Entrapment neuropathies of the pelvis following surgery can manifest with confusing clinical features and are therefore often underrecognized or underdiagnosed in a clinical examination. Historically, electrophysiologic evaluation has been considered the mainstay of diagnosis along with clinical examination (Petchprapa et al, Radiographics 2010). MR neurography has changed this approach. High resolution imaging of the nerves of the pelvis can provide additional information and often work as a problem-solving tool. MR imaging is noninvasive, operator independent and allows identification of the underlying cause of injury, differentiation between surgically related or unrelated causes, and can provide guidance for a specific therapy.

This lecture will revise the anatomy of the pelvic nerves, the technique of MR neurography, as well as to begin applications of not imaging in the pelvis. Special attention will be drawn to entrapment or compression neuropathies of the pelvis which may occur after of the surgery.

Diagnosis of entrapment neuropathies of the pelvis and hip requires familiarity with the normal MR imaging anatomy and awareness of the anatomic and pathologic factors that put peripheral nerves at risk for injury (Petchprapa et al, Radiographics 2010).

Nerves at risk include the (1) sacral plexus, the (2) femoral nerve, the obturator nerve, the (3) sciatic nerve, as well as the superior and inferior gluteal nerve.

The following chapters are excerpts from the book: Chhabra A, Andreisek G. Magnetic Resonance Neurography. Jaypee SP Medical Publsihers, London 2012

(1) The lumbosacral (LS) plexus is a series of nerve convergences and separations, which ultimately combine into large terminal nerves that supply the pelvis and lower extremity. It is comprised of lumbar plexus, sacral plexus, and the pudendal plexus, formed by ventral rami of lumbar (L1-4 +/- T12), sacral (S1-4 and lumbosacral trunk, L4-S1) and lower sacrococcygeal (S2-4 and C1) nerves, respectively. The LS plexus is relatively protected by the axial skeleton and entrapment neuropathy is much less common than brachial plexopathy. LS plexopathy may be caused by ischemic / diabetic neuropathy, entrapment/involvement by mass lesions, complications of pelvic or spine surgery, fracture / dislocations of lumbar spine and sacrum, peripartum neuropathy, retroperitoneal hematoma from trauma or poor coagulation status, psoas / retroperitoneal abscess, radiation, infectious/infiltrative disorders, and less commonly by idiopathic neuropathy/ganglionopathy, anomalous intramuscular course of the nerves, Tarlov cysts, piriformis syndrome, vasculitis, hereditary neuropathies, amyloidosis, and neurocutaneous syndromes. Peripartum neuropathy results from direct compression of the lumbosacral trunk during fetal descent and usually resolves in few months following delivery. Traumatic plexopathy is seen with spine, sacrum or pelvic fractures or during abdominopelvic and retroperitoneal surgeries such as low anterior resection of the rectum or aortic surgery. Iatrogenic trauma may cause compression, traction, and ischemic insult to the nerves, with the femoral and obturator nerves most frequently injured. Retroperitoneal hematoma causing LS plexopathy must be differentiated from isolated femoral nerve compression due to hematoma at the inguinal ligament or mild retroperitoneal hematoma with fascial tracking of blood to the femoral nerve sheath. Under the inguinal ligament, the femoral nerve is more susceptible to injury due to limited extraneural space and a relative paucity of microvasculature. Concomitant involvement of the obturator nerve, manifested as weakness of adduction also points to a lumbar plexus lesion and excludes an isolated femoral neuropathy.

(2) The femoral nerve, a mixed motor/sensory nerve, originates from the dorsal divisions of the lumbar nerve roots (L2,3,4) and descends through the fibers of the psoas major muscle and distally travels between the psoas major and the iliacus muscles, posterior to the iliacus fascia. Distally, it courses beneath the inguinal ligament as the lateral most structure in the femoral canal. Once in the thigh, it divides into anterior and posterior divisions. Femoral nerve may be affected by a diverse group of etiologies, such as lumbar plexopathy (pathology involving the dorsal divisions of the L2-4 nerve roots); trauma; nerve sheath tumors; diffuse peripheral polyneuropathies, such as hereditary motor sensory neuropathy, chronic demyelinating inflammatory neuropathy, idiopathic or autoimmune neuropathies; perineural compressive lesions; infectious/inflammatory; metabolic and; radiation neuropathies. MRN is best used for entrapment neuropathy or nerve injury. Entrapment neuropathy most commonly occurs immediately distal to the inguinal ligament, where it lacks sufficient protection. It may result from space occupying lesions, such as hematoma, iliopsoas bursitis, neoplasm, and psoas muscle abscess. Open injury to the femoral nerve may result from iatrogenic or non-iatrogenic causes. The non-iatrogenic injuries are usually caused by penetrating injuries. The iatrogenic injuries more commonly result from gynecologic surgeries due to the self-retaining retractors and hip replacement surgeries due to pressure from the dislocated hip, impingement by cement, or over retraction leading to stretch injury. A variety of other surgeries, such as appendectomy, renal transplantation, aortic surgery, etc have been implicated. In addition, the nerve may be injured inadvertently at the time of wound repair or intramuscular injection to the thigh.

(3) The sciatic plexus/nerve is formed from the ventral rami of L4–S3 nerve roots, which join to form three distinct components, the tibial nerve (medial), the common peroneal nerve (lateral) and the posterior cutaneous nerve (posterior femoral cutaneous nerve) of the thigh. After the plexus exits the pelvis through the greater sciatic foramen, descending below the piriformis muscle, the sciatic nerve is formed by the tibial and common peroneal trunks, which are enclosed in a common nerve sheath. The sciatic nerve enters the gluteal region and then continues down the thigh, between the adductor magnus muscle anteriorly and the gluteus maximus muscle posteriorly. Sciatic neuropathy may be caused by a lesion involving the plexus in the pelvis and lumbosacral region, gluteal region, hip or thigh regions, basically distal to the lumbosacral plexus but proximal to the bifurcation of the nerve into its distal branches. The long course of the sciatic nerve renders it vulnerable to injury. A common tunnel syndrome involving sciatic nerve is referred to as ‘Piriformis Syndrome.’ Piriformis syndrome is attributed to compression of the sciatic nerve in the pelvis by the piriformis muscle.