Endometriosis is the ectopic implantation of functional endometrial epithelium outside the uterine cavity [4]. It is most often found in the pelvis (ovaries, fallopian tubes, broad ligaments, recto-vaginal septum, round and bladder ligaments), extra-pelvic, notably abdominal (appendicular, ureteral, etc.) or extra-abomino-pelvic (cardiac, pulmonary, pleural, diaphragmatic, cerebral, parietal). Of these, parietal endometriosis is the most common¹. Endometriosis is parietal when the ectopic location is superficial to the peritoneum⁵. It may be superficial (subcutaneous tissue), intermediate (attached to the fascia) or deep (in the parietal muscle). Umbilical endometriosis is one of the most common parietal endometrioses.

Several theories have been put forward to explain this ectopic implantation, including reflux migration via the tubes, lympho-vascular migration theory and cellular metaplasia [5,6].

It is a chronic pathology that mainly affects women of childbearing age, with a peak around the age of 40. Diagnosis is often made later than the onset of symptoms, due to a lack of awareness of the pathology [3,4].

Umbilical endometriosis essentially manifests as a painful mass with a catamenial character. The cyclical nature of the pain is an important element of orientation, but its absence does not eliminate the diagnosis. It may be absent in 13% of cases [7]. Clinical signs may appear in the weeks or even years following surgery [4]. In our patient, the interval between the operation and the onset of symptoms was 12 months. The nodule may be located higher than the scar, which may be offset from the deep scar [1]. The classic presentation includes the triad of cyclical abdominal pain at the time of menstruation, a palpable mass and a history of caesarean section or other gynaecological surgery [7].

Ultrasound and CT images of scar endometriosis are not very specific. They may appear as fluid collections or as tissue. In fact, the appearance of lesions depends on hormonal impregnation, depending on the cycle. They may be solid, cystic or mixed [4,8].

MRI is the examination of choice in the search for deep endometriotic lesions indicating medical treatment associated with excision surgery [4]. It provides a better definition of the size of the mass, its relationship with neighbouring structures, particularly the deep plane, and infiltration into surrounding tissues, useful information for choosing the best method of closure of the fascial defect during surgery [5,11]. However, signal characteristics and contrast enhancement patterns are not specific, and it is not always possible to determine the true histological nature of soft tissue masses, with a few exceptions [6].

Different imaging modalities (ultrasound, MRI, CT scan) are not specific but are useful for determining the extent of disease and aiding surgical resection planning, particularly for large, recurrent lesions [6].

Once identified, the endometrioma must be completely excised and sent to pathology [9].  Treatment is based primarily on excision of the mass with safety margins of at least 1cm to reduce the risk of recurrence¹⁰. Hysterectomy is a last-line treatment suitable for patients who do not wish future fertility and who have not responded to more conservative management [11].

Even though ultrasound-guided fine needle aspiration can provide diagnosis, surgical excision is recommended for both diagnosis and treatment of an abdominal wall mass [12].

Finally, it should be noted that endometriosis has been shown to undergo a rare malignant transformation in 0.7-1.5% of all cases of endometrioma. It is therefore important to be proactive in the treatment of symptomatic endometriosis, particularly in atypical locations such as those adjacent to the abdominal wall. Some studies report that malignant transformations have a significant mortality rate, of up to 43% in the absence of treatment [11].