endometriosis is a common benign disease , which is defined by the presence of endometrial tissue outside the uterus that most often affects the pelvic peritoneum [ 1 , 2 ] . primary umbilical endometriosis ( pue ) is often not recognized based on its clinical appearance , and knowledge about the pathophysiology is scarce . we report here a rare case of a painful , livid coloured nodule in the umbilicus , which turned out to be pue . pue should be a differential diagnostic consideration in patients with a painful swelling in the umbilicus as is outlined in this report . a 47-year - old healthy female presented with a painful and livid coloured nodule in the umbilicus , which gradually evolved over the past 6 months . , she had a soft , painful swelling with a diameter of 2 cm in the umbilicus , which was irreducible by gentle digital pressure ( fig .  1 ) . ultrasonography revealed a hypodense nodule of 1.8 cm at the umbilicus ( fig .  2 ) . however , a definitive diagnosis could not be established thus far . under the provisional clinical diagnosis , irreducible umbilical hernia with probably strangulated fatty tissue in the hernia sac surgical exploration of the umbilicus was performed . under general anaesthesia , a sub - umbilical incision was made . to our surprise , a subcutaneous , lobulated mass was exposed , which was fixated on the bottom of the umbilicus ( fig .  3 ) . the nodule was excised , and histopathological examination revealed connective tissue fragments with irregular tubular formations surrounded by stromal cells . our patient was treated by excision of the swelling that turned out to be pue . two months after the surgery , she visited the outpatient clinic and reported complete recovery of the painful sensation and swelling in the umbilicus . figure  1:clinical presentation of a livid , soft , painful swelling in the umbilicus , which could not be reduced by gentle digital pressure . figure  2:ultrasonography of the umbilicus demonstrated a hypodense nodule of 1.8 cm in the umbilicus . figure  3:intraoperative image of dark discoloured , lobulated swelling at the bottom of the umbilicus that turned out to be pue . clinical presentation of a livid , soft , painful swelling in the umbilicus , which could not be reduced by gentle digital pressure . intraoperative image of dark discoloured , lobulated swelling at the bottom of the umbilicus that turned out to be pue . endometriosis is a benign disorder , which affects 610% of all women in the reproductive age . the umbilicus could be an extraordinary site of endometriosis and is affected in 0.54% of the women with endometriosis . secondary nature , but this disorder can also be spontaneous or  primary . in the english literature , possible explanations for pue could be the migration of endometrial cells to the umbilicus through the abdominal cavity , the lymphatic system or through the embryonic remnants in the umbilical fold such as the urachus and the umbilical vessels [ 35 ] . secondary umbilical endometriosis is caused by iatrogenic dissemination of endometrial cells , for instance , after a caesarean section or laparoscopic operation [ 1 , 2 , 4 ] . other causes of a painful swelling in the umbilicus are shown in table  1 [ 35 ] . table  1differential diagnosis of an umbilical nodule.benignmalignantcutaneous endometriosis / endosalpingiosissister mary joseph nodehaemangioma / vascular malformationmelanomaumbilical hernia / cicatrical herniasarcomasebaceous cystadenocarcinomagranulomalipomaabscesskeloidomphalomesenteric or urachus anomaly and/or infectiondesmoid tumourlymphomaendosalpingiosis : the presence of fallopian tube - like epithelium outside of the fallopian tube.sister mary joseph node : palpable nodule bulging into the umbilicus as a result of metastasis of a malignant tumour in the pelvis or abdomen . endosalpingiosis : the presence of fallopian tube - like epithelium outside of the fallopian tube . sister mary joseph node : palpable nodule bulging into the umbilicus as a result of metastasis of a malignant tumour in the pelvis or abdomen . in the majority of cases , the clinical manifestation of pue this could be associated with cyclical pain or a bleeding tendency from the umbilicus . instead of cyclical pain , patients could have  continuous umbilical pain , like in our case . little is known about the sensitivity and specificity of ultrasonography , ct scan and mri in diagnosing umbilical endometriosis . ultrasonography can provide some information about the size of the nodule and its adherence to surrounding tissues [ 5 , 6 ] . however , in our case , it was not possible to establish a definitive diagnosis with ultrasonography . furthermore , ct scan and mri are also of little value in diagnosing intra - abdominal endometriosis [ 7 , 8 ] . in patients with a symptomatic umbilical nodule due to pue , surgical exploration and excision of the nodule should be considered . the patient in our case had no symptoms of extensive intra - abdominal endometriosis such as dysmenorrhoea . when patients do have clinical symptoms of intra - abdominal endometriosis , referral to a gynaecologist is mandatory . treatment of umbilical endometriosis with hormones might be different approach for this condition and can diminish clinical symptoms temporarily . however , after the cessation of hormonal therapy , it is likely that the symptoms will reoccur . agonist of gonadotropin - releasing hormone and oral contraceptives are most frequently used hormones to treat endometriosis [ 1 , 3 ] . in conclusion , by increasing the awareness of pue as a potential diagnose in women with a painful , sometimes discoloured , umbilical swelling , we hope this condition will be recognized and treated optimally .
a female patient presented with a painful swelling in the umbilicus . 
 ultrasonography demonstrated a hypodense nodule of 1.8 cm . 
 surgical exploration revealed a subcutaneous , dark discoloured , lobulated swelling at the bottom of the umbilicus , which turned out to be primary umbilical endometriosis ( pue ) . 
 primary umbilical endometriosis is a rare and benign disorder , caused by the presence of ectopic endometrial tissue in the umbilicus , which can present as a painful , discoloured swelling in the umbilicus . 
 the clinical distinction between primary umbilical endometrioses and other causes of an umbilical nodule is difficult . 
 additional imaging modalities do not show any pathognomonic signs for establishing this diagnose . 
 surgical exploration and excision are a safe and definitive treatment of primary umbilical endometrioses . 
 this case highlights the importance of including pue in the differential diagnosis of women with a painful umbilical nodule .