A 28-year-old Caucasian woman, who had undergone caesarean delivery two months earlier, was referred from her gynaecologist to our institution to define a sonographic finding of an unknown uterine mass. The patient had metrorrhagia after parturition.
Suprapubic US detected a hypoechoic homogeneous 4 cm pelvic mass adjacent to the anterior wall of the uterine body (Fig.1) without any vascular signal on colour Doppler.
MR examination confirmed the presence of a well-defined fluid-filled mass with a smooth wall originating from the anterior wall of the uterine body (Fig. 2a). On T1- and T2-weighted images the mass content was always hyperintense, also on fat suppressed acquisitions, according to the presence of methaemoglobin (Fig. 2b, c). The MR diffusion weighted images demonstrated ADC restricted water diffusion due to haemorrhagic contents (Fig. 2d). Uterus was anteverted, normal in size, with regular endometrial thickness and normal myometrium. No peritoneal fluid was detected.
In 2 months follow-up MR the uterine mass appeared mildly smaller in size (3 vs 4cm) and T2-weighted image showed area of hypointense signal within the collection due to haemosiderin content (Fig. 3).
The diagnosis based on imaging findings was post-cesarean section bladder flap haematoma without sign of dehiscence.
Bladder flap haematoma (BFH) is defined as a blood collection between the bladder and the uterine body originating as a rare complication of a lower-uterine transverse incision during the caesarean section . The source of bleeding is usually injury of uterine vessels during surgery, although an occult dehiscence may be the underlying cause . Bleeding from the uterine incision is usually confined by the overlying peritoneum but can spread to the broad ligaments, retro-peritoneum and peritoneum .
BFH occurs in up to 50% of women who had undergone caesarean section and if the size is less than 4 cm it can be considered normal. If the collection size exceeds 5 cm, a dehiscence has to be supposed. BFH can be clinically characterised by anaemia, disurya, persistent lower abdominal pain, fever and the possible palpation of a pelvic mass [3, 4, 5].
US is considered the preliminary imaging modality and recognises the bladder flap haematoma like a hypoechoic fluid collection with regular wall and posterior reinforcement of the distal echoes in the vescico-uterine space . Using this imaging modality, it could be difficult the differential diagnosis with other conditions such as abscess, infected haematoma or endometrioma.
MRI is considered the best imaging modality for the evaluation of female pelvis for its high intrinsic contrast and multiplanar capability, without any exposure to ionising radiation . MRI allows to depict the site of the incision, to evaluate the integrity of serosal and endometrial layers . The bladder flap haematoma appears characteristically hyperintense on T1- and T2- weighted images due to its methaemoglobin content, an early haemoglobin catabolic product; sometimes it is possible to detect some hypointense spot within the collection due to the presence of haemosiderin, a later haemoglobin catabolic product.
The imaging studies are important in women with fever or significant blood loss after caesarean section, allowing the evaluation of uterine complication like dehiscence, BFH and superimposed infection .
The surgical treatment of this pathology includes: percutaneous drainage of febrile BFH, surgical transvaginal or laparotomical BFH evacuation and laparoscopical drainage .
When their size is less than 4 cm, BFH do not require any therapeutical procedure  while larger hematomas need an early identification in order to carry out the proper interventional treatment . Diagnostic imaging techniques therefore allow an early detection of pelvic haematoma, providing information concerning size and anatomic location, which are helpful in the treatment planning.
Differential Diagnosis List
Bladder flap haematoma
Subfascial uterine haematoma