Electomyography basics , with neurogenic and myopathic potential

1. EMGs and the
Electrodiagnostic Evaluation for
the Internist
The American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM)
Marketing Committee has coordinated this presentation as part of an AANEM initiative to
increase awareness of electrodiagnostic medicine and the disorders it helps to diagnose.

2. Outline
• What is an electrodiagnostic evaluation?
• Is an EMG study the same as an
electrodiagnostic evaluation?
• Who is qualified to be an electrodiagnostic
practitioner?
• What is the role and value of nerve conduction
studies (NCSs) and needle electromyography
(EMG)?
• When should I suspect a neuromuscular
disorder?

3. Outline
• What conditions are commonly
evaluated/diagnosed by NCSs/EMG?
• What are the technical details of these
studies?
• What are some limitations of EMG
studies?
• What can I expect from an EMG report?

4. What is an electrodiagnostic
evaluation?
• Similar format to a medical consultation
• Focused neuromusculoskeletal history and
physical examination
• Development of a differential diagnosis
• Examination of muscles and nerves using
nerve conduction studies (NCSs) and
needle electromyography (EMG)
• Formulation of a final diagnosis

5. Is an EMG study the same as an
electrodiagnostic evaluation?
• EMG is often used to mean the entire
spectrum of electrodiagnosis of nerve and
muscle disease
• Strictly speaking, an EMG refers to the
needle or surface examination of the
bioelectrical activity of muscle, which is one
component of the electrodiagnostic
consultation.

6. Who is qualified to be an
Electrodiagnostic Practitioner?
• Physicians trained in electrodiagnostic medicine
– MDs who receive specialized training in the diagnosis
and treatment of neuromuscular and neurological
diseases, and the application of particular
neurophysiologic techniques to the study of these
disorders
• Generally - Neurologists or Physical Medicine
and Rehabilitation (PM&R) physicians
American Association of Neuromuscular & Electrodiagnostic Medicine Position Statement

7. Electrodiagnostic Practitioners
• NCSs may be performed by trained non-
physicians under supervision of
electrodiagnostic medicine physicians
• Needle EMG component should be
performed by an electrodiagnostic
medicine physician
American Association of Neuromuscular & Electrodiagnostic Medicine Position Statement

8. What are the indications for
electrodiagnostic
evaluation/testing?
• Suspected neuromuscular disease
– nerve root pathology
– peripheral nerve/plexus pathology
– neuromuscular junction pathology
– muscle pathology

9. What is the value of NCSs/EMG?
• Confirm the clinical impression of a
neuromuscular disorder
• Rule out certain diagnoses
• Enhance patient care

10. Value of NCSs/EMG
• When neuromuscular disease is present,
electrodiagnostic testing can:
• Clarify the type of pathology (i.e.
neuropathy vs myopathy)
• Determine severity & extent of pathology
• Confirm site of pathology
• Estimate chronicity of pathology

11. Complaints Suggestive of
Neuromuscular Pathology
• Numbness or Tingling
• Decreased Sensation
• Pain or Cramping
• Weakness
• Gait difficulty
• Fatigue

12. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Generalized Neuropathies
• Axonal
• Demyelinating
– Acquired
» Acute: GBS
» Chronic: CIDP
– Hereditary
• Mixed
– Diabetic sensorimotor neuropathy

13. Polyneuropathies
• Polyneuropathies associated with many
medical conditions
• Multiple investigations often needed
• NCSs/EMGs: best initial test to clarify
underlying pathophysiology (i.e., axonal vs
demyelination)
• Results may help focus rest of work-up

14. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Mononeuropathy multiplex
– Vasculitic/ischemic neuropathies
– Demyelinating neuropathies
– Infectious neuropathies
– Neoplastic infiltration
– Granulomatous infiltration
– Compression neuropathy

15. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Focal Neuropathies
– Carpal Tunnel Syndrome (median
neuropathy at the wrist)
– Ulnar Neuropathy
– Peroneal Nerve Palsy
– Others: brachial plexus lesions, tarsal
tunnel syndrome, etc.

16. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Radiculopathy
– Cervical
– Lumbar
• Motor Neuron Disease
– Amyotrophic lateral sclerosis (ALS)
– Spinal muscular atrophy (SMA)

17. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Muscle Disease
– Inflammatory
• Polymyositis, Dermatomyositis
– Metabolic
– Hereditary or Congenital

18. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Neuromuscular Junction Disease
– Myasthenia Gravis
– Lambert Eaton Myasthenic Syndrome
– Botulism
– Medications

19. Disorders Diagnosed/Evaluated
by NCSs/EMG
• Generalized weakness in the critical care setting
• Acute/unexplained onset of respiratory failure
• Neuromuscular cause for failure to wean from
mechanical ventilation
• Neuromuscular diseases unique to critical care
setting
– Critical illness neuropathy/myopathy

20. Disorders Evaluated/Diagnosed
by NCSs/EMG
• Specialized electrodiagnostic expertise
can be useful in evaluation of:
– Ocular muscle weakness
– Speech difficulties due to weakness of
laryngeal muscles
– Disorders of movement and tone from central
nervous system disorders

21. Nerve Conduction Studies (NCSs)
Technical Information
• Peripheral nerves are stimulated with an
controlled electrical stimulus
• Responses recorded
– Compound motor action potential (CMAP)
– Sensory nerve action potential (SNAP)
– F wave
– H- reflex

22. Motor NCS Parameters
• Distal Latency
– determined by conduction velocity of the
nerve, neuromuscular junction & muscle
• Amplitude
– determined by number of muscle fibers
activated
• Proximal conduction velocity
– determined by conduction velocity of the
fastest fibers

23. Normal Median Motor Study
DL CV Amp
(msec) (m/s) (mV)
Wrist-APB 3.2 15.0
Elbow-Wrist 55 14.8

24. 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
sensory fibers activated

25. Normal Median Sensory Study
1 msec/div
Latency CV Amp
(msec) (m/s) (uV)
Wrist-D2 2.2 58 44.1

26. F Waves
• Useful to assess proximal nerve
conduction
• Helpful in the evaluation of:
– Radiculopathy
– Guillian-Barre Syndrome
– Peripheral neuropathy
– Other demyelinating neuropathies

27. H Reflexes
• Useful to assess proximal nerve
conduction
• Helpful in the evaluation of:
– Polyneuropathy
– S1 radiculopathy
– Upper Motor Neuron lesions

28. Neuromuscular Junction Testing
• Repetitive Nerve Stimulation
– Stimulate nerve with train of supramaximal
stimuli before and after exercise
– Record from muscle
– Attention to technical factors important
– More sensitive recording from proximal
muscles

29. Repetitive Nerve Stimulation:
Normal
3Hz stimulation

30. Repetitive Nerve Stimulation:
Myasthenia Gravis
3 Hz stimulation rate

31. Technical Details of Needle
EMG
• A needle electrode is placed into the
muscle
– needle is sterile and disposable
• Muscles examined depends on the clinical
question and may be modified depending
on NCS findings
• Detailed knowledge of anatomy is
necessary to identify specific locations

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

33. Spontaneous Muscle Electrical
Activity
• Examples:
– Fibrillation potential/positive waves
• indicates loss of muscle-nerve connection
• provides information about the chronicity of the
problem
– Fasciculation
• spontaneous motor unit potential, may indicate
irritability in the motor nerve cell

34. Fibrillation Potentials and
Positive Sharp Waves
Fibrillation Potentials Positive Sharp Waves

35. Fasiculations

36. EMG - Motor Unit Potentials
• Evaluated during early recruitment
• Morphologic parameters studied
– Amplitude
– Duration
– Phases

37. EMG - Recruitment
• Recruitment is the pattern of motor unit
firing when a muscle contracts
– Reduced Recruitment - Neuropathy
– Increased (early) Recruitment - Myopathy
– Dependent on patient cooperation and effort

38. Normal Recruitment Pattern

39. Specialized Testing
• Interference pattern analysis
• Quantitative motor unit analysis
• Single fiber analysis
• Segmentation studies
• Cranial nerve testing
• Brainstem and somatosensory evoked
potentials
• Pelvic Floor and respiratory muscles

40. Limitations of NCSs/EMG
• Generally not helpful in the
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

41. What to Expect From an
EMG Report
• A clinically and physiologically relevant
interpretation/diagnosis
• An outline of the localization, severity, and
acuity of the process
• Notation of other diagnoses that are
detected/excluded
• Explanation of any technical problems

42. What to Expect From an
EMG Report
• The reason for the referral is addressed
• Pertinent information that may affect
management is provided
– Need for re-evaluation in the future
– Urgent need for medical intervention

43. What to Expect From an
EMG Report
• Data obtained during the study: (NCS)
– Amplitude
– Distal latency
– Distance
– Conduction velocity
– Normal (Reference) data
– Side-to-side comparison (when appropriate)
– Limb temperature during the study

44. What to Expect From an
EMG Report
• Data obtained during the study: (EMG)
– Presence & type of abnormal spontaneous
activity
– Motor unit recruitment
– Motor unit morphology

45. EMG “Pearls”
• Electrodiagnostic studies are a
supplement to, and not a replacement, for
the history and physical examination
• Electrodiagnostic results are often time-
dependent
• Electrodiagnostic studies are not
“standardized” investigations and may be
modified by the practitioner to answer the
diagnostic question

46. American Association of
Neuromuscular & Electrodiagnostic
Medicine
The AANEM is a nonprofit organization dedicated to the advancement of
neuromuscular, musculoskeletal, and electrodiagnostic medicine.
For more information about the AANEM and the benefits of
membership:
Call (507) 288-0100
or
Visit www.aanem.org