Description and Clinical Features
The myometrium is the muscular portion of the uterus. It is bounded internally by the endometrium and externally by the serosa, a membrane derived from the peritoneum. The myometrium forms the bulk of the uterus from the cervix, the caudal portion of the uterus, through the body to the fundus. It is thickest during a woman’s menstrual years and atrophies substantially after menopause.
The uterine cervix is the portion of the uterus that connects the main body of the uterus to the vagina. The cervix is cylindrical with a length of approximately 3 cm. Its wall is formed by a thick layer of smooth muscle and fibrous tissue. A central canal runs through the cervix providing a conduit from the uterine cavity extending to the vagina. The opening of the cervix on the uterine cavity side is called the internal os, while the opening into the vagina is called the external os. The inferior aspect of the cervix protrudes a small distance into the vagina. Cysts within the cervix, termed nabothian cysts, occur frequently. They result from the retention of mucus within obstructed endocervical glands. These cysts are of little or no clinical significance unless very large or infected.
The arterial blood supply to the myometrium comes from the uterine artery, which arises from the internal iliac artery. Numerous branches of the uterine artery penetrate the uterus, forming the arcuate arteries that course within the myometrium.
The myometrium is moderately hypoechoic, especially in relation to the more echogenic endometrium. The normal myometrium is fairly homogeneous in echotexture, though in some cases the inner myometrium is less echogenic than the outer myometrium. The arcuate arteries and veins may be visible as serpiginous structures in the outer myometrium and may be calcified in older patients (Figure 27.1.1).
The normal uterus has a smooth, rounded outer contour superiorly. The configuration of the uterus is best assessed via an image in the coronal plane of the uterus. Such a view is most easily obtained by multiplanar reconstruction from a 3D volume acquisition (Figure 27.1.2).
The myometrium atrophies after menopause, becoming smaller and thinner than in women of reproductive age.
The cervix is identifiable at the caudal end of the uterus. The cervical myometrium is generally homogeneous, except where nabothian cysts are present (Figure 27.1.3).
Figure 27.1.1 Normal uterus. Sagittal transvaginal view of the uterus in four women. A: The myometrium (M) is homogeneous and hypoechoic relative to the endometrium (arrowheads). B: The inner myometrium (arrowheads) is less echogenic than the outer myometrium. Both are less echogenic than the endometrium (arrows). C: Arcuate vessels are seen as anechoic serpiginous structures (arrows) in the peripheral aspect of the myometrium. D: The arcuate arteries are calcified (arrows) in this elderly postmenopausal woman.
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