Hematoma of the uterine artery
Keywords: uterine, middle, artery, hematoma, injury, equine
Hemorrhage during foaling can involve the utero-ovarian arteries, external iliac arteries or uterine (middle uterine) arteries. Of these, rupture of the uterine arteries is most likely.
The location of the uterine arteries is shown below. In the left inset image, these vessels have been left in color to accentuate their location. In the right inset, the location of the right uterine artery within the mesometrium is indicated by the red arrow. Note its relationship to the ovaries (colored purple), the uterus (colored pink) and the bladder (colored yellow). A phantom hand is shown, examining the tract as though it would during transrectal palpation. One can appreciate how readily a hematoma in the mesometrium can be palpated during that examination.
Image size: 1023 x 480px. Right inset image highly modified from an unknown source.
During parturition, the foal's movement may draw the uterine artery in a caudal direction, trapping it against the shaft of the ilium. It is probable that direct tension or local vascular hypoxia exacerbated by tension, causes rupture of the vessel. Age-related degeneration processes are probably important as well because data suggests strongly that vascular rupture is more more common in older than younger mares. Although copper deficiency has been implicated in vascular rupture, it is relatively unimportant in most cases because balanced rations are available to well managed horses.
Although it is possible, most mares do not exsanguinate after rupturing the uterine artery. Instead, blood escapes into the mesometrial ligament, accumulating between its dorsal and ventral surfaces. The discomfort of normal parturition and placental expulsion probably masks the pain as hemorrhage. It is usually at foal heat breeding that a hematoma is discovered by chance while one is examining the uterus and ovaries. Once the large mass is discovered, ultrasonography will confirm the diagnosis. The typical ultrasonographic appearance of a mesometrial hematoma is illustrated elsewhere.
In the image below, a large hematoma has been removed from its position in the left mesometrial ligament.
Image size: 1845 x 1435px
The following image provides a ventral view of the tract shown above. The location of the hematoma in the left mesometrial ligament is clearly visible. This mare also has a large urolith in her bladder. A detailed view of that urolith is given elsewhere in LORI.
Image size: 1271 x 1435px
On rare occasions bleeding can dissect the mesometrium until it ruptures, with fatal results. A description of the potentially life saving treatment used in those cases is beyond the scope of this entry.
It is often suggested that mares with mesometrial hematomas should not be re-bred because of the threat of fatal hemorrhage during subsequent foalings. However, the purpose of mares on many stud farms is only to produce foals. Therefore some breeders continue to use affected mares notwithstanding. In the author's experience over many years, this practice has not led to the subsequent death of a broodmare through fatal hemorrhage. Therefore it is tempting to suggest that mesometrial hematomas are one-time accidents and once recovered, affected broodmares can again be used for breeding, with little fear of repeated hemorrhage. This philosophy is probably not suitable when there is a substantial emotional attachment to a mare.
In many mares, a palpable mass may remain in the mesometrium for months after foaling.
References:
Pascoe, R. R. Rupture of the utero-ovarian or middle uterine artery in the mare at or near parturition. Vet. Rec., 104:77, 1979.
Rooney, J. R. Internal hemorrhage related to gestation in the mare. Cornell Vet., 54:11, 1964.
Stowe, H. D.: Effects of age and impending parturition upon serum copper of Thoroughbred mares. J. Nutrition., 95:179, 1968.
