Published online before print August 18, 2004, 10.1148/radiol.2331020777
(Radiology 2004;233:19-34.)
© RSNA, 2004
Müllerian Duct Anomalies: Imaging and Clinical Issues1
Robert N. Troiano, MD and
Shirley M. McCarthy, MD, PhD
1 From the Departments of Radiology and Obstetrics and Gynecology, Weill Medical College of Cornell University, 1300 York Ave, New York, NY 10021 (R.M.T.), and Department of Radiology, Yale University School of Medicine, New Haven, Conn (S.M.M.). Received June 25, 2002; revision requested August 22; final revision received August 25, 2003; accepted September 29. Address correspondence to R.N.T. (e-mail: rnt2001@med.cornell.edu).
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ABSTRACT
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While estimates of the frequency of müllerian duct anomalies vary widely owing to different patient populations, nonstandardized classification systems, and differences in diagnostic data acquisition, these anomalies are clinically important, particularly in women who present with infertility. An understanding of the differences between these uterovaginal anomalies, as outlined in the most widely accepted classification system—that published by the American Fertility Society (AFS) in 1988—is imperative given the respective clinical manifestations, different treatment regimens, and prognosis for fetal salvage. Although the AFS classification system serves as a framework for description of anomalies, communication among physicians, and comparison of therapeutic modalities, there often is confusion about appropriate reporting of certain anomalies, particularly those with features of more than one class. Many of the anomalies are initially diagnosed at hysterosalpingography and ultrasonography; however, further imaging is often required for definitive diagnosis and elaboration of secondary findings. At this time, magnetic resonance imaging is the study of choice because of its high accuracy and detailed elaboration of uterovaginal anatomy. Laparoscopy and hysteroscopy are reserved for women in whom interventional therapy is likely to be undertaken.
© RSNA, 2004
Index terms: Genitourinary system, abnormalities • Genitourinary system, MR, 85.121411 • Genitourinary system, radiography, 85.1282 • Genitourinary system, US, 85.12981, 85.12983 • State of the Art • Uterus, abnormalities, 854.1411, 854.1413, 854.14783, 854.14784
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INTRODUCTION
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The true incidence and prevalence of müllerian duct anomalies are difficult to assess. Examination of different patient populations, nonstandardized classification systems, and differences in diagnostic data acquisition have resulted in widely disparate estimates, with a reported prevalence that ranges from 0.16% to 10% (1–9). A reflection of selection bias, a prevalence of 0.4% has been reported in women being evaluated with ultrasonography (US) because of nonobstetric indications (2), while a prevalence of 8%–10% has been reported in women being evaluated with hysterosalpingography because of recurrent pregnancy loss (6,7). The overall data suggest that the prevalence both in women with normal fertility (1,10,11) and in women with infertility (10–13) approximates 1%, and the prevalence in women with repeated pregnancy loss approximates 3% (10,11,14–16).
While the majority of women with müllerian duct anomalies have little problem conceiving, they have higher associated rates of spontaneous abortion, premature delivery, and abnormal fetal lie and dystocia at delivery (17,18). Most studies report an approximate frequency of 25% for associated reproductive problems, compared with 10% in the general population (18–20). Primary infertility in these women usually has an extrauterine cause and is not generally attributable to müllerian duct anomalies alone (18,21). In addition, cervical incompetence has been reported to be associated with these anomalies (3,14,22).
The majority of müllerian duct anomalies are considered to be sporadic or multifactorial in nature; however, polygenic and genetic patterns of inheritance have been described in the expression of these anomalies (23,24). Extrauterine and intrauterine environmental factors, such as exposure to ionizing radiation, intrauterine infections, and drugs with teratogenic effects such as thalidomide and diethylstilbestrol (DES), can also cause defects of the developing fetal genital tracts (18).
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EMBRYOLOLOGY
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At 6 weeks of development, the male and female genital systems are indistinguishable in appearance, constituting two sets of paired ducts: the paramesonephric (müllerian) ducts and the mesonephric (wolffian) ducts. In the absence of the testis-determining factor of the Y chromosome, the mesonephric ducts begin to degenerate and form a matrix for the developing paramesonephric ducts. Synchronously, the paramesonephric ducts develop bidirectionally along the lateral aspects of the gonads. The proximal segments of the uterovaginal canal, derived from coelomic epithelium, remain unfused and open into the peritoneal cavity to form the fallopian tubes. The distal segments, induced by or derived from the adjacent mesonephric ducts, progress caudomedially and join each other before contacting the posterior aspect of the pelvic urethra at the level of the sinusal tubercle. These distal segments of the uterovaginal canal give rise to the uterus and upper four-fifths of the vagina.
Initially separated by a septum, at 9 weeks the paramesonephric ducts fuse at their inferior margin forming the single lumen of the uterovaginal canal. Regression of the uterine septum has been proposed to be a result of apoptosis, mediated by the Bcl2 gene (25). Absence of this gene has been implicated in persistence of the septum. The classic theory of unidirectional regression hypothesizes that the septum regresses from the caudal to cranial aspect of the uterovaginal canal, with the uterus initially bicornuate in configuration. However, an alternative bidirectional theory has been proposed in which it is hypothesized that the process proceeds simultaneously in both the cranial and the caudal directions (26). This would explain anomalies such as a complete septum with a duplicated cervix or isolated vertical upper vaginal septum in an otherwise unremarkable uterus.
At week 12, the uterus exhibits its normally developed configuration: a fused external uterine contour of the myometrium and a triangular-shaped endometrium. Because the fallopian tubes are derived from a different cellular origin than are the uterus and mid- to upper vagina, they are rarely involved in müllerian duct anomalies.
During formation of the uterovaginal canal, the sinusal tubercle thickens and forms the sinovaginal bulbs of the primitive urogenital sinus, which gives rise to the lower 20% of the vagina. The uterovaginal canal remains separated from the sinovaginal bulbs by the horizontal vaginal plate. The vaginal plate elongates during the 3rd–5th month, and its interface with the urogenital sinus forms the hymen, which usually ruptures during the perinatal period (27–29).
The urinary and genital systems both arise from a common ridge of mesoderm arising along the dorsal body wall, and both rely on normal development of the mesonephric system. The ureters, renal calices, and collecting tubules are formed from the ureteral bud, which arises from the mesonephric ducts, which also induce formation of the kidneys. Hence, abnormal differentiation of the mesonephric and paramesonephric ducts may also be associated with anomalies of the kidneys. Renal agenesis is the most common associated anomaly, although crossed renal ectopy, cystic renal dysplasia, and duplicated collecting systems have all been described (26–30).
The ovaries arise from the mesenchyme and epithelium of the gonadal ridge and are not influenced by the formation of the mesonephric or paramesonephric ducts. The undifferentiated gonads are induced to develop by primordial germ cells that migrate from the yolk sac to the dorsal mesenchyme at 5 weeks. These germ cells induce cells of the mesonephros to form genital ridges, which in turn form primitive sex cords. If germ cells do not develop in the region of the gonads, the gonads do not form. Hence, ovarian development is a separate process from the formation of the uterovaginal canal and is not usually associated with müllerian duct anomalies (27).
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IMAGING TECHNIQUES
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Hysterosalpingography (HSG) is indicated in the early stages of evaluation of the infertile couple (31,32). The examination provides a morphologic assessment of the endometrial and endocervical canals and supplies important information regarding tubal patency. Characterization of uterine anomalies can be difficult, however, and there can be considerable overlap in findings, notably with regard to differentiation of a septate from a bicornuate uterus (33–36). The major limitations of the procedure are the ability to characterize only patent canals and the inability to evaluate the external uterine contour adequately. HSG also entails exposure to ionizing radiation in these typically young women.
US imaging should be performed during the secretory phase of the menstrual cycle, when the endometrial thickness and echo complex are better characterized (37). Imaging should not only focus on conventional sagittal and transverse imaging of the pelvis but also include orthogonal images along the long axis of the uterus to characterize the external uterine contour. Transabdominal US is usually best performed with a curved 4–1-MHz or 6–3-MHz transducer, although US is operator dependent and may be limited because of the patient’s body habitus, the uterine lie, and shadowing from peristaltic bowel loops. Endovaginal US should be performed with an 8–5-MHz endovaginal transducer; endovaginal US has the advantage of improved spatial resolution, although at the expense of a decreased field of view. US has a reported pooled accuracy of approximately 90%–92% (35,37,38). Hysterosonography, with infusion of saline into the endometrial canal, provides improved delineation of the endometrium and internal uterine morphology; however, it shares limitations similar to those of conventional endovaginal US and can only help evaluate patent endometrial canals (39). Three-dimensional US with surface- and transparent-mode reconstructions of the uterus has reported advantages over conventional two-dimensional scanning. In experienced hands, a sensitivity of 93% and a specificity of 100% have been achieved (40). The technique allows improved delineation of the external uterine contour and uterine volume (12,41). Further refinement of the technique and more universal experience and availability of the modality should help determine its precise role in the evaluation of müllerian duct anomalies.
Magnetic resonance (MR) imaging has a reported accuracy of up to 100% in the evaluation of müllerian duct anomalies (34,35,42). Although MR imaging is more expensive than US, its greater accuracy makes it more trusted by many gynecologists (35). Diagnostic laparoscopy, routinely used when HSG and US were the only available imaging modalities, is more expensive and invasive. MR imaging provides clear delineation of internal and external uterine anatomy in multiple imaging planes and, most important, reliable depiction of the external uterine contour. Complex anomalies and secondary diagnoses such as endometriosis can often be optimally characterized noninvasively.
Patients are best imaged with a phased-array MR surface coil. At our institutions, an inversion-recovery or gradient-echo image of the uterus in the sagittal plane is obtained initially to determine uterine lie. Fast spin-echo T2-weighted images are then acquired parallel to the long axis of the uterus to characterize the external uterine contour and are typically obtained in an oblique transverse or a coronal plane, depending on uterine lie. It is important to obtain the images through the long axis of the uterus immediately after the localizing image is acquired, otherwise urinary bladder filling may move the uterus such that the uterus no longer lies in the position demonstrated on the localizing image. If the fundal configuration is not well delineated on the T2-weighted images, T1-weighted images parallel to the long axis can aide in characterization of the external contour, owing to increased contrast between the myometrial fundal contour and the overlying fat. T1-weighted spin-echo sequences are performed with the following parameters: 600/16 (repetition time msec/echo time msec), 20–24-cm field of view, one signal acquired, 256 x 160 matrix, and 4-mm section thickness with a 1-mm gap. Fast spin-echo T2-weighted images (5000–7500/100–130) are acquired with a 20–24-cm field of view, three to four signals acquired, 256 x 256 matrix, echo train length of 16, bandwidth of 32 Hz, and 4-mm section thickness with a 1-mm gap. Further imaging of the pelvis with a transverse T1-weighted sequence and additional multiplanar fast spin-echo T2-weighted sequences may then be performed to fully evaluate the cervix, vagina, and ovaries. Finally, a coronal fast spoiled-gradient-echo image or a single-shot fast spin-echo T2-weighted image is obtained by using the body coil, with a large field of view to enable assessment of the kidneys (Fig 1).
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Figure 1a. (a) Sagittal inversion-recovery MR image (4500/130/150 [repetition time msec/echo time msec/inversion time msec]) is used to determine plane for obtaining images parallel to the long axis of the uterus. (b) Coronal single-shot fast spin-echo T2-weighted MR image (minimum repetition time, 180-msec echo time) of the retroperitoneum obtained to assess the kidneys shows congenitally absent left kidney.
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Figure 1b. (a) Sagittal inversion-recovery MR image (4500/130/150 [repetition time msec/echo time msec/inversion time msec]) is used to determine plane for obtaining images parallel to the long axis of the uterus. (b) Coronal single-shot fast spin-echo T2-weighted MR image (minimum repetition time, 180-msec echo time) of the retroperitoneum obtained to assess the kidneys shows congenitally absent left kidney.
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CLASSIFICATION OF MÜLLERIAN DUCT ANOMALIES
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The most basic classification of müllerian duct defects consists of agenesis and hypoplasia, defects of vertical fusion, and defects of lateral fusion. In 1979, Buttram and Gibbons (17) proposed a classification of müllerian duct anomalies that was based on the degree of failure of normal development, and they separated these anomalies into classes that demonstrate similar clinical manifestations, treatment, and prognosis for fetal salvage. Modified in 1988 by a subcommittee of the American Fertility Society (now the American Society of Reproductive Medicine) (43), the classification remains the most widely accepted schematization and addresses uterovaginal anomalies (Fig 2).
Class I anomalies consist of segmental agenesis and variable degrees of uterovaginal hypoplasia. Class II anomalies are unicornuate uteri that represent partial or complete unilateral hypoplasia. Class III is composed of uterus didelphys in which duplication of the uterus results from complete nonfusion of the müllerian ducts. Class IV anomalies are bicornuate uteri that demonstrate incomplete fusion of the superior segments of the uterovaginal canal. Class V anomalies are septate uteri that represent partial or complete nonresorption of the uterovaginal septum. Class VI anomalies are arcuate uteri that result from near complete resorption of the septum. Class VII anomalies comprise sequelae of in utero DES exposure. Because of the variability and overlap of features of associated cervical and vaginal malformations, these changes generally are not incorporated into the basic schematics and are reported as a subset of the primary uterine defect. Secondary classification systems also have been introduced that further dissect and elaborate on the original Buttram and Gibbons schema. Toaff et al (44) described nine subtypes of septate and bicornuate uteri that are characterized by the presence of a communication between two otherwise separate uterocervical cavities.
Isolated congenital anomalies of the fallopian tubes are rare, although agenesis, hypoplasia, and segmental narrowing have been reported (18). Congenital anomalies of the ovaries are extremely rare. Agenesis and partial development of the ovaries have been documented, often in association with müllerian duct and urinary tract anomalies. Supernumerary ovaries and accessory ovaries contiguous with the original ovary have been described, as have been ovaries located above the true pelvis (18,45).
It is important to note that the American Fertility Society classification system functions as a framework for the description of anomalies, for communication between clinicians, and for comparison among various therapeutic modalities. However, the most notable inherent deficiency of the classification is related to the description of anomalies that include features of two or more classes. These anomalies should be described according to their component parts and should not be categorized into the class that most closely approximates the dominant feature. Underscoring the importance for appropriate reporting is the repercussion on clinical approach and therapeutic regimen.
Septate Uterus
The septate uterus is the most common müllerian duct anomaly. This anomaly composes approximately 55% of müllerian duct anomalies (10,11,14) and results from partial or complete failure of resorption of the uterovaginal septum after fusion of the paramesonephric ducts. The septate uterus is associated with some of the poorest reproductive outcomes. The prevalence of septa in patients who have had recurrent spontaneous abortions (usually three or more) is well known, with reported spontaneous abortion rates ranging from 26% to 94% (pooled data, 65%) (3,4,11,15,16,20,46–48). It is difficult to assess reproductive outcome definitively, however, because the majority of studies to date have not been controlled (10). The septate uterus is also associated with the worst obstetric outcome of the müllerian duct anomalies, with overall premature birth rates ranging from 9% to 33% (pooled data, 20%) and fetal survival rates from 10% to 75% (pooled data, 30%) (3,4,30,46–49). The length of the septum does not appear to correlate with differences in obstetric outcome (50). Reproductive outcome has been shown to improve after resection of the septum, with reported decreases in the spontaneous abortion rate from 88% to 5.9% after hysteroscopic metroplasty (10,51–53).
Recurrent pregnancy loss in these patients was traditionally attributed to the fibrous and avascular nature of the septum, despite the lack of histologic data (14,54). This assumption was brought into question with the advent of pelvic MR imaging, when it was observed that the signal intensity of the septum was usually myometrial (ie, isointense to myometrium) (34,35). In one study (35), septal tissue obtained from five patients at hysteroscopic metroplasty was sent for pathologic review, and all specimens demonstrated smooth muscle, not fibrous tissue. In another study, in which MR signal intensity of the septum was evaluated along with pathologic specimens (55), all partial septa contained myometrium, whereas patients with complete septa had evidence of myometrium in the upper segment of the septum and fibrous tissue in the lower segment. In another prospective study (56), septal and nonseptal tissue samples were obtained from the posterior uterine wall at the time of Tompkin metroplasty. Multiple biopsies demonstrated increased amounts of muscular tissue and less connective tissue in the septum. The authors theorized that the decreased connective tissue may result in poor decidualization and implantation, while increased muscular tissue may result in increased contractility of the tissue, thereby predisposing the patient to spontaneous abortion. In addition to the inherent deficiencies of the composition of the septum, the overlying endometrium has been shown to be defective (57). A scanning electron microscopy comparison of the septal endometrium to the endometrium overlying the lateral uterine wall myometrium showed the septal endometrium to be irregular in morphology, with a decrease in sensitivity to preovulatory hormonal changes (58). Morphologic narrowing of the cavity by the septum, causing a reduction in endometrial capacity, has also been implicated in the cause of poor outcomes (48).
There is consensus that vascularity within the septum is abnormal. Inadequate vascularization and altered relationships between the endometrial and myometrial vessels and nerves are thought to be causative (14,54).
The septum, which arises in the midline fundus, is considered to be complete when it extends to the external cervical os. Extension of the septum to the upper vagina is seen in approximately 25% of cases (46). A partial septum is variable in length and may be mild or extend to the endocervical canal proximal to the external os. On rare occasions, complete duplication of the cervix can occur with a septum, and this anomaly is included in the spectrum of findings of the American Fertility Society classification of septate uteri. The external uterine contour may be convex, flat, or mildly (<1.0-cm) concave (10,34,35,42). The American Fertility Society and Toaff et al classifications do not specify the minimal depth of fundal indentation for differentiation of a septate from a bicornuate or a uterus didelphys. A cutoff of 1.0 cm was chosen after subjective evaluation by gynecologists at the time of laparoscopy and hysteroscopy and, while noted to be arbitrary, has been found to be reliable for differentiation from a bicornuate configuration (38,59) (Fig 3).
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Figure 3a. Uterine septum. (a) Laparoscopic image shows flat external uterine fundal contour. (b) Hysteroscopic image shows intervening septum (arrow). (Reprinted, with permission, from reference 43.)
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Figure 3b. Uterine septum. (a) Laparoscopic image shows flat external uterine fundal contour. (b) Hysteroscopic image shows intervening septum (arrow). (Reprinted, with permission, from reference 43.)
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The configuration of the external uterine contour is crucial for the differentiation of a septate from a bicornuate uterus, because widely different clinical and interventional approaches are assigned to each anomaly (60). A septate uterus is often treated with hysteroscopic resection of the septum. Bicornuate uteri rarely necessitate surgical intervention, although Strassman metroplasty with wedge resection of the medial aspect of each uterine horn and subsequent unification of the two cavities may be considered in selected patients with recurrent second- and third-trimester pregnancy losses (61). It is important to recognize that mild concavity of the external uterine contour should not be construed as a "partial" bicornuate configuration, because these patients may not be given the option of hysteroscopic metroplasty. When evaluating the uterus following hysteroscopic metroplasty, no residual septum or evidence of a residual septum measuring up to 1 cm in length is considered indicative of optimal resection (62).
HSG of a septate uterus can be used to evaluate the size and extent of septa (63,64); however, the diagnostic accuracy of HSG alone is only 55% for differentiation of septate from bicornuate uteri (36). An angle of less than 75° between the uterine horns is suggestive of a septate uterus, and an angle of more than 105° is more consistent with bicornuate uteri (36,63). Unfortunately, the majority of angles of divergence between the horns fall within this range, and considerable overlap between the two anomalies is noted (Fig 4). In addition, the presence of leiomyomas or adenomyosis within the septum may cause secondary distortion and widening of the angles of divergence of the uterine horns (Fig 5). It has been reported that when US is used in conjunction with HSG, the correct diagnosis can be made in 90% of cases (10,31,36,38).
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Figure 4a. HSG demonstration of septate versus bicornuate uteri. (a) Acute angle of divergence between uterine horns is most suggestive of a septate uterus (arrow). (b, c) Indeterminate angles of divergence may suggest either (b) septate uterus (arrow) or (c) bicornuate uterus (arrow). Final diagnoses were based on subsequent MR imaging results (not shown).
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Figure 4b. HSG demonstration of septate versus bicornuate uteri. (a) Acute angle of divergence between uterine horns is most suggestive of a septate uterus (arrow). (b, c) Indeterminate angles of divergence may suggest either (b) septate uterus (arrow) or (c) bicornuate uterus (arrow). Final diagnoses were based on subsequent MR imaging results (not shown).
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Figure 4c. HSG demonstration of septate versus bicornuate uteri. (a) Acute angle of divergence between uterine horns is most suggestive of a septate uterus (arrow). (b, c) Indeterminate angles of divergence may suggest either (b) septate uterus (arrow) or (c) bicornuate uterus (arrow). Final diagnoses were based on subsequent MR imaging results (not shown).
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Figure 5a. Septate uterus. (a) HSG image shows wide divergence of opacified endometrial cavities simulating a bicornuate configuration. (b) Corresponding coronal oblique fast spin-echo T2-weighted MR image (6000/120 [effective]) demonstrates insinuated leiomyoma (long arrow) within the septum, causing exaggerated separation of cavities. Note lateral wall myoma with cystic degeneration (short arrow) also causing distortion of left endometrial cavity.
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Figure 5b. Septate uterus. (a) HSG image shows wide divergence of opacified endometrial cavities simulating a bicornuate configuration. (b) Corresponding coronal oblique fast spin-echo T2-weighted MR image (6000/120 [effective]) demonstrates insinuated leiomyoma (long arrow) within the septum, causing exaggerated separation of cavities. Note lateral wall myoma with cystic degeneration (short arrow) also causing distortion of left endometrial cavity.
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At US, the echogenic endometrial cavities are separated at the fundus by the intermediate echogenicity of the myometrium in all partial septa and within the fundal segments of complete septa (Fig 6). The inferior segment of the complete septum is hypoechoic and reflects the caudal fibrous component. The external uterine contour must demonstrate a convex, flat, or mildly concave configuration and may best be appreciated on transverse images of the uterus; however, definitive characterization of the fundal contour remains a potential limitation. US has been reported to allow differentiation of a septate from a bicornuate uterus if a true orthogonal view along the long axis can be obtained. In this plane, a line is drawn between the apices of the endometrium at the level of the ostia. If the fundal indentation of the external uterine contour is below the interostial line or less than 5 mm above the line, the uterus is considered to be bicornuate or didelphic. The septate uterus is defined by a fundal indentation of more than 5 mm above the interostial line (10,38) (Fig 7).
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Figure 6. Coronal oblique reconstructed three- dimensional endovaginal US image of a partial uterine septum demonstrates mild indentation of the uterine fundus with no intervening cleft (short arrow) and septum separating endometrial cavities (long arrow). (Image courtesy of Anna Lev-Toaff, MD, Thomas Jefferson University, Philadelphia, Pa.)
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Figure 7. Classification criteria for US differentiation of septate from bicornuate uteri. A, When apex (3) of the fundal external contour occurs below a straight line between the tubal ostia (1, 2) or, B, 5 mm (arrow) above it, the uterus is bicornuate. C, When apex is more than 5 mm (arrow) above the line, uterus is septate.
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On MR images, the septate uterus is generally normal in size; however, each endometrial cavity appears smaller than the configuration of a normal cavity. In both partial and complete septa, the signal intensity within the fundal segment of the septum is isointense with the confluent adjacent myometrium of the walls of the anterior and posterior uterine body. In a complete septum, the upper fundal segment of the septum has signal intensity similar to that of myometrium and contains a low-signal-intensity linear band extending to the external cervical os on T2-weighted images (35,55) (Fig 8). Only a small subset of partial septa demonstrates an inferior linear low-signal-intensity band on T2-weighted images. The presence of leiomyomas and adenomyosis within the septum, as documented on MR images, further substantiates the myometrial composition of the septum (55) (Fig 9). Duplication of normal cervical zonal anatomy is seen in the rare instance of septate uteri with two cervices (Fig 10).
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Figure 8a. MR images of complete uterine septum. (a) Transverse oblique fast spin-echo T2-weighted image (7150/120) shows convex external uterine contour with upper myometrial segment (short arrow) and lower fibrous segment (long arrow) extending to external uterine os. (b) Transverse fast spin-echo T2-weighted image (6000/115) shows vertical septum extending to upper third of vagina (arrow). (Reprinted, with permission, from reference 43.)
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Figure 8b. MR images of complete uterine septum. (a) Transverse oblique fast spin-echo T2-weighted image (7150/120) shows convex external uterine contour with upper myometrial segment (short arrow) and lower fibrous segment (long arrow) extending to external uterine os. (b) Transverse fast spin-echo T2-weighted image (6000/115) shows vertical septum extending to upper third of vagina (arrow). (Reprinted, with permission, from reference 43.)
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Figure 9a. MR images of partial uterine septum. (a) Coronal oblique fast spin-echo T2-weighted image (6000/120) shows flat external uterine contour with prominent upper myometrial component (short arrow) and small lower fibrous component (long arrow). (b) Coronal oblique fast spin-echo T2-weighted image (6100/110) shows partial septum with insinuated leiomyomas (arrow). (c, d) Coronal oblique fast spin-echo T2-weighted images (6000/105) show partial septum with extensive adenomyosis (curved arrows). Note concave external uterine contour (straight arrow).
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Figure 9b. MR images of partial uterine septum. (a) Coronal oblique fast spin-echo T2-weighted image (6000/120) shows flat external uterine contour with prominent upper myometrial component (short arrow) and small lower fibrous component (long arrow). (b) Coronal oblique fast spin-echo T2-weighted image (6100/110) shows partial septum with insinuated leiomyomas (arrow). (c, d) Coronal oblique fast spin-echo T2-weighted images (6000/105) show partial septum with extensive adenomyosis (curved arrows). Note concave external uterine contour (straight arrow).
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Figure 9c. MR images of partial uterine septum. (a) Coronal oblique fast spin-echo T2-weighted image (6000/120) shows flat external uterine contour with prominent upper myometrial component (short arrow) and small lower fibrous component (long arrow). (b) Coronal oblique fast spin-echo T2-weighted image (6100/110) shows partial septum with insinuated leiomyomas (arrow). (c, d) Coronal oblique fast spin-echo T2-weighted images (6000/105) show partial septum with extensive adenomyosis (curved arrows). Note concave external uterine contour (straight arrow).
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Figure 9d. MR images of partial uterine septum. (a) Coronal oblique fast spin-echo T2-weighted image (6000/120) shows flat external uterine contour with prominent upper myometrial component (short arrow) and small lower fibrous component (long arrow). (b) Coronal oblique fast spin-echo T2-weighted image (6100/110) shows partial septum with insinuated leiomyomas (arrow). (c, d) Coronal oblique fast spin-echo T2-weighted images (6000/105) show partial septum with extensive adenomyosis (curved arrows). Note concave external uterine contour (straight arrow).
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Figure 10a. MR images of complete uterine septum with cervical duplication. Fast spin-echo T2-weighted images obtained in (a) transverse oblique (7300/120) and (b) coronal oblique (7216/105) planes show enlarged cervical segment with two distinct cervices, each with preserved zonal anatomy (arrows).
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Figure 10b. MR images of complete uterine septum with cervical duplication. Fast spin-echo T2-weighted images obtained in (a) transverse oblique (7300/120) and (b) coronal oblique (7216/105) planes show enlarged cervical segment with two distinct cervices, each with preserved zonal anatomy (arrows).
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An intercornual distance of less than 4.0 cm has been used to distinguish a septate from a bicornuate uterus (34). However, this measurement is a residuum of HSG criteria that were created to compensate for the inability of HSG to demonstrate the fundal contour and should not be used as a differentiating criterion with MR imaging. Furthermore, the angle will be broadened by leiomyomas and/or adenomyosis that involve the septum.
Arcuate Uterus
The arcuate uterus is characterized by a mild indentation of the endometrium at the uterine fundus as a result of near complete resorption of the uterovaginal septum. Classification has been problematic, because it remains unclear whether this variant should be classified as a true anomaly or as an anatomic variant of normal. In the original Buttram and Gibbons classification, the arcuate uterus was subclassified with the bicornuate uterus because it "most closely resembled a ‘mild’ form of bicornuate uterus" (17). On revision of the classification by the American Fertility Society, a separate class was designated, because the arcuate uterus can be distinguished from a bicornuate uterus on the basis of its complete fundal unification (43). Data regarding the reproductive outcomes of patients with an arcuate uterus are extremely limited and widely disparate. In small studies, both poor and good obstetric outcomes have been reported (65–67), although an arcuate configuration is generally thought to be compatible with normal-term gestation, with a quoted delivery rate of 85% (65). However, when all extrauterine factors for infertility have been excluded, hysteroscopic correction may be considered in selected patients with recurrent pregnancy loss who have a prominent or broad configuration of the fundal myometrium.
It has been proposed that when a ratio of less than 10% between the height of the fundal indentation and the distance between the lateral apices of the horns is calculated on the basis of HSG findings, an adverse reproductive outcome is not anticipated (10,63,67) (Fig 11). However, a defining depth of the indentation to differentiate an arcuate configuration from a broad septum has not been established.
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Figure 11. Diagram of arcuate uterus ratio. When ratio of height (H) to length (L) is less than 10%, an adverse reproductive outcome is not expected. (Reprinted, with permission, from reference 63.)
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On HSG images, opacification of the endometrial cavity demonstrates a single uterine canal with a broad saddle-shaped indentation of the uterine fundus (35) (Fig 12).
On US images, a normal external uterine contour is noted, with a broad smooth indentation on the fundal segment of the endometrium. The indentation may best be appreciated in the transverse plane with subtle, focal, superior duplication of the endometrial echogenic complexes. No division of the uterine horns is noted (35).
On MR images, the normal external uterine contour is maintained. The myometrial fundal indentation is smooth and broad, and the signal intensity of this region is isointense to normal myometrium. No low-signal-intensity fibrous component is appreciated (35) (Fig 13). Subtle indentation of the peripheral subserosal arcuate vessels at the level of the fundus may be noted on MR images. The orientation mimics the pattern seen in the septate uterus and, hypothetically, may indicate a degree of vascular aberrancy.
Bicornuate Uterus
The bicornuate uterus results from incomplete fusion of the uterovaginal horns at the level of the fundus and accounts for approximately 10% of müllerian duct anomalies. Patients with a bicornuate uterus and no extrauterine infertility issues usually have little difficulty conceiving. Spontaneous abortion rates are reported to range from 28% to 35% (pooled data, 30%) (3,46–48). Premature birth rates range from 14% to 23% (pooled data, 20%); and fetal survival rates, from 57% to 63% (pooled data, 60%) (3,45,47,48,68). The rates of spontaneous abortion and premature delivery have also been reported to reflect the degree of nonfusion of the horns. Spontaneous abortion rates and preterm delivery are reported to be higher in women with a complete bicornuate uterus than in those with a partial bicornuate uterus (3).
A bicornuate uterus consists of two symmetric cornua that are fused caudad, with communication of the endometrial cavities—most often at the level of the uterine isthmus. The intervening cleft of the complete bicornuate uterus extends to the internal cervical os (bicornuate unicollis), while the cleft of a partial bicornuate configuration is of variable length. A bicornuate bicollis uterus is associated with a duplicated cervix, although a degree of communication is maintained between the two horns. At least six variations of the bicornuate uterus have been described in the literature (44). Longitudinal upper vaginal septa are reported to coexist in 25% of bicornuate uteri.
Surgical intervention is usually not indicated, and the length of subsequent gestations often increases with increasing parity. Strassman metroplasty has been advocated in women with a history of recurrent pregnancy loss and in whom no other infertility issues have been identified (48,61). However, the benefits of metroplasty have never been formally studied in a prospective trial (46). The bicornuate uterus has been reported to have the highest associated prevalence—38%—of cervical incompetence among müllerian duct anomalies (69). Prophylactic placement of a cervical cerclage in selected patients has been reported to increase fetal survival rates (70).
On HSG images, the horns of the endometrial cavity are usually widely separated with an intercornual angle greater than 105°. Each horn has a fusiform appearance, with apices that taper and end in a single fallopian tube. However, because the radiographic appearance has such a large degree of overlap with that of the septate uterus and because the external uterine contour cannot be characterized, differentiation from the septate uterus is more often not possible (35,63,64).
On US images, a large fundal cleft must be documented with divergence of the uterine horns and associated echogenic endometrial complexes (35,38). Features such as extreme anteflexion or retroflexion and the presence and deformity caused by overlying leiomyomas may prove to be confounding (Fig 14).
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Figure 14a. Bicornuate uterus. (a) Transverse US image and (b) corresponding transverse oblique fast spin-echo T2-weighted MR image (7250/105) demonstrate external fundal cleft (straight arrow) with wide divergence of endometrial cavities. Note leiomyoma (curved arrow) in right lateral wall.
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Figure 14b. Bicornuate uterus. (a) Transverse US image and (b) corresponding transverse oblique fast spin-echo T2-weighted MR image (7250/105) demonstrate external fundal cleft (straight arrow) with wide divergence of endometrial cavities. Note leiomyoma (curved arrow) in right lateral wall.
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On MR images, the bicornuate uterus demonstrates a cleft of at least 1.0 cm of the external fundal uterine contour. The horns demonstrate normal uterine zonal anatomy. The endometrial-to-myometrial ratio and width are normal in appearance (35,42). Superimposed leiomyomas and adenomyosis are well demonstrated (Fig 15).
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Figure 15a. MR images of bicornuate uterus. Fast spin-echo T2-weighted images in (a) coronal oblique (5650/105) and (b) transverse (6000/130) planes provide two examples of bicornuate uteri and demonstrate wide divergence of uterine horns, with communication of endometrial cavities in the lower uterine body (arrow).
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Figure 15b. MR images of bicornuate uterus. Fast spin-echo T2-weighted images in (a) coronal oblique (5650/105) and (b) transverse (6000/130) planes provide two examples of bicornuate uteri and demonstrate wide divergence of uterine horns, with communication of endometrial cavities in the lower uterine body (arrow).
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Uterus Didelphys
Uterus didelphys, which constitutes approximately 5% of müllerian duct anomalies, is the result of nearly complete failure of fusion of the müllerian ducts. Each müllerian duct develops its own hemiuterus and cervix and demonstrates normal zonal anatomy with a minor degree of fusion at the level of the cervices. No communication is present between the duplicated endometrial cavities. A longitudinal vaginal septum is associated in 75% of these anomalies (71). Longitudinal vaginal septa may be complicated by defects in vertical fusion that result in a transverse vaginal septum and subsequent hematometrocolpos. Spontaneous abortion rates are reported to range from 32% to 52% (pooled data, 45%) (3,46–48). Premature birth rates range from 20% to 45% (pooled data, 38%); and fetal survival rates, from 41% to 64% (pooled data, 55%) (3,46–48). Strassman metroplasty, leaving the duplicated cervix intact in an attempt to prevent cervical incompetence, is a consideration for selected patients with recurrent spontaneous abortions and premature deliveries (48,61,72). As with the bicornuate uterus, however, the benefits of intervention remain unclear because no controlled trials have been performed (46).
Nonobstructive uterus didelphys is usually asymptomatic, while uterus didelphys with unilateral vaginal obstruction may become symptomatic at menarche and manifest as dysmenorrhea. Endometriosis and pelvic adhesions have an increased prevalence and are reported to be secondary to retrograde menstrual flow in the subset of patients with obstruction (73,74).
HSG demonstrates two separate endocervical canals that open into separate fusiform endometrial cavities, with no communication between the two horns. Each endometrial cavity ends in a solitary fallopian tube. However, if the anomaly is associated with an obstructed longitudinal vaginal septum, only one cervical os may be depicted, and it may be cannulated with the endometrial configuration mimicking a unicornuate uterus (63,64) (Fig 16).
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Figure 16a. Uterus didelphys. (a, b) HSG images show catheterization of two separate cervices with opacification of two widely divergent noncommunicating endometrial cavities (arrow).
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Figure 16b. Uterus didelphys. (a, b) HSG images show catheterization of two separate cervices with opacification of two widely divergent noncommunicating endometrial cavities (arrow).
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On US images, separate divergent uterine horns are identified, with a large fundal cleft. Endometrial cavities are uniformly separate, with no evidence of communication. Two separate cervices need to be documented.
MR imaging demonstrates two separate uteri with widely divergent apices, two separate cervices, and usually an upper vaginal longitudinal septum. In each uterus, the endometrial-to-myometrial width and ratio are preserved, as is normal uterine zonal anatomy (34,35,42). An obstructed unilateral vaginal septum may cause apparent marked deformity of the uterus according to the degree of associated hematometrocolpos (Fig 17).
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Figure 17a. Uterus didelphys. (a, b) Transverse fast spin-echo T2-weighted MR images (7216/130) show complete duplication of uterine horns (short arrows), with partial degree of fusion of adjacent cervices (long arrows).
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Figure 17b. Uterus didelphys. (a, b) Transverse fast spin-echo T2-weighted MR images (7216/130) show complete duplication of uterine horns (short arrows), with partial degree of fusion of adjacent cervices (long arrows).
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Unicornuate Uterus
Failure of one müllerian duct to elongate while the other develops normally results in the unicornuate uterus and accounts for approximately 20% of müllerian duct anomalies. A unicornuate uterus may be isolated, manifesting in 35% of patients, although it is usually associated with variable degrees of a rudimentary uterine horn. A noncavitary rudimentary horn without associated endometrium is seen in 33% of cases, and that with an endometrial segment is seen in 32%. A cavitary rudimentary horn is designated "communicating" if there is communication with the endometrium of the contralateral horn (10% of cases) and "noncommunicating" if there is no such communication (22% of cases) (17,34,75). The embryologic predominance of the unicornuate uterus to be on the right has not been explained (76). Spontaneous abortion rates are reported to range from 41% to 62% (pooled data, 50%) (46–48,76–78). Reported premature birth rates range from 10% to 20% (pooled data, 15%); and fetal survival rates, from 38% to 57% (pooled data, 40%) (46–48,76–78). Common obstetric complications include abnormal fetal lie and intrauterine growth retardation.
As compared with uterus didelphys, the increased spontaneous abortion rate and the decreased fetal survival rate of the unicornuate uterus are incompletely understood, as is the pathogenesis of pregnancy loss. While gestational capacity has been observed to be proportional to uterine muscular organ mass (77), the decrease in myometrial mass seen in a unicornuate uterus and in the horn of a didelphic uterus are common to both. It has been hypothesized that inadequate vascularization and compromised uteroplacental blood flow of the unicornuate uterus result from the decreased vascular contribution of the uterine and utero-ovarian arteries from the abnormal side (77).
As with uterus didelphys, the manifestation is usually incidental unless a noncommunicating rudimentary horn is present. Dysmenorrhea with hematometra may manifest at menarche in this subgroup. Resection of the noncommunicating horn is indicated, not only for symptomatic relief but also because ectopic pregnancy may occur in the horn by means of transperitoneal sperm migration (79). Eighty-nine percent of pregnancies that arise in a noncommunicating uterine horn end in rupture (79). In addition, the incidence of endometriosis is increased in this subgroup, similar to the case in an obstructed uterus didelphys (73,74). Resection of a communicating horn is also a consideration, because pregnancies that develop in the rudimentary horn rarely yield viable offspring (79,80). Surgical intervention in a rudimentary horn without associated endometrium is rarely indicated.
Renal abnormalities are more commonly associated with unicornuate uterus than with other müllerian duct anomalies and have been reported in 40% of these patients (8,30,48). The anomaly is always ipsilateral to the rudimentary horn. Renal agenesis is the most commonly reported abnormality, occurring in 67% of cases. Ectopic kidney, horseshoe kidney, cystic renal dysplasia, and duplicated collecting systems have also been described (30,79,81).
On HSG images, speculum inspection of the cervix demonstrates a small cervix and a poorly developed contralateral vaginal fornix. After instillation of contrast material, the endometrial cavity assumes a fusiform shape, tapering at the apex and draining into a solitary fallopian tube. The uterus is generally shifted off of midline. Filling of a small communicating rudimentary horn may be seen, although HSG cannot clearly delineate noncavitary and noncommunicating rudimentary horns (60,62) (Fig 18).
On US images, the isolated unicornuate uterus appears small, although the characteristically asymmetric ellipsoidal shape is difficult to appreciate (75). Deviation of a small uterus to one side of the pelvis may suggest the diagnosis. A rudimentary horn in the presence of a small uterus may confirm the diagnosis; however, the hypoplastic horn may simulate a prominent cervix and confound the findings. The identification of a cavitary uterine horn may be difficult to differentiate from other types of duplicated uterus. Three-dimensional US may help further characterize the anomaly (Fig 19).
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Figure 19a. Unicornuate uterus. (a) Transverse and (b) sagittal two-dimensional endovaginal US images demonstrate uterus without gross morphologic anomaly. (c) Three-dimensional reconstructed transverse oblique US image shows an abnormal lenticular shape of endometrial cavity (long arrow) with asymmetric tapering at the cornua (short arrow). (Image courtesy of Anna Lev-Toaff, Thomas Jefferson University, Philadelphia, Pa.)
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Figure 19b. Unicornuate uterus. (a) Transverse and (b) sagittal two-dimensional endovaginal US images demonstrate uterus without gross morphologic anomaly. (c) Three-dimensional reconstructed transverse oblique US image shows an abnormal lenticular shape of endometrial cavity (long arrow) with asymmetric tapering at the cornua (short arrow). (Image courtesy of Anna Lev-Toaff, Thomas Jefferson University, Philadelphia, Pa.)
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Figure 19c. Unicornuate uterus. (a) Transverse and (b) sagittal two-dimensional endovaginal US images demonstrate uterus without gross morphologic anomaly. (c) Three-dimensional reconstructed transverse oblique US image shows an abnormal lenticular shape of endometrial cavity (long arrow) with asymmetric tapering at the cornua (short arrow). (Image courtesy of Anna Lev-Toaff, Thomas Jefferson University, Philadelphia, Pa.)
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On MR images, the unicornuate uterus appears curved and elongated, with the external uterine contour assuming a banana shape. Uterine volume is reduced, and the configuration of the uterus is asymmetric. Normal myometrial zonal anatomy is maintained. The endometrium may be uniformly narrow or may assume a bullet shape, tapering at the apex. The endometrial-to-myometrial width and ratio are reported to be normal (34,35) (Fig 20). The appearance of the rudimentary horn is variable. When the endometrium is absent, the horn is of low signal intensity, with loss of normal zonal anatomy. When the endometrium is present, zonal anatomy may be preserved.
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Figure 20a. Unicornuate uterus. Transverse oblique T2-weighted MR images (6000/105) show (a) unicornuate uterus (short arrow) with no associated rudimentary horn and (b) unicornuate uterus with rudimentary horn (long arrow) and no associated endometrium.
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Figure 20b. Unicornuate uterus. Transverse oblique T2-weighted MR images (6000/105) show (a) unicornuate uterus (short arrow) with no associated rudimentary horn and (b) unicornuate uterus with rudimentary horn (long arrow) and no associated endometrium.
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Complex Uterine Anomalies
Because müllerian duct defects are the result of a spectrum of nonfusion, deficient development, and/or defective canalization, the process may become arrested at any point in development, especially if the bidirectional theory of septal regression is considered. Classification of these defects must address the component parts of the anomaly rather than simply assign it to the original American Fertility Society class most 
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ABSTRACT
INTRODUCTION
