Nerve conduction studies
- 1. Nerve Conduction Studies DR SANJOG CHANDANA (MIND)
- 2. Introduction • The electrical properties of peripheral nerves can be evaluated with externally applied stimuli and analysis of the consequent neurophysiologic responses. • Nerve conduction study (NCS) techniques permit stimulation and recording of electrical activity from individual peripheral nerves with sufficient accuracy, reproducibility, and standardization to determine normal values, characterize abnormal findings, and correlate neurophysiologic-pathologic features.
- 3. Used to • Diagnose focal and generalized disorders of peripheral nerves • To differentiate primary nerve and muscle disorders • Classify peripheral nerve conduction abnormalities due to axonal. Demyelination and conduction block • Prognosticate regarding clinical course and efficacy of the treatment
- 4. Nerve involvement • Axon degeneration / injury / loss • Demyelination ( conduction block / compression neuropathy )
- 5. Muscle involvement ( End organ ) • Normal • Denervated completely • Started getting re-iinervated
- 6. Surgical need for Electrodiagnostic studies • Preoperative: • Confirm the diagnosis • Localize the lesion • Determine the severity of axonal discontinuity • Intraoperative • Study involved segment • Nerve potential donors • Follow up • Re-innervation
- 7. Information to patients • Term “ electric shock” -not used • Small safe pulses over several points on the skin of the limbs and measuring the reponses obtained • No long term side effects.
- 8. Contraindications • Prescence of cardiac pacemakers : if NCS involving stimulation close to chest muscles • Stimulation intensities should be kept under 1 Hz and pulse width under 0.2 ms duration • Repetitive stimulation should be avoided • Patients with external pacing , guidewires – avoided.
- 9. Contraindication • Patients with bleeding disorders or who are anticoagulated – carefull / not a contraindication • Smallest gauge needle, superficial muscle tested, avoid deep muscles that could compress neurologic structures and avoid muscles with large vasculature near by
- 12. Nerve conduction studies ( NCS ) • Records electrical activities seen in a nerve when it is stimulated SENSORY SNAP MOTOR CMAP
- 13. Nerve conduction studies - Sensory
- 15. Sensory nerve action potential • C6 ( thumb and index finger) • C6-7 ( index and long finger ) • C8-T1 ( little and ring finger) • Recording from median , radial and ulnar more proximally
- 16. Sensory nerve action potentials ( SNAP) • Compound potential that represents the summation of all the recorded individual sensory fiber action potentials • Onset latency, peak latency, duration and amplitude are measured.
- 17. Sensory nerve action potentials ( SNAP) • Onset latency : Time from stimulus to the first deflection from baseline • Represents nerve conduction time from the stimulus to the recording electrodes for the large cutaneous fibers
- 18. Sensory nerve action potentials ( SNAP) • Peak latency : Measured at midpoint of the first negative peak
- 19. Sensory nerve action potentials ( SNAP) • Duration : Measured from the onset of the potential to the first baseline crossing • SNAP duration is much shorter than CMAP duration
- 20. Sensory nerve action potentials ( SNAP) • Amplitude : Measured from the baseline to the negative peak • Low SNAP amplitude indicates definite disorder of peripheral nerve
- 21. Sensory nerve action potentials ( SNAP) • SNCV : Measured by dividing the distance travelled by the onset latency • Meters / second
- 22. Sensory nerve action potentials ( SNAP) • Preganglionic injury , found proximal to the DRG, produces a complete distal sensory loss but preserves distal sensory conduction
- 23. Sensory nerve action potentials ( SNAP) • Post ganglionic injury, found distal to the DRG, also produces a complete distal sensory loss but no distal sensory conduction
- 24. Sensory nerve action potentials ( SNAP) • Smaller amplitude • Sensory fibers have lower threshold for stimulation than motor fibers • More sensitive for detecting mild / early disorders • ( compression neuropathy )
- 25. Nerve conduction studies ( NCS ) - Motor •
- 26. Nerve conduction studies ( NCS ) - Motor
- 27. Nerve conduction studies ( NCS ) - Motor • Median to APB/ FCR • Ulnar to ADM/ FCU • Radial to EIP/ EDC • Musculocutaneous to biceps • Axillary to deltoid • Spinal Accessory to trapezius
- 28. Nerve conduction studies ( NCS ) - Motor • CMAP
- 29. Nerve conduction studies ( NCS ) - Motor • Compound potential that represents the summation of all the recorded individual motor fiber action potentials • The latency, amplitude , duration, CMAP area and nerve conduction velocity are measured.
- 30. Nerve conduction studies ( NCS ) - Motor • Latency : Time from the stimulus to the initial CMAP deflection from the baseline • 3 processes occur • Nerve conduction time from stimulus site to the NMJ • Time delay across NMJ • Depolarizing time across the muscle
- 31. Nerve conduction studies ( NCS ) - Motor
- 32. Nerve conduction studies ( NCS ) - Motor
- 33. Nerve conduction studies ( NCS ) - Motor • Proximal latency – Distal latency • Divided by Time • = Motor Nerve conduction Velocity ( MNCV ) • Meters / Second
- 34. Nerve conduction studies ( NCS ) - Motor • Amplitude : Measured from the baseline to electrical negative peak ( seen above baseline • Causes of low CMAP • Axonal neuropathy • Demyelination with conduction block • Pre synaptic NMJ disorder
- 35. Nerve conduction studies ( NCS ) - Motor • Duration : Measured from initial deflection from baseline to first baseline crossing Duration is increased in demyelinating diseases
- 36. Nerve conduction studies ( NCS ) - Motor • CMAP Area: Measured between the baseline and the negative peak • Difference in CMAP area between distal and proximal stimulation sites signify conduction block from a demyelinating lesion
- 37. H reflex
- 38. CMAP - Significance • Amplitude depends on number of surviving axons • 50 -75 % decrease indicates a moderate axon loss • > 75 % decrease indicates a severe axon loss • Absent CMAP indicates no viable axons
- 39. CMAP - Significance • A progressive increase in the amplitude from a muscle on serial studies would signify re-innervation of that muscle.
- 45. • Sensory nerve action potentials drop by day 5 and reach their lowest by day 11 post injury • Motor amplitudes drop by day 3 and reach lowest by day 7 post injury • NCS done after 12 days of injury • Needle EMG – by 3 weeks.
- 46. Normal values • Compared to the values on opposite – unaffected side • Factors: • Temperature - The fastest motor nerve conduction velocity is reduced approximately by 1 m/s per Degree Celsius temperature fall • Limb Length or height • Age: Generally the sensory conduction halves with age.
- 47. Age related amplitude and velocity ( SNAP) Nerve Stim/ Rec site Amp (microV) Vel(m/sec) Amp (microV) Vel(m/sec) Amp (microV) Vel(m/sec) AGE 20 20 40 40 80 80 ULNAR N /LITTLE FINGER – WRIST 10 55 5 50 2.5 45 MEDIAN N =INDEX FINGER- WRIST 20 55 10 50 5 45 SUPERFICIAL RADIAL FOREARM- SNUFFBOX 30 55 15 50 7.5 45
- 48. ELECTROMYOGRAPHY • TO FIND OUT • NORMAL MUSCLE • A DENERVATED MUSCLE • RE-INNERVATING MUSCLE
- 49. Normal muscle – 3 phases of activity in EMG • The first phase: insertional activity , which occurs in response to a needle being placed in the affected muscle • The second phase: Occurs with the needle inside the muscle and with the muscle at rest • The third phase: occurs with the contraction of the muscle and increasing force.
- 50. Denervated muscles • Fibrillation potentials Seen early at 2 weeks in proximal muscles Late at 3-6 weeks In distal muscles There will also be some voluntary motor unit potentials – signifies better prognosis
- 51. Denervated muscles • Positive sharp waves • Both not seen if neuropraxia • Both will disappear if collateral sprouting occurs in 2-6 weeks
- 52. F wave • A response by retrograde stimulation from spinal cord
- 53. Re- innervating muscle • Early signs of muscle recovery may be detected on EMG • Occurrence of nascent potentials – small duration , small amplitude , polyphasic • Presence of unstable polyphasic potentials • Decreased number of fibrillation potentials • Increased number of motor unit potentials
- 54. • EMG recovery does not always ensure relevant clinical recovery
- 55. Example of interpretation SEVERITY OF CTS SNAP CMAP NEEDLE EMG ACTIVITY MILD PROLOGED LATENCY NORMAL NORMAL MODERATE PROLONGED LATENCY AND DECREASED AMPLITUDE PROLONGED LATENCY NORMAL SEVERE ABSENT PRLONGED LATENCY AND DECREASED AMPLITUDE ABNORMAL ACTIVITY
- 57. AXONAL DEGENERATION DEMYELINATION SENSORY OR MOTOR AMPLITUDES SMALL / ABSENT NORMAL / SLIGHTLY REDUCED DISTAL LATENCIES NORMAL PROLONGED CONDUCTION VELOCITIES NORMAL / SLIGHTLY REDUCED SLIGHTLY REDUCED F WAVE LATENCIES NORMAL / SLIGHTLY PROLONGED SIGNIFICANTLY PROLOGNED / ABSENT CONDUCTION BLOCK/ TEMPORAL DISPERTION NOT PRESNET PRESENT