1. Motor Nerve Conduction and F waves
AAPMR Assembly 2015
Workshop
William S. Pease, MD

2. Motor Nerve Conduction and F waves
Plan
Presentation of a few cases to illustrate basic principles and abnormalities.
Small group sessions to practice setup techniques, and some errors and their prevention.

3. Why we do it? Case 1: CC: “My hand tingles and feels weak” History and Clinical Exam of the patient
Numbness felt in all digits, mildly painful
Difficulty holding cup of coffee
Symptoms occasionally awaken him at night

4. Hand numbness and weakness
Differential Diagnosis:
Carpal tunnel syndrome/median neuropathy
Ulnar neuropathy
Elbow, wrist
Lower trunk of plexus (med and uln)
C8 root (radiculopathy)
CNS (cord, stroke)

5. Nerve Conduction Study (NCS) Why we do it! (Besides the fun!)
Is the nerve functioning normally?
(electrically)
If not,
what is the pathophysiology?
how severe is the dysfunction?
What is the distribution of the problem?
What is the likely outcome?

6. Ulnar Nerve at Elbow
Vulnerable
Segment

7. Ulnar motor Nerve conduction study
Measures the response from a muscle (ADQ)
Response is much larger than sensory
Typically measure several segments, multiple stimulation sites
Calculate NCV between stim sites
wrist
20 cm
10 cm
below elbow
above elbow

8. Ulnar motor Nerve conduction study
Ulnar motor NCS to ADM set up
Commonly used stimulation sites.

9. Ulnar motor Nerve conduction study
Ulnar motor NCS to First Dorsal Int (FDI) set up.
Use same stimulation sites.
Can record together with ADM using 2-channel amplifier

10. Ulnar motor Nerve Conduction Velocity
Latency times of responses
Stim Wrist- 2.9 ms
Stim below elbow- 6.6 ms
Stim above elbow- 9.3 ms
Forearm Velocity
200/( )=54 mm/ms
Elbow Velocity
100/( )=37 mm/ms (m/s)
Segmental slowing at elbow!
wrist
20 cm
10 cm
below elbow
above elbow

11. Ulnar motor responses Reduced amplitude at elbow (conduction block)
Top = wrist stim 2.9ms
Mid = below elbow 6.6ms
Low =above elbow 9.3ms
Reduced amplitude at elbow (conduction block)
Change in shape
Temporal dispersion
Both effects suggest myelin dysfunction
2.9ms
5 mV
6.6ms
4 mV
9.3ms

12. Ulnar Motor Study All three possible changes agree:
Focal slowing of conduction
Conduction block (neurapraxia)
Change is shape of response (wider)
Temporal dispersion
=Nerve entrapment (As good as it gets!)

13. Hand numbness 51 yo R handed F Numbness L D4,5 Weak grasp, pinch
No trauma, nonDM
Motor Summary Table
Site NR
Onset (ms)
O-P Amp (mV)
Site1
Site2
Delta-0 (ms)
Dist (cm)
Vel (m/s)
Left Ulnar FDI Motor (Abd Dig 5/FDI)
Wrist
3.8
5.0
Abd Dig 5/FDI
8.0 21
Below Elbow
7.2
3.5
3.4
18.5 54
Above Elbow
10.6
1.3
10.5 31
Right Ulnar FDI Motor (Abd Dig 5/FDI)
6.0

14. Ulnar conduction block MNCS A good prognostic sign
Anti Sensory Summary Table
Site NR
Peak (ms)
O-P Amp (µV)
Neg Dur (ms)
Site1
Site2
Delta-P (ms)
Dist (cm)
Ulnar Wrist Digit 5
3.7
5.0
1.09
Right Ulnar Anti Sensory (5th Digit)
Wrist
3.0
11.0
1.02
5th Digit
14.0
Friedrich & Robinson. Ulnar prognosis. Muscle Nerve 2011;43:596

15. Median Motor NCS setup

16. Med mot wr, elb

17. Peripheral Nerve Anatomy- Review
What is the physiology of the problem?
Fast conducting
First to stimulate

18. Motor Nerve Conduction
Record muscle fiber action potentials
As a group called Compound Muscle Action Potential (CMAP).
Much larger than SNAP and can interfere with the SNAP (ie, motor artifact).
Isolate a muscle related to nerve of interest.
Record from motor point of muscle (E1).
E2 electrode at distal tendon of muscle.

19. Stimulation
At the brachial plexus, one cannot isolate stimulus to a specific nerve
MUST isolate the recording site
hypothenar works well
Thenar does not

20. NCV Vocabulary: Measurements
Motor
Distal Latency (A)
Amplitude (B)
Baseline-to-peak
Duration (C)
Negative potential
Supra-maximal Stimulation is that which just barely gives full amplitude plus 10%
Note: Some labs use onset latency for sensory, as well as motor testing. A B C A
Sensory B C

21. Motor Nerve Conduction
Can easily do many different sites of stimulation, up to spinal nerves.
Allows accurate segmental testing for focal entrapment.

22. Nerve Stimulation How much current/voltage do I use to stimulate?
The basic language of nerve conduction testing;
Start with a relatively low stimulus, 10mA or 30V; 0.1ms pulse duration
Gradually increase stimulus watching the response.

23. Supra-Maximal Stimulation
Final intensity and duration varies widely depending upon nerve, disease and depth of location.
When response no longer increases with a stimulus 10% larger than the one before, then you are 10% over maximal response and have supramaximal stimulation
Hint: Watching the muscle twitch also gives visual feedback on stimulation.

24. Supramaximal Stimulation
Standard term all NCS.
Maximum refers to response, not stimulus.
Overstim is too common
10 mA
80 mA
mA = milliAmpere
Machine limit is100 mA for 1 ms (1000µs)

25. Supramaximal Stimulation in NCS produces the Maximum response from the nerve without excess stimulation, note also latency
Median motor wrist Stim
gradually increasing stimulus current
to supramaximal stimulation
15 mAmp
20 mAmp
24 mAmp
35 mAmp
39 mAmp
Wrist stimulation
Distal motor latency

26. Electrical Stimulation “Volume Conduction”
Correct intensity of stim Vs
Excessive stim intensity

27. Motor Response Excess Stim
Thenar Eminence
Median nerve
Median nerve
Plus Ulnar
nerve

28. Median motor NCS Numb/painful hands, +DM
Adjust the recording settings to fit the situation
When you see an unusual response, “ask why?”
Chronic reinnervation changing the motor end-plate zone

29. Case Example
CC: Right ankle dorsiflexion weakness with minimal paresthesias
HPI: 87 year-old physician with chronic lumbar pain and h/o prostate cancer, had recent weight loss. He had no history of diabetes or of neuropathy. Prostate had been excised.
PE: Ankle dorsiflexion and eversion of affected right side was 2/5 with normal inversion and knee flexion, and no other significant weakness noted. Minimal sensory loss in the dorsal foot.

30. Case Example-Motor NCS
Nerves
Latency
Amplitude
Distance
Velocity Ms mV Cm
m/s
R Fibular (EDB)
Ankle
6.3
0.2
31.5
Fib Head
15.6 13 34
Knee
No response
R Fibular
(Ant Tib)
3.1
3.5
6.9
0.5

31. First testing-6 weeks after onset Recording with 2-channel amplifier
Ankle stim-EDB
Fibula-EDB
Politeal-EDB (NR)
Fib-Tibialis Ant
Pop-Tibialis Ant
Strength ADF 2/5

32. Case Example-Motor NCS 3 months later
Nerves
Latency
Amplitude
Distance
Velocity Ms mV Cm
m/s
R Fibular (EDB)
Ankle
6.75
0.2 8
Fib Head
22.65 32
20.1
Knee
R Fibular
(Ant Tib)
2.85 4 6
3.1 10
31.7

33. Follow-up testing- 3 months later
Strength ADF 4+/5
Fib-Tibialis Ant
Pop-Tibialis Ant
Improved amplitude with proximal stimulation = improved strength!

34. Fibular Nerve Compression at the Knee-Etiology Crossing Legs
87 yo m
Electrodiagnostician
Acute fibular neuropathy
Ernest W. Johnson, M.D.

35. Fibular (Peroneal) Nerve Compression
“Drop foot”
Weakness of dorsiflexion and eversion
Paresthesia/sensory loss dorsal foot
Exclusions in differential diagnosis
Not CNS, reflexes WNL, Babinski -
Not cauda equina-opposite side normal
No incontinence

36. Fibular Nerve Recording Motor NCS Deep Branch
TibAnt
EDB

37. Accuracy means Reproducibility
Problems of interobserver differences in analysis of NCS responses continues to be seen, among experienced physicians in EDX.
Litchy W, et al. Proficiency of NC. M&N 2014;50:900

38. Kimura

39. F wave: A late response
But not a reflex
Requires only one axon

40. F wave latency is the most reproducible of NCS measures.
Has the fewest chances for us to make error:
eg, measurement, electrode placement, judgement
Pinheiro D. Reproducibility in NCS/F wave. Clin Neurophys 2008;119:2070

41. Median nerve F wave with 10 stimulations
Persistence is 80-90%
Simple wave-forms

42. F wave of median nerve, 16 stimulations What is persistence?
Some subjectivity or at least assumptions needed.
Campbell textbook, median F wave

43. F wave affected by the subject’s action
F wave persistence and amplitude both increase with imagery or actual muscle contraction, 3% MVC or more.

44. F wave affected by the subject’s action
F waves can be extinguished by learned non-use, such as during casting of the joint.

45. F wave Problems
Avoid contamination from inadvertent voluntary activities.
“a latency within a reasonable range for the investigated nerve, excluding spurious voluntary activity.”
Maintain supramaximal stimulation.
Decide about A waves.
How big is an F wave?
20 µV ?
50 µV ?
“If it looks like one to me” vv

46. F wave latency is height-dependent
H. Pan et al. / Clinical Neurophysiology 124 (2013) 183–189 (Kimura’s group in China)

47. F wave latency is height-dependent
Kimura J. EDX Med. 4th ed. 2013

48. Diabetic Neuropathy F wave most sensitive
Peroneal, tibial, and ulnar equally sensitive with abnormalities in 60-70%.
Symptomatic diabetics referred for EMG.
Tibial
A waves 28%
3% control
Persistence did not change with diabetes, stayed at 100% for tibial.

49. F wave Persistence % F wave/#stimulations
Study F min amp 50 µV
Listed here in order of nerve’s persistence
Peroneal (fibular) nerve may be the least useful.
Recording tibial most useful.
Peroneal ankle
60.5 ± (8)
Median wrist (Lower limit)
92.5 ± (77)
Ulnar wrist
97.7 ± (87)
Tibial ankle
100.0 ± (100)
H. Pan et al. / Clinical Neurophysiology 124 (2013) 183–189 (Kimura’s group in China)

50. Tibial F wave in diabetic 36 yF
All motor and sensory NCS are normal in both lower limbs.
Are the complex F waves (are some A waves?) indicative of neuropathy?

51. “A wave” shown preceding F waves Case is CMT2 Fastest F wave latency remains normal.

52. “A” waves can occur before or after the F wave: It depends upon axon conduction time

53. Chronic Neuropathy – CIDP Multi-focal myelin disruption
F wave Responses
Chronic Neuropathy – CIDP
Multi-focal myelin disruption

54. Case 1: s/p gbs, 4 yrs

55. Case 2: Type 2 DM

56. Case 3: Type 2 DM, 46yo M with h/o DMII x10yrs, poorly controlled, with paresthesias in the bilateral feet extending to the proximal calves x6mo. Denies LBP, weakness,
sensory changes in the hands.

57. Case 3: Type 2 DM46yo M with h/o DMII x10yrs, poorly controlled, with paresthesias in
the bilateral feet extending to the proximal calves x6mo. Denies LBP, weakness, sensory changes in the hands.

58. Tarsal Tunnel Syndrome
Tibial nerve compression at medial malleolus is relatively rare
NOT analogous to CTS
Anatomy of tendons and ligaments is very different
Associated with altered anatomy of foot, usually following trauma

59. Tarsal Tunnel Syndrome
EDX
Motor latency –
medial plantar n < 6 ms;
lateral plantar n - < 6.5 ms
If Medial Plantar comes within 0.5 ms of lateral plantar latency suspect medial plantar entrapment

60. Lateral plantar nerve entrapment
More frequently seen in diabetic peripheral neuropathy
CNAP will be reduced or absent with stimulation at foot sole (Lat Pl N)
Needle EMG abnormalities in abd dig V ped and lateral interosseus muscles

61. Tibial nerve Motor study
Abd Hall
Medial Plantar
Abd Dig Min
Lateral Plantar

62. Tibial Nerve Motor NCS

63. Nerve Conduction demonstrates Stabilization of diabetic neuropathy with treatment
Fig. 1. Changes in MCV in the median nerve (A) and tibial nerve (B) from baseline levels in
each group over 2 years. Values are mean±S.E.M. **Pb.01 between
groups using unpaired t test. Black squares, EP group (n=38); gray circles, C group (n=36).
Kawai. Journal of Diabetes and Its Complications 24 (2010) 424–432

64. References
Andersen. F-Wave Studies in Diabetes Mellitus. Muscle Nerve 20: 1296–1302, 1997
Campbell WW. Essentials of EDX Medicine. 2nd Ed
Friedrich. Ulnar prognosis. Muscle Nerve 2011;43:596
Litchy W, et al. Proficiency of NC. Muscle Nerve 2014; 50:900
Pinheiro D. Reproducibility in NCS/F wave. Clin Neurophys 2008;119:2070
Robinson L. EDX and outcome. Muscle Nerve 2015;52:321