Muscle denervation syndromes represent one cause of many different muscular disorders. Magnetic resonance imaging (MRI) is the main imaging modality which is used for the assessment of muscle denervation syndromes. In the early phase increased high T2 signal or increased STIR signal is the main imaging finding. Subsequent findings over time include atrophy of the muscle as well as increased T1 signal representing fatty atrophy. Differential diagnosis of increased high T2 signal on MRI carries an extremely broad differential. They include: trauma, early myositis ossificans, inflammatory myopathies such as dermatomyositis, polymyositis, eosinophilic myositis, proliferative myositis, or myositis associated with connecting connective tissue diseases, infectious myositis, infiltrating neoplasm, rhabdomyolysis, muscle infarction, sickle cell disease, or overuse syndromes.

A very frequent finding however is a neurogenic cause of muscular edema consistent with early denervation muscle syndrome. Many neurogenic disorders can cause a muscle denervation syndrome. These include but are not exclusively limited to:

1. systemic diseases, such as ischemia, vasculitis, toxic, endocrine and metabolic disorders such as diabetes amyotrophy, hereditary motor-sensory neuropathies, amyloidosis, hyperlipidemia, acute and chronic demyelinating inflammatory neuropathies, etc.

2. local conditions, such as plexopathy, nerve injury, perineural compressive lesions, adhesive neuropathy in failed tunnel cases, infections and nerve sheath tumors, etc.

3. neuropathies related to functional anatomical changes, such as habitual leg crossing, repeated typing and repetitive exercise, which may cause traction mononeuritis or compressive neuropathy in functional compartment syndromes.

The role of MRI in these conditions needs to be understand:

1. In systemic nerve diseases, MRI is not expected to make these diagnoses. However, MRI may be used to confirm the clinical suspicion by demonstrating abnormality of the innervating and regional muscle denervation changes or, exclude any structural cause for mononeuropathy in these cases.

2. In local conditions, MRI has a major role in these cases as it supplements information gained from the clinical exam and electrodiagnostic tests or provides information, which may not be possible to attain from other modalities.

3. In neuropathies related to functional anatomical changes, the role of MRI is limited. This group is mostly diagnosed on clinical basis and pressure catheter studies. However, rarely, MRI may be used in clinically confounding cases with imaging performed before and after the exercise/effort in question. Indirect signs such as significant prolongation of T2 signal intensity within the muscle compartment may aid in the diagnosis of compartment syndrome.

Muscle denervation syndrome, however, may also be caused by a variety of nerve diseases that can be divided into divided such that are based on the anatomic site (tissue) of affliction (neuromuscular junction or nerve- cell body, axon and myelin sheath) or functional involvement (sensory, motor or autonomic). Neuromuscular junction (NMJ) disorders are primarily related to acetylcholine receptor abnormalities. These disorders could be familial/congenital (myasthenia gravis) or acquired, such as drug induced, botulism or paraneoplastic (Eaton-Lambert syndrome). Unlike most neuromuscular disorders that present with predominant involvement of lower extremities early in the course of disease, the neuromuscular junction disorders present with facial and extraocular muscle weakness, normal tendon reflexes and sensation. The electromyography (EMG) and nerve conduction studies (NCV) are also normal in neuromuscular junction disorders. The nerve diseases distal to the NMJ disorders lead to motor weakness, sensory symptoms and autonomic symptoms depending upon the nerve involved. EMG and NCV studies are usually positive. With predominant axonal involvement, distribution of involvement and clinical presentation is usually distal>proximal, and legs>arms. Nerve biopsy may be useful in axonal and myelin disorders. The pathologist usually classifies the histologic alterations as axonal neuropathy, demyelinating neuropathy, inflammatory neuropathy, or as a process of the supporting and/or vascular tissues, such as vasculitis, storage disease, and infectious or neoplastic infiltration. In most specimens, the pathology features are not specific to a certain disease entity and must be correlated with clinical, electrodiagnostic, and imaging information to derive a correct diagnosis.

Myopathies (muscular dystrophy/myositis) may result from infectious, endocrine, metabolic, autoimmune, myoglobinuria or familial causes, such as limb-girdle syndrome. Myopathies present with isolated motor symptoms and the distribution is usually proximal and symmetric with absent tendon reflexes in the involved muscles. EMG is usually positive and on serologic exams, there are high creatine kinase levels in myopathies. Muscle biopsy is often used to make this diagnosis. MRI exams will show one or a combination of edema like signal intensity changes on T2-weighted fat suppressed MR images, fatty infiltration or atrophy depending upon the stage of the disease, similar to denervation muscle changes. However, the involved muscles may not correspond to single nerve territory and regional nerves will show normal signal intensity, thereby excluding neuropathy as the cause of muscle findings. Perimuscular fascial edema and enhancement may be seen with myositis, which is absent with muscle denervation changes.

This lecture will focus on the MR imaging features of muscle denervation syndromes, the time course for the increased T2 signal, as well as for potential differential diagnosis of a variety of etiologies imaging includes of course peripheral nerve pathologies. Thus, this lecture will also cover, in parts, the technique of peripheral nerve MR neurography.