EMG AND NCV--BASICS

when to do it?

*discrepancy between clinical findings and neuroradiologic imaging [magnetic resonance imaging (MRI), computed tomography (CT), myelogram] that often occurs in patients with back pain accompanied by extremity pain, numbness, or weakness
*all forms of nondestructive pathology noted on spinal imaging tests
*When imaging studies show multiple levels of spinal pathology consistent with nerve root compression
*certain patients with well-defined neurologic deficits and corresponding imaging studies to determine whether the deficits are acute or chronic
*To know if neuronal symptoms in the extremities are due to more peripheral neuronal entrapment (e.g., carpal tunnel syndrome in the arms or peroneal neuropathy in the legs) rather than an observed spinal pathology

The purposes of these tests are not only to document the presence of radiculopathy and differentiate it from more peripheral neuronal deficits, but also to determine its segmental level and to give some estimate as to degree and chronicity of the nerve root dysfunction.

There are 2 mechanisms by which the nerve fibres react to the variety of pathology

1.loss of myelin and axon both

2. demyelination with intact axon

EMG:- It is mainly affected by axon loss

NCV:- It is mainly affected by demyelination.

Needle EMG:
*the most useful electrodiagnostic procedure for the evaluation of radiculopathies

*A bipolar or monopolar needle electrode is inserted into a muscle, and the electrical activity in the muscle is measured under various functional conditions

*all EMG manufacturers use digital monitors

*A normally innervated muscle is quiet at rest, and the recording shows a flat or straight line

*If the muscle is denervated, as may occur in radiculopathy, there may be ongoing spontaneous activity at rest. The abnormal spontaneous waves may be in the form of fibrillation potentials, positive sharp waves, or fasciculations

*Fibrillation potentials and positive sharp waves tend to develop in the muscles within the myotome in a proximal to distal sequence.Following onset of an acute lesion, they may be found after 6 or 7 days in the paraspinal muscles, after 3 weeks in the proximal extremity muscles, and after 5 to 6 weeks in the distal extremity muscles. For that reason, sampling the paraspinal muscles as well as the limb muscles is important.

*spontaneous activity in the paraspinal muscles indicates that the lesion affects the fibers of the posterior primary ramus and is therefore within or near the intraspinal canal .

*Often, the paraspinal muscles do not show spontaneous activity even in patients with proven radiculopathies.

*mech. of polyphasic waves:When a muscle is denervated, adjacent nerve fibers will attempt to reinnervate the affected muscle fibers through sprouting of collateral axons, which then migrate to and make synaptic connections with the denervated muscle fibers. This phenomenon results in higher amplitude (giant) motor unit potentials with increased duration and number of phases (polyphasia).

Conclusions from EMG:

*the differentiation of acute from chronic denervation can be made. The acute changes result in the fibrillation potentials, positive sharp waves, and fasciculation during the resting phase, whereas the chronic changes result in the larger or giant motor unit potentials with polyphasia.
*severity of nerve dysfunction can also be estimated, although this is relatively crude. The quantity of denervation potentials allows some assessment of the extent of the radiculopathy, but perhaps more important is the nature of the recruitment and the degree to which a muscle is able to generate an interference pattern. A muscle that is affected only slightly may show abnormal potentials but still may be able to generate a full interference pattern. A more severely denervated muscle only generates a weaker interference pattern.

*the level of a radicular injury can be localized within one or two segments by means of EMG. This requires testing multiple muscles in the extremity and paraspinal muscles and correlating the pattern of abnormalities observed with the known muscular distribution of each nerve root.

H Reflex:

*monosynaptic spinal reflex with both motor and sensory pathways traveling through the S-1 nerve root by large diameter nerve fibers

*The reflex is elicited by recording the electrical response generated by the soleus muscle on stimulation of the posterior tibial nerve in the popliteal fossa

*Depending on age, leg length, and other factors, this stimulation is followed in approximately 23–32 milliseconds by a second contraction of the soleus muscle, which is the H reflex response (Fig. 4). The H reflex first increases in amplitude with low stimulus intensities and then decreases in amplitude as the stimulus intensity increases. An absent H reflex correlates well with an S-1 radiculopathy (2) and an absent Achilles deep tendon reflex.

*Problems with H reflex:

1.no changes in mils s-1 radiculopathy

2.experienced myographer and fastidious technique is required

3.affected only in s-1 root lesions

4.not synonymous with s-1 root since lesion anywhere along the sensory/ motor pathway or spinal cord can give ris eto the abnormal response 5.remains abnormal indefinitely, so not of any use in recurrent disc 6.absent in polyneuropathies and old patients

* it is complementary to EMG and confirms acute s-1 root injury even before EMG.

F Response:-
*The F responses are late responses that can be recorded from a muscle after maximal stimulation of its nerve

*The response is often inconsistent and of small amplitude when compared to the direct motor response and can show considerable variation in latency. For this reason, it is necessary to elicit at least ten responses for every tested nerve and to use the shortest or minimal latency

*The advantage of the F response is that it becomes abnormal immediately after an injury and may be the only early electrodiagnostic abnormality in patients with a radiculopathy

*F response studies are disappointing in patients with clinically unequivocal cervical and lumbar radiculopathies

NCV:-
*Motor and sensory nerve conduction studies are usually within normal limits in the patient with pure one-level radicu-lopathies

*Even when clinical examination strongly suggests a radiculopathy, it is still important to consider motor and sensory peripheral nerve conduction studies

*The primary shortcoming of EMG and F responses is their ability to detect a spinal cord lesion or a purely sensory radiculopathy

*TYPES:

Large mixed-nerve SEPs

Small sensory nerve evoked potentials

dermatomal somatosensory evoked response or potential (DSEP)Pudendal Evoked Responses

Magnetically Evoked Potentials

* BENEFITS:

1.directly assess the physiologic integrity of the spinal nerve roots, thereby providing information of both diagnostic and prognostic relevance

2.noncompressive radiculopathies

3.differentiating between radicular and more peripheral sources of neuronal symptoms or clinical deficits

4.documenting the presence of lesions affecting the spinal cord and the pudendal nerve that may be difficult to assess from the neurologic examination

*LIMITATIONS:-

1.does not detect all compressive radiculopathies

2.time dependent, Studies may be falsely negative if they are performed too early or too late in the course of the radiculopathy

3.with chronic radiculopathies, the EMG may be unrevealing, because muscles that had previously contained fibrillation potentials for a time after the onset of radiculopathy may have become completely reinnervated and therefore do not show any spontaneous activity

4.cause of the causative pathologic process cannot be determined, regardless of the electrodiagnostic procedure used in determining the radiculopathy

5.Although the root affected may have been determined, the anatomic site of the lesion, especially with regard to disk level, can only be estimated

6.In the end, it is the correlation of the patient's symptoms, clinical examination, electrodiagnostic consultation, and imaging studies that leads to the most accurate diagnosis of the patient with spine and extremity symptoms.