The peripheral nervous system is susceptible to a diverse array of pathologic insults, broadly categorizable into those entities intrinsic to the nerves themselves, either primarily arising within the nerve(s) or direct involvement of the nerve(s) secondary to a systemic process, and those processes external to the nerve(s) proper but affecting them extrinsically via mass effect, such as entrapment neuropathies. The soft tissue contrast inherent to high-quality MR imaging allows for outstanding visualization of the peripheral nervous system and surrounding structures. This review focuses on the use of MR imaging in the diagnosis and management of peripheral nerve disorders of the upper extremity.
Key points
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A solid knowledge of nerve anatomy coupled with high-quality, standard MR sequences is a strong tool in the evaluation of patients with upper extremity neuropathies.
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Patterns of MR muscle denervation help confirm and localize nerve pathology.
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MR imaging can often confirm clinically suspected entrapment sites while excluding extrinsic masses, ganglion cysts, and variant anatomy for preoperative planning.
Introduction
The peripheral nervous system is susceptible to a diverse array of pathologic insults, which may be broadly categorized into 2 categories: those entities intrinsic to the nerves themselves, either primarily arising within the nerve(s) or direct involvement of the nerve(s) secondary to a systemic process, and processes external to the nerve(s) proper but affecting them extrinsically via mass effect, such as entrapment neuropathies. Tumors (primary and metastatic), tumorlike conditions, and inflammatory conditions may also affect peripheral nerves. Lacking the osseous protection of the spine and cranium, the peripheral nervous system is also susceptible to traumatic injury.
In the past, clinicians depended largely on combined findings from a patient’s history, physical examination, and electrophysiologic testing to localize and characterize suspected peripheral neuropathies. Today’s clinicians are aided greatly by the tremendous advances in cross-sectional imaging that have occurred during the past several decades. Although remarkable contributions in ultrasound technology and improved user performance contribute greatly to the current evaluation and management of peripheral neuropathies, this review focuses primarily on the use of MR imaging in the diagnosis and management of peripheral nerve disorders of the upper extremity.
The soft tissue contrast inherent to high-quality MR imaging allows for outstanding visualization of the peripheral nervous system and surrounding structures. MR imaging examinations of the peripheral nervous system should ideally be performed on a 3T magnet paired with a surface coil, which allows for imaging of the entire suspected area of abnormality. For those examinations requiring imaging of structures away from the isocenter of the magnet, use of a dedicated transmit/receive coil is of great benefit in an attempt to mitigate known artifacts that can occur in this setting.
Standard sequences should include transverse, typically axial, T1, and fat-suppressed T2-weighted images. Similarly weighted coronal and sagittal images obliquely oriented along the long axis of the fibers or course of the suspected involved nerve(s) are also of great benefit for both diagnosis, for example, appreciating caliber change in the setting of entrapment neuropathy, and treatment planning, for example, preoperative localization for biopsy and/or operative resection. The authors strongly advocate the use of gadolinium in all cases, without contraindications, presenting for MR evaluation of suspected peripheral neuropathy because some inflammatory disorders may only be appreciated as subtle enhancement on postgadolinium sequences.
The median, radial, and ulnar nerves are susceptible to entrapment at multiple well-established sites between the arm and the wrist. The anatomy of each nerve is reviewed individually followed by a discussion of known entrapment neuropathies affecting that nerve.
Introduction
The peripheral nervous system is susceptible to a diverse array of pathologic insults, which may be broadly categorized into 2 categories: those entities intrinsic to the nerves themselves, either primarily arising within the nerve(s) or direct involvement of the nerve(s) secondary to a systemic process, and processes external to the nerve(s) proper but affecting them extrinsically via mass effect, such as entrapment neuropathies. Tumors (primary and metastatic), tumorlike conditions, and inflammatory conditions may also affect peripheral nerves. Lacking the osseous protection of the spine and cranium, the peripheral nervous system is also susceptible to traumatic injury.
In the past, clinicians depended largely on combined findings from a patient’s history, physical examination, and electrophysiologic testing to localize and characterize suspected peripheral neuropathies. Today’s clinicians are aided greatly by the tremendous advances in cross-sectional imaging that have occurred during the past several decades. Although remarkable contributions in ultrasound technology and improved user performance contribute greatly to the current evaluation and management of peripheral neuropathies, this review focuses primarily on the use of MR imaging in the diagnosis and management of peripheral nerve disorders of the upper extremity.
The soft tissue contrast inherent to high-quality MR imaging allows for outstanding visualization of the peripheral nervous system and surrounding structures. MR imaging examinations of the peripheral nervous system should ideally be performed on a 3T magnet paired with a surface coil, which allows for imaging of the entire suspected area of abnormality. For those examinations requiring imaging of structures away from the isocenter of the magnet, use of a dedicated transmit/receive coil is of great benefit in an attempt to mitigate known artifacts that can occur in this setting.
Standard sequences should include transverse, typically axial, T1, and fat-suppressed T2-weighted images. Similarly weighted coronal and sagittal images obliquely oriented along the long axis of the fibers or course of the suspected involved nerve(s) are also of great benefit for both diagnosis, for example, appreciating caliber change in the setting of entrapment neuropathy, and treatment planning, for example, preoperative localization for biopsy and/or operative resection. The authors strongly advocate the use of gadolinium in all cases, without contraindications, presenting for MR evaluation of suspected peripheral neuropathy because some inflammatory disorders may only be appreciated as subtle enhancement on postgadolinium sequences.
The median, radial, and ulnar nerves are susceptible to entrapment at multiple well-established sites between the arm and the wrist. The anatomy of each nerve is reviewed individually followed by a discussion of known entrapment neuropathies affecting that nerve.
Imaging of entrapment neuropathies
Entrapment neuropathies are common at the elbow and wrist. The most common entrapment in the upper extremity is the median nerve at the level of the carpal tunnel, followed by the ulnar nerve at the cubital tunnel. The purpose of MR imaging of entrapment neuropathy is to confirm the clinical suspicion and identify factors contributing to entrapment. Perpendicular (axial) and sagittal to the long axis of the nerve are the best planes for the evaluation of entrapment neuropathy. Axial images typically offer the best spatial resolution for evaluation of fascicular architecture. A high-resolution T1-weighted sequence is best for demonstrating the fine intrafascicular fat present in normal nerves. Peripheral nerves are surrounded typically by a thin rim of perineural fat that also may become effaced, scarred, or inflamed and edematous in the setting of entrapment neuropathies. The sagittal image plane parallel to the long axis of the nerve best depicts the location and severity of the entrapment. Fluid-sensitive sagittal sequences are most helpful in localizing the point of entrapment, because the nerves demonstrate enlargement and increased T2-weighted hyperintensity near the point of entrapment. The inflammatory response associated with entrapment, that is, neuritis, gradually decreases both proximal and distal to the point of entrapment. In severe nerve entrapments, the nerve demonstrates a decreased T2-weighted signal at the point of entrapment with increased T2-weighted signal proximal and distal to the point of compression, the so-called triple B sign of a bright, black, bright nerve. In some instances, 3-dimensional isovolumetric sequences may be helpful in obtaining true sagittal and axial planes to the nerves.
Anatomic variations are well-recognized as additional or potential factors predisposing nerve entrapment. These anatomic considerations are discussed herein as they pertain to the common entrapment sites.
Imaging of tumors and inflammatory neuropathies
Soft tissue tumors and ganglion cysts may cause neuropathy at the elbow or wrist. Tumors and tumorlike conditions affecting peripheral nerves may be either extrinsic to the nerve (metastasis or sarcoma) or intrinsic to the nerve, as in the setting of benign and malignant peripheral nerve sheath tumors. Similarly, ganglion cysts may be either intraneural or extraneural and are an additional well-recognized cause of peripheral neuropathy. Intraneural ganglion cysts have been reported at both the elbow and the wrist. It is important to distinguish between intraneural and extraneural ganglion cysts and the joint connection for surgical planning purposes.
MR imaging is useful in the evaluation of poorly localizable peripheral nerve pathology, such as inflammatory conditions. In this clinical setting, electromyography is often performed before MR imaging and may aid in localization and thus narrow the anatomic region to be covered. Diffuse inflammatory processes typically have a nonspecific MR imaging appearance with T2-weighted hyperintensity and variable nerve enlargement. As noted, these authors advocate the use of gadolinium in MR evaluation of all suspected peripheral neuropathies in the absence of contraindications. Postgadolinium imaging with fat saturation is particularly useful in evaluation of inflammatory neuropathy because most inflammatory neuropathies demonstrate no or minimal, thin peripheral postgadolinium enhancement. Normal peripheral nerves demonstrate no postgadolinium enhancement owing to the inability of gadolinium to cross an intact blood–nerve barrier. Inflammation can disrupt the endothelial tight junctions at the blood–nerve barrier, resulting in abnormal nerve enhancement.
Several tumorlike conditions of the peripheral nerves demonstrate relatively specific imaging findings. Chronic inflammatory polyneuropathies and mononeuropathies typically demonstrate marked fusiform nerve enlargement and T2 hyperintensity with no or minimal peripheral postgadolinium enhancement. Intraneural perineurioma is a benign lesion that typically presents in children and young adults. MR findings include fusiform enlargement, T2-weighted hyperintensity, and markedly diffuse postgadolinium enhancement of the nerve. Lipomatosis of nerve (fibrolipomatous hamartoma) has characteristic MR features of nerve enlargement with increased intraneural fat. It is most common in the median nerve and can be diagnosed confidently on MR without the need for a biopsy.
Median nerve
Anatomy
The median nerve is formed by contributions from the lateral (C5, C6, and C7) and medial cords (C8 and T1). The median nerve forms in the axilla and runs in the medial arm, adjacent to the brachial artery. The nerve passes through the antebrachial fossa and travels deep to the proximal margin of the pronator teres in the proximal forearm. The anterior interosseous nerve (AIN) branches off the median nerve in the proximal forearm. In the forearm, the median nerve is located deep to the flexor carpi radialis and flexor digitorum superficialis muscles. The palmar cutaneous branch of the median nerve originates in the distal forearm before the median nerve entering the carpal tunnel. The palmar cutaneous branch supplies sensory innervation to the lateral palm over the thenar eminence and does not pass through the carpal tunnel. The carpal tunnel contains the median nerve and 9 flexor tendons (flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus). The median nerve branches distal to the carpal tunnel into the digital nerve branches and provides sensation to the palmar thumb, index, long, and radial half of the ring finger.
The median nerve does not provide motor innervation in the arm. Motor branches to the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and palmaris longus are given off at the level of the elbow. The AIN branches from the median nerve in the proximal forearm and provides motor innervation to the deep muscles in the anterior compartment of the forearm: namely, the flexor pollicis longus, pronator quadratus, and radial aspect of the flexor digitorum profundus. Muscular branches from the median nerve provide motor innervation to all the remaining flexor compartment muscles with the exception of the flexor carpi ulnaris and a portion of the flexor digitorum profundus, both of which receive motor innervation from the ulnar nerve. The recurrent thenar branch of the median nerve supplies the thenar muscles of the hand, the first and second lumbricals, the opponens pollicis, the abductor pollicis brevis, and the flexor pollicis brevis (so-called LOAF muscles). The recurrent thenar branch has been referred to as the “million dollar branch” because inadvertent iatrogenic injury during carpal tunnel surgery may lead to significant morbidity and a subsequent lawsuit.
Pathology
Carpal tunnel syndrome
The median nerve at the carpal tunnel is the most common site of upper extremity nerve entrapment. Patients typically present with classic symptoms of median neuropathy of the hand and are most often diagnosed clinically and treated without the need for advanced imaging techniques. Any mass in the region can result in median nerve compression at the wrist/palm and can again be either intraneural or extraneural. Intraneural causes include peripheral nerve sheath tumors and lipomatous overgrowth/enlargement of nerve (fibrolipomatous hamartoma). Extraneural causes include ganglion cysts, lipomas, heterotopic ossification, and vascular malformations, to name a few. MR imaging may be particularly helpful in patients with symptoms suggestive of carpal tunnel syndrome when they have a mass lesion, in those patients with atypical presentations primarily, or in those who have not responded or recurred after primary carpal tunnel release ( Fig. 1 ).
The pathophysiology of idiopathic carpal tunnel syndrome is not known entirely. Synovial hypertrophy and fibrosis occurs and ultimately results in increased carpal tunnel pressure and compression of the median nerve. The fibrosis is accompanied by vascular proliferation and hypertrophy. When advanced, this subsynovial connective tissue fibrosis may decrease the normal gliding mechanics of the flexor tendons in the carpal tunnel that may exacerbate median nerve irritation. Patients often present with classic symptoms of median neuropathy at the wrist (including paresthesias of the radial 3 ½ digits, which are worse at night or when driving a car and improved with shaking the hand). Patients with classic symptoms are most often diagnosed on clinical grounds and treated without the need for advanced imaging techniques.
Numerous studies have evaluated the utility of MR imaging in evaluating patients with carpal tunnel syndrome. T2-weighted quantitative mapping, diffusion-weighted, and diffusion tensor sequences are advanced techniques that have been reported in the evaluation of idiopathic carpal tunnel syndrome. MR features associated with the clinical diagnosis of carpal tunnel syndrome include thenar muscle denervation edema and/or atrophy, decreased cross-sectional diameter of the nerve in the carpal tunnel compared with diameter proximal to the tunnel, bowing of the flexor retinaculum, and hyperintensity of the nerve proximal and distal to the compression.
Anterior interosseous nerve syndrome
AIN syndrome is rare. It is not typically associated with mass effect. Although entrapment (such as by fibrous bands) has been put forth, more commonly the cause of AIN syndrome is often unknown or is of an inflammatory nature. MR imaging may support the diagnosis when the characteristic pattern of muscular denervation change is present ( Fig. 2 ). The AIN innervates the flexor pollicis longus, pronator quadratus, and flexor digitorum profundus of the index and middle fingers. Isolated edema in the pronator quadratus should be interpreted with caution, because it is found commonly in asymptomatic individuals and does not necessarily indicate denervation changes. In AIN syndrome the nerve typically demonstrates nonspecific T2-weighted hyperintensity.
