Incarcerated obturator hernia: pitfalls in the application of ultrasound

Obturator hernia is defined as an abdominal wall hernia protruded through the obturator canal, and usually consists of a hernia sac containing small bowel. The entity is relatively rare, but a significant cause of small bowel obstruction, especially in thin, elderly females [1–3]. It is also reported to occur more often in the multipara. The female predominance of the entity has been supposed to be the results of pregnancy which leads to relaxation of the pelvic peritoneum and a wider and more horizontal obturator canal. The herniated intestinal segment is often incarcerated and strangulated because the hernia orifice is small, about 1 cm in diameter [2]. Consequently, early recognition of the entity is crucially important because delay in diagnosis and treatment is often associated with its high mortality and morbidity.

Clinical symptoms and signs are usually consistent with acute or recurrent bowel obstruction, but in the majority of the cases no symptoms characteristic to the obturator hernia can be obtained. Howship–Romberg sign, which stands for pain radiating along the medial aspect of the thigh when the leg is extended or abducted because of obturator nerve compression by the hernia contents, has been reported to be recognized in less than half the patients [1–3]. It may not be checked preoperatively unless obturator hernia is considered as one of the disorders to be differentiated. A groin mass is rather difficult to recognize with palpation because the herniated mass locates deep in the femoral triangle, concealed beneath the pectineus muscle. Plain abdominal X-rays usually show the evidence of small bowel obstruction, but are not diagnostic for obturator hernia. Consequently, obturator hernia has been a diagnostic challenge before CT of the pelvis becomes a standard diagnostic tool for the entity. With CT of the pelvis, the herniated segment covered with the hernia sac is demonstrated as a soft tissue mass or opacified loop between the pectineus muscle and the obturator externus muscle [4–8].

On the other hand, the application of ultrasound for obturator hernia has been limited, but a few investigators reported that ultrasound was useful for the early recognition of the entity [9–13]. In our case series, ultrasound directly showed an incarcerated obturator hernia, and was evaluated as applicable and useful for the recognition of the entity. The opportunities for sonographers to encounter with some cases of obturator hernia may increase, as ultrasound has become widely used for the evaluation of acute abdomen including bowel obstruction, Therefore, it is required to clarify the ultrasonographic features of incarcerated obturator hernia, and the pitfalls in the application of ultrasound for the entity.

The ultrasonographic features of incarcerated obturator hernia are indicated as followed. (1) The herniated intestinal segment is visualized as a cystic mass with thickened wall posterior to the pectineus muscle in the femoral triangle. The pectineus muscle can be clearly delineated with a high-frequency probe, and is an important landmark to differentiate an obturator hernia from a femoral hernia in the ultrasound examination (Fig. 3). In cases of femoral hernia, a herniated segment is visualized anterior to the pectineus muscle in the subcutaneous space (Fig. 4). (2) The evidence of small bowel obstruction is demonstrated by showing dilated proximal small bowel and collapsed distal bowel. (3) Peritoneal fluid may increase depending on the severity of bowel obstruction. (4) Peristaltic activity of the incarcerated intestinal segment is supposed to depend on the degree of incarceration. In our case series, each incarcerated intestinal segment was observed as an akinetic, cystic one. However, obturator hernias with mild incarceration may show peristalsis when they are observed with ultrasound. Rettenbacher et al. [14] reported that peristalsis within hernia was absent during the sonographic investigation in 13 (76%) out of 17 cases of incarcerated abdominal wall hernia and 24 (38%) of 63 cases of non-incarcerated one [14]. They also reported that 18 (78%) out of 23 incarcerated abdominal wall hernias had detectable blood flow on color Doppler sonography. Practically, the significance of color Doppler ultrasound is not determined for the assessment of the viability of hernia contents.

In our case series, we also indicated pitfalls for applying ultrasound for incarcerated obturator hernia. In the Case 1, the examiner made a misdiagnosis by careless interpretation of the ultrasonograms and a belief that the palpated bulge in the femoral triangle stood for an incarcerated femoral hernia. In the Case 3, the examiner overlooked the obturator hernia in the initial examination because he did not hit the entity at the time and consequently, skipped scanning around the femoral triangle. These misdiagnoses were related with lack of knowledge or lapse of memory on the entity, and therefore, expected to be preventable by routine scanning around the groin and femoral triangle in addition to adequate education and training for examiners. Generally speaking, ultrasonography as well as the physical examinations are dependent on the examiners’ skill and experience and consequently, may fail to demonstrate the incarcerated obturator hernia in the femoral region. Therefore, it is important to manage the operator-dependent nature of ultrasound properly by developing the competency of sonographers and the systematic ways of scanning.

Ultrasound has been increasingly used for acute abdominal disorders in recent years and consequently, has been evaluated as useful for the diagnosis of bowel obstruction. Furthermore, our case series suggest that it can be applied for the recognition of incarcerated obturator hernia. The early recognition of the entity will consequently reduce its morbidity and mortality and also reduces the cost for CT scan and other imaging modalities. Therefore, important are the use of ultrasound to make a rapid evaluation for acute abdominal disorders including bowel obstruction, and routine scanning around the groin and femoral triangle so as not to overlook the entity when ultrasound is applied for the evaluation of small bowel obstruction.