This chapter should be cited as follows:
Djokovic D, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419623

The Continuous Textbook of Women’s Medicine Series – Gynecology Module

Volume 10

Ultrasound in gynecology

Volume Editors: Professor Antonia Testa, Agostino Gemelli University Hospital, Rome, Italy
Professor Simona Fragomeni, Agostino Gemelli University Hospital, Rome, Italy

Chapter

Benign Myometrial Pathology (Myomas)

First published: November 2024

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INTRODUCTION

Uterine leiomyomas (myomas or fibroids) are benign monoclonal neoplasms that arise from the myometrial smooth muscle cells, and are characterized by an excessive deposition of the extracellular matrix consisting of collagen, proteoglycan, elastin and fibronectin.¹ The usual-type leiomyomas (also referred to as classic, conventional, typical or ordinary leiomyomas) are the most common pelvic tumors in women.^1,2 Their prevalence increases during the reproductive period, being diagnosed in up to 70% of white women and >80% of women of African descent by the age of 50 years.²

The pathology is highly heterogeneous and different leiomyomas, even within the same uterus, can result from different genetic mutations and develop through different pathogenic pathways under the influence of autocrine, paracrine and endocrine growth factors and hormone receptor signaling.^3,4,5,6,7,8 In comparison to the malignant, genetically distinct and remarkably less frequent leiomyosarcomas,⁹ leiomyomas stand at the opposite end of the pathological spectrum of uterine smooth muscle tumors (Table 1).¹⁰ In between, there are several leiomyoma variants with atypical histological features,¹¹ as well as the smooth muscle tumors of uncertain malignant potential (STUMP) that exhibit various combinations of cytological atypia, increased mitotic index and tumor cell necrosis, surpassing classic leiomyomas and leiomyoma variants, but not meeting the defined histological criteria for leiomyosarcoma.¹⁰

The leiomyoma number, size and location determine their clinical relevance. Based on the location of the lesion, the International Federation of Gynecology and Obstetrics (FIGO) classification describes eight types of leiomyomas, as well as the association of ≥2 leiomyoma types,¹² allowing appropriate disease mapping (Figure 1). When symptomatic, these tumors can be clinically manifested by heavy and/or prolonged menstrual bleeding, dysmenorrhea, dyspareunia, bulk-related symptoms (such as pelvic pressure or pain and urinary tract or bowel symptoms), infertility and obstetric complications (including miscarriage, fetal growth restriction, malpresentation, placental abruption, preterm labor and birth).

Although the definitive diagnosis of leiomyoma is histological, as it is generally available only after myomectomy or hysterectomy, imaging studies (particularly ultrasound) serve as a basis for counseling, monitoring and therapeutic (pharmacological and/or surgical) management of affected patients. This chapter provides insights into the ultrasound features of uterine leiomyomas, including a description of the classic and atypical ultrasound morphologies, as well as ultrasound aspects that may be useful in differentiating leiomyomas from other uterine/pelvic lesions that may cause differential diagnostic dilemmas.

ULTRASOUND EVALUATION OF PATIENTS WITH SUSPECTED OR KNOWN MYOMAS

Ultrasonography is the first-line imaging tool used for the evaluation of myometrium and myometrial pathology due to its availability, reliability, patient tolerance and cost-effectiveness.^13,14 In women with suspected or known leiomyoma(s), two-dimensional (2D) ultrasound examination of the myometrium is performed using a transvaginal and/or transabdominal approach.¹⁵ While high-resolution transvaginal ultrasound is generally preferred, allowing for detailed evaluation of the myometrium, transabdominal ultrasound may be necessary for the detection and characterization of lesions that extend beyond the small pelvis.¹⁶ Vascularization of the myometrium and myometrial lesions is assessed using color or power Doppler ultrasound. Power Doppler is superior to color Doppler for detecting small vessels with low blood-flow velocity.¹⁶ Three-dimensional (3D) ultrasound permits offline manipulation and examination of images, which may facilitate access to a second opinion from an expert examiner.¹⁶ In addition, a 3D multiplanar view of the uterus offers excellent definition of the uterine cavity and the junctional zone that are altered by some types of myoma.¹⁷

The objectives of ultrasound assessment in patients with myomatous uteri are: (1) to identify all myometrial and other (endometrial and adnexal) lesions; (2) to confirm the number, location, position and size of the leiomyoma(s); (3) to determine myometrial penetration for each identified submucous leiomyoma and minimal distance between the lesion and serosa in the case of FIGO 2 leiomyomas; (4) and to distinguish leiomyomas from other uterine/pelvic lesions (e.g. adenomyomas, other myometrial tumors, endometrial polyps and ovarian solid lesions resembling pedunculated subserous leiomyomas). By providing clinically relevant information (including surgically important data), ultrasound does not only contribute to the correct diagnosis, but also facilitates the choice of the most appropriate treatment strategy and indirectly reduces the rate of complications in cases in which a surgical approach is chosen.

Appropriate use of ultrasound requires a standardized medical lexicon and scanning technique, as well as consistent reporting of ultrasound findings. To promote unambiguous communication between imagers, clinicians and researchers, the Morphological Uterus Sonographic Assessment (MUSA) group provided a consensus statement on terms, definitions and measurements to describe the sonographic features of the myometrium and uterine masses.^16,18 This is currently the most comprehensive and systematic proposal available, which can be used in both clinical practice and research to reduce intra- and interobserver variability, to assess the effect of medical/surgical treatment of leiomyomas and other myometrial pathology, and to compare ultrasound imaging with other imaging techniques. The MUSA recommendations for reporting on the myometrium in general clinical practice are summarized in Table 2.

Following the initial 2D or 2D and 3D ultrasound examination, additional ultrasound and other studies, such as contrast-enhanced/saline-infusion sonography (SIS), magnetic resonance imaging (MRI) or hysteroscopy may be appropriate to perform, depending on the clinical indications and ultrasound findings. For instance, MRI evaluation may be appropriate in circumstances in which ultrasound is limited (e.g. in the presence of numerous myomas, very enlarged uterus, high body mass index, coexistence of myomas and other uterine/pelvic lesions and/or uncertain nature of the uterine tumor).^13,19 Furthermore, when accurate characterization of an intracavitary or partial intracavitary myoma is required, SIS should be considered. High-quality evidence suggests that SIS is as effective as hysteroscopy for characterizing submucosal myomas, both SIS and hysteroscopy being superior to non-enhanced transvaginal ultrasound.^20,21 SIS defines the extent to which myomas protrude into the uterine cavity and, simultaneously, the depth of penetration into the myometrium. Thus, SIS offers information analogous to that of the combined use of hysteroscopy and transvaginal ultrasound.²² To predict complex hysteroscopic myomectomy, incomplete removal of the lesion, long operative time, fluid overload and other major complications,^23,24 SIS provides valuable information for adequate classification of intracavitary (FIGO 0, 1 and 2) leiomyomas using the STEPW classification/scoring system,²⁴ which is assigned on five submucosal myoma features: size, topography, extension of the base, penetration and lateral wall position (Table 3, Figure 2).

Knowledge of the ultrasound characteristics of typical leiomyomas, degenerating leiomyomas and variants of leiomyomas is of particular importance for proper clinical guidance of patients. Through training and increasing individual experience, sonologists should become familiar with ultrasound patterns and specific maneuvers during the dynamic ultrasound examination, which will be presented below, to help differentiation between leiomyomas (especially atypical) and other uterine/pelvic lesions.

ULTRASOUND FEATURES OF DIFFERENT LEIOMYOMA TYPES

Typical leiomyomas

Sonographically, a typical uterine leiomyoma is a well-defined, round, oval or lobulated lesion within the myometrium, which may protrude outside the uterine corpus and/or into the uterine cavity, or it can be a completely extracorporeal lesion attached to the myometrium (Figures 1–4). The lesion echogenicity varies (Figures 3), according to the proportion of muscle cells and fibrous stroma within the lesion.¹⁶ Myomas can be hypo-, iso- or hyperechoic compared to the myometrial echogenicity. Usually, their echogenicity is similar to that of the myometrium, or they are hypoechoic, with some hyperechogenicity internally, even without calcifications. Acoustic shadows at the edge of the lesion and/or internal fan-shaped shadowing are typically present (Figure 4). They result from the interface between the tumor smooth muscle cells, extracellular matrix and normal myometrium.²⁵ On power or color Doppler imaging, predominant peripheral (i.e. circumferential flow around the lesion) is a typical finding as well (Figure 4C). The MUSA group recommends that myomas which do not exhibit the typical ultrasound features described here should be labeled as sonographically atypical (Figure 5).

Degenerating leiomyomas

Secondary to a loss of blood supply, leiomyomas may undergo degeneration, which may be spontaneous (most frequently attributable to rapid tumor growth) or induced (due to infarction following a destructive treatment such as uterine artery embolization).²⁶ Pain is the most common symptom that leads to clinical and ultrasound examination. Myoma degeneration is categorized as red, hyaline, hydropic, cystic/myxoid or calcific.

Red degeneration, typically observed in pregnancy, is an initial manifestation after infarction.²⁷ It may be sonographically unremarkable or result in a homogeneously low echogenicity, with a hyperechogenic lesion rim and absent internal vascularity.²⁸ Similarly, after induced infarction, myomas commonly become homogeneously hypoechogenic with a hyperechogenic rim and no or minimal internal vascularity and acoustic shadows.^16,29,30 Hyaline degeneration is the most frequently observed degeneration type and represents a late manifestation after myoma infarction.^17,31 It results in mixed echogenicity and hypoechogenic cystic areas (Figure 6). Hemorrhage within myoma and edema may also cause mixed echogenicity.¹⁶ Cystic or myxoid degeneration has regular cystic lacunae filled with hypoechoic fluid as a sonographic hallmark.^16,32 Cystic degeneration may occur in malignant uterine smooth muscle tumors (Figure 7).^16,19,33 Hypoechoic lacunae in a myometrial tumor, especially if they are irregular or within a previously unknown lesion, generally present differential diagnostic dilemmas. On the other hand, calcifications in myomas are generally benign ultrasound indicators.^16,17 They are hyperechoic and very often cause acoustic shadows.

Leiomyoma variants

Benign histologic variants of leiomyomas, i.e. those with no or low recurrent and/or metastatic potential, are a heterogeneous subgroup of myometrial smooth muscle tumors. They exhibit the same symptoms and signs as usual-type leiomyomas. There are no imaging modalities that unequivocally distinguish histologic variants from classic leiomyomas.¹⁶ Definitive diagnosis is made by pathological examination following myomectomy or hysterectomy. Cellular and highly cellular leiomyoma, leiomyoma with bizarre nuclei, mitotically active leiomyoma, dissecting leiomyoma and leiomyoma with increased cellularity, no atypia or mitotic figures and increased vascularity may have the same clinical, ultrasound and macroscopic pathological features as do the usual-type leiomyomas.³⁴ However, they may exhibit increased vascularity on power and color Doppler examination, since this characteristic is related to the lesion cellularity (Figure 8).³⁵ Cystic degeneration can be observed.³⁶ Sonographically, lipoleiomyoma is a hyperechogenic mass partly surrounded by a hypoechogenic rim.^16,37 A cotyledonoid leiomyoma and cotyledonoid dissecting leiomyoma have been described as myometrial lesions with placenta-like echogenicity on ultrasound, but may also be cystic.^16,38 Even rarer forms, such as epithelioid leiomyomas and palisading/neurilemmoma-like leiomyomas, are particularly insufficiently characterized by ultrasound.¹⁶

Intravenous leiomyomatosis, disseminated peritoneal leiomyomatosis and benign metastasizing leiomyoma

Disseminated peritoneal leiomyomatosis and metastasizing leiomyoma are characterized by tumor extrauterine spread. This growth pattern is not pathognomonic of malignancy.¹⁰ In intravenous leiomyomatosis, disseminated peritoneal leiomyomatosis and benign metastasizing leiomyoma, the lesions have the same ultrasound characteristics as do the usual-type leiomyomas.^16,39,40 They are generally multiple tumors, located outside the uterine borders.

DIFFERENTIATING LEIOMYOMAS FROM OTHER UTERINE AND PELVIC LESIONS

Leiomyomas vs other mesenchymal tumors

The vast majority of leiomyomas have typical ultrasound features and are easily recognized even by less experienced operators (Figures 1–4). This is not the case for degenerating and/or atypical tumors (Figures 5, 6 and 8). On the other hand, mesenchymal malignancy of the uterus (sarcomas) can present with some ultrasound features typical of benign leiomyomas, such as internal shadows and fan-shaped shadowing.^41,42 An incorrect diagnosis is made in up to 20% of all uterine sarcoma cases.⁴² The correct differential diagnosis between benign leiomyoma, STUMP and uterine sarcoma is of utmost importance for appropriate patient counseling and treatment, and especially to avoid procedures like morcellation, which would lead to iatrogenic dissemination of occult malignancy.¹⁹ Current prospective international scientific research is, therefore, targeted at improving imaging-based prediction of malignant potential in patients with uterine mesenchymal tumors.

No clinical, imaging and intraoperative macroscopic findings unequivocally differentiate between benign leiomyomas and uterine sarcomas.¹⁹ A rapidly enlarging uterine mass is not a reliable sign of a uterine sarcoma in premenopausal patients.⁴³ In most studies, large uterine/myometrial tumor size has not been found to be associated with an increased uterine sarcoma risk.^44,45,46 Uterine sarcomas typically appear as solid masses with inhomogeneous echogenicity, sometimes with irregular cystic areas, and very occasionally with fan-shaped shadowing.⁴² Most are moderately or very well vascularized,^42,47 although they also can be poorly vascularized lesions.⁴⁷ Thus, in all women, and especially in symptomatic patients with abnormal genital bleeding, malignancy should be considered and ruled out if ultrasound shows a (large) inhomogeneous vascularized uterine lesion with irregular cystic areas, without shadows and calcification (Figure 7).^36,42,47

Leiomyoma vs adenomyosis/adenomyomas

Leiomyoma and adenomyosis are the most frequent pathological conditions affecting the myometrium.¹⁸ It is important to distinguish leiomyomas from adenomyosis/adenomyomas, especially when surgical excision is planned, because enucleation of adenomyomas, which unlike myomas do not have a pseudocapsule, is a more challenging task.

Adenomyosis may be dispersed within the myometrium (diffuse adenomyosis), or it may be present in a restricted part of the myometrium (focal adenomyosis).^16,18 Adenomyoma is a subtype of focal adenomyosis characterized by compensatory hypertrophy of the surrounding myometrium.¹⁶ Not infrequently, adenomyoma vs leiomyoma appears as a diagnostic dilemma after ultrasound detection of a myometrial nodule. Additionally, adenomyosis may manifest in the form of a large cystic formation or group of cysts within the myometrium (adenomyotic cysts or cystic adenomyomas) (Figure 9A).⁴⁸ Lacunar spaces in degenerating leiomyomas can resemble and be confused with cystic adenomyomas (Figure 9B).

As a general although not always applicable rule, leiomyomas are well-defined round/oval or lobulated lesions, with a smooth contour, hypo- or hyperechoic rim, edge shadows and/or internal shadows (often fan-shaped shadowing), while adenomyotic lesions are ill-defined (with the exception of some adenomyomas), have an irregular contour, often fan-shaped shadowing, but no edge shadows.^16,18 Leiomyoma vascularization is typically circumferential, whereas, adenomyotic lesions are characterized by translesional flow.⁴⁹

Intracavitary myomas vs endometrial polyps

The International Endometrial Tumor Analysis (IETA) group provided a consensus statement on terms, definitions and measurements that are recommended to be used to describe ultrasound characteristics of the endometrium and uterine cavity on gray-scale sonography, color flow imaging and SIS.⁵⁰ The typical ultrasound features of endometrial lesions and intracavitary leiomyomas, have been studied and described by the IETA group in pre- and postmenopausal women, with and without abnormal uterine bleeding.^51,52 Combined use of the grayscale sonography and color flow imaging is usually helpful in differentiating between intracavitary leiomyomas and polyps (Figure 10), while SIS provides precise information about the topography of the lesion (Figure 2B and 2C). Endometrial polyps are commonly seen as intracavitary uniform hyperechogenic formations or non-uniform formations with no cysts or regular cysts (Figure 10A); the endometrial midline is commonly undefined, particularly after menopause. The endometrial–myometrial junction is usually regular. In cases with detectable color-Doppler signals, there is typically a single vessel without or with branching (Figure 10A). On the other hand, in the presence of an intracavitary leiomyoma, the endometrium often appears uniformly hyperechogenic or non-uniformly heterogeneous without cysts, the endometrial midline undefined and the endometrial–myometrial junction interrupted (Figure 10B). On color Doppler, there is circular flow in over 50% cases with detectable color Doppler signals (Figure 2 and 10B). Circular flow is visualized more often in premenopausal than in postmenopausal women. Thus, characterization of the vascularization of intracavitary lesions can be of particular importance in differentiating between usual-type leiomyomas with circumferential flow and typical endometrial polyps supplied by a single vessel (Figure 10).

Subserous pedunculated leiomyomas vs adnexal tumors

A differential diagnostic dilemma may arise in the case of subserous pedunculated leiomyomas (FIGO 7), which spread to adnexal areas or may be intraligamentary (i.e. located between the folds of the broad ligament). Ovarian tumors, which may have a similar ultrasound morphology to that of a leiomyoma, reduced vascularization on power or color Doppler, and cause acoustic shadowing, include, for example, ovarian fibromas, thecomas, fibrothecomas and Brenner's tumor.^53,54 Non-gynecological tumors observed in adnexal areas (for example, some solid appendicular lesions) may also resemble pedunculated leiomyomas or solid ovarian neoplasms.⁵⁵ In addition to identifying the vascular pedicle of the leiomyoma originating from the uterus, it is important to localize the ipsilateral (healthy) ovary. The pressure produced by the ultrasound probe resulting in separation of the leiomyoma from the ovarian parenchyma (sliding sign) helps to distinguish this from an ovarian tumor.

ULTRASOUND DATA INTERPRETATION AND CONCLUDING REMARKS

In patients with uterine leiomyomas, ultrasonography provides important quantitative and qualitative characterization of these lesions. The data obtained by taking anamnesis and conducting clinical and ultrasound examinations should be interpreted integrally. Leiomyomas are common and they may not be the only reason for the patient's symptoms. Therefore, a complete gynecological ultrasound examination should always be performed meticulously, seeking to identify all possible reasons that contribute to the patient's clinical presentation. Most leiomyomas have typical ultrasound features. If atypical findings are observed, complementary investigations should be conducted, including additional imaging, and laboratory and/or histological evaluations, the latter avoiding tumor reduction techniques to prevent dissemination of occult malignancy.

PRACTICE RECOMMENDATIONS

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