Session 7 Neurophysiology Flashcards
What are nerve conduction studies used for?
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
what are nerve conduction studies measuring?
conduction velocity, measure compound nerve APs- sum of APs of different nerve fibres within nerve
what is conduction velocity affected by?
diameter of axon- larger=faster
degree of myelination
what is dispersion?
wider signal produced if nerve stimulation is proximal- longer distance
what is phase cancellation?
some signals can be lost if stimulation proximally, every nerve fibre has a different conduction velocity
main problems and artefacts in nerve conduction studies?
- 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
problems with electromyography recordings?
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
clinical uses of electroencephalography (EEG)?
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
advantages of EEG?
cheap
Excellent temporal resolution-good measurement precision with respect to time
Diagnosis of a few specific epilepsy syndromes
drawbacks of EEG?
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
what are the technical problems that interfere with EEG recordings?
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
how is an EEG performed?
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
locations of electrodes in EEG?
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
EEG rhythms?
delta up to 4
theta 4-8
alpha 8-13
beta >13 frequency/Hz
characteristics of normal EEG in adult in wakefulness?
alpha rhythm (8-13 frequency/Hz)
posterior regions head
eyes closed
what does normal EEG vary with?
age
alertness
characteristics of rhythmic EEG activity?
Frequency Amplitude Quantity Morphology Reactivity Variability Topography Phase relationships
evoked potentials in EEG?
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
how are recordings made in nerve conduction studies?
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
aim of nerve conduction studies?
Transmit information from AHC to muscle and from sensory receptors to spinal cord. Detects electrical and chemical changes (conduction velocity)
what does the EMG machine record in nerve conduction studies?
records velocity and amplitude of action potential
amplitude reflects no. axons
how are motor nerve conduction studies performed?
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)
what is latency in motor nerve compound motor AP studies?
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
recordings from sensory neurones in nerve conduction studies?
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
why must supramaximal values be reached in nerve conduction studies?
this is where amplitude doesn’t then increase with further stimulation, and allows reproducibility, but can be painful for patient
what is EMG?
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
factors affecting EMG?
Composition of motor units
o Number of fibers
Abnormalities of fibers
Spontaneous activity of denervated muscle fibers
what is insertion potential in EMG?
Electrical activity caused by the needle being inserted and damaging fibers
End plates fire spontaneously
Should settle down – if not reposition
abnormalities in EMG?
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
how is better spacial resolution obtained in EEG?
using dural electrodes rather than scalp electrodes
