prolonged Luteal Activity Persistence of luteal activity in the non-pregnant mare is a major cause of
subfertility and is the main cause of anoestrus during the breeding season. Traditionally,
the term `prolonged dioestrus' has been used to describe a condition where the function of
the corpus luteum continues beyond its normal cyclical lifespan of 15/16 days, resulting
in the maintenance of elevated circulating progesterone concentrations for longer than
expected. Recently, Ginther, in reviewing the condition, has suggested that the term
`prolonged luteal activity' should be used, as `persistent dioestrus' implies that the
corpus luteum persists, whereas it is possible that others are formed sequentially from
dioestrous ovulations. These occur in up to 20% of oestrous cycles in Thoroughbred mares
(less frequently in ponies) and are not accompanied by oestrus; the cervix will remain
pale in colour, dry and tightly closed. If dioestrous ovulations occur late in the luteal
phase they will be refractory to the effect of endogenous luteolysins, resulting in a
persistent luteal phase. True persistence of the corpus luteum occurs in approximately 20% of ovulations. These
mares present great difficulty to the stud manager as they can be assumed incorrectly to
be pregnant. Diagnosis Treatment Pyometra can also be the cause of prolonged luteal activity. Pyometra (see also later)
is the accumulation of substantial quantities of inflammatory exudate in the uterus
causing its distention. When the endometrium is severely damaged, there is extensive loss
of surface epithelium, severe endometrial fibrosis and glandular atrophy causing a
prolonged luteal phase, presumably due to interference with the synthesis or release of
PGF2alpha. This is in contrast to mild endometritis with collection of small amounts of
intraluminal uterine fluid, which is more likely to cause premature release of PGF2alpha
and luteolysis. Pseudopregnancy (the term used to describe a syndrome in which non-pregnant mares that
have been bred do not return to oestrus) occurs if there is early embryonic death after 15
days of gestation with persistence of the corpus luteum verum resulting in a prolonged
luteal phase. The cervix remains tightly closed and the uterus is tense and tubular. It is
differentiated from pregnancy by the absence of a conceptus on ultrasound examination. If
early fetal death occurs after endometrial cup formation at 36 days, mares will either
become anoestrus or come into oestrus. However, in the latter, follicular luteinization
without ovulation is thought to occur and therefore the oestrus is not fertile; this will
last until the endometrial cups regress spontaneously at 90--150 days. There is currently
no practical way of destroying endometrial cups prematurely. 3) Anovulatory Haemorrhagic Follicles The most common form of ovulation failure in mares is when the preovulatory follicle
fails to rupture or collapse, before filling with blood; the follicle in the mare normally
fills with blood after ovulation. The condition is known as 'haemorrhagic anovulatory
follicle syndrome'. In one recent study, 12 cases occurred in eight mares during 213
ovulatory intervals monitored by ultrasound. Where this occurs, the preovulatory follicle fills with blood and is initially
recognised, using transrectal ultrasound, by the presence of scattered free-floating
echoic spots within the follicular antrum. As the blood coagulates, the ultrasonic
appearance varies from a speckled to a uniformly echoic mass. Occasionally anechoic
sinuses appear as the serum is compartmentalised by the fibrin lattice network. These
structures can be as large as 8--10cm, occasionally much larger, and develop an outer wall
of luteal tissue. Functionally, they gradually regress in the same way as a normal corpus
luteum, but they remain visible ultrasonically over subsequent oestrous cycles. No
treatment is usually necessary. Sometimes they may also fail to regress around day 14 to
15 of the cycle and persist. The condition is difficult to diagnose on palpation alone. Ultrasonographic imaging is useful, although the structures may have a similar
appearance to that of a granulosa-theca-cell tumour (GTCT): the anechoic areas are
separated by trabeculae and are similar to those of a multicystic GTCT. The diagnosis of a
haemorrhagic follicle may be made on the basis of clinical signs, namely maintenance of
cyclicity, a normal contralateral ovary, the presence of an ovulation fossa and speed of
enlargement and regression of the ovary with time. The cause of these haemorrhagic follicles is not known. Similar structures are seen
under continued equine chorionic gonadotrophin (eCG) stimulation during days 40--150 of
pregnancy. 4. Ovarian tumours Ovarian neoplasia is uncommon in the mare although many types of tumour have been
described, with the granulosa theca cell tumours (CTCTs) being by far the commonest. Of
the other types, teratomata and cystadenomata are the next most frequently identified.
Teratomata, which are composed of different tissue types, are difficult to differentiate
clinically from GTCTs. GTCTs arise from the sex cord stromal tissue within the ovary and are frequently
hormonally active, producing variable amounts of steroids which cause behavioural changes
and alteration to normal cyclical activity. Mares may exhibit nymphomania, anoestrus or
aggressiveness with signs of virilism (clitoral enlargement, stallion-like conformation).
There appears to be no breed predisposition for GTCTs, and there is a wide range of age
distribution. One grossly enlarged ovary (> 10cm diameter) and with the opposite ovary
small and inactive, together with behavioural changes and raised serum testosterone levels
are usually sufficient evidence to confirm the diagnosis of a GTCT. However, there can be other reasons for a large ovary: In mares with GTCTs, behavioural changes alone can be misleading since not all affected
mares show virilism, and tumours other than GTCTs can also result in elevated plasma
testosterone values. It has been the author's experience that occasionally owners express
the opinion that their mare is `awkward' when in oestrus, and request veterinary
treatment. Frequently such mares are required to perform to a high level, e.g. advanced
dressage. Examination during the period of abnormal behaviour has shown them to be in
dioestrus with two normal ovaries. Owner pressure to perform an ovariectomy on suspicion
of a GTCT should be resisted, at least until the mare has been monitored throughout one
complete cycle. Transrectal ultrasonography generally assists a diagnosis: often the GTCT appears as a
large (7--40cm) mass, spherical and with a multicystic or `honeycomb' appearance. However,
there is no typical ultrasonographic appearance of GTCTs, since they can vary from being
uniformly homogenous to having one or several large fluid-filled cysts. The echogenicity
of the cyst wall differentiates it from persistent, large anovulatory follicles.
Teratomata, melanomata and dysgerminomata are solid neoplastic lesions appearing uniformly
echogenic. However, the ultrasonic appearance of some GTCTs seen by the author can be
similar to that of luteinized, unruptured (`haemorrhagic') follicles. Ultrasonography is
therefore best used as an aid in the evaluation of enlarged ovaries along with the
history, clinical and laboratory findings. It is important to diagnose accurately the reason for the enlarged ovary. For example,
in one report, 39% (11 out of 28) of surgically excised enlarged ovaries did not warrant
removal. It was concluded that histopathological examination of ovarian tissue and plasma
hormone concentrations are needed for a definitive diagnosis. Recently, it has been suggested that the identification of the hormone inhibin may be
more reliable than testosterone in confirming the presence of a GTCT. The secretion of
high amounts of inhibin by the neoplastic granulosa cells inhibits follicle-stimulating
hormone (FSH) secretion, and is thought to be the reason for atrophy of the contralateral
ovary. Unilateral ovariectomy is the only satisfactory treatment for GTCTs, since the prospect
of breeding from the mare is extremely poor unless the neoplastic ovary is removed. Most
mares return to normal cyclical ovarian activity during the next breeding season and are
fertile. Most GTCTs are benign and unilateral although a bilateral case has been reported.
Metastasis of the tumour is rare, but does occur. Cyst adenomas are rarer, but can appear ultrasonically similar to a GTCT, but the ovary
tends to be composed entirely of numerous large cystic structures. Teratomas depending on their composition, have marked echoic areas in their stroma
related to calcified deposits of bone, teeth and hair. Uterine Abnormalities 1. Uterine Cysts Uterine cysts are the most common type of uterine lesion identified in the mare. Two distinct morphological types recognised: They can be diagnosed at post-mortem examination; however, the use of ultrasonography
has shown that the incidence is much greater than was originally suspected. The relationship between subfertility and uterine cysts is not clear. Some authors
suggest that uterine cysts can reduce pregnancy rates. However, it is difficult to be sure
of their primary role as they are a common sign of uterine disease in general, including
senility and previous endometritis. The author's experience is only mares severely affected have a reduced pregnancy rate. Uterine Cysts: Effect on Fertility Direct effect on subfertility ? Larger lymphatic cysts may impede the early mobility of the conceptus, whilst later in
pregnancy, contact between the cyst wall and yolk sac or allantois may prevent absorption
of nutrients. Endometrial cysts can be confused with an early conceptus. Differentiation is based on previous cyst mapping, but also the early mobility of the
conceptus, the presence of specular reflections, the conceptus's spherical appearance and
growth rate. 2. Intraluminal Free Fluid Collections The main use of ultrasonography in the subfertile mare is the diagnosis of intraluminal
fluid accumulation. Only when large volumes of fluid are present is rectal palpation of
any value. The approach to a mare with intraluminal fluid is the subject of the next
lecture. 3. Pyometra Ultrasound is useful in confirming pyometra which on rectal palpation can resemble a
pregnancy. Pyometra describes the accumulation of large volumes of inflammatory exudate in
the distended uterus. It must be distinguished from the smaller, and intermittent,
accumulations of fluid that can be detected by ultrasonography in acute endometritis.
Pyometra occurs because of interference with natural drainage of fluid from the uterus
which may be due to cervical adhesions or an abnormally constricted, tortuous or irregular
cervix. In some cases, the fluid accumulates in the absence of cervical lesions presumably
due to an impaired ability to eliminate the exudate. Other predisposing factors are
chronic infection with P. aeruginosa or fungi. Some clinicians restrict the term `pyometra' to cases where, in addition to the
accumulation of exudate within the uterine lumen, the corpus luteum persists beyond its
normal lifespan. Some mares with pyometra have normal, regular cyclical ovarian activity.
Persistence of the corpus luteum is probably due to the failure of the synthesis and/or
release of prostaglandins from the uterus. Mares which have prolonged luteal activity have
the greatest endometrial damage. The mare with pyometra seldom shows overt signs of
systemic disease even when there is up to 60 litres of exudate in the uterine lumen. Very
occasionally there is weight loss, depression and anorexia. A vulval discharge is often
observed, especially at oestrus, which may vary in consistency from watery to cream-like.
Although the culture of endometrial swabs can sometimes result in the growth of mixed
organisms or sometimes no bacterial growth at all, in most cases the organism isolated is
S. zooepidemicus. Diagnosis Due to the lack of systemic illness, cases of pyometra have often become chronic before
treatment is sought. In such cases the prognosis is poor because of severe endometrial
damage which is unlikely to be able to sustain a normal pregnancy. Treatment Many cases can be significantly improved without surgery by repeated large-volume
lavage with warm saline. Initially, PGF2alpha can be used to induce luteolysis of the corpus luteum if present,
which should allow the cervix to relax sufficiently for digital exploration for the
presence of any adhesions. Oestradiol or PGE2 may also help relax the cervix. The
broad-spectrum combination of antibiotics used to treat endometritis should be infused
after repeated large-volume lavage and oxytocin to achieve drainage of exudate, and an
endometrial biopsy is useful in assessing the degree of endometrial damage. The mare must
be treated as a susceptible mare if she is to be mated. 4. Air in the Uterus Recognised as multiple hyperchoic reflections, air should not be seen in the uterus of
normal mares more than 2 hours after breeding, but may be seen for a few hours following
artificial insemination. 5. Neoplasia, Abscesses, Haematomas Other forms of less common uterine abnormality such as neoplasia, abscesses and
haematomas can occasionally be recognised. |
