Sciatic nerve MRI

Sciatic nerve tumors on MRI

Sciatic nerve involvement by neoplasms can present as either benign or malignant processes.

Benign tumors, such as plexiform neurofibromas (Plexiform NF), are frequently associated with type 1 neurofibromatosis (NF1). They are characterized by infiltration of a major nerve trunk and extension beyond its borders. On MRI sequences (e.g., STIR mode), these lesions appear as pronounced lobular thickening of the sciatic nerve, resembling a "bag of worms" (bag of worms). The target sign is highly characteristic of them.

Primary malignant lesions, known as malignant peripheral nerve sheath tumors (MPNST), account for up to 10% of all soft tissue sarcomas. On MRI, they are visualized as large masses originating from the sciatic nerve trunk. Signs of a high-grade tumour include small foci of hyperintense signal on T1-WI corresponding to hemorrhages, as well as areas of necrosis that do not enhance with contrast medium. Distal to the tumor location, the sciatic nerve itself may be thickened and demonstrate pathological contrast enhancement.

General MRI features of nerve neoplasms

When analyzing tomograms, it is important to pay attention to a number of characteristic imaging features that help suspect the neurogenic nature of the tumor:

• Fusiform morphology (Fusiform morphology): the lesion is elongated along the long axis of the nerve.
• Split-fat sign (Split-fat sign): the growing tumor displaces the surrounding adipose tissue in the intermuscular space, pushing aside adjacent muscles.
• Entering/exiting nerve sign: allows for tentative differentiation of the tumor type. In neurofibromas, the nerve typically enters and exits strictly in the center of the lesion, whereas in schwannomas, the trunk is often located eccentrically.
• Fascicular sign (Fascicular sign): the presence of ring-shaped hypointense areas within the lesion on T2-weighted images, corresponding to bundles of nerve fibers.
• Target sign (Target sign): on T2-WI, a decreased signal is determined in the center of the lesion (reflecting the accumulation of fibrous tissue), and a hyperintense signal (myxoid content) is seen at the periphery.

It should be remembered that when degenerative processes develop within the tumor ('ancient' changes — 'ancient' change), the imaging appearance can change dramatically. In such cases, fluid-filled cavities, hemorrhages, and calcifications form, which conceals the true neurogenic nature of the lesion.

Ischiofemoral impingement syndrome

The sciatic nerve can suffer not only from its own tumors, but also from compression by surrounding structures. Ischiofemoral impingement is a condition predominantly affecting women, in which there is a narrowing of the space between the ischial tuberosity (ischial tuberosity) and the lesser trochanter of the femur (lesser trochanter).

The threshold value for making the diagnosis on MRI is considered to be the narrowing of this space to a level of less than 15 mm. Due to repetitive compression, edema of the quadratus femoris muscle develops, which over time is replaced by its atrophy. On axial T2-weighted fat-suppressed images (T2-FS), the formation of a bursa can be visualized within the narrowed space.

Pronounced edema of the adjacent muscles leads to secondary irritation of the sciatic nerve, which clinically manifests as sciatica (sciatica). Nevertheless, in some patients, significant narrowing of the ischiofemoral space and muscle edema may be asymptomatic.

Class and exam questions

What else to study on this topic

• Peripheral nerve pseudotumors and Morton's neuroma
• Imaging manifestations of type 1 neurofibromatosis (NF1)
• Double-line sign in avascular necrosis of the femoral head
• Transient osteoporosis and bone marrow edema syndrome
• Thermal ablation and percutaneous osteosynthesis of pelvic bones

Other topics in the "Musculoskeletal System" section

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