Electroencephalography

Question 1

Delta waves

Answer 1

• 01-3Hz
• lowest frequencies
• associated with deep sleep or highly relaxed state

Question 2

Theta waves

Answer 2

• 4-8Hz
• slow activity
• associated with creativity, daydreaming, meditation, prayer

Question 3

Alpha waves

Answer 3

• 8-12Hz
• major rhythm in relaxed adults
• associated with relaxed but alert state
• not actively processing information
• best seen with eyes closed

Question 4

Beta waves

Answer 4

• 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

Question 5

Gamma waves

Answer 5

• above 36Hz
• associated with simultaneous processing of information from different areas of the brain
• good 40Hz activity = good memory
• poor 40Hz activity = learning difficulties

Question 6

How are the electrodes placement?

Answer 6

• 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

Question 7

How many types of derivation are used in EEG?

Answer 7

3 = bipolar, common average reference, common reference derivation

Question 8

Bipolar derivation

Answer 8

differential inputs connected serially in pairs in anterior-posterior lines

Question 9

Common average reference derivation

Answer 9

all electrodes linked together by a high resistance to a common point which is used as a reference for all channels

Question 10

Common reference derivation

Answer 10

• 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)

Question 11

What does a normal EEG look like?

Answer 11

• 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)

Question 12

What does an EEG look like in an altered mental status?

Answer 12

• 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

Question 13

Mild encephalopathy define

Answer 13

• clouding of consciousness and confusion

- associated with slowing of posterior dominant rhythm

Question 14

More severe encephalopathy define

Answer 14

• coma

- associated with high amplitude irregular delta activity

Question 15

Very severe encephalopathy EEG

Answer 15

All activity drops below 20uv, sometimes in suppression-burst patterns

Question 16

Extreme encephalopathy EEG

Answer 16

No cerebral activity is seen

Question 17

Can recovery always be made?

Answer 17

• for more severe, very severe and extreme = full recovery may not be made and are very serious
  UNLESS
• cause is drug intoxication

Question 18

Use of EEG in paroxysmal disorders

Answer 18

• 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

Question 19

Features of seizures

Answer 19

• brief loss of consciousness
• staring spell
• convulsions
• automatic behaviours such as pulling at hair/clothing/lip biting/ undressing

Question 20

EEG Waves in epilepsy

Answer 20

• spikes = duration of <70ms
• sharp waves = 70-200ms
• distinct from background activity

Question 21

Activating procedures

Answer 21

• hyperventilation
• intermittent photic stimulation (IPS)
• sleep

Question 22

IPS Response

Answer 22

• 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)

Question 23

Strobe flashing

Answer 23

• self sustained photoparoxysmal response

Question 24

LORETA

Answer 24

• 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

Question 25

Evoked potential Tests

Answer 25

• 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

Question 26

3 main types of evoked potential tests

Answer 26

1. visual evoked potentials (VEP) (patient sits before a screen on which an alternating checkerboard pattern is displayed)
2. brainstem auditory evoked potentials (BAEP) (patient hears a series of clicks in each ear)
3. 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