This document discusses electromyography (EMG) and nerve conduction studies (NCS), which are electrodiagnostic tests that examine nerves and muscles. EMG uses needle electrodes to record muscle activity at rest and during contraction, providing information about motor unit configuration. NCS stimulates peripheral nerves and records muscle and sensory responses, assessing latency, amplitude, and conduction velocity to evaluate for axonal loss or demyelination. Together, EMG and NCS can localize neuropathy or radiculopathy, determine chronicity and severity, and differentiate neuromuscular disorders like amyotrophic lateral sclerosis or myasthenia gravis. While valuable for diagnosing focal neuropathies, they have limitations for central, musculoskeletal

1. Electromyography (EMG) and Nerve
Conduction Studies (NCS)
Dr. Preeti Ghodge (M.P.T)

2. Electromyography (EMG) and Nerve
Conduction Studies (NCS)

3. Electro-diagnostic testing
Components - examination of nerves and muscles:
 Needle electromyography (EMG)
 Nerve conduction studies (NCS)

4. Indications
Suspected neuromuscular disease:
• Nerve root pathology
• Peripheral nerve/plexus pathology
• Neuromuscular junction pathology
• Muscle pathology

5. EMG/NCS Diagnose
• Neuromuscular Diseases
• It can differentiate:
Normal
Radiculopathy
Plexopathy
Neuropathy(focal , peripheral/poly)
Myopathy
Widespread denervation(MND)
Disorder of neuromuscular transmission(MG)
• It can rule out other diagnoses

6. Value
• Time course of disease
Acute vs. chronic vs. acute & chronic
• Anatomical location of pathology
Root
Plexus
Nerve(axonal,demyelinating)
Neuromuscular junction
Muscle
Specific sites
• Distribution
Polyradiculopathy
Trunk, Cord
Mono vs. Multiplex vs. distal symmetric
• Severity of disease

7. Limitations :
• Generally not helpful in evaluation/diagnosis of:
 Pain from joint disease
 Fibromyalgia or myofascial pain syndromes
 Central nervous system disorders
 Disorders that do not arise from the neuromuscular system

8. Electromyography
• Recording and analyzing the action potential of a muscle.

9. Electromyography Prerequisites
 Prior clinical examination
 Rough plan of test
 CPK levels
 Hemophilia & bleeding disorders
 Large muscles

10. Needle Electromyography
• Needle electrode is inserted into muscle
Needle is disposable, single use
Concentric (both recording and reference in same
electrode)
• Multiple muscles are accessible for examination
• Combination of muscles tested
Dependent upon clinical question
• Level of discomfort is mild

11. Needle Electromyography: Data
• Insertional Activity
• Spontaneous Activity
• Interference Pattern

12. EMG:
Spontaneous Activity
10 msec/div, timebase
2MV/vertical segment
Positive Sharp Waves Fasciculation Potential
Fibrillation Potential
Neurogenic Motor Unit Motor Unit Changes

13. EMG data

14. Muscle is studied at rest & at different levels of
sustained, voluntary contraction
• At rest , the muscle should be silent –any spontaneous activity
may signal a nerve abnormality
• During activity, the electrical shape and pattern of the response
can distinguish between nerve and muscle disease

15. Parameters evaluated
• Motor Unit Configuration :
Muscle is volitionally activated at different force levels
Single motor units are assessed (motor unit action potential
or MUAP-reflecting activity of all muscle fibers innervated by
single motor axon)
Assess amplitude,duration,morphology
Essentially three patterns:
Normal
Neuropathic (high amplitude,long duration,polyphasic)
Myopathic (low amplitude,short duration,polyphasic

16. Motor Unit Recruitment and interference pattern:
• Normal: graded recruitment of initially small then larger
MUAPs, full interference pattern
• Reduced Recruitment: large MUAPs at high firing rates,
incomplete interference pattern .seen with neuropathic
conditions
• Increased (early) Recruitment .Seen in myopathy
• Dependent on patients co-operation and effort

17. Components of an Electro-diagnostic Machine

18. Gain – Sweep representation
Duration (ms) x axis
Latency
(mV)
y
axis
Gain = Latency
Sweep = Duration

19. Nerve Conduction Studies
• Peripheral nerves stimulated with controlled electrical stimulus
(supra-maximal)
Generally useful for distal/extremity nerve only e.g.
median/ulnar/radial in UL, tibial/peroneal/sural in LL
More proximal nerves can be tested in specific situations
• Current depolarises nerve membrane & initiates action potential in
axon
When all possible axons in nerve depolarised,get “supra-
maximal” response
• Action potential travels along nerve
Speed on conduction depends on integrity of “insulating”
myelin membrane to allow saltatory conduction
Conduction along non-myelinated fibers much slower

21. Motor nerve

22. Propogation of action potential

23. Nerve Conduction Studies
• Responses are recorded
Motor studies: Compound motor action Potential (CMAP)
Sensory studies: Sensory nerve action potential(SNAP)
F-wave
H-reflex
• Assess large myelinated fibers only
not useful for small fiber disorders
• Assess for:
Axonal processes (axon loss, Wallerian degeneration)
Focal demylinating processes (slowing,conduction block)

24. Motor NCS Parameters
 Distal Motor Latency (DML)
• Time from stimulation to onset of muscle contraction
• Determined by conduction velocity of the nerve , neuromuscular
junction &muscle.
• Prolonged in demyelinating neuropathies, with compression
 Amplitude
• Determined by number of muscle fibers activated
• Varies with stimulus intensity ,impedance,skin temperature
• Reduced with axonal neuropathies, drop from distal to proximal
site (conduction block) in demyelinating
 Proximal conduction velocity (CV)
• Determined by conduction velocity of the fastest fibers
• Derived from ditance between distal & proximal site divided by
latency difference
• Marked reduced in demyelinating neuropathies

25. Normal Median Motor study

27. Worked motor study
• Initial stimulation of median nerve at
wrist
• Sub maximal stimulation gives small
CMAP
 Not all axons depolarised & so
not all motor units activated.

29. Worked motor study
 Supra-maximal stimulation gives
“best” amplitude CMAP possible
 Amplitude
• Marked of number of motor units
activated & thus how many axons
activated
• Reduction suggests axonal loss
 Distal latency
• Reflects time for action potential
to travel down segment of nerve
plus time for neuromuscular
transmission
• Any prolongation suggests
problem in this segment of nerve.

31. Worked motor study
• More proximal stimulation
• Look for:
• Any change in morphology
• Any drop in amplitude (“conduction
block”) which reflects demyelination or
other focal process affecting nerve across
the segment studied
• Can calculate conduction velocity from
distance (measured from active stimulus
point/cathode site in cm or mm) divided by
time (latency difference between the two
stimulus sites)

32. Worked motor study
• The report usually presents the relevant figures
Distal latency
Distal amplitude
Velocity across segments
Proximal latency (compared with distal)
Nerve / Sites Latency
ms
Amp
mV
Velocity
m/s
R MEDIAN -APB
Wrist 3.50 6.8
Elbow 7.30 6.2 59.2

33. Sensory nerve

34. Pre and post-ganglionic lesions

36. Sensory NCS Parameters
 Onset and Peak latencies
 Conduction velocity
• Determined by velocity of a very few fast fibers
 Amplitude
• Determined by the number of large (myelinated) sensory
fibers activated

37. Normal Median Sensory Study

39. Worked sensory study
• Generally only single stimulus site
• Can be recorded
orthodromically(same way as nerve
conduction) or antidromically
•Much smaller than motor
responses(micro vs. millivolts) & so
more prone to artifact

41. Worked sensory study
• Record
-Amplitude(again marker of number of axons)
-Latency
-Conduction velocity

42. Late Responses
• F-Wave Latency
Retrograde “rebound” motor impulse, travels full length of
motor axon and back
Information about proximal nerve segments
Limited sensitivity/specificity
Helpful in the evaluation of radiculopathy, GBS, other
demyelinating neuropathies, peripheral neuropathy

43. H-Reflex
• Follows muscle stretch reflex arc
Afferent path= sensory Ia fibers
Efferent path= alpha motor neurons
• Side to side latency most valuable
• Helpful in the evaluation of polyneuropathy,S1 radiculopathy,
upper motor neuron lesions

44. F Waves: Normal Median

45. Algorithm for each nerve

46. Putting it together

47. Value of NCS
• Define & localise focal neuropathies
Especially carpal tunnel syndrome, ulnar neuropathies,wrist
and foot drop
• Define severity & pathophysiology of peripheral neuropathies
Axonal
Demyelinating

48. Summary
• Confirms provisional diagnosis
• Efficacy of intervention
• Study the prognosis of the disease

49. • Electro-physiological Studies enhances the
Diagnostic Value and Prognostic ability

50. THANK YOU..