1. CARPAL TUNNEL SYNDROME Nerve Conduction Study
Katie Doney –

2. Motor Pathways
Ventral Horn
CNS
Motor Neuron
Muscle

3. Motor Pathways Possible places of dysfunction
CNS
EEG Electroencephalography
PET Positron Emission Tomography
fMRI Functional Magnetic Resonance Imaging

4. Motor Pathways Possible places of dysfunction
2. Muscle
Needle EMG Electromyography
CNS
EEG Electroencephalography
PET Positron Emission Tomography
fMRI Functional Magnetic Resonance Imaging

5. Motor Pathways Possible places of dysfunction
2. Muscle
Needle EMG Electromyography
CNS
EEG Electroencephalography
PET Positron Emission Tomography
fMRI Functional Magnetic Resonance Imaging
3. Peripheral Nerve
Nerve conduction study

6. Peripheral Nerve MEDIAN ULNAR Made up of: Sensory nerves Motor nerves
Myelin
Glial cells
MEDIAN
ULNAR

7. Median Nerve Pathology in Carpal Tunnel Syndrome
Compression of the median nerve results in
Demyelination
Axonal degeneration
Decreased blood supply
These manifest as:
- Numbness
- Pain
- Parasthesiae
- Weakness
- Problems with fine manipulative skills

8. Nerve Conduction Study
Stimulate nerve and record outcome.
2 main types
1. Motor (record compound muscle action potential)
2. Sensory (record compound sensory action potential)
-Orthodromic (stimulate at finger, record on elbow/wrist)
-Antidromic (stimulate at elbow/wrist, record on finger)
-Radial-median (stimulate at wrist, record on thumb)
-Palmar (stimulate at palm, record on elbow/wrist)
Most sensitive
Least sensitive

9. Changes to CMAP in Carpal Tunnel Syndrome
Latency: Normal < 4.9 ms
Amplitude: Normal ≥ 5 mV
Shape: Normal curve
Pathology: - demyelination
- axonal degeneration
Time
Response amplitude

10. 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

11. 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 non-pathologically due to:
Strength of initial stimulus

12. 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

13. Conduction Velocity
Importance: Eliminating wrong localisation of problem
Method: 1 sw r
Latency wrist 2 se r
Latency elbow

14. Conduction Velocity Calculations: Velocity = Method: Distance Distance
Lelbow – Lwrist =
Time r sw 1
Distance
Latency wrist r se 2
Latency elbow

15. Conduction Velocity Normal value > 50 m/s Other sources of error
– stimulus position
- estimation of nerve course
- latency measurement

16. Problems with comparing features to normal values
Features change with
Temperature
Age
Age 20 80
Latency
Age CV
• Solution: Repeat measurements on ulnar nerve as a control. This also gives additional evidence to rule out generalised neuropathy.

17. 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:
- Latency < 3.6 ms
- Velocity > 50 m/s
- Amplitude > 15 mV
- Latency < 3.1 ms
- Velocity > 54 m/s
- Amplitude > 10 mV
MEDIAN SENSORY (ANTIDROMIC)
ULNAR SENSORY (ANTIDROMIC)

18. An example: Right Antidromic Ulnar
Normal:
-Lat < 3.1 ms
-Amp > 10 uV
…. Ulnar OK – not generalised neuropathy

19. 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.

20. 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.

21. 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