1. CHAPTER 6 -
THE ELECTRODIAGNOSTIC
EXAMINATION OF THE SPINAL CORD

2. I. Brief review of the spinal cord anatomy
II. Electrodiagnostic examination
A. Nerve conduction studies
A. Motor NCS
B. Sensory NCS
C. Mixed NCS
B. Needle electrode examination
C. Special tests
III. Pathophysiology
A. EDX changes with spinal cord disorders
B. Coeexisting Spinal Cord and Peripheral Nerve Lesions
C. Specific Spinal Cord disorders
OUTLINE

3. I. SPINAL CORD ANATOMY

4. I. SPINAL CORD ANATOMY

5.  The basic EDX:
A. Nerve conduction study (NCS) – motor, sensory and mixed
B. Needle electrode examination (NEE)
C. Special studies – F-wave, H-reflex
II. THE ELECTRODIAGNOSTIC
EXAMINATION (EDX)

6.  1. Motor NCS
 A mixed nerve or a “pure” motor
nerve is stimulated at one or two
points along its course while the
resulting response generated by a
muscle it innervates is recorded,
preferably with surface electrodes
 Compound muscle action potential
(CMAP) – a function not only of the
motor nerve fibers stimulated, but
also of the muscle fibers that
produced it, and of the intervening
neuromuscular junctions
 Thus, it assesses motor nerve
fibers only indirectly
A. NERVE CONDUCTION STUDIES

7. 2. Sensory NCS
Directly assesses the sensory
component of mixed nerves,
or “pure” sensory nerves
Sensory nerve action
potentials (SNAP) is indicative
of an abnormality of the
peripheral sensory fibers or of
the DRGs from which they
derive
No magnification effect =
microvolts
A. NERVE CONDUCTION STUDIES

8.  Several measurable components of an NCS
 Amplitude
 Duration
 Latency
 Conduction velocity
A. NERVE CONDUCTION STUDIES

9.  Amplitude
 Height of the response
 Measured from baseline to
peak OR peak-peak
 Indicator of:
 the number of axons capable of being
stimulated
 Conducting impulses between the
stimulating and recording points
 Relative rate of conduction along those
axons (together with the duration)
 It conveys information regarding
the status of neuromuscular
transmission and of the muscle
fibers of the recorded muscles
(in motor NCS)
 Recorded in millivolts (CMAPs)
or microvolts (SNAPs)
A. NERVE CONDUCTION STUDIES

10.  Duration:
 Time period between the onset and termination of the response
 Measured in milliseconds
 Indicator of the relative rates of conduction
 Related to amplitude – as the duration increased, the amplitude
must decrease if the area under the curve is to remain constant
A. NERVE CONDUCTION STUDIES

11.  Distal or Peak Latency
 A RATE measurement
 Determined from the moment of stimulation to either the onset of
the response (distal latency) or to the peak of response (peak
latency)
A. NERVE CONDUCTION STUDIES

12.  Conduction Velocity
 A RATE measurement
 Obtained by stimulating the nerve at two points along its course,
subtracting the latency elicited on distal stimulation (distal latency)
from that obtained on proximal stimulation (proximal latency), then
dividing the difference (in ms) into the distance (in mm) between the
two stimulation points
 Measured in meters per second
A. NERVE CONDUCTION STUDIES

13.  Essentially assesses the entire motor unit from the AHCs to
the muscle fibers peripherally
 Three phases:
 Insertion
 Rest phase
 MUAP activation
B. NEEDLE ELECTRODE EXAMINATION
(NEE)

14.  Insertion Phase
 Refers not only to the single instance in which the needle
recording electrode is thrust through the skin into a particular
muscle, but also to each of the several times it is advanced
within that muscle
B. NEEDLE ELECTRODE EXAMINATION
(NEE)

15.  At-Rest Phase
 The needle is held fixed in a muscle that is not being
contracted
 Various types of abnormal potentials can be observed,
collectively called spontaneous activities
 Fibrillation potentials
 Most commonly seen
 Non-specific in nature
 Produced by a single denervated muscle fibers
 Fasciculation potentials
 Generated by motor units or portions of motor units
 Larger than the typical fibrillation potentials
 Motor unit must be intact to generate fasciculation potentials
 Evidence of motor unit irritation (versus motor unit disintegration)
 Fire irregularly
B. NEEDLE ELECTRODE EXAMINATION
(NEE)

16.  At-Rest Phase
 Complex repetitive discharges
 Caused by repetitive firing of a group of muscle fibers, two of which
serve as primary and secondary pace makers, respectively
 Non-specific
 Rarely seen with any disorder of less than 6 months duration
B. NEEDLE ELECTRODE EXAMINATION
(NEE)

17.  Motor Unit Action Potential Activation Phase
 The patient voluntarily contracts the muscle in which the
needle recording electrode has been inserted
 2 MUAP firing patterns
 Reduced MUAP recruitment
 A significant number of motor units in the muscle being assessed by NEE do not fire on
attempted AHC activation
 Unequivocal evidence of LMN lesion, and its severity has high correlation with the clinical
weakness of the recorded muscle
 If the MUAPs are firing in substantially decreased numbers at 25 to 35 Hx, the
muscle will appear weak on clinical examination
 If only one or two MUAPs fire on maximal activation at faster than 10 Hz, the
muscle will essentially be paralyzed
 Incomplete MUAP activation
 Seen whenever the patient is requested to vigourously contract the muscle and the MUAPs
fire in reduced numbers, but at a slow-to-moderate rate
 Not a sign of LMN lesion – can be from pain, conversion reaction, malingering
B. NEEDLE ELECTRODE EXAMINATION
(NEE)

18.  F-waves
 Caused by stimulation-induced impulses traveling anti-dromically
along motor axons, causing the AHCs to backfire and thereby sending
impulses back down the motor axons to the recorded muscle
 Assess solely motor axons and their parent AHC within the spinal
cord
 Elicited by supramaximal stimulation
 H-reflexes
 A spinal monosynaptic reflex initiated by nerve stimulation
 Elicited by submaximal stimulation of a mixed nerve
 Very sensitive in two PNS disorders: S1 radiculopathy and generalized
polyneuropathies
C. SPECIAL STUDIES

19.  The large myelinated axons assessed by EDX have only two
reactions:
 Axon degeneration (axon loss)
 Nerve fibers are interrupted at the lesion site and the entire distal portion
of the nerve fibers undergo Wallerian Degeneration
 Focal demyelination
 The only alteration to the nerve fiber is that the myelin at the lesion site is
damaged to varying degrees
 Portion distal to the point of the lesion are intact and conduction is
normal in all respects distal to the focal demyelinating injury
III. PATHOPHYSIOLOGY

20.  Superior to the level of the lesion
 Generally all aspects of the EDX examination are NORMAL
 At the level of the lesion
 Motor axons
 CMAPs are low in amplitude
 Reduced MUAP recruitment with chronic neurogenic changes
 Sensory axons
 SNAPs are low in amplitude, unelicitable, normal in SCD that affect AHCs
only
 Inferior to the level of the lesion
 Motor NCS – normal
 Sensory NCS – normal
 NEE – abnormal MUAP activation
 Fibrillation potentials – should not be observed
ELECTRODIAGNOSTIC CHANGES WITH
SPINAL CORD INJURIES

21.  These may involve nerve fibers derived from spinal cord
segments situated rostral or caudal to the involved segments,
or from the damage segments themselves
 Superior to an SCD:
 Crutch palsy – infraclavicular plexopathy caused by excessive
pressure placed on terminal nerves in the axilla as the result of
crutches that are ill fitted or used improperly
 Ulnar neuropathies
 Carpal tunnel syndrome – wheelchair users
 At the level of and SCD
 Trauma, ischemia
 Caudal to the SCD
 Traction and compression
 Most commonly involved: ulnar nerve along the elbow in quadriplegic
patients and common peroneal nerve at the fibular head in both
quad and para patients
COEXISTING SPINAL CORD AND
PERIPHERAL NERVE LESIONS

22.  Anterior Horn Cell Diseases
 Poliomyelitis and Post-polio syndrome
 Acute Polio
 SNAPS are normal
 CMAPS are normal, low in amplitude or unelicitable depending on the segment involved and
the muscle being elicited
 NEE – fibrillation potentials and reduced MUAP recruitment are noted in muscles innervated
by the involved segments
 MUAPs are normal in configuration because there are no collateral sprouting yet
 Remote poliomyelitis (at least 12 months duration)
 SNAPs are normal
 Unelicitable or very low amplitude CMAPs
 May be normal – indicative of the total number of muscle fibers in the
recorded muscle responding to the stimulus rather than the the total number
of nerve fibers supplying the recorded muscles  reflects the substantial
amount of collateral sprouting
 NEE – fibrillation and fasciulation may be seen
 Post-polio syndrome
 No distinctive EDX changes (same as in remote polio)
SOME SPECIFIC SPINAL CORD
DISORDERS

23.  Anterior Horn Cell Diseases
 Amyotrophic Lateral Sclerosis
 Characteristically manifests UMN as well as LMN changes
 Often begins in the C8-T1 segments (unilateral or bilateral painless hand
wasting) or L5 spinal cord segment (painless foot drop)
 SNAPS are normal for age
 CMAPS are normal or abnormal depending on whether the appropriate
segments are involved and the severity of axon loss)
 NEE: goal is to find widespread fibrillation potentials and chronic
neurogenic MUAP changes with at least some fasciculation potentials
SOME SPECIFIC SPINAL CORD
DISORDERS

24.  Anterior Horn Cell Diseases
 Kennedy’s Disease
 Sex-linked recessive AHC disorder involving sensory and motor neurons in
both bulbar and spinal cord segments
 NCS suggests pure sensory polyneuropathy or neuronopathy
 NEE abnormalities indicative of very chronic AHC disease (prominent
MUAP changes with sparse numbers of fibrillation potentials)
SOME SPECIFIC SPINAL CORD
DISORDERS

25.  Spinal Cord Injuries
 The EDX examination has a limited role in the initial assessment of
these injuries
 Can be helpful in demonstrating the presence or absence of
coexisting plexopathies
SOME SPECIFIC SPINAL CORD
DISORDERS

26.  Intraspinal Canal Neoplasms
 Radicular pain is the common reason for referral for EDX
examination
 EDX findings typically are nonspecific in regard to etiology but their
presence frequently initiates the appropriate neuroimaging studies
SOME SPECIFIC SPINAL CORD
DISORDERS

27.  There are 3 basic parts of the EDX
 NCS
 NEE
 Special tests
 EDX can differentiate a spinal cord injurry and a peripheral
nerve lesion
 EDX examination can help differentiate or confirm a pathology
SUMMARY

28. FIN
CHAPTER 6 -
THE ELECTRODIAGNOSTIC
EXAMINATION OF THE SPINAL CORD