Key points

Introduction

The evaluation of patients suspected of having a myopathy begins with a thorough history and clinical examination. This process leads to the elaboration of a clinical impression, based on symptoms, progression, family history, and examination findings. Further diagnostic tests are then ordered using a hypothesis-driven approach to add laboratory evidence in support of or against the clinical suspicion. Electrodiagnostic (EDX) studies, in this respect, are an extension of the physical examination and may help establish the diagnosis of myopathy.

EDX studies, however, are not always needed to diagnose a myopathy. This is particularly true in the pediatric and, occasionally, the adult population. Oftentimes, patients with inherited myopathies present with characteristic phenotypes, and, possibly, a positive family history. In these cases, it is reasonable to proceed directly to genetic testing. In addition, at times, the diagnosis ultimately requires a muscle biopsy, regardless of the EDX study results. Therefore, if clinical suspicion for a myopathy is high, generally corroborated by elevated creatine kinase (CK) levels, it is often reasonable to skip or limit the extent of the EDX studies. Finally, EDX studies may be normal in selected muscle diseases (certain endocrine, metabolic, congenital, and mitochondrial myopathies). Thus, in the appropriate clinical context, normal EDX studies do not necessarily rule out the presence of a myopathy.

EDX studies are most useful to diagnose a myopathy when further data are needed to exclude alternative diagnoses, confirm the presence of a muscle disease, and narrow down the differential. The role of EDX studies is summarized in Box 1 . First of all, the results of nerve conduction studies (NCSs) and electromyography (EMG) are used to exclude neuromuscular conditions that may mimic a myopathy (such as motor neuron disease and neuromuscular junction disorders or, occasionally, motor neuropathies). Second, EMG is often able to confirm the diagnosis of a muscle disorder, when motor units with characteristic morphology and recruitment pattern are identified ( Figs. 1 and 2 ). In such cases, EMG may also add diagnostic information relating to the location, type, and severity of the underlying process. For example, the presence of abnormal spontaneous activity may help narrow down the differential among different myopathic processes ( Box 2 ). Finally, EMG may be useful in identifying target muscles for biopsy. This is particularly helpful when the only clinically weak muscles are not easily accessible for biopsy (such as the gluteal muscles, the hip flexors, or the paraspinals). The yield of a muscle biopsy increases when a weak (Medical Research Council grade 4 of 5), but not end-stage, muscle is biopsied. EMG analysis allows the evaluation of multiple sites and the identification of affected muscles that are not weak on neurologic examination.

Physiologic model of motor units in myopathies. The pathologic process in myopathies results in dysfunction and dropout of individual muscle fibers located randomly within the motor unit. Motor neurons and motor axons are not affected. As a result, each MUAP is generated by fewer motor fibers. MUAPs become polyphasic, short in duration, and low in amplitude.

Morphology and recruitment pattern of MUAPs in myopathies. Myopathies are characterized by the presence of polyphasic, short-duration, low-amplitude MUAPs. Because each small motor unit is able to generate only a reduced amount of force compared with normal, with little muscle contraction, many MUAPs are recruited.

Introduction

The evaluation of patients suspected of having a myopathy begins with a thorough history and clinical examination. This process leads to the elaboration of a clinical impression, based on symptoms, progression, family history, and examination findings. Further diagnostic tests are then ordered using a hypothesis-driven approach to add laboratory evidence in support of or against the clinical suspicion. Electrodiagnostic (EDX) studies, in this respect, are an extension of the physical examination and may help establish the diagnosis of myopathy.

EDX studies, however, are not always needed to diagnose a myopathy. This is particularly true in the pediatric and, occasionally, the adult population. Oftentimes, patients with inherited myopathies present with characteristic phenotypes, and, possibly, a positive family history. In these cases, it is reasonable to proceed directly to genetic testing. In addition, at times, the diagnosis ultimately requires a muscle biopsy, regardless of the EDX study results. Therefore, if clinical suspicion for a myopathy is high, generally corroborated by elevated creatine kinase (CK) levels, it is often reasonable to skip or limit the extent of the EDX studies. Finally, EDX studies may be normal in selected muscle diseases (certain endocrine, metabolic, congenital, and mitochondrial myopathies). Thus, in the appropriate clinical context, normal EDX studies do not necessarily rule out the presence of a myopathy.

EDX studies are most useful to diagnose a myopathy when further data are needed to exclude alternative diagnoses, confirm the presence of a muscle disease, and narrow down the differential. The role of EDX studies is summarized in Box 1 . First of all, the results of nerve conduction studies (NCSs) and electromyography (EMG) are used to exclude neuromuscular conditions that may mimic a myopathy (such as motor neuron disease and neuromuscular junction disorders or, occasionally, motor neuropathies). Second, EMG is often able to confirm the diagnosis of a muscle disorder, when motor units with characteristic morphology and recruitment pattern are identified ( Figs. 1 and 2 ). In such cases, EMG may also add diagnostic information relating to the location, type, and severity of the underlying process. For example, the presence of abnormal spontaneous activity may help narrow down the differential among different myopathic processes ( Box 2 ). Finally, EMG may be useful in identifying target muscles for biopsy. This is particularly helpful when the only clinically weak muscles are not easily accessible for biopsy (such as the gluteal muscles, the hip flexors, or the paraspinals). The yield of a muscle biopsy increases when a weak (Medical Research Council grade 4 of 5), but not end-stage, muscle is biopsied. EMG analysis allows the evaluation of multiple sites and the identification of affected muscles that are not weak on neurologic examination.

Physiologic model of motor units in myopathies. The pathologic process in myopathies results in dysfunction and dropout of individual muscle fibers located randomly within the motor unit. Motor neurons and motor axons are not affected. As a result, each MUAP is generated by fewer motor fibers. MUAPs become polyphasic, short in duration, and low in amplitude.

Morphology and recruitment pattern of MUAPs in myopathies. Myopathies are characterized by the presence of polyphasic, short-duration, low-amplitude MUAPs. Because each small motor unit is able to generate only a reduced amount of force compared with normal, with little muscle contraction, many MUAPs are recruited.

Electrodiagnostic approach

A practical EDX approach for patients with suspected myopathy is outlined in Box 3 (adapted from other sources).

Box 3

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