Extragenital endometriosis is less common, but has been described in almost every area of the female body including the bowel, bladder, lungs, brain, umbilicus, and surgical scars [3]. Due to its varied presentations, endometriosis remains a difficult condition to diagnose and treat.

The patient had regular, heavy and painless menstrual periods and did not wish for any treatment for such. She had two previous spontaneous vaginal deliveries and had no history of abdominal pain, dyspareunia or infertility. She was not using any form of hormonal contraception. Her medical history was not significant and she never had any abdominal surgeries.

Clinical examination revealed that the patient had a 2 cm × 2 cm firm nodule at the umbilicus, which appeared to be covered by a reddish brown discharge. Suspecting that she had an infection, the patient was swabbed and given a five-day course of oral broad-spectrum antibiotics. She showed up on check up two months later with no relief of symptoms. She then underwent an ultrasound scan that showed a 15-mm thin-walled cyst, approximately 5 mm below the skin surface. The key clinical feature that led to the correct diagnostic hypothesis of umbilical endometriosis was the temporal association of the bleeding with her menstrual period.

The patient was offered both medical and surgical management and she opted to have depot injections of Zoladex (AstraZeneca UK, Goserelin acetate, 3.6 mg subcutaneously, monthly). The swelling continued to persist in spite of three doses of Zoladex, and the patient then requested surgical excision. The risk of recurrence and scar endometriosis were explained to her.

The patient successfully underwent excision of the nodule with accompanying umbilical reconstruction. Histology confirmed the diagnosis of endometriosis and revealed the presence of endometriotic glands with mucinous type metaplasia and extravasation of the mucinous secretion into the adjacent stroma (Figure ​(Figure1).1). No epithelial atypia was seen and the excision appeared complete. The patient was seen six weeks after the surgery and found to be asymptomatic with a normal umbilicus. Before being discharged, the patient was again reminded of the risk of recurrence.

There has been great speculation about the pathogenesis of this phenomenon and several theories have been proposed. Latcher has classified these theories into three main categories: the embryonal rest theory, which explains endometriosis adjoining the pelvic viscera by Wollfian or Mullerian remnants [4,5]; the coelomic metaplasia theory, which states that the embryonic coelomic mesothelium dedifferentiates into endometrial tissue under stimulus such as inflammation or trauma [7]; and the migratory pathogenesis theory, which explains the dispersion of endometrial tissue by direct extension, vascular and lymphatic channels, and surgical manipulation. Still others suggest cellular proliferation of endometrial cells from initial extraperitoneal disease along the urachus [8,9]. The real mechanism still remains a mystery.

These patients are usually in the reproductive age group and present commonly with swelling, pain, discharge or cyclical bleeding from the umbilicus. There may be associated symptoms of coexistent pelvic endometriosis. These lesions are usually bluish-black in colour and become painful, larger and bleed about the time of menses. They range in size from 0.5 cm to 3 cm, but can enlarge to even more enormous sizes [4].

While the diagnosis is primarily clinical, magnetic resonance imaging (MRI) can be useful in evaluating patients with suspected endometriosis. Endometriomas appear homogeneously hyperintense on T1-weighted sequences [10]. MRI also has an advantage over laparoscopy for evaluating pelvic and extraperitoneal diseases, as well as lesions concealed by adhesions.

Histological findings are characterized by irregular glandular lumina embedded in the stroma with a high cellular and vascular component resembling the stroma of functional endometrium. A fairly recent study has suggested a distinctive dermatoscopic feature in cutaneous endometriosis -- that of comprising small red globular structures called 'red atolls' [11].

Differential diagnosis of umbilical nodules should include pyogenic granuloma, hernia, residual embryonic tissue, primary or metastatic adenocarcinoma (Sister Joseph's nodule), nodular melanoma, and cutaneous endosalpingosis.