Sciatic neuropathy: findings on magnetic resonance neurography

Injuries of the sciatic nerve are common causes of pain and limitation in the lower limbs. Due to its particular anatomy and its long course, the sciatic nerve is often involved in diseases of the pelvis or leg. In recent years, magnetic resonance neurography has become established as an important tool for the study of peripheral nerves and can be widely applied to the study of the sciatic nerve. Therefore, detailed knowledge of its anatomy and of the most prevalent diseases affecting it is essential to maximizing the accuracy of diagnostic imaging.

the nerve or plexus. The use of a TE > 66 ms serves to avoid the magic angle phenomenon, which is related to anisotropy of the peripheral nerve and can simulate signal abnormality. These sequences provide greater spatial and contrast resolution for neural study. MRN reveals peripheral nerve abnormalities by identifying changes in signal intensity, notably on T2-weighted images, together with changes in the cross-sectional area and course of the nerve, as well as disorganization or absence of the typical fascicular pattern. Another focus of MRN-based peripheral nerve studies is the denervation of muscles, an abnormality that can manifest as a pattern of edema or, in chronic cases, hypotrophy of the muscles innervated by the sciatic nerve (1)(2)(3)(4) .
Clinically, lesions or diseases of the sciatic nerve manifest as pain of varying intensity in the lower lumbar region, with irradiation to the gluteal region and to the posterior region of the ipsilateral lower limb. Those manifestations can be accompanied by changes in sensitivity or motor deficits.
Another MRI technique that can be used for the study of neural pathways is tractography, which involves diffusionweighted acquisition-typically diffusion tensor imaging (DTI)-and is classically used in studies of the central nervous system (5)(6)(7) . Recent studies have demonstrated that this technique can also be used in the study of peripheral nerves (8) , although we found no studies with an emphasis on the specific evaluation of sciatic nerve abnormalities in humans.

INTRODUCTION
Sciatic nerve injuries are common causes of pain and limitation in the lower limbs. Detailed knowledge of the anatomy of the sciatic nerve is essential for the recognition of alterations and diseases involving the nerve (1,2) .
In recent years, magnetic resonance imaging (MRI) has established itself as an important tool for the study of peripheral nerves, especially after the development of protocols including sequences optimized for this purpose, generally referred to as magnetic resonance neurography (MRN). Ideally, MRN involves the use of 3.0 T magnets and the protocol includes the following: graded fluid signal-sensitive sequences with fat suppression; a TE > 66 ms; thin slices; and acquisition in the best anatomical planes for study of crush injury and sciatic nerve traction injury showed that DTI tractography, using routine clinical 1.5 T MRI scanners, is a promising tool in the assessment of sciatic lesions (9,10) . In those studies, DTI was able to differentiate between nerves with nerve damage and control group nerves. In addition, the values and time curves of fractional anisotropy and the eigenvalue lambda (perpendicular) correlated well with the histological findings of Wallerian degeneration and with functional recovery.
The aim of this essay is to illustrate the imaging aspects of neuropathies and the anatomy of the proximal segment of the sciatic nerve by MRN. Among the causes of neuropathies, we illustrate those that are neoplastic, compressive, traumatic, hereditary, iatrogenic, or idiopathic in nature.

NORMAL ANATOMY OF THE PROXIMAL SCIATIC NERVE
The lumbosacral plexus is composed of ventral rami of the L4-S3 nerve roots, which join to form the tibial (medial) nerve, the common (lateral) peroneal nerve, and the posterior cutaneous nerve of the thigh. The sciatic nerve consists of the tibial and common peroneal components, which are encased in a common sheath and exit the pelvis through the sciatic notch (3,4) , as depicted in Figure 1. Immediately before leaving the pelvis, the sciatic nerve has an intimate relationship with the ventral surface of the piriformis muscle.

NEOPLASTIC CAUSES
Benign or malignant neoplasms can arise from nerve fibers or from the cuff of the sciatic nerve sheath (Figures 2, 3, 4, and 5). However, the nerve can be affected because it is contiguous to or compressed by neoplasms in adjacent tissues ( Figure 6 and 7). Another form of sciatic nerve involvement is perineural dissemination of neoplasms, which is particularly common in cases of prostate cancer (11)(12)(13) .
On MRN, a typical finding is an expansile lesion involving the sciatic nerve. Here, we illustrate examples of primary sciatic nerve neoplasms as well as secondary involvement of the sciatic nerve in cases of neoplasms in the region of the pelvis and proximal thigh.

HEREDITARY CAUSES
Chief among the hereditary causes of sciatic neuropathies is Charcot-Marie-Tooth disease, which is a spectrum of diseases related to alterations currently described in more than 30 genes (14) . In imaging studies, the finding typical of Charcot-Marie-Tooth disease is diffuse nerve hypertrophy. Clinically, patients with Charcot-Marie-Tooth disease present muscular weakness, pain, and a variety of deformities related to muscular atrophy (15) . Figure 8 illustrates the case of a 46-year-old patient with pain in the lower limbs and image findings typical of Charcot-Marie-Tooth disease.

COMPRESSIVE CAUSES
As previously noted, the proximal segment of the sciatic nerve has an intimate relationship with the piriformis muscle. Variations include the sciatic nerve coursing anterior to, posterior to, or within the ventral portion of the piriformis muscle. When it is related to an abnormality in the piriformis muscle, neuropathy of the proximal segment of the sciatic nerve can be called piriformis syndrome, although there is controversy in the literature about the existence of this causal relationship (16,17) .

B A
In compressive sciatic neuropathy, MRN findings include changes in the path, thickness, or signal of the sciatic nerve, with or without abnormalities of the piriformis muscle anatomy. We illustrate two examples of piriformis muscle alteration with neuropathy of the proximal portion of the sciatic nerve (Figures 9 and 10).

TRAUMATIC CAUSES
Because of its particular anatomy and long course, the sciatic nerve can be affected by direct trauma or pelvic ring fracture, which are common in automobile accidents. Dislocations of the coxofemoral joint, which occur in cases of high-energy trauma, are also common causes of sciatic neu-  ropathy (18) . In such cases, imaging findings include, other than the changes typical of neuropathy, alterations to the surrounding soft tissues, such as muscle bruising. The clinical correlation, if available, reveals a history of trauma, thus confirming the diagnosis.
We illustrate a case of neuropathy caused by direct trauma resulting from a fall from standing height ( Figure 11).

IATROGENIC CAUSES
One of the main iatrogenic causes of sciatic nerve neuropathy is radiotherapy for pelvic neoplasms, including prostate, gynecological, and colorectal cancer, resulting in radiation-induced neuropathy, as depicted in Figure 12 (11) . The A B development of neural changes related to radiotherapy tends to occur at 5-30 months after treatment, its incidence peaking between months 10 and 20. In radiation-induced neuropathy, the imaging findings are nonspecific and include changes in the thickness and signal intensity of the affected nerve. The clinical correlation with a history of radiotherapy is fundamental to suggesting this diagnosis. Clinically, radiation-induced neuropathy is initially characterized by sensory manifestations, mainly pain and paresthesia, in some cases evolving to muscle weakness (19)(20)(21) .
Neuroma ( Figure 13) is a common sequela of amputation, causing pain that is usually refractory to pharmacological treatment and sometimes disabling. Although its pathophysiology is still poorly understood, there are measures that have proven effective in reducing its incidence. Such measures

A B
include implantation of the nerve stump into the ventral aspect of a muscle or capping the nerve stump with epineural flap. More rare and serious causes, such as ischemic neuropathy secondary to the placement of an aortoiliac stent (22) , can also be demonstrated ( Figure 14). Like radiation-induced neuropathy, ischemic neuropathy presents nonspecific MRN findings, the clinical correlation also being fundamental.

CONCLUSION
The sciatic nerve is often affected in diseases of the pelvis or lower limbs, as well as by lesions that originate within the nerve itself. Its course is long, which predisposes it to various types of compression. MRN is an important tool for the evaluation of peripheral nerve diseases and should be widely used for the study of the sciatic nerve whenever possible. Detailed knowledge of its anatomy and of the imaging aspects of the main diseases affecting it is fundamental to optimizing imaging studies of the sciatic nerve.