Electrodiagnostic (EDX) studies play a key role in the evaluation of patients with neuromuscular disorders. Among these studies are included nerve conduction studies (NCSs), repetitive nerve stimulation, late responses, blink reflexes, and needle electromyography (EMG), in addition to a variety of other specialized examinations. NCSs and needle EMG form the core of the EDX study . They are performed first, and usually yield the greatest diagnostic information. NCSs and needle EMG are complementary, and therefore are always performed together and during the same setting. Performed and interpreted correctly, EDX studies yield critical information about the underlying neuromuscular disorder and allow use of other laboratory tests in an appropriate and efficient manner. Likewise, the information gained from EDX studies often leads to specific medical or surgical therapy. For example, a patient with a peripheral neuropathy clinically, who is subsequently found to have an acquired demyelinating neuropathy with conduction blocks on EDX studies, most often has a potentially treatable condition.
In practice, EDX studies serve as an extension of the clinical examination and should always be considered as such. Accordingly, a directed neurologic examination should always be performed before EDX studies in order to identify key clinical abnormalities and establish a differential diagnosis. With numerous nerves and literally hundreds of muscles available, it is neither desirable for the patient nor practical for the electromyographer to study them all. In each case, the study must be individualized, based on the neurologic examination and differential diagnosis, and modified in real time as the study progresses and further information is gained.
NCSs and EMG are most often used to diagnose disorders of the peripheral nervous system ( Figure 1–1 , Box 1–1 ). These include disorders affecting the primary motor neurons (anterior horn cells), primary sensory neurons (dorsal root ganglia), nerve roots, brachial and lumbosacral plexuses, peripheral nerves, neuromuscular junctions, and muscles. In addition, these studies may provide useful diagnostic information when the disorder arises in the central nervous system (e.g., tremor or upper motor neuron weakness). Occasionally, information from the EDX study is so specific that it suggests a precise etiology. In most cases, however, the exact etiology cannot be defined based on EDX studies alone.
Motor neuronopathy
Amyotrophic lateral sclerosis
Spinal muscular atrophy
Infectious (poliomyelitis, West Nile virus)
Monomelic amyotrophy
Sensory neuronopathy
Paraneoplastic
Autoimmune
Toxic
Infectious
Radiculopathy
Disk herniation
Spondylosis
Neoplastic
Infarction
Infectious
Inflammatory
Plexopathy
Radiation induced
Neoplastic
Entrapment
Diabetic
Hemorrhagic
Inflammatory
Neuropathy
Entrapment
Polyneuropathy
Demyelinating
Axonal
Mononeuritis multiplex
Neuromuscular junction disorders
Myasthenia gravis
Lambert–Eaton myasthenic syndrome
Botulism
Toxic
Congenital
Myopathy
Inherited
Muscular dystrophy
Congenital
Metabolic
Acquired
Inflammatory
Toxic
Endocrine
Infectious
Localization of the Disorder is the Major Aim of the Electrodiagnostic Study
The principal goal of every EDX study is to localize the disorder . The differential diagnosis is often dramatically narrowed once the disorder has been localized. Broadly speaking, the first order of localization is whether the disorder is neuropathic, myopathic, a disorder of neuromuscular transmission, or a disorder of the central nervous system (CNS). For example, in patients with pure weakness, EDX studies can be used to localize whether the disorder is caused by dysfunction of the motor neurons/axons, neuromuscular junctions, muscles, or has a central etiology. The pattern of nerve conduction and especially EMG abnormalities usually can differentiate among these possibilities and guide subsequent laboratory investigations. For example, a patient with proximal muscle weakness may have spinal muscular atrophy (i.e., a motor neuron disorder), myasthenic syndrome (i.e., a neuromuscular junction disorder), or polymyositis (i.e., a muscle disorder), among other disorders, including those with central etiologies (e.g., a parasagittal frontal lesion). EDX studies can easily differentiate among these conditions, providing key information to guide subsequent evaluation and treatment, which differ markedly among these diseases.
Once the localization is determined to be neuropathic, myopathic, a disorder of the NMJ or of the CNS, EDX studies can usually add other important pieces of information to localize the problem further ( Figure 1–2 ). For instance, the differential diagnosis of a patient with weakness of the hand and numbness of the fourth and fifth fingers includes lesions affecting the ulnar nerve, lower brachial plexus, or C8-T1 nerve roots. If EDX studies demonstrate an ulnar neuropathy at the elbow, the differential diagnosis is limited to a few conditions, and further diagnostic studies can be directed in a more intelligent manner. In this situation, for instance, there is no need to obtain a magnetic resonance imaging scan of the cervical spine to assess a possible cervical radiculopathy because the EDX studies demonstrated an ulnar neuropathy at the elbow as the source of the patient’s symptoms.
In a patient with a CNS disorder who is mistaken as having a peripheral disorder, the EDX study often correctly suggests that the localization is central. For example, transverse myelitis may mimic Guillain–Barré syndrome, or a small acute cortical stroke may mimic the pattern of a brachial plexopathy. In settings such as these, the EDX study is often the first test to suggest that the correct localization is central rather than peripheral.
Neuropathic Localization
Neuropathic is probably the most common localization made on EDX studies. Neuropathic literally means a disorder of the peripheral nerves. However, in common usage, it includes the primary sensory and motor neurons as well. EDX studies are particularly helpful in neuropathic conditions. First, in conjunction with the history and examination, they can usually further localize the disorder to the neurons, roots, plexus, or peripheral nerve. In the case of peripheral nerve, further localization is usually possible to a single nerve (mononeuropathy), multiple individual nerves (mononeuropathy multiplex) or all nerves (polyneuropathy). In the case of a single nerve, the exact segment of nerve responsible for the problem may be localized in some cases.
In the case of neuropathic lesions, EDX studies often yield further key information, including the fiber types involved, the underlying pathophysiology, and the temporal course of the disorder ( Figure 1–3 ).
Information About the Fiber Types Involved and the Underlying Nerve Pathophysiology can be Gained, which then Further Narrows the Differential Diagnosis
In the case of neuropathic disorders, the involved fiber types and the underlying pathology can usually be determined. First, EDX studies are more sensitive than the clinical examination in determining which fiber types are involved: motor, sensory, or a combination of the two. Sensorimotor polyneuropathies are common and suggest a fairly large differential diagnosis. On the other hand, predominantly motor or predominantly sensory neuropathies are rare and suggest a much more limited set of disorders. For instance, a patient with numbness in the hands and feet and diminished reflexes may be diagnosed with a peripheral neuropathy. However, if EDX studies demonstrate abnormal sensory nerve conductions with completely normal motor nerve conductions and needle EMG, then the differential diagnosis changes from a peripheral neuropathy to a pure sensory neuropathy or neuronopathy, which has a much more limited differential diagnosis.
Second, EDX studies often can define whether the underlying pathophysiology is demyelination or axonal loss. Although most demyelinating neuropathies have some secondary axonal loss and many axonal loss neuropathies have some secondary demyelination, EDX studies usually can differentiate between a primary demyelinating and a primary axonal neuropathy. Because EDX studies usually can make this differentiation quickly and non-invasively, nerve biopsy is essentially never required to make this determination. Furthermore, the differentiation between primary axonal and primary demyelinating pathology is of considerable diagnostic and prognostic importance, especially in the case of polyneuropathies. The vast majority of polyneuropathies are associated with primary axonal degeneration, which has an extensive differential diagnosis. In contrast, the number of true electrophysiologic primary demyelinating neuropathies is extremely small. They are generally subdivided into those that are inherited and those that are acquired. EDX studies can typically make that determination as well. The finding of an unequivocal primary demyelinating polyneuropathy on EDX studies often leads quickly to the correct diagnosis and, in the case of an acquired demyelinating polyneuropathy, often suggests a potentially treatable disorder.
Assessing the Degree of Axonal Loss versus Demyelination has Implications for Severity and Prognosis
A nerve that has sustained a demyelinating injury often can remyelinate in a very short time, usually weeks. However, if there has been substantial axonal loss, whether primary or secondary, the prognosis is much more guarded. The rate of axonal regrowth is limited by the rate of slow axonal transport, approximately 1 mm per day. Clinically, axonal loss lesions can rarely be differentiated from demyelinating ones, especially in the acute setting. For example, in a patient who awakens with a complete wrist and finger drop, the etiology usually is compression of the radial nerve against the spiral groove of the humerus. However, the paralysis could result from either conduction block (i.e., demyelination) or axonal loss, depending on the severity and duration of the compression. Clinically, both conditions appear the same. Nevertheless, if the injury is due to axonal loss, it has a much worse prognosis and a longer rehabilitation time to recovery than a similarly placed lesion that is predominantly demyelinating in nature. EDX studies can readily differentiate axonal from demyelinating lesions.
Assessment of the Temporal Course can Often be Made
For neuropathic conditions, there is an orderly, temporal progression of abnormalities that occurs in NCSs and needle EMG. A combination of findings often allows differentiation among hyperacute (less than one week), acute (up to a few weeks), subacute (weeks to a few months), and chronic (more than a few months) lesions. The time course suggested by the EDX findings may alter the impression and differential diagnosis. For example, it is not uncommon for a patient to report an acute time course to his or her symptoms, whereas the EDX studies clearly indicate that the process has been present for a longer period of time than the patient has been aware of.
Conversely, the temporal course described by the patient may impact the interpretation of the EDX findings. For instance, the finding of a normal ulnar sensory nerve action potential recording the little finger, in a patient with numbness of the little finger, has very different implications depending on the time course of the symptoms. If the symptoms are truly less than one week in duration, the normal ulnar sensory response could indicate an ulnar neuropathy (with incomplete wallerian degeneration), a proximal demyelinating lesion, or a lesion at the level of the nerve root or above. On the other hand, if the symptoms have been present for several weeks or longer, the same finding would indicate either a proximal demyelinating lesion or a lesion at the level of the nerve root or above. These temporal changes underscore the electromyographer’s need to know the clinical time course of symptoms and signs in order to ensure an accurate interpretation of any electrophysiologic abnormalities .
Myopathic Localization
In the case of myopathic (i.e., muscle) disease, EDX studies can also add key information to further define the condition ( Figure 1–4 ). First, the distribution of the abnormalities may suggest a particular diagnosis: are they proximal, distal or generalized? Most myopathies preferentially affect proximal muscles. Few myopathies, such as myotonic dystrophy type I, affect distal muscles. Some very severe myopathies (e.g., critical illness myopathy) can be generalized. In rare myopathies, there is prominent bulbar weakness; accordingly, EDX abnormalities may be most prominent in the bulbar muscles. Most myopathies are fairly symmetric; the finding of asymmetry either clinically and/or on EDX studies can be very helpful in narrowing the differential diagnosis. For example, inclusion body myositis may present asymmetrically, whereas polymyositis and dermatomyositis do not.
