Session 7 Neurophysiology

Question 1

What are nerve conduction studies used for?

Answer 1

Localisation of problem
o	Anatomical location
o	Which nerves are affected
o	Site of entrapment
Pathophysiology of problem
o	Motor- e.g. motor neurone disease
o	Sensory- e.g. diabetes- peripheral neuropathy
o	Motor and sensory
o	Myelin affected e.g. multiple sclerosis
o	Axons affected
Severity of problem
Temporal course of problem
o	Hyperacute
o	Acute
o	Subacute
o	Chronic
Disease course
o	Progression
o	Stable
o	Improving, so can do serial measurements

Question 2

what are nerve conduction studies measuring?

Answer 2

conduction velocity, measure compound nerve APs- sum of APs of different nerve fibres within nerve

Question 3

what is conduction velocity affected by?

Answer 3

diameter of axon- larger=faster

degree of myelination

Question 4

what is dispersion?

Answer 4

wider signal produced if nerve stimulation is proximal- longer distance

Question 5

what is phase cancellation?

Answer 5

some signals can be lost if stimulation proximally, every nerve fibre has a different conduction velocity

Question 6

main problems and artefacts in nerve conduction studies?

Answer 6

• patient discomfort: electrical stimulation of nerve unpleasant to painful and though stimulation strength of stimulus used can be varied, using suboptimal stimulation risks artefactually low readings that can mean an inconsistency error, basically giving an electric shock
• patient factors: movement– noise in signal, limb (especially feet) too cold e.g. diabetic patient – check with thermometer and warm gently in water before test, peripheral odema increases distance from recording electrode to nerve or muscle, inaccessible area,syncope before or during
• equipment failure: machine, electrodes, stimulator

-Electrical interference- signals are small so have to be amplified- CMAPs less affected than SNAPs as amplitudes bigger (mV)
-Wrong Question-“go for the money” operator needs to be guided by clinical information
-Over calling abnormalities without clinical correlation
Very sensitive test/ wide range normal values – mild, subclinical changes may be of no significance

Question 7

problems with electromyography recordings?

Answer 7

 Technical
o Electrical interference
o Inaccurate needle placement
o Non homogeneous muscle- must sample enough areas
o Each electrode track only gives a local picture-operator dependent, clinical
o Wrong question
o Unpleasant and invasive procedure
o Risk of haematoma- must check if patients are on aspirin or warfarin
o Difficult and less informative if muscle is wasted e.g. muscular dystrophy- fat replacement of muscle means difficult interpretation

Question 8

clinical uses of electroencephalography (EEG)?

Answer 8

To identify specific epilepsy syndromes- changes prognosis and treatment
To localise the area of brain where a seizure starts- Implications for surgery
To distinguish epilepsy from other ‘funny turns’
To distinguish some neurological and psychiatric conditions

Question 9

advantages of EEG?

Answer 9

cheap
Excellent temporal resolution-good measurement precision with respect to time
Diagnosis of a few specific epilepsy syndromes

Question 10

drawbacks of EEG?

Answer 10

Poor spatial resolution
Poor detection of deep brain activity
Need training and experience to perform
Need training and experience to interpret
EEG changes with age and wakefulness

Question 11

what are the technical problems that interfere with EEG recordings?

Answer 11

Biological Artifacts: Blink, eye movement, ECG, EMG (muscle movement)
Environmental: Electrical interference (signals detected are very tiny so huge amplification used)
Technical:Lead misplacement, Lead disconnection
Patient co-operation required

Question 12

how is an EEG performed?

Answer 12

we’re measuring electrical activity in the brain by measuring the sum of APs of all neurones within a certain distance of the recording electrode. APs of neurons with same spatial orientation recorded, and main contribution neurons close to surface
surface electrical activity measured using scalp electrodes or dural electrodes
deeper activity using depth electrodes
electrodes placed in a pattern= montage: even nos on right side, odd on left
use a differential amplifier to produce each channel or trace of activity
Each amplifier has two inputs
An electrode is connected to each of the inputs

Question 13

locations of electrodes in EEG?

Answer 13

all labelled according to position
odd on left side of brain, even on right
frontal, frontopolar, temporal, central, parietal, occipital, auricular
Bipolar
Each channel represents the difference in voltage between two adjacent channels
Referential
Each channel represents the difference in voltage between an electrode and a designated reference electrode (often midline)
Average reference
Outputs of all amplifiers are summed and averaged – the average signal used as the common reference for each channel

can localise abnormalities based on electrode placement in montage

Question 14

EEG rhythms?

Answer 14

delta up to 4
theta 4-8
alpha 8-13
beta >13 frequency/Hz

Question 15

characteristics of normal EEG in adult in wakefulness?

Answer 15

alpha rhythm (8-13 frequency/Hz)
posterior regions head
eyes closed

Question 16

what does normal EEG vary with?

Answer 16

age

alertness

Question 17

characteristics of rhythmic EEG activity?

Answer 17

Frequency
Amplitude
Quantity
Morphology
Reactivity
Variability
Topography
Phase relationships

Question 18

evoked potentials in EEG?

Answer 18

Visual evoked potentials-tests the pathway of the visual pathways to the visual areas
of the brain
Somatosensory evoked potentials-Tests the pathway of the sensory nerves to the sensory areas
of the brain

Question 19

how are recordings made in nerve conduction studies?

Answer 19

Surface electrodes over the skin
A stimulator sends electrical impulses to the nerves
EMG machine records electrical response: Velocity and Amplitude
Sensory nerve action potential (SNAP)
when performed on a sensory nerve
Motor nerve compound motor action potential (CMAP)
When performed on a motor nerve

Question 20

aim of nerve conduction studies?

Answer 20

Transmit information from AHC to muscle and from sensory receptors to spinal cord. Detects electrical and chemical changes (conduction velocity)

Question 21

what does the EMG machine record in nerve conduction studies?

Answer 21

records velocity and amplitude of action potential

amplitude reflects no. axons

Question 22

how are motor nerve conduction studies performed?

Answer 22

Recording electrode over center muscle belly
Reference electrode over distal tendon
Stimulator over nerve to muscle
Cathode closest to recording electrode
increase stimulation slowly to ensure all axons stimulated
Current 20-30mA for supra maximal stimulation (i.e. when increasing current has no effect on size of CMAP – all nerve fiber have already been excited)

Question 23

what is latency in motor nerve compound motor AP studies?

Answer 23

Time for impulse to travel from stimulation site to recording site
 Velocity = distance/time
 Can be measured across a segment to see whereabouts conduction is altered
Reflects only fastest conducting fibers
o Myelination
• Number of muscle fibers that fire is proportional to the number of motor axons
o Loss of axons (will NOT alter latency – think!)
o Conduction block (loss of conducting axons)
o NMJ disorders or myopathies
 Duration – synchrony
• Increases if some muscle fibers are slowed
• E.g. due to myelination

Question 24

recordings from sensory neurones in nerve conduction studies?

Answer 24

Sensory neurones
 Record from purely sensory part of the nerve e.g. fingers
 Recording electrode is most proximal
o Because neurone is sensory you can measure conduction in both directions
 Latency
o Time from stimulus to SNAP
 Amplitude less than for motor neurones
 The closer the electrode to the source the greater the amplitude
 Supramaximal: 5-30mA
 Lower stimulation threshold
 Amplitude – number of fibers that depolarise is proportional to the proximity of the recording electrode to nerve
 Duration – shorter than CMAP
o Measure of synchrony

Question 25

why must supramaximal values be reached in nerve conduction studies?

Answer 25

this is where amplitude doesn’t then increase with further stimulation, and allows reproducibility, but can be painful for patient

Question 26

what is EMG?

Answer 26

Records electrical activity of skeletal muscle (upon contraction)
 Generates action potential
 Membrane potential of muscle cells is about -90mV
Potentials
 Sum of electrical activity as motor units are activated
Uses needle electrodes
 Give a local picture of only a few electrodes
 Surface electrodes are not as good
o Give a very general picture – not specific enough
o Biofeedback

Muscle at rest should be electrically silent
Action potential is produced by voluntary contraction
 Interference pattern produced

Question 27

factors affecting EMG?

Answer 27

 Composition of motor units
o Number of fibers
 Abnormalities of fibers
 Spontaneous activity of denervated muscle fibers

Question 28

what is insertion potential in EMG?

Answer 28

 Electrical activity caused by the needle being inserted and damaging fibers
 End plates fire spontaneously
 Should settle down – if not reposition

Question 29

abnormalities in EMG?

Answer 29

 Nerve damage
o Motor units lose controlling alpha neurone (degree proportional to number of motor neurones damaged)
o Single muscle fibers released from motor unit fire spontaneously
 Fibrillation occurs producing positive sharp waves
• Positive – fall below line
• Not visible through skin
o Denervated fibers are viable for 7 to 10 days
o Muscle fibers that are released from one motor unit are taken up by others
 Produces abnormally large motor units and motor unit action potentials
o Sound produced by machine indicates the strength of the contraction
o Decreased number of motor units
 Interference patterns produced are less at full contraction
 Duration of motor unit action potential increases

 Muscle damage
o Decreased duration of motor unit action potential
o Decreased amplitude of motor unit action potential
o In severe disease the number of motor units decreases

Question 30

how is better spacial resolution obtained in EEG?

Answer 30

using dural electrodes rather than scalp electrodes