Electroencephalography Flashcards
Delta waves
- 01-3Hz
- lowest frequencies
- associated with deep sleep or highly relaxed state
Theta waves
- 4-8Hz
- slow activity
- associated with creativity, daydreaming, meditation, prayer
Alpha waves
- 8-12Hz
- major rhythm in relaxed adults
- associated with relaxed but alert state
- not actively processing information
- best seen with eyes closed
Beta waves
- above 12Hz
- associated with eyes open
- listening and thinking during analytical thinking
- divided into low (12-15Hz), mid (15-18Hz) and high (>18Hz)
- associated with increased concentration levels
Gamma waves
- above 36Hz
- associated with simultaneous processing of information from different areas of the brain
- good 40Hz activity = good memory
- poor 40Hz activity = learning difficulties
How are the electrodes placement?
- 10-20 international system is the standard naming and positioning scheme for EEG
- based on iterative subdivision of arcs on the scalp from craniometric reference points
- pre-auricular points
- vertex = intersection of longitudinal and lateral points
How many types of derivation are used in EEG?
3 = bipolar, common average reference, common reference derivation
Bipolar derivation
differential inputs connected serially in pairs in anterior-posterior lines
Common average reference derivation
all electrodes linked together by a high resistance to a common point which is used as a reference for all channels
Common reference derivation
- one electrode (e.g. central mid-line (Cz)) is used as a common reference for all channels
- Ear electrodes may be used as reference electrodes)
What does a normal EEG look like?
- alpha rhythm in 2 hemispheres very similar in frequency
- significant theta (4-8Hz) activity present in children and adolescents
- delta activity rarely seen whilst awake over age of 5 years
- alpha frequency slows with age
- consistent difference in alpha frequency between hemispheres of 0.5 to 1Hz is significant
- alpha amplitude asymmetry can be used up to 1.5 to 1 (right to left) or 1.25 to 1 (left to right)
What does an EEG look like in an altered mental status?
- cause may be metabolic, toxic, inflammatory, anoxic or degenerative
- EEG provides quantification not differentiation
- good correlation between severity of EEG changes and severity of encephalopathy and clinical state
Mild encephalopathy define
- clouding of consciousness and confusion
- associated with slowing of posterior dominant rhythm
More severe encephalopathy define
- coma
- associated with high amplitude irregular delta activity
Very severe encephalopathy EEG
All activity drops below 20uv, sometimes in suppression-burst patterns
Extreme encephalopathy EEG
No cerebral activity is seen
Can recovery always be made?
- for more severe, very severe and extreme = full recovery may not be made and are very serious
UNLESS - cause is drug intoxication
Use of EEG in paroxysmal disorders
- most useful method for evaluating epilepsy
- may detect epileptiform abnormalities between seizures
- can demonstrate focal abnormalities
- can diagnose epilepsy in patients with periods of prolonged twilight state or abnormal behaviour
- can localise epileptogenic zone prior to surgery
Features of seizures
- brief loss of consciousness
- staring spell
- convulsions
- automatic behaviours such as pulling at hair/clothing/lip biting/ undressing
EEG Waves in epilepsy
- spikes = duration of <70ms
- sharp waves = 70-200ms
- distinct from background activity
Activating procedures
- hyperventilation
- intermittent photic stimulation (IPS)
- sleep
IPS Response
- intermittent photic stimulation
- 15Hz most effective frequency so this rate is avoided in discos and films
- can occur in photic driving/ flash rate/ harmonics but also can occur naturally (shadows from regularly spaced trees when driving)
Strobe flashing
- self sustained photoparoxysmal response
LORETA
- low resolution electromagnetic tomography
- method of computing the spatial distribution of EEG activity from the surface potentials
- relies on inverse transformation process in same way as CT
- resolution limited by small number of electrodes and uncertainty of structure and geometry
Evoked potential Tests
- similar equipment used as in sensory nerve conduction studies
- measure electrical activity in certain brain areas in response to stimulation of specific sensory nerve pathways
- often used to make diagnosis of MS as indicate dysfunction along these pathways that is too subtle to be noticed or to show up on neurological examination
- demyelination in MS causes nerve impulses to be slowed/garbled/halted altogether
- wired placed on scalp over brain areas being measured
- examiner provokes specific types of stimulation and records responses
- harmless, painless and very sensitive in detecting lesions/damaged areas
- diagnostic info derived from amplitude and timing of averaged responses
3 main types of evoked potential tests
- visual evoked potentials (VEP) (patient sits before a screen on which an alternating checkerboard pattern is displayed)
- brainstem auditory evoked potentials (BAEP) (patient hears a series of clicks in each ear)
- sensory evoked potentials (SEP) (short electrical impulses administered to an arm or leg)
Takes 2 hours to do all 3 types, results interpreted by neurologst/ neurophysiologist with specialised training
