By: Sridhar Nadamuni
Sciatica is generally characterized by low back pain (LBP) down the sciatic nerve along the leg(s) with associated symptoms of nerve root entrapment. The leg pain may be triggered by cough, sneeze, or strain. Prompt detection of sciatica is essential for effective management. However, diagnosis is a challenge due to non-specific pain originating in other structures in the lower back. Magnetic resonance imaging (MRI) is often used as the reference standard. However, positive MRI findings may be found in asymptomatic individuals, while patients manifesting nerve root symptoms may have normal MRIs. Currently there is no gold standard for sciatica diagnosis, and tracing the origin of chronic sciatica is a challenge.
In a study funded in part by GE Healthcare, researchers led by Sandip Biswal, MD, Associate Professor of Radiology at the Stanford University School of Medicine, used a novel diagnostic approach based on 18F-FDG PET/MRI for the investigation of metabolic and structural abnormalities underlying the painful lesions of sciatica. The approach has been shown to significantly reduce motion-induced errors by combining PET and MRI, and facilitated the detection of specific pain sources (herniated disc, degenerated facet joints, and spinal nerve impingement) in sciatica. 
The 18F-FDG PET is highly sensitive in detecting abnormal increase in metabolism triggered by painful inflammation. MRI provides high spatial resolution and superior soft-tissue contrast. Based on morphological insights obtained by MRI combined with the increased 18F-FDG uptake, the researchers identified spinal nerve impingement caused by a herniated disc in half of all the patients investigated. The remaining patients showed lesions associated with abnormally high 18F-FDG uptake in the leg muscles, peripheral nerve, pars, and facet joints, with minimal defects detected by MRI. Therefore, the combination of metabolic testing by 18F-FDG PET and anatomic investigations facilitated by 3.0T MRI was used to identify the origin of sciatic pain in a limited number of patients. The combination technique represents a potential diagnostic tool for the pathological inflammation in chronic sciatica. In this study, the diagnosis of sciatica triggered by spinal nerve impingement was based on the increased 18F-FDG uptake suggesting a potentially painful lesion, while the MRI indicated the lesion type associated with pathologies such as tumor, facet synovitis, herniated disc or other causes. Even when MRI revealed multiple defects, the highly localized and increased uptake of 18F-FDG was diagnostic of pain.
In the absence of any abnormal MRI findings, a high 18F-FDG absorption by leg muscles may suggest a herniated disc and nerve impingement manifesting as edema or atrophy. It may also suggest muscle defect mimicking spinal sciatica, due to infectious or non-infectious causes, ischemia, or trauma, and may be confirmed or denied by local anesthesia to ascertain the pain origin. However, large study populations are needed to establish the diagnostic role of 18F-FDG PET/MRI in sciatica and its predictive value in treatment.
Magnetic Resonance Neurography (MRN) for Sciatic Neuropathy
MRN utilizes MRI sequences designed for imaging peripheral nerves by using 3.0 T magnets and graded fluid signal-sensitive sequences with fat suppression and an echo time (TE) exceeding 66 ms. MRN has been used to delineate the etiology of sciatic neuropathy associated with cancer, nerve compression, trauma, and heredity, and even to analyze iatrogenic and idiopathic factors. 
MRN facilitated detection of peripheral nerve defects based on altered signal intensity, especially T2-weighted images. It also aided investigations into muscle denervation manifesting as edema or muscle hypotrophy associated with the sciatic nerve. MRN can be used to image lesions or pathologies of the sciatic nerve associated with varying degrees of pain in the lower back, radiating to the gluteal and lower limbs on a single side, along with altered sensitivity or movement disorders. The 3.0T high-resolution MRN facilitated the detection of sciatic neuropathy accurately via quantitative and qualitative analysis of the sciatic nerves and regional skeletal muscles.  Sciatic neuropathy was characterized by nerve-to-vessel signal intensity (SI) ratios of 0.89 or higher along with elevated T2 hyperintensity, enlarged and abnormal morphology compared with the normal nerves. Fatty infiltration, edema and atrophy were the other manifestations suggesting neuropathy. 4,5
The sciatic nerve may be compressed by benign or malignant changes occurring either in the nerve sheath or adjacent tissues, or even metastatic dissemination from cancers such as those involving the prostate. , The effect of such neoplastic changes involving the sciatic nerve may be primary or secondary, and detected by MRN typically as expanding lesions.4
Sciatic neuropathy may be triggered by hereditary pathologies such as Charcot-Marie-Tooth disease characterized by extensive nerve enlargement in imaging studies. The symptoms include muscle atrophy, weakness, and pain.4,
Due to the close proximity of the sciatic nerve to the piriformis muscle, sciatic neuropathy can be induced by a defective piriformis muscle, although the causal relationship has yet to be established clearly. However, MRN revealed changes in sciatic nerve phenotype or signals in sciatic neuropathy triggered by nerve compression, which may or may not be associated with abnormal piriformis.4
Sciatic neuropathy can also result from trauma, either directly following an accidental injury or pelvic fracture. MRN revealed hyperintense signals and extensive thickening of the sciatic nerve suggesting traumatic neuropathy.4
Sciatic nerve damage may also be detected several months after radiotherapy for cancers of the pelvis, reproductive organs, colon and rectum, although MRN may be non-specific and highlight changes in the nerve thickness and signal intensity.4 Similarly, MRN findings may be non-specific in cases of ischemic neuropathy.
Thus, pathologies of the lower limb or pelvis often affect the sciatic nerve in addition to intrinsic lesions. MRN facilitates the evaluation of sciatica based on the anatomical and imaging aspects, for appropriate interventions.
Evidence suggests that an active lifestyle rather than bed rest provides relief. The effectiveness of painkillers or treatment with non-steroidal anti-inflammatory drugs (NSAIDS), acupuncture, epidural steroids, spinal interventions, traction, physical or behavioral therapies, and multidisciplinary approaches is not clearly established. However, the NICE guidelines for patients with acute and severe sciatica recommend treatment with epidural injections using local anesthesia and steroids, or even spinal decompression. Over-the-counter medications including aspirin, NSAIDs, or even opioids may be used, but only for the shortest period based on patient risk factors, and may not always work in serious cases. Also, steroids may not be effective in case of sciatica due to herniated or ruptured disc, in which case surgery may be an option. The NICE guidelines also recommend radio-frequency denervation in patients with low back pain associated with the tibial component of the sciatic nerve.
In carefully selected patients diagnosed with lumbar disc prolapse, surgical discectomy may ameliorate symptoms that are refractory to conservative care.
In any clinical decision of sciatica, MRI findings should be combined with other modalities such as 18F-FDG, and evaluated in conjunction with the constellation of pain symptoms occurring below the knee, along with worsening leg pain compared with back pain, neural tension and neurological abnormalities. Using MRI exclusively as an independent reference standard is an unreliable indicator, and can often lead to a mismatch in clinical symptoms and diagnostic findings. In any case, treating the cause rather than symptoms of sciatica is a sensible management option.
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 18F-FDG PET/MRI in Chronic Sciatica: Early Results Revealing Spinal and Non-spinal Abnormalities. The Journal of Nuclear Medicine. 2018 Jun;59(6):967-972. doi: 10.2967/jnumed.117.198259
 Sciatic neuropathy: findings on magnetic resonance neurography. Radiologia Brasileira. doi: 10.1590/0100-3984.2015.0205. Accessed August 15, 2018.
 3-T high-resolution MR neurography of sciatic neuropathy.American Journal of Roentgenology.doi: 10.2214/AJR.11.6981. Accessed August 17, 2018.
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