CARPAL TUNNEL SYNDROME
Nerve Conduction Study
www.teleemg.com
Katie Doney – kdoney@med.usyd.edu.au
Motor Pathways
Muscle
CNS
Motor Neuron
Ventral Horn
Motor Pathways
Possible places of dysfunction
1. CNS
1. EEG
Electroencephalography
2. PET
Positron Emission Tomography
3. fMRI
Functional Magnetic Resonance
Imaging
Motor Pathways
Possible places of dysfunction
2.
Muscle
Needle EMG
Electromyography
1. CNS
1. EEG
Electroencephalography
2. PET
Positron Emission Tomography
3. fMRI
Functional Magnetic Resonance
Imaging
Motor Pathways
Possible places of dysfunction
2.
Muscle
Needle EMG
Electromyography
1. CNS
1. EEG
Electroencephalography
2. PET
Positron Emission Tomography
3. fMRI
Functional Magnetic Resonance
Imaging
3.
Peripheral Nerve
Nerve
conduction study
Peripheral Nerve
• Made up of:
- Sensory nerves
- Motor nerves
- Myelin
- Glial cells
MEDIAN
ULNAR
Median Nerve Pathology
in Carpal Tunnel Syndrome
• Compression of the median nerve results in
Demyelination
Axonal
degeneration
• These manifest as:
Decreased
blood supply
- Numbness
- Pain
- Parasthesiae
- Weakness
- Problems with fine manipulative skills
Nerve Conduction Study
• Stimulate nerve and record outcome.
• 2 main types
Least
sensitive 1. Motor (record compound muscle
action potential)
2. Sensory (record compound sensory
action potential)
-Orthodromic (stimulate at finger, record on elbow/wrist)
Most
sensitive
-Antidromic (stimulate at elbow/wrist, record on finger)
-Radial-median (stimulate at wrist, record on thumb)
-Palmar (stimulate at palm, record on elbow/wrist)
Changes to CMAP in Carpal
Tunnel Syndrome
-Latency: Normal < 4.9 ms
Pathology: - demyelination
-Amplitude: Normal ≥ 5 mV
- axonal degeneration
-Shape: Normal curve
Response
amplitude
Time
Nerve Conduction Study
• Measures Latency and Amplitude
– Latency is the time between the artefact and
the initiation of the compound action potential
– The artefact occurs when you press the button
to stimulate the electrode
– Latency increases pathologically due to:
– Axonal degeneration
– Demyelination
– Latency increases non-pathologically due to:
– Length of axon
Nerve Conduction Study
• Measures Latency and Amplitude
– The amplitude of the curve shows the strength
of the compound action potential
– The area under the curve decreases
pathologically due to:
– Number of axons involved
» Diameter of nerve
– The area under the curve decreases nonpathologically due to:
– Strength of initial stimulus
Nerve Conduction Study
• Motor Nerve Conduction Study Setup:
– Stimulating electrode to set up action
potentials in median nerve from wrist or
elbow
– Recording electrode on abductor pollicus
brevis to record compound muscle action
potential
– Techniques to increase effectiveness of
stimulation
– Earth
Conduction Velocity
• Importance: Eliminating wrong localisation
of problem
• Method:
r
sw
1
Latency
wrist
r
se
2
Latency
elbow
Conduction Velocity
• Calculations:
• Method:
Velocity =
Distance = Distance
Time
Lelbow – Lwrist
r
sw
1
Distance
Latency wrist
r
se
2
Latency elbow
Conduction Velocity
• Normal value > 50 m/s
• Other sources of error
– stimulus position
- estimation of nerve course
- latency measurement
Problems with comparing features
to normal values
• Features change with
– Temperature
– Age
Latency
CV
Age
20
80
Age
• Solution: Repeat measurements on ulnar nerve as a control.
This also gives additional evidence to rule out generalised
neuropathy.
Compound Antidromic Sensory
Action Potential
Setup:
- stimulate at wrist/elbow and record at
finger
- gain set higher
- signal very small so average random
noise
• Normal values:
MEDIAN SENSORY (ANTIDROMIC)
ULNAR SENSORY (ANTIDROMIC)
- Latency < 3.6 ms
- Velocity > 50 m/s
- Amplitude > 15 mV
- Latency < 3.1 ms
- Velocity > 54 m/s
- Amplitude > 10 mV
An example:
Right Antidromic Ulnar
Normal:
-Lat < 3.1 ms
-Amp > 10 uV
…. Ulnar OK – not generalised neuropathy
An example:
Right Antidromic Median
Normal:
-Lat <3.6 ms
-Amp >15 uV
-CV >50 m/s
…. In median nerve, evidence of demyelination and axonal
degeneration between wrist and finger, but not between wrist
and elbow. Suggestive of carpal tunnel syndrome.
An example:
Right Thenar Median
Normal:
- Lat < 4.9 ms
- Amp ≥ 5 mV
- CV > 50 m/s
…. Motor portion of median is OK. Mild to moderate
carpal tunnel syndrome.
Take home message!
• In carpal tunnel syndrome there will be a
longer latency between the wrist and the
finger/thenar eminence but NOT between
the elbow and the wrist
• The ulnar nerve will not be effected in
carpal tunnel syndrome because it does
not pass through the carpal tunnel