Electromyogram

Technique, principles, results normal and pathological:

- The electrodiagnosis consists in studying the muscular answer after an electric stimulation indirect of Muscle. - Detection or electromyogram studies the potentials emitted by the muscle at the time of voluntary contractions of this one. One observes some potentials of driving units normally low frequency leaf at the time of a moderate contraction and a phenomenon of temporal and space “recruitment” at the time of a stronger contraction (more driving units are activated and beat at a higher frequency as the effort is more constant). This analysis is followed on a screen, and is printed, but also rests on an analysis with the ear of the muscular potentials

One associates with it the study nervous speeds of conduction (VCN) stimulation and detection. One can study the driving VCN ou/et sensitive. The normal values of the VCN are of approximately 50 m/s.

- In the event of central attack, there are no anomalies of the EMG. This examination indeed has interest only in the peripheral attacks of the peripheral nervous system.

- In the event of attack neurogene one observes a fibrillation on the layout of rest. During the contraction, there is a temporal summation without space summation: one with the aspect known as of “accelerated simple layout”. The VCN precociously and are strongly lowered in attacks tronculai-LMBO (polynévrites axonales, in particular) and much less in the radicular attacks. In the event of attack originally muscular (myosite, myopathy), the layout of rest remains normal and at the time of the contraction the layout grows rich in a too fast way with a particular aspect of the electric potentials which are of small size, polyphase, jagged.

Precautions

The interpretation of a EMG requires a great practice on behalf of the doctor, a co-operation of the patient and clinical information sufficient. There is neither counter-indication, nor particular monitoring after the examination. The needles used are disposable, the only counter-indication of the examination in detection is represented by an effective anticoagulant treatment or severe disorders of the Hémostase.

Indications

The EMG makes it possible to originally distinguish an attack peripheral neurogene from an attack myogene, assistance to differentiate an attack tronculaire from a radicular attack by the study of the VCN. It can make it possible to specify the diffusion and the topography of a peripheral attack. It makes it possible to detect a neuropathy beginner and/or latent. It can have a prognostic interest by its repeatability to follow, for example, the evolution of a Syndrome of Guillain-Barred or a Syndrome of the carpel tunnel. Interest also in the study of the blocks neuromusculaires and in particular in the myasthenic description of the phenomenon or block.

Its cost is of 120-150 euros

Technical sides

The electrodiagnosis gives an access to all the nervous trunks and all the rather surface muscles. The protocol of examination i.e. the choice of the various territories to be explored and the various techniques to be applied depends on the clinical Tableau. The electromyogram answers questions which rise from a given clinical picture and these questions or at least the private clinic must be correctly specified by the clinician: with the electromyographist then to know to lead his examination by choosing territories and techniques correctly.

Velocity measurement of driving nervous conduction (VCNM): it consists in stimulating a nervous trunk (median, cubital…) using an electric shock in short and to collect downstream the answer of a distal muscle belonging to the driving territory of the stimulated nerve. The nerve is stimulated in two points of its way. One collects a later answer for the site of stimulation proximal than during distal stimulation: the difference of the latency time between the two answers corresponds to the time of conduction on the nervous trunk between the two points of stimulation. The length of the nerve between the two points of stimulation is measured directly on the skin. The report/ratio of this distance to the time of conduction then represents the speed of driving nervous conduction along this segment of examined nerve.

The speed of nervous conduction is ensured by the sheath of myéline insulating sleeve surrounding the axons and consisted the cells of Schwann. The displacement of the nerve impulse is carried out in manner saltatoire between the nodes of Ranvier i.e. the interstices between the cells of Schwann: it will be all the more fast as the cells will be large more drawing aside the distance between two nodes. The reduction the speed of driving nervous conduction translates the existence of a disorder of the myelinic function along driving nervous fibers.

The VCNM varies according to the nerves and parallel to the progressive myelinisation of the nerves it increases with the age: it is mature between 3 and 5 years (it is about 20 to 25 m/s at the newborn of 50 m/s in the adult).

The latency of the answer to the point of distal stimulation is variable according to the age and the explored nerve. It constitutes an important index of distal conduction.

Velocity measurement of sensitive nervous conduction (VCNS): One stimulates the skin between two circular electrodes placed around the fingers and one once again collects the answers distal and proximale on the nerve corresponding to the stimulated cutaneous zone. As for the VCNM, the VCNS is equal to the relationship of the distance between the two points of collection and of the time of conduction between these two points.

It varies according to the nerves and maturation parallel to the progressive myelinisation of the nerves between 3 and 5 years: it is about 20 to 25 m/s; at the newborn of 60 m/s in the adult.

The deceleration of the VCNS and the lengthening of distal latency translate a disorder of the myelinic function along sensitive fibers.

References

muscular contraction
University off Vienna: biomedical course Engineering, simple (EMG) Electromyography
electronic Assembly of recording of EMG

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