CT and MRI in The Evaluation of Extraspinal Sciatica

Anatomy

A comprehensive understanding of the structure of the lumbosacral plexus and the sciatic nerve is crucial when assessing extraspinal sciatica. The lumbar plexus, located behind the psoas muscle, comprises nerves stemming from L1–L4.

By combining with other nerves, the lumbosacral trunk gives rise to the sacral plexus, with the sciatic nerve as its prominent component. Traversing the gluteal region, the sciatic nerve courses between the ischial tuberosity and greater trochanter before dividing into the common peroneal and tibial nerves just above the knee.

Trauma, infection, inflammation, tumors, and vascular issues are among the various factors that can lead to extraspinal sciatica.

Traumatic Causes

Sciatic nerve injuries resulting from trauma can exhibit a wide range of findings, varying from disruption of axonal conduction while maintaining anatomical continuity to complete loss of nerve trunk continuity.

In the early stages, these injuries can be caused by laceration, stretching, or compression, while later stages may involve the encasement of the nerve by heterotopic ossification. To precisely assess the location and severity of the injury,

Magnetic Resonance Imaging (MRI) is an effective diagnostic tool. T2 weighted MR images can reveal high signal intensity in the nerve fibers, increased nerve dimension, deformation, or total loss of nerve integrity, depending on the injury’s severity.

Intramuscular Injection

Injecting drugs into muscles can lead to peripheral nerve injuries. These injuries can happen when the injection is too close to the nerve or when the injected drug accumulates in the spaces where the sciatic nerve travels between the piriformis, gemellus, quadratus, and obturator muscles.

Other possible causes include direct damage from the needle, compression by scar tissue, and the harmful effects of neurotoxic chemicals in the injected drug on the nerve fibers.

Surgical Interventions

Extraspinal sciatica primarily stems from hip joint surgery. The sciatic nerve can sustain damage during pelvic surgery due to factors like incorrect patient positioning, surgical dissection, or excessive pressure exerted by the surgeons.

Fracture

Fractures of the sacrum, sacroiliac joint, acetabulum, femur, or femoral head often lead to the development of sciatica because the lumbosacral plexus is located near these structures. Consequently, the sciatic nerve can sustain injuries in such cases.

Haematoma

A gluteal region hematoma is often caused by trauma, hip surgery, hemophilia, or anticoagulation therapy. It can damage nerves directly or indirectly through pressure or vasa vasorum compression.

Appearance after trauma, signal changes, and size reduction are indicative of hematoma. MRI signals vary based on the hematoma’s contents. Acute hematoma appears as a low-intensity mass with possible edema.

Subacute hematoma shows hyperintensity on T1-weighted images and changes in signal intensity over time. Chronic hematoma exhibits hypointensity due to hemosiderin accumulation.

Infection-Related Causes

Abscess

Abscesses in the gluteal and pelvic regions are infrequent but can be associated with infections in the gastrointestinal and urinary tracts. They have the potential to spread to the sacral plexus through regular anatomical routes.

These abscesses can directly impact the lumbosacral plexus and sciatic nerve, leading to sciatica. MRI scans demonstrate distinct signal characteristics for the abscess content and the fibrovascular rim. Inflammation and contrast enhancement are commonly present surrounding the abscess.

Inflammatory Causes

Sacroiliitis

Sacroiliitis is a prevalent occurrence in seronegative spondyloarthropathies. Patients typically develop gradual-onset pain that is alleviated with physical activity but aggravated during nighttime.

Sciatica can arise either from referred pain or inflammation near the sacroiliac joint that directly affects the nerve. CT and MRI examinations offer conclusive diagnostic insights. CT scans identify cortical erosion and joint alterations, while MRI captures inflammatory changes, facilitating early detection and assessment of disease activity.

Primary Tumours of the Sciatic Nerve

Schwannomas are common primary tumors of the sciatic nerve, while pelvic region schwannomas are rare. They have similar imaging features to solitary neurofibromas.

Neurofibromas and schwannomas show comparable CT and MRI characteristics. Malignant peripheral nerve sheath tumors are more prevalent in patients with neurofibromatosis type 1, particularly after radiation therapy.

Differentiating between benign and malignant neural tumors can be challenging based on imaging, but progressive enlargement suggests a malignant nerve sheath tumor.

Tumors Causing Compression or Invasion of the Sciatic Nerve

Sciatic pain can be caused by various conditions such as intra-abdominal or intrapelvic masses, both benign and malignant, originating from nearby soft tissues and bones along the path of the sciatic nerve. Additionally, lymphomas can also affect the sciatic nerve and result in sciatic pain.

Intra-Abdominal and Intrapelvic Benign and Malignant Tumors

Compression or invasion from intra-abdominal or intrapelvic masses can affect the lumbosacral plexus. The most common malignant cause is colorectal carcinoma, while endometriosis is frequently encountered as a benign cause.

Additionally, the sciatic nerve can be locally invaded by tumors originating from the uterus, prostate, or ovaries. It can also be compressed by uterine leiomyoma, adenomyosis, or a retroverted uterus.

Malignant soft-tissue tumors

Metastatic involvement of skeletal muscle is rare, primarily from lung carcinomas. Commonly affected muscles include the diaphragm, rectus abdominis, deltoid, psoas, and intercostal muscles.

Intramuscular metastasis appears as a low-attenuation mass on CT and shows distinct signal characteristics on MRI. It causes muscle expansion, possible peritumoral edema, and may exhibit hemorrhage, necrosis, and calcification.

Soft-tissue Sarcomas

A histopathological examination is necessary to obtain a definitive diagnosis. Soft tissue sarcomas and intramuscular metastases share similar MRI characteristics, appearing as isohypointense lesions on T1-weighted images and hyperintense lesions on T2-weighted images.

However, soft tissue sarcomas typically exhibit fewer instances of necrosis, peritumoral edema, and lobulation compared to metastatic lesions.

Malignant Tumors Originating from Bones

The pain caused by tumors that result in sciatica shares similarities with malignant soft tissue tumors, such as an insidious onset, persistent and progressive nature, worsening at night, and lack of relief from changing position.

The pain characteristics are vital for diagnosing sciatica related to tumors. Most tumors causing sciatica are located in the pelvis and proximal femur. CT and MRI scans not only offer clear visualization of the tumor but also provide detailed information about its proximity to the sciatic nerve.

Benign Tumors of Bones and Soft Tissues

Various benign tumors can impact the lumbosacral plexus and cause sciatica by exerting pressure on its components. Lipoma, osteochondroma, and ganglion cyst are the most commonly observed benign tumors associated with sciatica.

However, other benign tumors have also been documented. Although the appearance of many benign tumors lacks specificity, specific imaging features can provide clues for a more accurate diagnosis.

Lymphoma

Lymphomas can impact the sciatic nerve by compressing it with enlarged lymph nodes, infiltrating nearby soft tissues, or directly invading the nerve.

Radiological findings may show muscle enlargement, density changes on CT scans, and signal intensity changes on MRI scans. Contrast attenuation patterns can vary. While direct invasion of the sciatic nerve is rare, it has been documented.

Vascular Causes

Enlargement of the iliac artery and its branches, whether due to aneurysmal or pseudoaneurysmal conditions, can result in compression of the sciatic nerve and subsequent sciatica.

Ischemia caused by vasa vasorum compression may also play a role in aneurysm-related sciatic pain. In rare cases, sciatica can also be caused by direct pressure from arteriovenous malformations or fistulas.

Other Causes

Sciatica can occur due to various causes. Compression of the sciatic nerve by endometriosis typically takes place at the sciatic notch, resulting in distinct pain patterns.

Piriformis syndrome is characterized by abnormal conditions of the piriformis muscle leading to sciatic nerve entrapment. Pregnancy-related sciatica can be caused by either direct nerve compression or vascular compression resulting from the gravid uterus.

Post-radiation neuropathy, though rare, can cause nerve damage characterized by diffuse thickening or focal mass-like lesions. Osteoarthritis-related sciatica stems from mechanical compression of the sciatic nerve caused by degenerative changes in the sacroiliac and hip joints.