Basics of electro myo graphy study (EMG)

Electromyography (EMG) is a vital technique in the field of bioelectrical signal analysis. It involves capturing muscle activity through surface or needle electrodes for diagnostic purposes. EMG signals can be analyzed to detect various muscle conditions, such as myopathic or neuropathic lesions, using numerical parameters. The spatial frequency bandwidth of surface EMG signals is crucial for detailed muscle activity reconstruction, with appropriate inter-electrode distances being essential for accurate mapping. In the context of neuro-monitoring, EMG plays a role in intra-operative detection of adverse events and predicting postoperative outcomes, especially when used complementarily with other modalities like motor evoked potentials. Overall, EMG serves as a valuable tool for understanding muscle function, diagnosing muscle disorders, and enhancing neuro-monitoring practices

1. electromyography (EMG) Dr Joe antony Junior resident Physical medicine and rehabilitation KGMu , Lucknow

2. contents Part a Introduction Differences b/w EMG and NCV EMG Machine and electrodes Recording methods and settings Components of EMG Insertional activity Spontaneous activity Part b Components of EMG Voluntary/exertional activity (MUAP) Recruitment Single fiber EMG Jitter Fiber density 2

3. Introduction EMG is the process by which an examiner puts a needle into a particular muscle and studies the electrical activity of that muscle. This electrical activity comes from the muscle itself – no shocks are used to stimulate the muscle. Easy emg : a guide to emg and ncs , jay weiss 3

4. Difference between emg and ncs EMG NCS Electrode is placed in form of needle inside muscle Surface electrodes over skin No use of electric shocks , intrinsic electrical activity of muscles are used Electrical stimulus applied through electrodes Muscle activity measured directly and nerve function measured indirectly Nerve function measured directly Easy emg : a guide to emg and ncs , jay weiss 4

5. Electrodiagnostic machine Performs both EMG and NCS It has, Display system Electrodes Analysing unit Amplifier Filters Easy emg : a guide to emg and ncs , jay weiss 5

6. Electrodes EMG needs three type of electrodes, Needle electrodes (Active electrodes) Surface electrodes Reference electrodes Ground electrodes Easy emg : a guide to emg and ncs , jay weiss 6

7. Needle electrodes Monopolar needle Concentric/Standard needle Bipolar needle Stainless steel with insulation except distal 0.2 to 0.4mm Stainless steel canula with a wire in center. Cannula with two steel or platinum wires Picks up 360 degree field ( larger potential recorded) Picks up 180 degree field ( lesser potential recorded Much smaller area Registers between surface electrode and tip Registers between wire and shaft Registers potential difference between two wires Lesser diameter-lesser discomort Lesser noisy Larger diameter- extremely uncomfortable Cheaper- preferred commonly Clearer signal Easy emg : a guide to emg and ncs , jay weiss 7 Monopolar Concentric Bipolar

8. Recording technique Select the muscle as per diagnosis Instruct the patient how to contract and relax the muscle Identify the muscle as the patient is contracting and relaxing Locate the insertion point slightly away from motor point Insert the needle quickly Sharp MUPs On minimal contraction confirm that needle is in proper position 8

9. Recording settings Sweep speed – 5-10ms/division Amplification 50microvolt / division – spontaneous activities 200 millivolt/ division – MUPs Filter setting 20-10,000 Hz 9

10. Components of emg Insertional activity Examination of muscle at rest Analysing the motor unit in exertion Recruitment 10

11. Insertional activity 11

12. Normal insertional activity Introduction of needle into muscle  brief burst of electrical activity Lasts slightly exceeding needle movement(0.5 -1s) Atleast 4-6 brief needle movements are made in 4 quadrants of each muscle Appears as positive or negative high frequency spikes in cluster 12

13. Increased insertional activity Lasts more than 300msec Increased insertional activity may occur when there is muscle pathology P resence of positive sharp waves and sometimes fibrillation potentials, That are apparent only on insertion and do not persist. 13

14. Altered insertional activities Increased Reduced Periodic paralysis Myopathies Atrophied muscle 14 Denervated muscle Myotonia Local trauma

15. Examination muscle at rest/ spontaneous activity 15

16. SA generated near the NMJ Endplate noise (Miniature End Plate Potential) Endplate spikes (End Plate Potential) SA generated from muscle fibers Fibrillation potentials Positive sharp waves Complex repetitive discharges Myotonic discharges SA generated from motor neurons Fasciculation potentials Doublets, triplets, multiplets Myokymic discharges Cramp discharges Neuromyotonic discharges 16

17. Spontaneous activity generated nearby nmj End plate noise (miniature end plate potential- MEpp ) End plate spike (end plate potential- epp ) 17 Normal SA

18. 18

19. Spontaneous activity generated from muscle fiber Fibrillation potentials Positive sharp waves Complex repetitive discharges Myotonic discharges 19

20. Fibrilation potentials Biphasic/triphasic waves with initial positivity low amplitude(10–100 μ V) Regular 0.5 to 10 Hz 1-5 msec Rain drop on tin roof 20

21. Positive sharp waves Recorded from muscle fibre with unstable resting potentials Initial sharp positivity followed by a long negative phase Amplitude 20-200 micro volt Few millisec - 100ms 1-15 Hz firing pattern is regular “ Saw tooth appearance ” Dull pop sound 21

22. Mechanism of fibs and PSW Denervated muscle fibre leads to Hypersensitivity to Ach Increased no. of Ach receptors Depolarisation Fibrillations and PSW 22

23. Fibs and sharp waves are seen in 1. Neurogenic d/o – Anterior horn cell d/s, radiculopathy, axonal neuropathy 2. NMJ d/o – Botulism, Myasthenia gravis 3. Myogenic d/o – Myositis, muscular dystrophy, trauma Density of fibs doesn’t correlate with degree of nerve damage 23

24. Grading of fibs or sharp positive waves 0 - none 1+ - single train of potential in atleast 2 areas 2+ - moderate no. of potentials in 3 or more areas 3+ - many fibs/sharp waves in all areas 4+ - full interference pattern of fibrillations/ sharp waves 24

25. Complex repetitive discharges Repetitive and synchronous firing of group of muscle fibres local muscular ‘arrhythmia’ high frequency (20–150 Hz) regular multi-serrated, repetitive discharges. abrupt onset and termination discharges identical in morphology Machine-like sound Ephaptic spread among denervated fibers Initiated by pacemaker muscle fiber circus movement 25

26. Complex repetitive discharges Myogenic conditions Polymyositis Muscular dystrophy Neurogenic conditions Poliomyelitis ALS SMA neuropathies 26

27. Myotonic discharges Action potentials of muscle fibres firing for a prolonged period after external excitation Waxing and waning of both amplitude and frequency 20 to 150 Hz 2 types of potentials Positive waves Brief spikes D ive bomber sound 27

28. Myotonic discharges Seen in myotonic dystrophy, myotonia congenita, and paramyotonia congenita acid maltase deficiency, polymyositis, or myotubular myopathy hyperkalemic periodic paralysis 28

29. Spontaneous activity generated from motor unit Fasciculation potentials Doublets, triplets, multiplets Myokymic discharges Cramp discharges Neuromyotonic discharges 29

30. Fasciculations Single , spontaneous, involuntary discharge of an individual motor unit 0.1 to 10 Hz Random and irregular Not under voluntary control Corn popping Seen in MND SMA Radiculopathies Polyneuropathies Entrapment neuropathies 30

31. Doublets, triplets, multiplets Spontaneous MUPs that fire in groups of 2,3 or multiple potentials Occur because of spontaneous depolarisation of motor unit or its axon Seen in hypocalcemia , hyperventilation,MND 31

32. Myokymic discharges Spontaneous bursting Repetitive discharges of same MUP Fixed pattern and rhythm Associated with “worm – like quivering” of muscles “Marching soldiers/ machine gun” 32

33. Myokymic discharges Facial myokymia – MS, pontine glioma, bell’s palsy Limb myokymia – radiculopathies, entrapment neuropathies, radiation plexitis 33

34. Neuromyotonia High -frequency (150–250 Hz) repetitive discharges of a single MUAP Wane in amplitude and frequency Not influenced by voluntary activity syndromes of continuous motor-unit activity (CMUA) Potassium channel disorders c/c neuropathies Peripheral N irritation during surgery Pinging sound 34

35. Cramp Discharge High-frequency discharges of MUAPs Abrupt onset and cessation benign ( eg , nocturnal calf cramps, post-exercise cramps) neuropathic, endocrinologic, and metabolic conditions Salt depletion Hypocalcemia Pregnancy Uremia Myxedema 35

36. Thank you Reference Easy EMG: a guide to EMG and NCS, Jay Weiss 36

37. contents Part a Introduction Differences b/w EMG and NCV EMG Machine and electrodes Recording methods and settings Components of EMG Insertional activity Spontaneous activity Part b Components of EMG(continued) Voluntary/exertional activity (MUAP) Recruitment Single fiber EMG Jitter Fiber density 37

38. Voluntary / exertional activity/ motor unit action potential ( muap ) Amplitude Rise time Duration phases 38

39. MUAP Occurs on voluntary contraction Represents synchronous discharge of all muscle fibres supplied by single motor neuron Yield information about integrity of motor unit 39

40. Duration Reflects the number of muscle fibers It indicates the degree of synchrony of firing among all individual muscle fibers w length, conduction velocity and membrane excitability Normal duration is 5–15 milliseconds long-duration MUAPs - dull and thud short-duration MUAPs - crisp and sharp 40

41. Factors affecting duration Number of muscle fibers in a motor unit 2. Dispersion of their depolarization over time (Temporal Dispersion) Longitudinal and transverse scatter of endplates (territory of the motor unit) Distance Conduction velocity 3. Synchrony of different muscle fibers in a motor unit 4. Age – increased age increased duration 5. Temperature- decreased temp increased duration 6. Muscle studied- MUAP is shorter in duration for proximal and bulbofacial muscles than distal muscles. 41

42. Altered duration reduced Myopathies NMJ d/o increased MND Axonal neuropathies c/c radiculopathies Neuropathies 42

43. Amplitude 100 μ V -2 mV reflects only those few fibers nearest to the needle Factors associated with increased amplitude proximity of the needle number and diameter of muscle fibers synchronized firing 43

44. Altered amplitude Increased amplitude Neuropathic diseases with reinervation Decreased amplitude Myopathic diseases 44

45. Phases Phase – portion of MUP b/w departure and return to baseline Measure of how synchronously muscle fibers in a motor unit fire Inverted triphasic potential ( + - + ) MUP with > 4 phases – Polyphasic potential Turns –directional changes without crossing the baseline Polyphasia & turns - “desynchronization” 45

46. Rise time Duration from initial + ve to subsequent- ve peak Indicator of distance of needle electrode from MF Slow rise time – Resistance of intervening tissue An acceptable rise time is 0.5 milliseconds or less 46

47. Recruitment Recruitment refers to the Orderly addition of motor units so as to increase the force of a contraction. 47

48. Recruitment A contraction becomes stronger in two ways: the firing motor units I ncrease their rate of firing Additional motor units commence firing Analysis should begin with the patient being told to think about contracting the muscle being analyzed . Observe for the firing of a single MUAP. It usually begins to fire at 2–3 Hz in an irregular pattern. 48

49. Normally the motor unit will fire in a regular pattern at about 5Hz. At around 10 Hz another MUAP will be recruited to fire. The new motor unit (MU) will initially fire at about 5 Hz. The normal firing rate of most motor units, before additional units are recruited, is 10 Hz. To calculate the firing rate of the MU, note how many times a MU with an identical morphology repeats across a screen set at 100 msec/screen (sweep speed of 10 msec/division). Multiply that number by ten to get the motor unit firing per 1000 msec or one second. Hz indicates cycles per second. 49

50. Altered recriutments Neuropathic recruitment In severe neuropathic lesions, when there are few functional motor units, we can see motor units firing at 30 Hz before a second motor unit in that area is recruited Seen in Neuropathies Radiculopathies Motor neuron disease Nerve trauma Few motor units fire at an increased rate Myopathic recruitment Large number of motor units are ‘recruited’ for a minimal contraction. Individual muscle fiber contribution to each motor unit is reduced Since myopathic motor units cannot increase their force output, they quickly recruit additional motor units to increase the force of a contraction. 50

51. Singel fiber EMG Method of recording action potential of a single muscle fiber Selectivity in SFEMG is achieved by Small recording area Setting a low frequency filter Filter setting is 500 Hz – 10 kHz Inserted at 20-30 degree to skin Closer the needle to MF, higher the amplitude and shorter the rise time EDC, Frontalis, Biceps, I DO, Tib. Ant – MC used 51

52. Jitter SF needle usually records from SF Possible to position the needle to record from 2 or more MF of same motor units A pair of SF potential Triggering potential Slave potential Time interval between two potential varies from one discharge to another This interpotential variability is known as JITTER 52

53. Fiber density Refers to number of fibers from one motor unit that is with in a radius of about 300 micrometer square of single fibre needle Nl fibre density 1.2- 1.8 Increased fibre density – manifest early reinnervation 53

54. SFEMG IN NEUROLOGIC DISEASES SFEMG is helpful in Neuromuscular transmission disorders Neuropathies Myopathies 54

55. Neurogenic transmission disorders Jitter – increased Fiber density – normal Neurogenic disorders Jitter – increased Fiber density – increased Myopathies Jitter – increased / decreased Fiber density – increased 55

56. Thank you Reference Easy EMG: a guide to EMG and NCS, Jay Weiss 56