Teaching Case

An 83-year-old male presented with a 2 year history of progressive right leg weakness. He underwent electro-conductive nerve study and the findings were suggestive of right sciatic nerve lesion/injury proximal to fibres sub-serving the right semimembranosus muscle.
An outpatient pelvic MRI was performed, findings are described below.

The pelvic MRI demonstrates a large pelvic mass in the region of the right internal iliac artery measuring 15cm in the widest axis and extending through the greater sciatic notch. The centre is low signal on all sequences with some layering of different signal patterns. The findings are consistent with aneurysm of the right internal iliac artery.
Urgent admission was arranged. CT angiogram was performed for further vascular assessment and to guide further management. The CT confirmed the presence of a large right internal iliac aneurysm. No abdominal aortic aneurysm. After vascular multi-disciplinary team discussion, the patient underwent embolization of the inflow vessel with a single AmplatzerTM Vascular Plug II. Complete occlusion of the vessel was confirmed with completion angiography. Unfortunately the patient died three days later with chest infection.

The prevalence of the internal iliac aneurysm is quite rare and it is about 0.04-0.4% [1, 2]. The patients might present with pelvic organs dysfunction (3), abdominal pain, urological, or neurological symptoms, groin pain, hip or buttock pain [4, 5], and gastrointestinal symptoms [1, 6]. There is high mortality associated with the rupture of the internal iliac aneurysm (75%); therefore early management is mandated [2].

Leg pain and neurological symptoms are mostly related to disc and facet joint degenerative changes. However, the extra spinal sacral plexus and sciatic nerve entrapment might be due to other pathologies within the pelvis or in the deep gluteal space. The latter lies between the greater sciatic notch and gluteal muscles superiorly and the common origin of the hamstrings muscles and posterior thigh at the level of ischium inferiorly. It is bounded laterally by the tensor fascia lata muscle and linear aspera - and medially by the greater and minor sciatic foramina. The gluteus maximus lies posteriorly and anteriorly the abductor and external rotators of the hip and the posterior aspect of the femoral neck.

The term “deep gluteal syndrome” has emerged recently as a generic name for all the extra spinal causes of sciatica [7]. These can be grouped into traumatic; inflammatory; infectious; gynaecological; tumorous or vascular such as persistent sciatic artery or aneurysm of the internal iliac artery as in this case.

Iatrogenic causes are mostly related to radiotherapy or post total hip replacement surgery due to stretching or injury of the nerve. Other causes include sciatic nerve entrapment from formation of fibro-vascular bands in the deep gluteal space secondary to acute or chronic inflammation [7]. Another under diagnosed entity is the pyriformis syndrome which is a neuromuscular condition and part of deep gluteal syndrome.

Most patients would normally have spinal MRI first to look for spinal cause of sciatic neuropathy. Pelvic MRI is considered the gold standard modality for assessment of the extra spinal causes of sciatic pain as most of these are difficult to assess with CT. Imaging of the deep gluteal region should include the sacral plexus origin and the proximal sciatic nerve so from L3 to below the ischial tuberosities. CT angiogram should be considered when a vascular abnormality identified to guide further management [1].

In conclusion, pelvic MRI should always be considered to look for causes of sciatic nerve entrapment in patient with equivocal lumbar MRI.