Electromyography (EMG) is a science that originated in Europe and the United States, and began to be used in clinical practice abroad in the 1950s and in China around the 1980s. It is now recognized as an extension of clinical examination of neuromuscular diseases, and can provide important information for the determination of the localization, characteristics and degree of lesions of neuromuscular diseases in clinical diagnosis. Histochemistry, molecular biology, genetic testing and imaging cannot replace the examination techniques.
Electromyography is a test that records bioelectrical activity of neuromuscles to determine the functional status of neuromuscular diseases and is used for the diagnosis of neuromuscular diseases. EMG examination includes direct recording of muscle potential (electromyography), stimulation of neuromuscular evoked potentials (nerve conduction velocity, repetitive electrical stimulation of nerves, visual, auditory brainstem, somatosensory evocation, etc.). EMG mainly addresses lesions below the anterior horn cells: anterior horn cells, nerve roots, nerve plexus, peripheral nerves, neuromuscular junctions (presynaptic modulus and posterior membrane) heads and muscles. Evoked potentials (ABR, SEP, VEP, etc.) can help in the diagnosis of some central diseases:
Visual evoked potentials (VEP) clinical applications
1.Multiple sclerosis (MS): the main abnormalities are prolonged P100 latency, decreased wave amplitude and disappearance of normal waveform. vep has the most practical value for MS diagnosis and can detect subclinical lesions and other multi-system damage, and the abnormality rate can reach 90% in patients with clinically confirmed MS, even without obvious visual impairment.
2, VEP is more sensitive to optic nerve demyelinating lesions, the abnormal rate of the acute phase of optic neuritis is up to 90% or more; such as optic neuromyelitis optica VEP is abnormal; VEP is abnormal when diabetes attacks the optic nerve.
3, Parkinson’s disease, progressive neurogenic muscular atrophy, etc., can appear VEP latency prolongation.
Auditory brainstem evoked potentials (ABR)
1, posterior cranial fossa tumor: the abnormality rate is 75-92%, among which the abnormality of auditory neuroma is almost 100%. Early lesions may have no hearing impairment, (sometimes only dizziness, headache, ataxia and other non-specific manifestations) but BAEP has abnormal changes, which is more sensitive than CT in early diagnosis. Many studies and literature show that BAEP is the most effective method to diagnose auditory neuroma except for posterior cranial fossa angiography; brainstem tumor abnormalities are 85-90%, and BAEP is superior to CT in diagnosing brainstem lesions due to the defects of CT in showing brainstem lesions.
2, cerebrovascular disease; BAEP changes are related to the location, nature and severity of the lesion, with a higher rate of abnormality in cerebral bridge lesions, BAEP disappearance mainly shows 3-5 wave abnormalities or disappearance, vertebrobasilar system TIA or plastic stroke usually BAEP normal.
3. Coma and brain death: Since BAEP is rarely affected by drug intoxication or metabolic abnormalities, it is valuable to identify comatose patients from drugs or organic brainstem lesions, and it is also valuable to determine the prognosis of comatose patients. With clinical and electrophysiological indices such as BAEP and EEG, the diagnosis of brain death can be made.
4, clinical audiology: mainly used to determine whether the hearing of infants and children and difficult to test subjects are sound, hearing loss – look at that stimulus intensity BAEP disappeared, first from 80db-70-60-50db on which level to report. If 50db BAEP disappears, report hearing loss at 50db. Generally normal people can elicit BAEP at 30 db. the younger the month of infants and children, the higher the response threshold. One month after birth is 30 db, June is 20 db, December is 16 db, two years old is 12 db, and five years old is 8 db. If the threshold is found to be elevated, the most likely cause is immature brainstem development.
5. Localization of abnormal auditory function and identification of cochlear and retrocochlear lesions. Wave I:Auditory nerve fibers occur. Wave II:Cochlear nucleus (extended brain). Wave III:superior olivary nucleus (pontocerebrum). Wave IV:Extradural nucleus of the thalamus (pons). V wave: inferior colliculus (midbrain).
6. Brainstem auditory evoked potentials do not fully reflect the activity of all parts of the auditory system. It is possible that certain diseases may violate the neural pathways reflected in the brainstem without affecting the auditory conduction pathways.
7, multiple sclerosis (MS): generally suspicious of MS, clinicians are a full set of evoked potentials – BAEP, VEP, SEP examination.
Somatosensory evoked potentials (SEP): Traditional sensory conduction velocities are performed only in the distal part of the limb and rarely involve the proximal nerves. Somatosensory evoked potentials are different and can measure the full length of the input nerve. Therefore SEP can measure not only central conduction, but also has value for peripheral nerves, especially its proximal abnormal findings.
1, cerebral hemispheres and brainstem lesions: SEP abnormalities are related to lesions, cortical lesions are manifested by wave amplitude changes in cortical potentials, mostly with reduced disease side; subcortical lesions are manifested by wave amplitude changes in cortical potentials with delayed latency;
2, MS (multiple sclerosis): SEP can detect its subclinical lesions, and can make localized diagnosis of lesions.
3, spinal cord lesions: whether the SEP of spinal cord lesions appears abnormal depends on whether the posterior cord of the spinal cord is involved and whether there is deep sensory impairment in the clinic, especially in the case of spinal cord trauma, demyelination and degenerative disease, the SEP can also determine the extent and degree of lesions.
4. Coma and brain death: SEP of the upper extremity is more valuable than BAEP in determining the prognosis of coma.
Finally, there is a clinical idea to share with you: these electrodiagnostics are good for clinical diagnosis when they are positive, but they cannot completely exclude the diagnosis of certain diseases when they are negative. After all, electrodiagnosis also has its limitations.