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Use of Ultrasonography in the Evaluation of the Non-Pregnant Mare

Walter Zent

Hagyard Equine Medical Institute, Lexington, KY, USA

In preparation for the ultrasonographic evaluation of the non-pregnant mare, a manual transrectal examination of the reproductive tract should be performed. This palpation allows the examiner to arrange the mare’s reproductive tract so as to make the ultrasonographic examination easier. The tract should be arranged so that it is free of any intestine and the ovaries should be positioned so that they are not behind the broad ligament. The operator should be able to follow the tract from one ovary to the other without any obstruction. This will allow the examination to be done without the interference of abdominal structures that can make the examination difficult and in some instances cause confusion in interpretation. More importantly, this examination allows the operator to become aware of possible abnormalities that will need closer visualization when the ultrasonographic examination is being performed.

In order to properly perform an ultrasonographic examination of the reproductive tract, it is very important that the mare be properly restrained. The amount of restraint that is required will differ greatly depending on the temperament of the mare and the quality of the available help. Older mares that have been examined multiple times often need very little restraint, while younger, more fractious individuals may need considerable restraint. It is very important that the animal is handled so that the safety of the animal, help, and veterinarian is maintained. Remember that the veterinarian is often the only professional involved and therefore the person responsible for the proper execution of the procedure. If stocks are not available, the mare is frequently positioned with the hindquarters in a stall door, in order to prevent too much lateral movement and give some protection to the operator and equipment. If the mare is not accustomed to being palpated, some form of minor sedation can be helpful. The author would rather not use ace-promazine for sedation as it will cause a profound loss of uterine tone and thus make examination difficult. Xylazine (0.5–1.1 mg/kg) will cause much less uterine relaxation and will provide ample sedation in most instances. Detomidine (0.02–0.04 mg/kg IV) is also a good choice. If the mare resists transrectal examination by persistent straining or if the operator must do extensive manipulation of the reproductive tract, a small dose of N-butylscopolammonium bromide (Buscopan, 20–40 mg IV) can be given to relax the rectum and make examination easier on the operator and the patient.

When ultrasonography is being performed, it is important for the operator to hold the probe in a manner that will allow the head of the probe to have full contact with the ovaries and uterus. The author believes that this can best be done by grasping the probe in a way that the index finger is placed along the dorsal surface of the probe, with the thumb on one side and the remaining fingers on the other, being careful not to cover the crystals on the bottom. The middle finger can be used as a guide to move the probe along the anterior edge of the uterus so that the probe will be easily centered over the uterus while the examination of the horns is being performed (Figure 14.1).

**Figure 14.1** Proper method of gripping probe for good visualization of the uterus.

The examination should always be done in the same manner so that all of the structures are examined thoroughly and the operator will be able to cover the entire tract. The author palpates left handed so it is easiest for him to start at the right ovary, and then examine the right horn, left horn, left ovary, body, and cervix, ending with the vagina. It is very important that the operator makes sure that the entire uterus is examined. If the probe is held correctly, the horns of the uterus will appear as a cross-section of a piece of sausage on the screen and can easily be followed from one ovary to the other (Figures 14.2A and 14.2B). The body of the uterus will be seen longitudinally (Figures 14.3A and 14.3B). When scanning the body of the uterus, sometimes the body is wider than the width of the probe and parts of the lateral areas can be missed. In order to make sure that the operator has covered the entire uterus, the probe is moved from left to right. This is particularly important when mares are being examined for very early pregnancies.