MR Imaging of Entrapment Neuropathies of the Lower Extremity

Abstract

Entrapment neuropathies can manifest with confusing clinical features and therefore are often underrecognized and underdiagnosed at clinical examination. Historically, electrophysiologic evaluation has been considered the mainstay of diagnosis. Today, cross-sectional imaging, particularly magnetic resonance (MR) imaging and specifically MR neurography, plays an increasingly important role in the work-up of entrapment neuropathies. MR imaging is a noninvasive operator-independent technique that allows identification of the underlying cause of injury, differentiation between surgically treatable and untreatable causes, and guidance of selective diagnostic anesthetic nerve blocks. Pathologic conditions affecting the lumbosacral plexus and major motor and mixed nerves of the pelvis and hip include neuropathies of the lumbosacral plexus, femoral nerve, lateral femoral cutaneous nerve, obturator nerve, and sciatic nerve; piriformis muscle syndrome; and injury of the gluteal nerves. Diagnosis of entrapment neuropathies of the pelvis and hip with MR imaging requires familiarity with the normal MR imaging anatomy and awareness of the anatomic and pathologic factors that put peripheral nerves at risk for injury.

High-Resolution 3-T MR Neurography of the Lumbosacral Plexus

“…It courses inferiorly between the psoas major and iliacus muscles, posterior to the iliac fascia, and runs beneath the inguinal ligament and into the thigh, where it splits into anterior and posterior divisions; the saphenous nerve derives from the posterior division (15)(16)(17). The femoral nerves are consistently seen at MR neurography, which depicts areas that are symmetric in terms of hyperintensity and size at the level of the iliopsoas crotch and isointense (including their anterior and posterior divisions) at the site of the inguinal ligament (Fig 6) (9,18). …”

“…It courses under the gluteus maximus, branches into the perineal and inferior cluneal nerves, and descends in the posterior thigh over the long head of the biceps femoris to the back of the knee, where it pierces the deep fascia and accompanies the small saphenous vein to the middle back of the leg. The terminal segments of the posterior femoral cutaneous nerve communicate with the sural nerve (9,12,21). The sciatic plexus and sciatic nerves are consistently seen at MR neurography, normally with symmetric morphologic characteristics and signal intensity, given the limitations of inconsistent fat suppression (Fig 8).…”

“…It then branches into the superior and inferior nerves: The inferior gluteal nerve originates from the L5-S2 nerve roots and exits the pelvis through the greater sciatic foramen, inferior to the piriformis muscle (9). Because of their small size, the gluteal nerves are not visualized at MR neurography unless they are abnormally enlarged.…”

“…The lumbrosacral plexus may be locally involved by extrinsic compression or infiltration or associated with systemic conditions, such as metabolic, autoimmune, ischemic, and inflammatory disorders and vasculitis (9). Causes of local involvement include neoplasms, such as benign and malignant peripheral nerve sheath tumors; lymphoma; malignancies, such as cervical cancer, uterine cancer, colorectal cancer, mesenchymal tumors, and metastatic infiltration; tumor variants, such as perineuroma and amyloid; entities related to the psoas major muscle or greater sciatic notch, such as hematoma, abscess, and phlegmon; endometriosis; and aortic aneurysm.…”

Bildgebung des Plexus lumbosacralis

“…Die Unterscheidung zwischen sensiblen und motorischen Ausfällen und die klinischneurologische Erfassung der betroffenen Regionen und Muskulatur helfen dabei, die lädierten Nerven aufzusuchen und die Pathologie zu finden [12]. Das klinische Bild kann komplex sein durch eine Kombination von Muskelgruppenausfällen, sensorischen Veränderungen und (selten) Darm-und Blaseninkontinenz sowie sexueller Dysfunktion [9,24]. Eine lumbosakrale Plexopathie kann durch primäre Veränderungen der neuralen Strukturen selbst oder Kompression oder Infiltration von außen verursacht werden [27].…”