neurophysiology - EEG

• a. A recording of summated excitatory postsynaptic potentials(EPSPs) and inhibitory postsynaptic potentials(IPSPs) generated by synchronous activity at numerous synapses of cortical neurons
• b. Action potentials do not have an important role

• Rhythmic EEG Activity: caused by oscillating EPSPsand IPSPs and is due to the following:
• 1. Intrinsic membrane properties of neurons
• 2. Synaptic connections of neurons
• 3. Thalamocortical inputs (sleep spindles are one of the best documented examples of rhythmic activity caused by projections of thalamic pacemaker cells in the nucleusreticularis)

a. Temporal alpha: independent rhythm over temporal regions as asynchronous bursts; normal in elderly

b. Frontal alpha: predominant over anterior head regions;may be seen following arousal from sleep or toxic-metabolic insult

• c. Paradoxical alpha: alpha activity appearing with eyeopening and mental alerting and disappearing with eyeclosure
• 1) Occurs most commonly with sedation
• 2) Occurs in context of awakening without immediatevisual fixation

d. Slow variant: rhythm frequencies of 3.5 to 6.5 Hz which may be seen together with the normal alpha frequency, rare

e. Fast variant: rhythm frequencies of 14 to 16 Hz

Used in digital recordings where one electrode is referenced to the sum of four surrounding electrodes C3- (T3+Cz+F3+P3)

b. Good for detecting localized waveforms

syncopal attacks

moyamoya disease

Activates epileptiform discharges in 30% to 50% ofpatients with absence seizures and 6% to 10% of patientswith focal seizures

Myoclonic seizures and Juvenile myoclonic epilepsy

downward deflection

- Occur with visual scanning (visual evoked potentials) --> you look at the pt and see the pt is scanning

- Occipital positive sharp sawtooth transients that have amplitudes less than 50 μV

- Do persist in dark (may be related to eye movements)

- Morphologically looks like POSTS

• a. 7 to 11-Hz arch-shaped rhythm most obvious in centro-parietal area
• b. Best observed in bipolar montage
• c. Attenuates with contralateral movementd. Enhanced by immobility and hyperventilation
• e. Seen in more than 10% of EEGs, predominately inyoung adults

Mu rhythm.

• a. Burst of low-amplitude alpha or theta activity
• b. May be associated with mental activity

• a. About 5% of sleep time is spent in stage 1 sleep
• b. Slow eye movements (SEMs) which are less than 0.5 Hz
• c. Attenuation of background
• d. Dropout of alpha or replacement by theta activity
• e. Enhancement of beta activity

Stage 1 sleep, Slow eye movement and dowsiness

• a. About 45% of time is spent in stage 2 sleep
• b. Characterized by sleep spindles and/or K complexes
• c. POSTS (positive occipital sharp transients of sleep): Triangular waves at irregular intervals (usually >1second)
• d. Vertex (V) waves: Negative sharp transients at vertex with a frequency less than 2 Hz
• e. Sleep spindles: 11 to 15-Hz waveforms with more than 0.5 second duration
• f. K complexes: Negative sharp waves similar to V waves and Followed by slower positive component, more than500 milliseconds

• a. 25% of time is spent in stages 3 and 4 sleep
• b. May still see POSTS, sleep spindles, K complexes
• c.Stage 3: less than 50% delta activity
• d. Stage 4: more than 50% delta

• a. 25% of time spent in REM sleep
• b. Low-amplitude EEG
• c. Sawtooth waves (medium amplitude, theta waves) overcentral region
• d. Decreased muscle activitye. Increased heart rate
• f. Epileptiform activity is most likely to be suppressed in REM sleep

Rapid eye movement (REM) sleep. Note the sawtooth waves, medium-amplitude sawtooth-appearing theta waves in frontal and central head regions and the REMs recorded in prefrontal electrodes, Fp1 and Fp2.

Stage 2 sleep:

Rapid eye movement (REM) sleep:

• Stage 1 sleep: attenuation of background and sloweye movements (SEMs)
• Stage 2 sleep: spindles and V waves
• Deep sleep (stages 3 and 4): delta activity
• Rapid eye movement (REM) sleep: low-amplitude EEG, sawtooth waves, decreased muscle tone, increased heart rate

• a. 5 to 7-Hz spike-and-wave pattern in young adults
• b. Brief, bisynchronous bursts (lasting up to 2 seconds) of low-amplitude repeating spike and slow wave (the latter consisting of a higher amplitude and more widely distributed slow component than the small spike component,which may be buried in the slow wave)
• c. Higher incidence of epilepsy, especially in males with high-amplitude, anterior-predominant 6-Hz spike-and wave waveforms occurring while awake (WHAM)
• d. May be induced by the administration and withdrawal of sedatives and diphenhydraminee. Two variants: occipital form is usually benign, frontalform is commonly associated with epilepsy
• 1) FOLD (female, occipital, low amplitude [<40 μV],drowsiness): benign, may be increased by diphenhydramine,prevalence of epilepsy about 30%
• 2) WHAM (wake, high amplitude, anterior predominance, male): prevalence of epilepsy about 80%

• 
• a. Small amplitude, short duration (<65 milliseconds)
• b. Sharply contoured, one or two phases
• c. Unilateral or bilateral, noted predominantly in anterior and mid-temporal regions
• d. Do not disturb background
• e. Disappear with deeper levels of sleep

• a. Single spike-like or 6 to 11-Hz mu-like wave forms
• b. Negative polarity, with an amplitude up to 200 μVc. Present over temporal regions
• d. Occur primarily in older adults during drowsiness and light sleep

Eyes have a dipole, with the cornea relatively positive

a) Upward eye movements produce downward deflection on a bipolar montage

- Alpha rhythma slows to an average of 9 Hz at 60 years, 8 to 9 Hz at 100 years

- Beta activity may be seen in half of those older than 60, more common in women

- May see a slight increase in theta activity in persons older than 75 years

• - Premature infants have active sleep and quiet sleep with a tracé alternant pattern.
• _{Premature infants less than 30 weeks of conceptual age shows. Discontinuous tracing (Bursts of medium- to high-amplitude, mixed frequency, mostly delta, may appear as occipital rhythmic delta activity).As brain matures, intervals between the bursts gradually shorten, and the tracing changes into a less discontinuous pattern}: the “tracé alternant” (periods between the burstsare shorter in duration)

- May also see delta brushes, occipital slow waves, and frontal sharp transients in premature infants

• 
• very preterm infants born at less than 27 weeks gestation

• - Slowing
• - Assymetry
• - supression
• - epileptiform activity

five different types:

1. Theta slowing may reflect mild to moderate disturbance of cerebral function, may be focal or generalized.

• 2. Polymorphic delta activity (PDA)
• _{- Associated with white matter lesions}
• ^{- Focal PDA usually signifies a focal lesion such as infarct,tumor, or abscess}
• _{- Generalized PDA is usually seen in association with adiffuse encephalopathy or a degenerative cognitive disease}

• 3. Intermittent rhythmic delta activity (IRDA)
• ^{- Enhances with drowsiness, attenuates with alerting, anddisappears with sleep (whereas PDA usually persiststhrough sleep and alerting)}
• - _{Distant rhythms originating from deep or midline structures projecting to the surface(including subcortical gray matter, brainstem, diencephalon,mesial surfaces of frontal lobes, periventricularregions) by direct compression or distortion (e.g., herniation)may produce these bisynchronous slow waves}

^{IRDA in frontral called --> “frontal intermittent rhythmic delta activity” (FIRDA)may be seen in association with noncommunicatinghydrocephalus, herniation syndromes causing pressureon deep midline structures.}

_{IRDA appearing maximal in occipital areas --> occipital intermittent rhythmic delta activity” (OIRDA)and may be seen in association with lesions (e.g., medulloblastoma) involving posterior fossa structures}

Persistent asymmetry of alpha activity by 50% or beta activity by 35% is considered abnormal

Spike: duration < 70 milli-seconds, Sharp wave: duration > 70 milli-seconds

Moderate-amplitude rhythmic delta activity in temporal area --> Epileptogenic significance similar to temporal sharp waves

Central-temporal (rolandic-sylvian) spikes.

1) Various forms of periodic complexes: sharp waves, spikes, or spike-and-wave discharges

2) Spike-and-sharp-wave discharges coincide with myoclonic jerks

Hypsarrhythmia.

• 1) Tonic phase
• a) Low-amplitude fast activity (>10 Hz), followed by higher amplitude and lower frequency rhythmic activity
• b) Lasts about 10 to 20 seconds

• 2) Clonic phase
• a) High-amplitude spike or polyspike-and-wave complexes of 1 to 4 Hz
• b) Lasts about 30 seconds
• c) Postictal phase: low-amplitude, slow activity

EEG recording from a 60-year-old man with multifocal seizures secondary to multiple embolic ischemic events. The tracing shows a seizure discharge in the midline leads (mainly Cz) associated with clonic movements of the feet. Note also generalized slowing of background activity in the delta range.

3-Hz spike-and-wave pattern (typical spike-and-wave pattern).

• . Absence status epilepticus ^{_{Less regular continuous 3-Hz spike-and-wave pattern than in typical absence seizures. Common in elderly patients without previous history of absence seizures}}
• . Atypical absence status epilepticus _{^{Continuous or clusters of repetitive epileptiform generalized discharges, consisting of spike-and-wave, with no reactivity to stimuli. Patient may be alert between clusters}}
• . Simple partial status epilepticus
• . Complex partial status epilepticus

• PLEDs (periodic lateralized epileptiform discharges).
• _{1. Biphasic or multiphasic spikes, sharp waves, slow waves, or rhythmic fast activity}
• _{2. Occur in periodic fashion of 1 to 3 Hz}
• _{3. May evolve into seizure pattern}
• 4. Usually resolve in 1 to 4 weeks
• _{5. May occur in vascular insults, herpes simplex encephalitis, epilepsy, subdural hematoma, focal infection (abscess or cerebritis), and other disorders, including toxic-metabolic conditions such as hypoglycemia, uremia, alcohol withdrawal}

BiPLEDs: independent PLEDs bilaterally over hemispheres, usually occur in diffuse or bilateral focal disease processes

Chronic PLEDs: rare and can occur with structural lesions, e.g., those associated with tuberous sclerosis

Triphasic waves

• - Not associated with seizures
• - Frontal predominance (highest amplitude) with 1.5 to 2.5-Hz frequency, less commonly posterior predominance
• - Sharply contoured, bilateral, and symmetric; often with an apparent anterior-posterior phase lag
• - Associated with hepatic coma (34%-50% of patients); other causes include renal failure, anoxia, electrolyte disturbances, encephalitis.

• - asymmetry more than 50%
• - more likely to represent epileptiform activity (than symmetric bilateral triphasic waves)
• - almost always associated with a structural abnormality rather than ametabolic-toxic disorder

Generalized periodic sharp waves in a patient with Creutzfeldt-Jakob disease

- Occur at 0.5 to 1-second intervals

- Seen also in metabolic encephalopathies (e.g., hypothyroidism), inflammatory disorders, hypoxia, and with use of baclofen, interferon,lithium, or ifosfamide

• a. Triphasic waves
• b. PLEDs
• c. Generalized periodic sharp waves
• d. Generalized periodic epileptiform discharges
• e. Generalized periodic slow-wave complexes

• 1) May be seen in status epilepticus
• 2) Also seen after cardiopulmonary arrest at a frequency of 1 to 2 Hz, --> associated with myoclonus and a poor prognosis

1) High-voltage slow-wave complexes lasting from 0.5second to seconds and occurring every 4 to 15 seconds

2) Seen in subacute sclerosing panencephalitis (SSPE)

- The slower the frequency of the bursts, the worse the prognosis

- May also occur with deep anesthesia, drug overdose,or hypothermia

Alpha coma: unresponsive to external stimuli, 3 days after cardiopulmonary arrest, indicates a poor prognosis, Can also be seen in brainstem strokes and drug overdose,

<sup>- Posterior-dominant alpha coma pattern may be seen in setting of a brainstem lesion.

- Anterior-dominant (or generalized) invariant alpha coma pattern may be seen in setting of a global insult such as cardiac arrest or bilateral thalamic infarcts</sup>

Beta coma: Beta activity superimposed on delta, Seen in drug overdose(often barbiturate or benzodiazepine intoxication)

Spindle coma: Resembles sleep EEG but little response to afferent stimuli, Results from head trauma, brainstem lesions, drugs, hypoxia, Associated with good prognosis

The posterior dominant background (alpha rhythm) is at 7 Hz, slower than the alpha frequency range. No other generalized slow activity is present. Since this patient is older than 8 years, and is clearly awake, this is abnormal. This represents (electrographically) the mildest degree of diffuse encephalopathy.

a posterior dominant background is present, but it is only at 6-7 Hz, and bursts of generalized polymorphic delta activity (this one lasting 2-3 s) are present --> Mild diffuse encephalopathy

Continuous slowing, generalized. The record is dominated by generalized polymorphic delta activity. When this is "continuous" (greater than 80% of the recording), it usually goes along with a severe diffuse encephalopathy.

Alpha coma. not posterior dominant, is continuous, and is non-reactive. If the entire record is present and the patient is known to be comatose, this qualifies as alpha coma.

• Beta coma. ( activity is noted at 15-22 Hz) --> in spindle come is 13-16 Hz.
• _{Note that this pattern could be seen in an awake patient, so that the term "beta coma" is reserved for patients known to be comatose.}

Spindle coma. 13-16 Hz. associated with coma is even more continuous than shown here, and unreactive.

Triphasic waves

• 1. Minimum of 8 electrodes
• 2. Inter-electrode impedances less than 10,000 Ω and more than 100 Ω
• 3. Sensitivity of a least 2 μV/mm for a least 30 minutes of recording
• 4. Test integrity of entire recording system by tapping on electrodes
• 5. Inter-electrode distances of at least 10 cm
• 6. Appropriate filter settings
• 7. No EEG reactivity to external stimuli
• 8. Performed by a qualified technologist
• 9. Exclusion of drug overdose, anesthetic agents, hypothermia