Indication
- Identify location of areas of abnormal function
- Seizure foci
- Space occupying lesions,
- Identify degree of abnormal function.
- As a guide to depth of anaesthesia
- In theatre and in intensive care.
- EEG or cerebral function monitor can give a guide to the establishment of satisfactory burst suppression if barbiturates are used to reduce cerebral activity.
- Head injured patients may show subclinical seizures.
- In one study, of 144 monitored head injured children, 30% showed seizure activity correlating with duration of ITU stay (O’Neill et al., 2015).
- Continuous EEG - who to monitor?
- High risk
- Acute brain injuries
- Recent SE
- Also occur in medical and surgical ICU
- Toxic, electrolyte and metabolic abnormalities
- 5-20% with toxic-metabolic encephalopathy had NCSz
- NCSz detected using continuous EEG (cEEG) in different intensive care populations
- Why monitor?
Study | Population | EEG | N | % of NCSz |
Medical ITU pts without known brain injury with unexplained low GCS | cEEG | 201 | 5% | |
All pts with moderate to severe HI | cEEG | 94 | 11% | |
Vespa et al 1999 | Neuro-ICU with stroke and ICH | cEEG | 109 | 16% |
Traumatic brain injury: raised ICP
Intracerebral haemorrhage: increase mass effect, worse NIHSS
MCA occlusion: increased infarct volume
- Monitor some vascular procedures such as in aneurysm surgery and carotid endarterectomy.
- Continuous EEG shows immediate asymmetries produced by ischaemia
- Although SSEP can be more sensitive, nevertheless with SSEP it takes time for averaging to record the warning changes.
- EEG ‘raw’ or processed ‘cerebral function analysing monitor’ (CFAM)
- Showing an acute drop in activity can be a sensitive guide for the need of shunt insertion during carotid endarterectomy.
- As the CFAM displays the previous two seconds of processed EEG continuously, the response is almost immediate.
Pros
- Give the best overview of interictal epileptiform activity because they sample extensive areas of the cranium,
Limitation
- Low sensitivity due to intervening high-resistance tissue
- Poor ability to sample activity from deep structures.
Scalp EEG records
- Function
- Cons
- Mainly from cortical gyri
- Does not give details of deep brain structures such as the hippocampus and basal ganglia
- Spatial resolution is low
- Changes often non- specific, but time resolution is superior to functional imaging.
Interictal EEG done
- Frequency of abnormal interictal EEG: 50-60%
- With further increase in yield by repeated or prolonged recordings that sample drowsiness and sleep.
- Routine EEGs aim to answer the following questions:
- Whether there is an abnormality of background rhythm globally and/or focally,
- Are there any interictal epileptiform discharges (IEDs; sharp waves, spikes, spike-and-wave complexes)?
- Are the IEDs diagnostic of an idiopathic generalized syndrome (i.e. not appropriate for surgery)?
- Whether there are focal interictal epileptic discharges
- If so, where, and whether there are generalized spike–wave discharges.
- Are the IEDs confined to one hemisphere or bilateral?
- If unilateral, are IEDs confined to one area/lobe or are they multifocal?
Scalp video/EEG telemetry
- To find out
- The precise features of habitual seizures and ictal EEG patterns
- The time taken for discharge to reach scalp surface electrode can vary depending on its distance from the site of ictal onset.
- Video EEG is done in conjunction with sleep deprivation and reduction/cessation of AEDs to maximize the chance of recording a seizure during the observation period.
- The goals of video EEG are to:
- Lateralize and localize seizure onset
- To allow comparison with neuroimaging findings
- Further characterize the interictal discharges and correlate ictal EEG with behavior
- Detect, characterize and quantify the patients habitual seizures—are they having more than one type?
Example
- Surface EEG from an adult patient who presented with seizures:
- Standard 10– 20 electrode positions used—
- Odd numbers left, even right:
- F = frontal,
- Fp = frontopolar,
- P = parietal,
- O = Occipital,
- T = Temporal
- X1- X2— spare channel, used here for electrocardiogram (ECG) lead 1.
- ‘Alpha’ rhythm
- For example, in channel 16 (T5- O1):
- Normal
- The slow activity in channels 9, 10, and 11 is the result of damage from an underlying glioma in this region.
- The ‘compressed spectral density’ top right here does show the location of the slow activity— measured at position of green line in this trace— but such frequency maps can be misleading at times with surface EEG recordings.
Common EEG rhythms
Name | Description |
Alpha | Frequency 8–13 Hz |
Delta | Frequency < 4 Hz |
Beta | Frequency 14–40 Hz |
Theta | Frequency 4–8 Hz |
Gamma | Frequencies > 40 Hz |
Lambda | Diphasic sharp transient occurring over the occipital regions of the head of waking subjects during visual exploration. The main component is positive relative to other areas. Time-locked to saccadic eye movement. Amplitude varies but is generally below 50 mV. |
Fast | Activity of frequency faster than alpha (i.e. beta and gamma activity) |
Slow | Activity of frequency slower than alpha (i.e. theta and delta activity) |
Sharp | A transient, clearly distinguished from background activity, with pointed peak at a conventional paper speed or time scale and duration of 70–200 ms |
Spike | A transient, clearly distinguished from background activity, with pointed peak at a conventional paper speed or time scale and a duration from 20 to 70 ms |
- Slow brain activity
- Indicate
- Tumours
- Frontal area may suggest damage to the deep midline structures of the brain.
- Characteristic EEG patterns
Pattern | Description |
Sleep spindle | Burst at 11–15 Hz but mostly a 12–14 Hz generally diffuse but of higher voltage over the central regions of the head, occurring during sleep |
Periodic lateralized epileptiform discharges | Sharp transients such as sharp waves or spikes, which repeat in a periodical or semiperiodical fashion. They have either a regional or a lateralized distribution. They may also occur independently over both hemispheres. The epileptiform discharges often have multiple phases and a complex morphology. The main component is negative. |
Burst suppression | Pattern characterized by bursts of theta and/or delta waves, at times intermixed with faster waves, and intervening periods of low amplitude (below 20 mV). This EEG pattern indicates either severe brain dysfunction or is typical for some anesthetic drugs at certain levels of anesthesia. |
Frontal intermittent rhythmical delta | Fairly regular, approximately sinusoidal or sawtooth waves, mostly occurring in bursts at 1.5–2.5 Hz over the frontal areas of one or both sides of the head. Most commonly associated with unspecified encephalopathy. |
Occipital intermittent rhythmical delta | Fairly regular or approximately sinusoidal waves, mostly occurring in bursts at 2–3 Hz over the occipital areas of one or both sides of the head. Frequently blocked or attenuated by opening the eyes. |
Rhythmic temporal theta burst of drowsiness | Characteristic burst of 4 ± 7 Hz waves frequently notched by faster waves, occurring over the temporal regions of the head during drowsiness. |
- Burst suppression
- Bursts of 8–12Hz electrical activity (lasting 1–10 s) that diminish to 1–4Hz prior to intervals of electrical silence (no excursions ≥ 5 microvolts, lasting > 10 s)
- Used as an endpoint for titrating neuroprotective drugs such as barbiturates, propofol… e.g. for
- Temporary clipping during cerebrovascular surgery,
- Traumatic or intracranial hypertension refractory to lower tier interventions.
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