Endometriosis

Endometriosis is a common and clinically important problem in women of childbearing age. It is classically defined as the presence of functional endometrial glands and stroma outside the uterine cavity and musculature ¹. This is distinct from adenomyosis, in which endometrial tissue is confined to the uterine musculature. It may vary from microscopic endometriotic implants to large cysts (endometriomas).

Epidemiology

Typically endometriosis presents in young women, with a mean age of diagnosis 25 - 29 ⁴, although it is not uncommon among adolescents. Up to 5% of cases are diagnosed in post menopausal women.

Overall prevalence has been difficult to ascertain ⁵. In asymptomatic women undergoing laparoscopic tubal ligation endometriosis is identified in 1-7% whereas in women undergoing laparoscopic investigation for primary infertility prevalence ranged between 17-50%, versus 5-21% for investigation of pelvic pain ⁶. It is important to realise that endometriosis may be asymptomatic, especially if disease is isolated to the peritoneum. 

Potential risk factors include family history and short menstrual cycles. Racial predisposition remains controversial ^5,7

Clinical presentation

Symptoms

• infertility : 20% of infertile women undergoing laparoscopic investigation have endometriosis while 30-50% of women with endometriosis are infertile ¹⁵.
• pelvic pain : including dyspareunia, dysmenorrhea, chronic pelvic pain, urinary symptoms and rectal discomfort and dyschezia; pain is not always cyclic ¹²
• unusual symptoms :
  • gastrointestinal involvement : catamenial diarrhoea, rectal bleeding and constipation
  • vesical involvement : urgency, frequency, haematuria
  • thoracic involvement : pleuritic chest pain, pneumothorax, pleural effusions or cyclic hemoptysis

Examination findings

• non specific
• tenderness along the uterosacral ligaments, cul-de-sac +/- thickening or nodularity
• rectovaginal masses
• adnexal tenderness / masses
• stage of disease does not necessarily correlate with the severity of the symptoms ¹⁶

Pathology

The pathogenesis of endometriosis remains unclear and is subject to much debate; potential mechanisms include:

• transplantation of endometrial cells (via retrograde menstruation, lymphatic or vascular dissemination, iatrogenic implantation) with probable immune / hormonal / inflammatory mediators ⁸. Supporting this theory is that up to 90% of women have bloody peritoneal fluid during the perimenstrual period ⁹
• retroperitoneal deep endometriosis may originate from metaplasia of Mullerian remnants located in the rectovaginal septum ¹⁰
• induction theory whereby shed endometrium releases substances that induce undifferentiated mesenchyma to form endometriotic tissue ²

See a nice illustration of  Theories of endometriosis

Macroscopic appearances vary depending on the duration of disease and depth of penetration. Endometriosis is divided into superficial and deep (penetrating into the retroperitoneal space or the wall of the pelvic organs to a depth of at least 5mm ³, and comprises nodules, cysts and secondary scarring. 

Nodules vary size from a few millimeters to 2cm in diameter. The amount of pigment appears to increase with the age of the lesion. Initially they appear as white plaques, non-pigmented clear vesicles or red petechia/ or flame-like areas. As they age the colour changes to bluish / brownish lesions. These are referred to as “powder burns”, representing haemolysed blood encased in fibrotic tissue ¹¹. Additionally, appearance not only varies with age but also with the part of the menstrual cycle.

Endometriotic cysts (endometriomas or "chocolate cysts") most commonly occur in the ovaries and are the result of repeated cyclic haemorrhage within a deep implant. Often there is complete replacement of ovarian tissue. The cyst walls may become thick and fibrotic with dense adhesions, with lining that varies in contour (smooth to shaggy) and colour (pale to brown).

Adhesions and fibrosis can distort normal pelvic anatomy and lead to obliteration of the pouch of Douglas.

Superficial endometriosis - Sampson syndrome

Location 

The most common locations for endometriotic deposits are the ovaries and pelvic peritoneum. Less common locations include C section scars (scar endometriosis), deep subperitoneal tissues, gastrointestinal tract, bladder, chest and subcutaneous tissues.

Pouch of Douglas, uterosacral ligament and torus uterinus are the most common pelvic sites of involvement ¹³ Deep pelvic endometriosis is divided into :

• anterior cul-de-sac
  • endometriosis of the bladder detrusor with associated adhesion and anteflexed uterus
  • vesicovaginal septal involvement typically more caudal
• posterior cul-de-sac
  • retroperitoneal lesions and dependent intraperitoneal locations that may result in infiltrating lesions
  • adhesions between anterior rectal wall and posterior vaginal fornix
  • rectovaginal septal involvement
• pelvic side wall
  • including ureteral lesions said to arise from extension of pelvic foci and ovarian endometriosis
• gastrointestinal tract
  • implantation occurs between 12 - 37% of patients
  • rarely proximal to the terminal ileum ¹
  • rectosigmoid > appendix > caecum > distal ileum
• urinary tract
  • involvement is typically asymptomatic except with severe pelvic disease ²⁰
  • bladder > distal ureter  

Extra-abdominal locations include :

• chest
  • uncommon
  • almost exclusively right sided
  • usually in the setting of longstanding (> 5 years) pelvic endometriosis
• cutaneous / disease
  • scars
  • abdominal wall and recesses (e.g. inguinal hernias)
  • cervix : associated with cone biopsy
  • vulva

Radiographic features

General

Radiologic evaluation of small endometriotic implants is limited; therefore, the radiologist's role is generally to identify and evaluate endometriomas. 

Although laparoscopy continues to be the gold standard for the diagnosis of endometriosis, MRI is increasingly being used, especially to evaluate deep disease, with high sensitivity (90 %)  and specificity (91%) ²⁰. US predominantly is used to evaluate the ovaries, and to generally asses the pelvis in the work up for pelvic pain or infertility. 

Ultrasound 

Although there is some variation in the sonographic appearance of endometriomas, the classic endometrioma is a homogenous, focal lesions with low level echos, reminiscent of the testicle. They may be uni or multilocular, contain thin or thick septations and may contain wall nodules (a finding also found in ovarian neoplasms). If these mural nodules are hyperechoic these have a high predictive value for endometrioma over non-endometrial lesions ²². As opposed to many other ovarian cysts, endometriomas do not resolve. 

Ultrasound is poor at detecting peritoneal implants ²¹, and although it is unable to detect adhesions, it is able to dynamically assess mobility and fixation.

MRI

Technique : pelvic MRI protocol - endometriosis

MRI has a greater specificity for the diagnosis of endometriomas than the other non invasive imaging techniques ¹ and thus has a role to play in the evaluation of adnexal masses, as well as assessing for response to medical therapy (see below) potentially eliminating the need for followup laparoscopy. Typically the lesions that can be detected with MRI are those that contain blood products ²³. 

• haemorrhagic “powder burn”
  • lesions appear bright on T1 fat saturated sequences.
• small solid deep lesions
  • may be hyperintense on T1 and low on T2
• adhesions and fibrosis
  • iso-intense to pelvic muscle on both T1 and T2 weighted images
  • spiculated low signal intensity stranding that obscures organ interfaces ¹
  • distortion of normal anatomy
    • posterior displacement of the uterus and ovaries
    • angulation of bowel loops
    • elevation of the posterior vaginal fornix
  • loculated fluid collections
  • hydrosalpinx
• endometriomas
  • < 5 mm: early stage disease; > 15 mm: advanced disease
  • shading sign ²⁵: may be less likely to respond to medical treatment ²⁸
  • low T1 and T2 due to tissue and haemosiderin laden macrophages ¹
  • diagnostic criteria:
    • multiple cysts with T1 hyperintensity OR
    • one or more cysts with high T1 and shading on T2

• uterosacral involvement ¹³
  
  • normal uterosacral ligaments are smooth and of regular contour
  • irregular margins
  • asymmetry
  • nodularity and thickening medially (> 9 mm) ¹³
  • altered T2 signal : iso-intense (50%), hypo-intense (40%) or hyper-intense (10%) c.f.   myometrium
  • if bilateral uterosacral involvement  with additional involvement torus uterinus involvement results in an arciform abnormality
• vaginal involvement
  • loss of hypo-intense signal of posterior vaginal wall on T2WI
  • thickening, nodules and / or masses also potentially seen
• pouch of Douglas
  • partial to complete obliteration
  • suspended or lateralised fluid collections
• rectovaginal septum
  • nodules or masses that passed through the lower border of the posterior lip of the cervix
• gastrointestinal tract
  • MRI has a low sensitivity (33%) for detecting rectal lesions ¹³ due to artefacts related to rectal content.  sensitivity may be increased with the use of  water enema, endovaginal coils and phased array coils ²⁰
    • rectal wall thickening
    • anterior displacement of the rectum
    • abnormal angulation
  • loss of fat plane between uterus and bowel
  • inflammatory response due to repeated haemorrhage can lead to adhesions, strictures and bowel obstructions
• urinary tract
  • bladder
    • localised or diffuse bladder wall thickening
    • signal intensity abnormality, nodules or masses usually located at the level of the vesicouterine pouch
    • involvement of bladder mucosa is rare
• chest
  • catamenial pneumothorax
  • haemothorax
  • lung nodules
• cutaneous tissues
  • nodules
• malignant transformation
  • solid enhancing components

Limitations of MRI

Despite all the advantages of MRI over all other imaging modalities, it nonetheless has a number of limitation, including:

• non pigmented lesions will not be hyperintense on T1, and thus harder too see
• small foci may have variable signal intensity:
  • may appear similar to normal endometrium: low T1, high T2
  • hypo-intense on all sequences
  • hyper-intense on all sequences ¹
• plaque like implants are difficult to delineate ²⁶
• adhesions cannot be directly identified, usually relying on distortion of normal anatomy to imply their existence ²⁶

Complications

Malignant transformation

Malignant transformation is rare, occurring in less than 1% of cases. It is usually in the form of endometrioid carcinoma, or less commonly clear cell carcinoma.

Treatment and prognosis

Treatment of endometriosis can be “expectant”, medical or surgical.

Medical treatment

Targets hormonal regulation, and includes medication with:

• danazol (a synthetic androgen) : suppresses oestrogen production
• gonadotropin releasing hormone (GRH) analogues : control the menstrual cycle
• OCP : suppress cyclical haemorrhage

Surgery

• laparoscopic (conservative surgery)
  • adhesionolysis
  • partial cystectomy for resection of anterior cul-de-sac involvement provided ureteric re-implantation does not need to be performed
  • uterosacral ligament excision
  • used in combination with vaginal approach for vaginal disease
• laparotomy
  • hysterectomy and oophorectomy
  • bowel involvement

Differential diagnosis

Differential considerations on MRI include

• dermoid cysts
  • endometeriomas have homogenous high signal intensity on T1 which does not suppress on T1FS
  • unlike a dermoid which has signal drop out on fat suppression images and chemical shift artifact
• mucinous lesions: e.g ovarian mucinous tumours
  • increased signal on T1 but less intense than fat or blood