Uterine leiomyomata are benign tumors that consist of smooth muscle and connective tissue. They are the most common tumors encountered in gynecologic practice and are commonly referred to as fibroids (Figure 33-8). It is estimated that leiomyomata are present in 20% of women older then 35 years. Such tumors are estrogen dependent and usually regress after menopause. Leiomyomata are most prevalent in black women and other dark-skinned groups. The usual clinical picture is a palpable mass in a middle-aged woman, but leiomyomata may be associated with excessive menstrual bleeding and pelvic pain. They may cause infertility due to distortion of the isthmic portion of the tube. In addition, fibroids can contribute to uterine dystocia and disruption of endometrial interfaces, or to pelvic obstruction in the laboring patient.
Fibroids. A: Transabdominal sonogram (TAS) of large uterine fibroid (arrow) extending from uterine fundus. B: Transverse TAS of degenerated fibroid (arrow) with cystic regions. C: Transvaginal sonogram of intramural fibroid appearing as hypoechoic area of the uterine corpus (arrow). D: Transvaginal sonogram of pedunculated subserosal fibroid (curved arrow) extending from the left uterine corpus by a broad pedicle (straight arrow). E: Transvaginal color Doppler sonogram of a vascular intramural fibroid. F: Transvaginal color Doppler sonogram of the vascular rim of a submucosal fibroid. G: Transvaginal color Doppler sonogram of a relatively hypovascular fibroid. H: Transvaginal sonogram of cystic mass within uterus containing low-velocity, low-impedance flow. I: Gross specimen of (H). J: Photomicrograph of (I) showing tortuous vessels with muscular media. This was found to be a leiomyosarcoma.
The fibroid is important to the sonologist for 2 reasons. First, as a neoplasm, fibroids do have a small malignant potential. More commonly, however, the clinician needs to differentiate a palpable mass of uterine origin from an adnexal mass. For these reasons, detailed sonographic examination of fibroid tumors is warranted.
Leiomyomata usually develop in the myometrium of the upper contractile fundal and corporeal portions of the uterus. Only 3% of the leiomyomata are of cervical origin. Microscopically, these tumors arise from the smooth muscle and connective tissue that surround the smaller vessels coursing within the outer layers of the myometrium. Intramural leiomyomata cause the uterus to contract, and the resultant compression of these tumors is believed to displace them either toward the peritoneal surface to form subserous nodules or toward the endometrial cavity to produce submucous nodules. Intraligamentary nodules can arise by extrusion of the intramural nodule retroperitoneally into the areolar tissue between the leaves of the broad ligament. Also, rarely, leiomyomata can develop from fibromuscular structures in the round ligament or from those surrounding the vessels.
Leiomyomata of the uterus are usually multiple and of various sizes. A solitary nodule is found in only 2% of patients; the number of tumors in a uterus may reach hundreds. The size ranges from microscopic to massive, with 100 pounds being the largest single fibroid reported.14 Microscopically, each nodule is delineated by a pseudocapsule through which vascular channels enter and arborize within the tumor. As the tumor increases in size, it may eventually outgrow the blood supply, and central ischemia is followed by various stages of degeneration.
Degenerative processes may be benign or malignant, asymptomatic or symptomatic. Asymptomatic, benign degenerative processes include atrophic, hyaline, cystic, myxomatous, lipomatous, and carneous degeneration.
The symptomatic group includes carneous degeneration, infarction, and infection. Under the effect of strong uterine contractions, rotation of nodules within the pseudocapsule may shear supplying vessels and result in necrobiosis of the tumor. This process is most frequently seen during pregnancy.
Both subserous and submucous nodules may become pedunculated and undergo torsion of the pedicle, with subsequent infarction, degeneration, necrosis, and potential infection. For some pedunculated nodules whose circulation is occluded, attachment to the omentum or intestine allows the entry of new vessels and a revitalized blood supply. Under such circumstances, the pedicle may atrophy and the nodule may become completely detached from the uterus, giving rise to a so-called parasitic fibroid.
Submucous fibroids are prone to necrosis because their blood supply is frequently insufficient to support the tumor mass. More importantly, their exposed position subjacent to the uterine lumen predisposes them to ascending infection. Pelvic inflammatory diseases may involve adjacent fibroids by direct extension or through the lymphatics; curettage can injure submucous nodules and introduce bacteria. Occasionally, when the fibroid is infected, the central core may be filled with purulent material.
Malignant change in the tumor is a generative, not a degenerative, process. Although the occurrence of leiomyosarcoma in preexisting leiomyomata is 0.2% or less, the prevalence of leiomyomata results in this form of sarcoma being the most common malignant stromal uterine tumor (25%). Malignancy in a fibroid is seldom diagnosed preoperatively because there are no characteristic symptoms to distinguish this entity from preexisting fibroid nodules. Sudden accelerated growth in a previously static tumor and postmenopausal enlargement should suggest the possibility of a superimposed malignant process.
The clinical manifestations of fibroids are variable and depend on the size and number of the tumors, age of the patient, proximity of the tumor to the endometrial cavity, mobility of the fibroid (sessile or pedunculated), and presence or absence of degenerative processes. Submucous tumors typically encroach on the endometrium and distort the endometrial cavity. Because of pressure necrosis, alteration in the vascular architecture of the endometrium may occur and cause excessive menstrual bleeding to be the presenting symptom. When submucous fibroids outgrow their blood supply, surface necrosis, slough, and bloody discharge may result. Pain is not a common symptom except in the presence of degenerative changes or torsion of the pedicle of a subserous nodule. Pelvic discomfort due to pressure on the surrounding organs may be present with large tumors, but often the only symptoms are abdominal enlargement and a palpable mass. Pedunculated submucous leiomyomas ("fibroid polyps") can be partly or completely extruded through the cervical canal and can cause infection, necrosis, and ascending endometritis. This event may also be associated with invasion of the uterus. Larger fibroids, particularly of the intraligamentous type, may compress the ureter with resultant hydroureter and hydronephrosis.
The typical sonographic appearance of leiomyomata consists of mildly to moderately echogenic intrauterine mass(es) that cause nodular distortion of the uterine outline. Small intramural or submucous leiomyomata may be recognized by their distortion of the normally linear central endometrial interfaces. The solid nature of a fibroid often may cause an indentation on the bladder or rectum.
The echogenicity of a fibroid depends on the relative ratio of fibrous tissue to smooth muscle. With a more fibrous component, there is increased echogenicity of the nodule. The sonographic texture of fibroids also depends on the type and presence of degeneration and on the vascular supply. Interfaces between the normal myometrium and the pseudocapsule of the mass can sometimes be demonstrated. With TVS, the distortion of the endometrial lumen associated with the fibroid can be demonstrated. In some fibroids, the whorled internal architecture can be appreciated. The whorled appearance corresponds to bundles of smooth muscle and connective tissue that are arranged in a concentric pattern. In some cases, leiomyomas are only minimally echogenic and appear as cystic masses, except that their posterior wall is not as prominent as expected. Irregular anechoic areas may be seen within leiomyomata if cystic degeneration has occurred. Calcific degeneration within a leiomyoma is quite common and can be recognized as clusters of high-level echoes that are associated with distal acoustical shadowing.
The most common cause of calcification within the uterus is calcific degeneration within a fibroid. Twenty-five percent of one series of 75 cases of fibroids had calcifications.14 The pattern of calcification differed from a few small foci to a large rim of globular calcification. If the calcification is extensive and located along the anterior portion of the fibroid, it may prohibit complete sonographic delineation of the mass. Intrauterine calcifications can also be encountered in uterine sarcomas, but this condition is much less common than in leiomyomata.
Other types of degeneration within leiomyomas that produce sonographically recognizable changes in uterine texture include cystic, myxomatous, and hyaline degeneration. Among these, hyaline degeneration is the most common and appears as anechoic areas within a fibroid. Areas of hyaline degeneration can be distinguished from areas of cystic degeneration in that areas of cystic degeneration will usually demonstrate distal wall enhancement. Accelerated enlargement of a fibroid after menopause may indicate sarcomatous degeneration. There do not appear to be any sonographic signs that distinguish benign from malignant changes, however.
Leiomyomata that are pedunculated can be confused with other adnexal masses if their pedicle is not visualized. The most common location of pedunculated leiomyomas is superior to the uterine fundus. In most cases, an echogenic interface corresponding to the tissue plane connecting the fibroid and the fundus can be delineated. Pedunculated subserosal fibroids can also extend into the broad ligament, and thus appear as an extrauterine mass. The typical whorled configuration of the fibroid usually can be recognized, however. To ascertain whether or not a mass is connected to the uterus by a pedicle, applying slight pressure to the mass while performing TVS has been suggested. The mass will move with the uterus if the two are connected. This procedure should be performed by an experienced sonographer or sonologist and dynamically observed with real-time sonography.
Submucous leiomyomata may be difficult to differentiate from intramural leiomyomata. Both may produce distortion of the endometrial interfaces. Submucosal fibroids are best documented by TVS or by sonohysterography. Submucosal leiomyomas tend to be more echogenic than endometrial polyps.
Fibroids can be particularly difficult to detect with TAS in the retroflexed uterus. Because the uterine fundus curves posteriorly in the retroflexed uterus, this area may be relatively hypoechoic. Appropriate gain settings and time-gain compensation curves should be used; a hypoechoic area within the fundus of a retroflexed uterus could be technical in origin rather than a fibroid.
Serial sonographic evaluation of leiomyomata can be of significant clinical value. Follow-up scans of the fibroid uterus of a pregnant woman can help assess the growth and accelerated degeneration of this mass. A study of fibroids during pregnancy revealed that the size of most fibroids remains stable during pregnancy.15 Because fibroids should regress after menopause, serial sonograms can objectively document enlargement or regression of leiomyomata in the older woman. Due to its ability to portray larger areas of interest, TAS is needed in fibroids that enlarge the uterus to over 8 to 10 weeks in size. When the uterus is smaller than this, TVS is recommended. Depiction of the endometrial interfaces by TVS is particularly helpful in identifying myometrial masses by their displacement of this interface.
Color Doppler sonography may be used to assess flow within the fibroid and its response to medical treatment. In general, well-vascularized fibroids tend to be more responsive to medical treatment than hypovascular ones. Fibroids that parasitize major vessels for their blood supply may have a low-impedance, high-diastolic flow similar to that seen in some malignant pelvic masses.
Sonographic Mimics of Fibroids
Occasionally, solid masses that are adjacent to the uterus appear as masses within the uterine contour (Figure 33-9). This finding has been referred to as "the indefinite uterus sign."16 In such a setting, a retrouterine mass may be misdiagnosed as an enlarged uterus. The most common solid masses to simulate the sonographic appearance of a fibroid are the solid ovarian tumors. In particular, cystadenofibromas can calcify, simulating the appearance of a fibroid. Metastases that settle and enlarge in the cul-de-sac, such as those associated with breast tumors, can also produce apparent enlargement of the uterine contour.
Transvaginal sonography can be helpful in distinguishing fibroids from other adnexal or ovarian masses. In particular, TVS is helpful in identifying the ovaries in patients with fibroids because it usually is difficult to distinguish a fibroid from the ovary by palpation. We have also been able to distinguish a mass, representing a tubal carcinoma, from a fibroid by using TVS.