ncs study - basics of nerve conduction study
- 1. Objectives • Motor nerve conduction studies • Sensory nerve conduction studies • Principles of stimulation • Important basic patterns • Review of cases
- 2. Overview • Peripheral nerves are easily stimulated and brought to action potential • Motor, sensory and mixed nerves studied • Nerves studied the most – Upper extremity: median, ulnar, and radial – Lower extremity: peroneal, tibial, and sural • Motor nerve responses range in milivolts (mV) • Sensory nerve responses range in microvolts (μV)
- 3. Motor Conduction Studies • Belly-tendon montage • Active electrode G1 is placed over center of muscle belly (motor endplate) • Reference electrode G2 is placed over muscle tendon • Stimulator is placed over the nerve (cathode placed closest to G1) • Gain is set at 2-5 mV per division • Duration of electrical impulse is set at 200 ms • Normal nerve requires a current in the range of 20-50 mA for supramaximal stimulation
- 5. Motor Conduction Studies • Compound MuscleAction Potential (CMAP) – Summation of all individual muscle fiber action potentials – Biphasic potential with initial negative (upward) deflection • Supramaximal stimulation – current increased to the point where CMAP no longer increases in size (all nerve fibers have been excited) • Latency,Amplitude, Duration, and area of CMAP are measured
- 6. • Latency – time from stimulus to the initial CMAP deflection from baseline – Measurements in ms; reflect the fastest conducting motor fibers • Amplitude – from baseline to negative peak – Reflects number of muscle fibers that depolorize – Low CMAP result from axon loss, conduction block, NMJ d/o, myopathies • Area – baseline to the negative peak – measured by EMG machines – Differences in CMAP areas between distal and proximal stimulation sites helps evaluate for conduction block • Duration – from initial deflection from baseline to the first baseline crossing – Measure of synchrony (some motor fibers conduct slower than the others causing increased duration, i.e. in demyelinating diseases) Compound Muscle Action Potential
- 7. Compound Muscle Action Potential
- 8. Conduction velocity • Motor conduction velocity – measure of the speed of the fastest conducting motor axons in the stimulated nerve – Velocity = Distance/Time in m/s • Cannot be calculated by single stimulation due to multiple parts of conduction – Conduction time along motor axon to NMJ – NMJ transmission time – Muscle depolarization time • Thus two stimulation sites are used to calculate accurate conduction velocity – Final conduction time used = proximal latency – distal latency = (A+B+C+D)- (A+B+C) = D
- 10. Sensory Conduction Studies • Sensory responses are very small (1-50 μV) • Electrical noise and technical factors are more significant • Only nerve fibers are assessed • Gain is set at 10-20 μV per division • Normal sensory nerve requires current in the range 5-30 mA • Sensory conduction velocity can be calculated with one stimulation site • SNAP duration is shorter compared with CMAP duration (1.5 ms vs 5-6 ms)
- 12. Sensory Nerve Action Potential
- 13. SNAP Onset vs Peak Latency
- 14. CMAP vs SNAP
- 15. Sensory Antidromic vs Orthodromic Recording • Nerve depolarized=> conduction occurs equally in both directions • Antidromic – stimulating toward the sensory receptor • Orthodromic – stimulating away from the sensory receptor • Latency and conduction velocity should be identical with either method – Amplitude is higher in antidromic stimulation • Antidromic technique is superior – higher amplitude
- 16. Sensory Antidromic vs Orthodromic Recording
- 18. Lesions proximal to DRG
- 19. Principles of stimulation • Supramaximal stimulation – current increased to the point where CMAP no longer increases in size (all nerve fibers have been excited) • Submaximal stimulation – current is low • Co-stimulation- current is too high and depolarizes nearby nerves
- 21. Important basic patters • Neuropathic lesions – Axonal vs demyelinating – Axon loss: toxic, metabolic, genetic conditions or physical disruption – Demyelination: dysfunction of myelin sheath can be seen with entrapment, compression, toxic, genetic, immunologic causes
- 22. Axonal loss • Most common pattern on NCS • Reduced amplitude is the primary abnormality associated with axonal loss • Conduction velocity and latency are normal vs mildly slowed; marked slowing does not occur • CV does not drop lower than 75% of lower limit of normal • Latency prolongation does not exceed 130% of the upper limit of normal • Exception – hyperacute axonal loss (nerve transection/nerve infarction) NCS within 3-4 days are normal – Wallerian degeneration between 3-5 days for motor n; 6-10 for sensory n. – With distal stimulation amplitude is normal; with proximal stimulation amplitude is lowered and simulates conduction block aka pseudo-conduction block
- 23. Axonal loss
- 24. Axonal loss
- 25. Axonal loss
- 26. Demyelination • Myelin is essential for saltatory conduction • Marked slowing of CV (<75% of lower limit of normal) • Marked prolongation of distal latency (>130% of the upper limit of normal) • If CV and latency is at the cutoff – look at the amplitude • In demyelinating d/o sensory amplitudes are low/absent – d/t temporal dispersion/phase cancelation • Reduced amplitudes in demyelinating lesions is due to conduction block or secondary axon loss in late stage of disease
- 27. Demyelination
- 28. Conduction Block • Seen in acquired demyelinating diseases • Reduced amplitudes between proximal and distal stimulation sites • Drop in CMAP area by >50% • Temporal dispersion and phase cancelation in demyelinating diseases can look like conduction block but if CMAP area drops by >50%, this is due to conduction block
- 29. Conduction Block
- 30. Conduction Block
- 31. Conduction Block
- 32. F waves • Stimulation of the motor nerves towards the spinal cord and recording at the muscle belly • F waves are brought on by supramaximal stimulation, have varying latencies and morphology. • F waves are usually prolonged in demyelinating neuropathies such as AIDP/CIDP
- 33. H reflexes • EMG correlate of ankle reflex (tibial nerve), less commonly in the forearm • Stimulation of 1a sensory fibers of the tibial nerve towards the spinal cord and recording at the gastrocnemius muscle belly • H waves are suppressed by supramaximal stimulation, have constant latencies • Useful for S1 radiculopathies
- 34. NCS Patterns • Radiculopathy - will have normal sensory conduction studies and abnormal motor NCS – The sensory root is presynaptic and therefore not tested on NCS – With the exception to superficial fibular nerve which is affected in L5 radiculopathy (in real life) • Plexopathy should have abnormal sensory conduction studies • Low motor amplitudes only – think of motor neuron disease, myopathy, and LEMS(Lambert Eaton Myasthenic syndrome) • LEMS - very low motor conduction amplitudes, in absence of other findings – Post exercise facilitation – increase in motor amplitude after short exercise • Martin Gruber anastomosis – anatomic variant in 30% of the population – Median nerve partial innervation of ulnar innervated muscles (ADM, FDI) – Distal median motor amplitude is smaller than proximal – Distal ulnar motor amplitude is significantly larger than proximal
- 35. 35 NORMAL EMG
- 36. 36 In neurogenic lesion or in active myositis, the following spontaneous activity is noted Positive sharp wave: A small potential of 50 to 100 µV, 5 to 10 msec duration with abrupt onset and slow outset. Abnormal MUPs
- 37. 37 Fibrillation Potentials Positive Sharp Waves
- 38. 38 Fibrillation potential: these are randomly occurring small amplitude potentials or may appear in runs.The audioamplifier gives sounds, as if somebody listen sounds of rains in a tin shade house.These potentials are generated from the single muscle fiber of a denervated muscle, possibly due to denervation hypersensitivity to acetyl choline.
- 39. 39 Fasciculation potentials: These are high voltage, polyphasic, long duration potentials appear spontaneously associated with visible contraction of the muscle.They originate from a large motor unit which is formed due to reinnervation of another motor unit from the neighboring motor unit.
- 42. 42 NEUROPATHY
- 44. 44 MYOPATHY
- 45. 45 MUP NORMAL NEUROGENIC MYOPATHIC Duration msec. 3 – 15 msec longer Shorter Amplitude 300 – 5000 µV Larger Smaller Phases Biphasic / triphasic Polyphasic May be polyphasic Resting Activity Absent Present Present Interference pattern full partial Full Analysis of a motor unit potential (MUP)
- 46. 46 MUP Myopathy Normal Neuropathy Duration < 3 msec 3 – 15 msec > 15 msec Amplitude < 300 µV 300-5000 µV > 5 mV configuration polyphasic triphasic Polyphasic Typical MUAP characteristics in myopathic, neuropathic & normal muscle
- 47. 47
- 48. Case 1 1. Axonal neuropathy 2. Demyelinating neuropathy 3. Conduction block at the fibular head
- 49. Case 2 1. L5 radiculopathy 2. Peripheral neuropathy 3. Conduction block at the fibular head
- 50. Case 3
- 51. Case 4
- 52. Case 5
- 53. Case 6 1. It suggests acquired demyelinating polyneuropathy 2. It suggests axonal polyneuropathy 3. It is a normal peroneal motor study for age
- 54. Case 7 1. Ulnar neuropathy at the wrist 2. Ulnar neuropathy at the elbow 3. Medial cord or lower trunk plexopathy
- 55. Case 8 1. A conduction block in the forearm 2. Abnormal temporal dispersion in the forearm 3. Normal median sensory study 4. Carpal tunnel syndrome
- 56. Case 9
- 57. Case 10 1. Carpal tunnel syndrome 2. Multifocal motor neuropathy 3. Lower trunk brachial plexopathy
- 58. Case 11
- 59. Case 12
- 60. Case 13
- 61. Case 14 1. Electrical artifacts 2. A-waves 3. Excess patient movement with each stimulation
- 62. Case 15
- 63. References • Preston and Shapiro. 2013. Electromyography and Neuromuscular Disorders. Third Edition. • Clinical Neurophysiology Board Review Q&A.
Editor's Notes
- #14: Each have their own advantages and disadvantages. Onset latency represents fastest conducting fivers and can be used to calculate CV. However, difficult to precisely place the latency marker on the initial deflection from baseline. Peak latency – easily marked, no inter-examiner variation; cannot be used to calculate conduction velocity.
- #15: CMAP amplitude usually is measured in millivolts, whereas SNAPs are small potentials measured in the microvolt range (note different gains between the traces). CMAP negative peak duration usually is 5 to 6 ms, whereas SNAP negative peak duration is much shorter, typically 1 to 2 ms.
- #18: Volume conducted motor potential
- #19: Bipolar cells outside spinal cord near the intervertebral foramina. Any lesion of the nerve root leaves dorsal root ganglion and its peripheral axon intact. SNAPS remain normal in lesions proximal to the dorsal root ganglia including lesions of the nerve roots, spinal cord, and brain
- #23: In the median nerve, for instance, the largest-diameter (and accordingly the fastest) myelinated fibers conduct at a velocity of approximately 65 m/s. At the other end of the normal range, there are slower fibers that conduct as slowly as 35 m/s.