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Abstract
The peripheral nervous system is composed of multiple cell types and elements that subserve diverse motor, sensory, and autonomic functions. Peripheral neuropathy and polyneuropathy are terms that describe syndromes resulting from diffuse lesions of peripheral nerves, usually manifested by weakness, sensory loss, pain, and autonomic dysfunction. Mononeuropathy indicates a disorder of a single nerve often resulting from local trauma, compression, or entrapment. Mononeuropathy multiplex signifies focal involvement of two or more nerves, usually as a result of a generalized disorder such as diabetes mellitus or vasculitis. Neuritis is typically reserved for inflammatory disorders of nerves resulting from infection or autoimmunity.
Myopathies are diseases of skeletal muscle which are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or wasted. It can be caused by inherited genetic defects (e.g., muscular dystrophies), or by endocrine, inflammatory (e.g., polymyositis), and metabolic disorders.
Muscle and nerve biopsies can be extremely useful in the evaluation of patients with myopathies and neuropathies. The interpretation of a nerve biopsy requires correlation of histological changes, with clinical information including the results of electrophysiological investigations.
The major indications for nerve biopsy are when vasculitis or amyloidosis are strongly suspected. Additional indications for nerve biopsy include other autoimmune inflammatory conditions, possible infectious processes (e.g., leprosy), and tumor infiltration (e.g., perineurioma). Less commonly, nerve biopsy may be warranted to diagnose uncommon forms of hereditary neuropathy when DNA testing is not available or is negative.
At present, muscle biopsy is an essential part of the diagnostic investigation of most categories of muscle diseases, including inflammatory and many metabolic and congenital myopathies, as well as many of the muscular dystrophies.
Muscle biopsy is somewhat complex in that an optimal outcome requires coordination of the clinician, surgical team, pathologist, and technical staff in the pathology laboratory.
The changes that can be demonstrated in muscle using conventional imaging techniques are limited. Fat atrophy may be apparent on plain radiography, while US can be used to identify different patterns of involvement in patients with primary muscle disorders or neurogenic disorders. Also, it can be used to guide muscle biopsies by selecting relatively spared muscles.
CT has been extensively used to distinguish patterns of selective muscle involvement in muscular dystrophies. MRI techniques have shown that MRI has a higher sensitivity than CT for identifying early fatty replacement in muscles, and provides better anatomical details.
MRI of the muscle can be useful in for many reasons. Just as EMG can be useful in demonstrating objective evidence of muscle dysfunction, MRI can provide clear evidence of organicity, assisting the ordering physician in deciding which patients are likely to benefit from a muscle biopsy. Once the decision is made to proceed with muscle biopsy, MRI is valuable in selecting which muscle is most likely to reveal a pathologic diagnosis and can minimize false-negative biopsies. MRI may be valuable in distinguishing between conditions with similar presentations, especially in conditions with phenotypic variability.
There is accumulating evidence that muscle MRI can be a valuable additional tool for diagnosing inherited neuromuscular disorders.
DMD is an X-linked disorder characterized by progressive weakness of skeletal muscle.
The diagnosis of DMD is generally suspected on muscle biopsy that shows absence of dystrophin on immunohistochemical staining, and it is confirmed by genetic screening for mutations in the dystrophin gene. Muscle MRI allows one to evaluate the progression of muscle involvement as it evolves over time.There is selective muscle involvement.
The pattern of selective muscle involvement in individuals with Becker MD, a milder form that is also secondary to mutations in the same gene, is similar to the one observed in DMD, but milder, in keeping with the observed clinical course.
Congenital muscular dystrophy is an uncommon genetically determined, relatively nonprogressive necrotizing myopathy. This dystrophy is characterized by hypotonia, multiple joint contractures, and generalized muscular weakness, with more severity proximally than distally.
The overall involvement of muscle in congenital dystrophy can be reliably depicted on MR imaging, The sartorius and gracilis muscles are relatively spared in the thighs. Histologic features of this disorder are typical of muscular dystrophy, consisting of fibrosis and fat replacement with marked variability in fiber size, although usually without active fiber necrosis and regeneration.
LGMDs are a genetically heterogeneous group of disorders that are characterized clinically by predominant proximal muscle weakness affecting mainly the hip girdle, elevated creatinine kinase levels, and dystrophic changes on muscle biopsy.
At the thigh level, patients with LGMD2A and LGMD2I both have predominant involvement of the adductor magnus and the posterior thigh muscles; however, there is more substantial involvement of muscles of the anterior compartment in LGMD2I, and a significant hypertrophy of the sartorius and gracilis muscles. At the calf level, patients with LGMD2I have variable involvement of the calf muscles, with predominant involvement of the posterior muscles, but without the striking differential involvement between the medial and the lateral head of the gastrocnemius observed in LGMD2A.
Emery-Dreifuss muscle dystrophy is characterized by slowly progressive weakness and early contractures of the elbows and Achilles tendons, with rigidity of the spine and invariable cardiac involvement.
Muscle MRI can help to identify distinct patterns of muscle involvement in the two forms (the X-linked form and the dominant form) While patients with the X-linked form have minimal involvement at thigh level, patients with the dominant form often have a moderate to severe selective involvement of the vastus lateralis and intermedius, which in some cases is associated with involvement of the adductor magnus.
Myotonic muscular dystrophy is inherited as an autosomal dominant trait. The disease affects several organ systems and is characterized by myotonia, weakness and wasting of muscles, Common findings in MRI include increased thickness of the subcutaneous fat associated with a decrease in muscle thickness.
Inflammatory myopathies are acquired primary diseases of the striated muscles characterized by muscle weakness, inflammatory changes, and frequently elevated levels of serum muscle enzymes. Together with inclusion body myositis, dermatomyositis and PM are the most common diseases of the striated muscle, skin, and surrounding connective tissue. Despite the underlying autoimmune dysfunction, each entity has unique clinical and histologic features.
Polymyositis is an immune-mediated syndrome secondary to defective cellular immunity that is most commonly associated with other systemic autoimmune diseases. Muscle biopsy is the definitive test not only for establishing the diagnosis of polymyositis but also for excluding other neuromuscular diseases. In polymyositis, inflammation is the histologic hallmark of the disease. The endomysial infiltrates are mostly in foci in the fascicles, initially surrounding healthy muscle fibers and finally invading these cells and resulting in phagocytosis and necrosis.
MRIs show signal intensity abnormalities of muscle due to inflammation, edema, or scarring. Images may be used to guide muscle biopsy.
Dermatomyositis is considered to be the result of a humoral attack against the muscle capillaries and small arterioles
Findings on muscle biopsy can be diagnostic. Although inflammation is the histologic hallmark of dermatomyositis, polymyositis, and inclusion-body myositis, dermatomyositis is the only disease that shows perifascicular atrophy.
MRI may be useful in assessing for the presence of an inflammatory myopathy in patients without weakness. Chest radiography should be obtained at the time of diagnosis and when symptoms develop.A barium swallow allows evaluation of esophageal dysmotility. US of the muscles has been suggested for evaluation but has not been widely accepted. CT scanning is useful in the evaluation of potential malignancy that might be associated with inflammatory myopathy.
IBM is characterized clinically by the insidious onset of slowly progressive proximal and distal weakness, CT or MRI imaging of muscles may be useful in helping diagnose difficult cases. Findings involve selective atrophy of the quadriceps and forearm flexors.
Muscle biopsy sample shows myopathic changes with varying degrees of inflammation, predominantly within the endomysium.
Congenital myopathies are a group of disorders defined largely on the basis of the pathologic findings within muscle. Most of these conditions share common clinical features, including onset in early life, nonprogressive or slowly progressive course, proximal or generalized muscle weakness, and hypotonia. They include:
In central core myopathy, the characteristic histological features are the structural alterations within the center of muscle fibers. MRI shows a consistent pattern of selective muscle involvement in the thighs and lower legs.
On routine histochemistry, the nemaline rods appear as small, red-staining bodies in the subsarcolemma and occasionally perinuclear regions Patients with nemaline myopathy have highly heterogeneous findings on muscle MRI corresponding to the wide range of clinical phenotypes and genes that have been reported.
While in centronuclear myopathy, muscle biopsies reveal myonuclei in the center of muscle fibers, Muscle MRI show a characteristic progressive sequence with early involvement of the ankle plantar flexors and subsequent signal changes within the hamstring muscles and, finally, the anterior thigh.
Medical imaging is playing an increasingly important role in the diagnosis of disorders affecting the peripheral nerves. Plain radiographs, US, CT, all have limited role in diagnosis of Peripheral Nerve Diseases
Recently, MR neurography enables the physician to examine the peripheral nerve for anatomic abnormalities. The intrinsic contrast properties of adipose tissue and both endoneurial and axoplasmic fluid allow distinct MR imaging of peripheral nerves.
Typical MR peripheral nerve imaging protocols use high-resolution spin-echo T1-weighted images to show anatomic detail and fat-suppressed T2-weighted or STIR images to detect abnormal nerve signal intensity.
DW MR neurography may provide improved contrast between the nerves of the brachial plexus and the surrounding tissues. Diffusion tensor imaging and tractography of peripheral nerves, is based on the principle that water molecules have anisotropic diffusion properties in white matter fiber tracts, compared with isotropic diffusion in surrounding tissues.
More recently, the 3D STIR SPACE sequence is mainly used to rapidly image the postganglionic nerve roots of the entire brachial plexus. It is useful for the initial screening of patients with neoplastic conditions involving the brachial plexus and it is a valuable adjunct in the depiction of nerve site compressions.