Stereo-EEG and Intracranial EEG
Why would doctors need to record seizures from inside the brain?
Most EEG tests record brain electrical activity using electrodes placed on the scalp.
For many people, that provides enough information.
But scalp EEG has limitations.
Electrical activity generated deep inside the brain can be difficult to detect clearly at the scalp. Signals also have to pass through:
brain tissue
cerebrospinal fluid
skull
and scalp
before reaching an external electrode.
When someone has drug-resistant focal epilepsy and epilepsy surgery is being considered, doctors sometimes need much more precise information about where seizures actually begin.
One method is intracranial EEG, often abbreviated to iEEG.
Instead of recording entirely from outside the head, electrodes are temporarily placed:
on the surface of the brain
or inside selected brain regions.
One of the most important modern forms of intracranial EEG is:
stereo-electroencephalography — SEEG.
King's College Hospital and other UK epilepsy-surgery centres use SEEG as part of specialist presurgical investigation for selected people with difficult-to-localise epilepsy.
SEEG is not an ordinary EEG
A routine EEG involves electrodes attached to the scalp.
SEEG is very different.
It requires a neurosurgical procedure.
Thin depth electrodes containing multiple electrical contacts are passed through small openings in the skull into carefully selected brain regions.
The electrodes then record electrical activity directly from those areas while the person is monitored in hospital.
The Northern Care Alliance describes SEEG as a specialised EEG investigation used for some people being considered for epilepsy surgery when doctors need to identify more precisely where seizures begin.
Why would somebody need SEEG?
SEEG is generally considered when the earlier presurgical investigations have produced a strong reason to suspect focal epilepsy but have not answered the surgical question precisely enough.
For example:
MRI may identify an abnormality, but doctors may not know whether the seizures actually begin there.
Or:
scalp EEG may suggest a general region, but not enough detail to determine whether surgery can be performed safely.
Or:
different tests may point towards slightly different areas.
Or:
MRI may be normal even though the history and EEG strongly suggest focal epilepsy.
SEEG can then investigate specific competing possibilities.
SEEG is a hypothesis-driven investigation
Electrodes are not placed randomly throughout the brain to see what happens.
Before SEEG, the epilepsy team develops one or more hypotheses about:
where seizures might begin
which networks may be involved
how seizures may spread
and which areas need to be sampled.
That hypothesis is built from earlier evidence such as:
seizure semiology
scalp video EEG
MRI
PET
SPECT
MEG
neuropsychological testing
functional MRI
and other specialist investigations.
The ILAE places intracranial EEG within the wider multidisciplinary presurgical evaluation rather than treating it as a standalone test.
SEEG cannot record from the entire brain
This is extremely important.
Each electrode records electrical activity only from the brain tissue around its contacts.
Even with several depth electrodes, doctors are sampling selected regions, not monitoring every neuron or every possible seizure-onset site.
SEEG therefore works best when the previous investigation has narrowed the possibilities enough for the team to ask specific questions.
For example:
Does the seizure begin in the hippocampus or neighbouring temporal cortex?
Or:
Does activity begin in the suspected frontal lesion or in a nearby network?
Or:
Are apparently bilateral scalp EEG changes actually spreading rapidly from one focal region?
What does a depth electrode look like?
A SEEG depth electrode is a very thin flexible electrode containing several recording contacts along its length.
Because there are multiple contacts, one electrode can sample electrical activity from several points along the planned trajectory.
The Northern Care Alliance describes SEEG electrodes as fine wires with multiple contact points capable of recording electrical activity from different locations along the electrode.
How many electrodes are inserted?
There is no universal number.
The number depends on:
the suspected seizure network
brain anatomy
imaging
previous EEG
and the questions the team needs to answer.
One NHS SEEG service reports that many of its patients receive approximately 6 to 15 depth electrodes, but this is a local example rather than a rule that applies to every centre or every person.
Some cases need fewer.
Others require more extensive sampling.
How does the team decide where to put them?
The planning can be extremely detailed.
Neurologists, neurophysiologists, neuroradiologists and neurosurgeons may review:
recorded seizures
MRI anatomy
possible lesions
PET or SPECT abnormalities
neuropsychology
functional anatomy
and blood vessels.
They then create individual electrode trajectories.
Each trajectory needs to reach the intended brain region while avoiding important structures, particularly blood vessels.
The UK Health Research Authority notes that SEEG trajectories require careful planning because incorrect placement could damage vessels or important brain tissue.
Why are blood vessels so important?
The electrode has to pass from the skull to its target inside the brain.
Along that route are:
arteries
veins
and smaller blood vessels.
The planned trajectory is designed to avoid them as much as possible.
Some epilepsy-surgery programmes therefore obtain detailed vascular imaging before implantation.
UCLH describes angiography as part of its intracranial EEG planning pathway to visualise blood vessels before electrodes are inserted.
The exact imaging and surgical-planning method varies between centres.
Is a robot used?
Often, yes.
Modern SEEG implantation frequently uses stereotactic navigation and robotic assistance to position electrodes extremely accurately.
At the Northern Care Alliance, small skull openings are made and electrodes are inserted using robotic guidance.
Robot assistance does not mean a machine independently decides where electrodes should go.
The clinical team determines the plan.
The robotic system helps the neurosurgeon follow those planned trajectories accurately.
What does “stereo” mean in stereo-EEG?
In this context, stereotactic refers to using three-dimensional coordinates to locate precise targets within the brain.
The neurosurgical team combines:
brain imaging
spatial navigation
planned coordinates
and specialised equipment
to guide each electrode along its intended three-dimensional path.
This makes it possible to sample brain regions that cannot simply be reached by placing electrodes on the cortical surface.
Is SEEG performed while the person is awake?
Electrode implantation is generally performed under general anaesthetic.
The person is asleep during the operation.
UCLH and the Northern Care Alliance both describe implantation of intracranial/SEEG electrodes under general anaesthesia.
Once the person has recovered from the operation, the electrodes remain temporarily in place while prolonged recording takes place on the ward.
What happens immediately after implantation?
Practice varies between centres.
A person may initially recover in:
a recovery unit
high-dependency area
or specialist neurological ward.
Imaging such as CT may be performed to check:
electrode position
and for immediate complications such as bleeding.
The Northern Care Alliance describes performing CT after placement before continued monitoring on its video-telemetry unit.
What happens during the SEEG admission?
Once connected, the electrodes continuously record electrical activity from the implanted brain regions.
Video is recorded at the same time.
The team wants to record the person's typical spontaneous seizures.
The recordings can show:
which electrode contacts change first
how electrical activity evolves
how activity spreads
what the person does at the same time
and whether several seizure types share the same origin.
This can be far more precise spatially than scalp EEG for the selected regions being sampled.
How long does SEEG monitoring last?
There is no fixed duration.
Monitoring usually continues until the clinical team has collected enough information to answer the diagnostic question, provided it remains safe to continue.
UCLH describes intracranial recordings commonly lasting around one to two weeks.
King's reports that intracranial telemetry can range from roughly 5 to 21 days, depending on the case.
The Northern Care Alliance says many of its SEEG admissions are around 10 days, with longer admissions occasionally required.
These figures describe individual NHS services rather than guaranteed lengths of stay.
Why might the admission take several days?
Doctors need to record enough seizures to know that what they are seeing represents the person's usual epilepsy.
One seizure may not always be enough.
For example, someone may have:
more than one seizure type
apparently different starting patterns
seizures arising from more than one region
or very infrequent events.
The team may need several representative seizures before drawing conclusions.
Medication may sometimes be reduced
As with ordinary inpatient video telemetry, doctors may temporarily reduce antiseizure medication to increase the likelihood of recording seizures.
This is done only under specialist hospital supervision.
King's and the Northern Care Alliance both describe medication reduction as something that may occur during prolonged monitoring when clinically appropriate.
The decision depends on:
seizure type
usual frequency
risk of clusters
risk of status epilepticus
medication
and the information already obtained.
Do not reduce medication at home to “help” the test
No.
Medication changes made during SEEG occur in an environment where:
seizures are continuously monitored
trained nurses are present
rescue medication is available
and doctors can respond to complications.
The Northern Care Alliance specifically warns that reducing or stopping antiseizure medication can increase the risk of longer or more intense seizures and describes having emergency treatment plans in place during SEEG.
A person should not reproduce this medication reduction independently.
Why video still matters when electrodes are inside the brain
SEEG tells doctors what the sampled brain regions are doing electrically.
Video shows what is happening clinically.
The team can therefore connect:
electrical activity
with:
actual symptoms and behaviour.
For example:
an electrode contact begins showing seizure activity;
seconds later the person reports déjà vu;
then responsiveness changes;
then automatisms begin;
then activity spreads into another region.
That sequence can provide important information about the seizure network.
What is the seizure-onset zone?
The seizure-onset zone is the region where recorded seizure activity appears to begin.
SEEG can help identify it.
But epilepsy surgery is more complicated than simply:
“Find the first electrode that changes and remove that spot.”
Specialists also need to understand:
the broader epileptogenic network
how reproducible the onset is
whether more than one region is involved
and whether the suspected tissue can be safely treated.
The ILAE continues to study how intracranial EEG connectivity and network analysis may improve localisation of epileptogenic networks.
The seizure-onset zone and epileptogenic zone are not identical concepts
These terms can easily become confused.
The seizure-onset zone refers to tissue where seizures are observed to begin during recordings.
The epileptogenic zone is a broader surgical concept: the brain tissue that would need to be treated to achieve seizure freedom.
Doctors cannot simply “see” the entire epileptogenic zone with one test.
It is inferred by combining:
intracranial EEG
imaging
seizure symptoms
neuropsychology
surgical anatomy
and outcome evidence.
This is one reason epilepsy-surgery decisions are made by multidisciplinary teams.
Can SEEG investigate deep brain structures?
Yes.
This is one of its major advantages.
Depth electrodes can sample regions such as:
hippocampus
amygdala
insula
deep sulci
and other structures difficult to assess directly using surface electrodes.
This can be particularly valuable when scalp EEG cannot clearly distinguish between several deep or closely connected regions.
SEEG can examine both sides of the brain
Where clinically necessary, trajectories can be planned in both hemispheres.
For example, specialists may need to determine whether apparent temporal seizures:
consistently begin on one side
arise independently from both sides
or spread so rapidly that scalp EEG makes the origin difficult to identify.
Bilateral implantation does not automatically mean the person has generalised epilepsy.
It may simply be required to test competing focal hypotheses.
Why scalp EEG can sometimes look bilateral
Electrical activity spreads through connected brain networks.
A seizure beginning in one focal region may spread very quickly.
By the time activity becomes clearly visible on the scalp, it may appear:
widespread
bilateral
or poorly localised.
SEEG can sometimes show an earlier focal pattern that was difficult to recognise externally.
But this depends on whether the true onset region was sampled by the implanted electrodes.
What if SEEG finds seizures beginning from several places?
That information is important even if it means conventional resective surgery becomes less straightforward.
The MDT may conclude that:
there is one dominant treatable network
there are several independent seizure-onset regions
seizure onset is too widespread
the important tissue cannot safely be removed
another treatment should be considered
or additional investigation is required.
A test does not fail simply because it does not lead to resection.
It may prevent an operation unlikely to help.
Can SEEG prove that surgery will work?
No.
SEEG can substantially improve understanding of:
seizure onset
spread
and functional anatomy.
But no presurgical test can guarantee seizure freedom after an operation.
Surgical outcome depends on many factors, including:
epilepsy cause
completeness of treatment
seizure network
lesion type
whether all relevant epileptogenic tissue can be safely treated
and individual biology.
SEEG helps improve the evidence used to make the decision.
It does not provide certainty.
What is electrical stimulation mapping?
The same implanted electrodes can sometimes be used to deliver carefully controlled small electrical currents.
While the person is awake, the team may ask them to:
speak
name pictures
move a limb
report sensations
or perform other tasks.
Electrical stimulation can help determine whether particular contacts are close to brain regions involved in functions such as:
movement
sensation
language
or other important abilities.
UCLH and the Northern Care Alliance both describe functional stimulation through implanted electrodes as part of selected intracranial EEG assessments.
Why map function?
Suppose SEEG suggests seizures begin in a particular area.
That does not automatically mean the area can safely be treated.
The same or neighbouring tissue may participate in:
speech
movement
sensation
vision
memory
or another important function.
Functional mapping helps the surgical team understand the potential consequences of treating that region.
The goal is not simply:
remove seizure tissue.
It is:
maximise the likelihood of seizure improvement while protecting important brain function.
Can stimulation cause seizure symptoms?
It can.
Electrical stimulation may occasionally reproduce:
a familiar aura
movement
sensory change
or sometimes a person's typical seizure.
This can provide additional information about the network being tested.
Because stimulation can provoke seizures, it is carried out under specialist supervision.
The Northern Care Alliance describes bedside functional stimulation during SEEG and notes that typical seizure symptoms can occasionally be produced during testing.
SEEG is not the same as deep brain stimulation
The names can sound similar.
SEEG uses temporarily implanted electrodes primarily to record and investigate epilepsy.
Deep brain stimulation — DBS involves permanently implanted stimulation electrodes used therapeutically in selected people.
SEEG electrodes are normally removed after the required information has been collected.
DBS is a treatment system intended to remain implanted.
They are different procedures.
SEEG is not the same as RNS either
Responsive neurostimulation — RNS — uses implanted electrodes as part of a long-term therapeutic system that detects abnormal electrical activity and delivers stimulation.
SEEG is predominantly a temporary diagnostic/presurgical investigation.
SEEG may help identify brain regions relevant to later:
resection
ablation
disconnection
or neuromodulation.
But it is not itself the same treatment.
Can anything be treated through the SEEG electrodes?
Some centres can perform a procedure called radiofrequency thermocoagulation — RFTC through selected SEEG contacts in carefully selected circumstances.
Electrical current is used to create a small controlled thermal lesion between selected contacts.
The Northern Care Alliance describes SEEG-guided thermocoagulation as something that may be considered in some patients after the diagnostic recording.
This is not automatically performed during every SEEG admission.
It is a separate clinical decision.
RFTC should not be confused with the main purpose of SEEG
SEEG's primary purpose is usually to obtain diagnostic information about the seizure network.
The fact that limited treatment can sometimes be delivered through electrodes does not mean every implantation is therapeutic.
For many people, the next step after SEEG may instead be discussion of:
resective surgery
laser ablation
another surgical procedure
neuromodulation
further medical treatment
or no operation.
What are subdural grids?
SEEG is not the only type of intracranial EEG.
Another approach uses electrodes placed on the surface of the brain.
These may be called:
subdural grids
strips
or surface electrodes.
UCLH describes two broad intracranial EEG approaches: electrodes covering the brain surface and depth electrodes placed into the brain using SEEG.
Why would grids be used instead of SEEG?
Surface electrodes can provide dense recordings over a particular area of cortex and can be useful when detailed mapping of the exposed cortical surface is required.
SEEG, by contrast, is particularly suited to sampling:
deep structures
multiple separated brain regions
networks involving both superficial and deep areas
and bilateral hypotheses.
The most appropriate approach depends on the surgical question.
Is SEEG safer than grid monitoring?
Both are invasive neurosurgical procedures and both have risks.
An international ILAE-associated comparative study examined more than a thousand eligible intracranial EEG cases.
In that observational dataset, complications were less frequent with SEEG than with subdural electrode evaluation.
However, treatment selection was not random, patient populations differed, and the choice of technique must still be individualised.
It would therefore be wrong to say that one technique is automatically the correct choice for every person.
What are the main risks of SEEG?
The most important procedural risks include:
bleeding inside the skull
infection
injury to brain tissue
neurological deficit
cerebrospinal fluid leakage
risks associated with anaesthesia
and risks associated with deliberately recording seizures.
The exact risk varies according to:
number and position of electrodes
individual anatomy
health
surgical technique
and centre.
How common is bleeding?
Bleeding is uncommon but is one of the principal serious risks.
The Northern Care Alliance's current patient information describes intracranial bleeding as occurring in less than 1 in 100 cases at its service.
Birmingham Women's and Children's NHS information similarly describes the risk of brain bleeding during paediatric SEEG as approximately 1%.
These figures are useful examples, but an individual centre should provide its own current risk figures because complication rates can vary between populations and procedures.
Can bleeding cause permanent problems?
Rarely, yes.
A significant haemorrhage can damage brain tissue.
Depending on location and severity, possible consequences could include:
weakness
language problems
visual problems
or other neurological deficits.
Occasionally additional surgery may be required to treat a significant bleed.
This is why electrode trajectories and blood vessels are planned so carefully.
Infection
Electrodes temporarily pass from outside the skull into the brain.
That creates an infection risk.
Hospitals therefore use measures such as:
sterile surgery
wound care
head dressings
monitoring
and other infection-prevention procedures.
UCLH describes keeping the head bandaged during intracranial monitoring partly to reduce infection risk.
Any signs of infection during or following the admission require medical assessment.
There are also risks from the seizures themselves
To obtain useful information, the team often needs to record typical seizures.
Those seizures can bring risks including:
falls
injury
clusters
prolonged seizures
or status epilepticus.
This is another reason monitoring takes place on a specialist ward.
Staff can:
observe continuously
provide first aid
administer rescue medication
and restart or adjust treatment when necessary.
What is it like being monitored?
The experience varies between hospitals.
Someone may spend several days largely confined to:
a bed
chair
and monitored room
because the electrodes are connected to recording equipment and seizures need to remain visible on video.
The Northern Care Alliance advises patients to remain within the camera view as much as possible and describes specialist nursing and daily equipment checks during SEEG monitoring.
This can be:
boring
tiring
uncomfortable
emotionally difficult
and stressful.
Those aspects are part of the procedure too.
Can someone wash normally?
Restrictions vary.
Because electrodes, dressings and connections need to remain protected, normal showering and hair washing may not be possible during monitoring.
One NHS SEEG unit advises patients that they cannot shower, bathe or wash their hair during the implanted recording period but can wash within their private bathroom.
The local team provides specific instructions.
What about privacy?
Video recording is part of the investigation.
Clinical cameras are normally positioned to observe seizures while preserving privacy in bathroom and toilet areas.
The Northern Care Alliance states that its SEEG videos are treated as confidential medical records and that separate permission is sought for uses such as teaching, research or publication.
Patients can ask the centre:
who can view the recordings
how long they are kept
and how they are protected.
How are the electrodes removed?
Once sufficient information has been obtained, the electrodes are removed.
UCLH states that SEEG electrodes are generally removed using local anaesthesia in its programme.
Other centres may use:
local anaesthetic
general anaesthetic
or a combination depending on circumstances.
The removal procedure is usually considerably shorter than the implantation procedure.
Does surgery happen immediately afterwards?
Not necessarily.
With SEEG, the electrodes are normally removed and the results are then analysed.
The team may need to review:
multiple seizures
hundreds or thousands of EEG channels or contacts
imaging
stimulation results
and the rest of the presurgical investigation.
The case is then usually reconsidered at an epilepsy-surgery multidisciplinary team — MDT meeting.
What happens at the MDT?
The MDT may include:
neurologists or epileptologists
neurosurgeons
neurophysiologists
neuroradiologists
neuropsychologists
specialist nurses
and other professionals.
The team asks:
Where did the seizures begin?
Was the pattern consistent?
How did they spread?
Does it fit the MRI and other investigations?
Is the suspected epileptogenic network treatable?
What important functions may be at risk?
What treatment offers the best balance of benefit and harm?
King's describes epilepsy surgery as a multidisciplinary programme combining several specialist investigations, including SEEG, before treatment decisions are made.
Possible outcomes after SEEG
SEEG can lead to several different conclusions.
Doctors may identify a clear focal region that could be considered for treatment.
They may find a network suitable for:
resection
disconnection
laser ablation
or another surgical approach.
They may determine that the seizures arise from an area that cannot safely be treated.
They may find several independent seizure-onset regions.
The recording may remain inconclusive.
Or the results may suggest that another treatment strategy is preferable.
Every one of these outcomes can provide useful information.
SEEG does not automatically lead to surgery
This is important.
Agreeing to SEEG means agreeing to an investigation.
It does not mean agreeing to subsequent brain surgery.
After the results are available, the epilepsy team should explain:
what was found
what remains uncertain
treatment options
likely benefits
risks
and alternatives.
The person then decides whether any proposed treatment is acceptable.
The Northern Care Alliance explicitly describes discussing SEEG results at an MDT and then giving the patient time to consider the risks and benefits before deciding whether to proceed with any operation.
What if SEEG does not find one clear seizure source?
That is not necessarily because something went wrong.
Possible explanations include:
seizures arise from a wider network
there are multiple onset regions
the true onset zone was not sampled
different seizures begin differently
activity spreads extremely rapidly
or current techniques cannot define the network accurately enough.
The outcome may therefore be:
surgery cannot currently be recommended safely.
Avoiding an operation unlikely to work can itself be an important result.
Can SEEG be repeated?
Occasionally further invasive investigation may be considered, but repeat SEEG is not a routine next step whenever the first study is inconclusive.
The MDT would need to consider:
why the first study did not answer the question
whether there is a substantially better new hypothesis
whether additional information has become available
and whether another invasive procedure is justified.
Can someone have SEEG with a normal MRI?
Yes.
MRI-negative focal epilepsy is one situation in which SEEG may be particularly valuable.
If other evidence suggests that seizures arise from a limited network but MRI does not reveal a visible lesion, invasive recording may help determine whether a surgically treatable seizure-onset region exists.
A normal MRI therefore does not automatically exclude epilepsy-surgery assessment.
Related Information Hub page:
MRI and Epilepsy: What Doctors Are Looking For
Can SEEG be used if scalp EEG appears bilateral?
Sometimes.
Scalp EEG may show activity on both sides because seizure activity:
spreads rapidly
arises from deep structures
or is difficult to localise externally.
If the wider presurgical evidence still supports specific focal hypotheses, specialists may use SEEG to test them.
But genuinely multifocal or widely distributed epilepsy may make focal surgery less appropriate.
The decision depends on the complete evidence.
SEEG can investigate a network, not just a lesion
Modern epilepsy surgery increasingly recognises epilepsy as a network disorder.
A structural lesion may be important, but the seizure-generating network can include:
the lesion
surrounding cortex
connected structures
and pathways through which activity spreads.
SEEG can help show the temporal sequence of electrical activity across selected parts of that network.
The ILAE's Epilepsy Surgery Commission continues to investigate intracranial EEG connectivity as a method for understanding and localising epileptogenic networks.
Why might the symptoms and SEEG location appear different?
The first symptom somebody notices is not always produced by the precise tissue where electrical activity originally started.
A seizure may begin silently in one region before spreading into a network that produces:
fear
déjà vu
movement
language difficulty
or another conscious symptom.
SEEG can sometimes reveal that electrical onset precedes the first reported symptom.
This is one reason doctors combine seizure semiology with electrical recordings rather than relying on symptoms alone.
What can SEEG tell doctors about auras?
If a person experiences their familiar aura during SEEG, clinicians can examine:
which contacts were active first
which regions became involved when the aura appeared
and whether the aura progressed into another seizure state.
This can help link a subjective symptom with a specific recorded network.
Related Information Hub page:
Auras and Epilepsy: The Seizure Before the Seizure?
What if the person does not remember the seizure?
That does not prevent SEEG from being useful.
The electrical recording continues regardless of whether the person:
recognises the seizure
presses an event button
remembers it afterwards
or is asleep.
Staff and later EEG review can identify seizures that the person did not report.
This is particularly important in epilepsy involving:
impaired consciousness
amnesia
or nocturnal events.
SEEG and memory
Placing electrodes in temporal or hippocampal regions may allow specialists to study electrical activity involving memory networks.
However, SEEG by itself does not replace formal neuropsychological assessment.
Memory risk before surgery is assessed using a combination of:
seizure location
structural imaging
functional investigations
neuropsychology
and sometimes stimulation mapping.
Related Information Hub page:
Neuropsychological Testing and Epilepsy
SEEG and language
Where the suspected seizure network lies near language-related cortex, electrical stimulation through implanted contacts can sometimes help identify language function.
This contributes to answering:
Could treatment of this region damage speech or language?
SEEG mapping is only one component of that assessment.
Functional MRI and neuropsychology may provide additional information.
Our next planned page covers this wider subject:
Functional MRI and Brain Mapping Before Epilepsy Surgery
Children can have SEEG too
SEEG is used in specialist paediatric epilepsy-surgery programmes when appropriate.
Great Ormond Street Hospital describes SEEG as one form of invasive monitoring used to confirm:
the exact region causing a child's seizures
and the location of areas involved in important functions such as movement or speech.
Birmingham Women's and Children's NHS Foundation Trust likewise uses SEEG within its invasive epilepsy monitoring pathway.
Decisions in children also take account of:
age
development
epilepsy syndrome
brain development
cognition
and the possible impact of continued seizures.
Why would someone agree to an invasive test when there is no guarantee of surgery?
Because the unanswered question may be impossible to resolve reliably with non-invasive testing alone.
Without SEEG, clinicians might know:
this person has disabling drug-resistant focal epilepsy
but still not know:
where it begins precisely enough to treat it safely.
SEEG may turn that uncertainty into:
a possible surgical treatment
a different treatment option
or evidence that surgery would be too risky or unlikely to succeed.
Each of those outcomes can change care.
Why might someone decide not to have SEEG?
SEEG is optional.
A person may decide the potential information is not worth:
invasive surgery
hospital admission
seizure provocation
or procedural risk.
The Northern Care Alliance notes that declining SEEG may reduce the likelihood of being offered resective epilepsy surgery when the invasive information is necessary to identify a safe target.
But the decision belongs to the person after informed discussion with the clinical team.
Questions to ask before SEEG
Useful questions include:
Why do you think I need intracranial EEG?
What question has the non-invasive testing failed to answer?
What are your current hypotheses about where my seizures start?
Which parts of my brain will you sample?
How many electrodes are you planning?
Will electrodes be on one side or both sides?
What imaging will you use to plan the trajectories?
What are this centre's current complication rates?
How long do you expect me to be monitored?
Will my epilepsy medication be reduced?
What is the emergency plan if I have prolonged or repeated seizures?
Questions about functional mapping
Useful questions include:
Will you perform electrical stimulation mapping?
Which functions are you trying to locate?
Could stimulation trigger one of my seizures?
What happens if the seizure-onset region overlaps language or movement cortex?
Will my neuropsychological results be considered alongside the stimulation findings?
Questions about the result
Useful questions include:
Did my seizures all begin in the same place?
How certain are you about the seizure-onset zone?
Did different seizure types start differently?
Did the SEEG findings agree with my MRI and scalp EEG?
Is there one treatable epileptogenic network?
Does important brain function overlap the proposed treatment area?
What treatment options do the findings support?
What are the chances of seizure freedom or meaningful improvement?
What are the risks of the proposed treatment?
What happens if I decide against further surgery?
The most important message
Stereo-EEG is one of the most specialised investigations used in epilepsy care.
It is not a routine diagnostic EEG.
Thin depth electrodes are temporarily implanted into carefully selected brain regions to record seizures directly from within the brain.
It is generally considered when:
focal epilepsy is drug resistant
surgery is being investigated
non-invasive tests have not localised the seizure network precisely enough
and there is a clear clinical question that invasive recording may answer.
Electrode placement is based on a pre-existing hypothesis built from:
seizure history
video EEG
MRI
functional imaging
neuropsychology
and other tests.
The electrodes can then show:
where recorded seizures begin
how they spread
whether different seizures share the same network
and how the suspected seizure region relates to important brain function.
But SEEG has limitations.
It samples selected parts of the brain rather than the entire brain.
It cannot guarantee that subsequent surgery will produce seizure freedom.
And because it is an invasive neurosurgical procedure, it carries risks including:
bleeding
infection
neurological injury
and risks associated with provoking seizures during monitoring.
If SEEG identifies a convincing, safely treatable seizure network, it may open the door to:
resection
ablation
disconnection
or another targeted treatment.
If it shows that the seizure network is too widespread or overlaps brain function that cannot safely be sacrificed, that information is equally important.
The purpose of SEEG is therefore not simply:
“put electrodes in the brain and find the bad spot.”
It is to answer a carefully planned question about a person's individual epilepsy network so that any later treatment decision is based on the strongest evidence possible.
Sources and further reading
Northern Care Alliance NHS Foundation Trust — Stereo Electro-Encephalography.
Detailed current NHS patient guidance covering SEEG electrode implantation, robotic guidance, monitoring, electrical stimulation, medication reduction, electrode removal, risks and the MDT process.
University College London Hospitals — A guide for patients considering epilepsy surgery.
Explains intracranial EEG using both surface grids and SEEG depth electrodes, typical prolonged recording, functional stimulation and electrode removal within a UK epilepsy-surgery programme.
King's College Hospital — Epilepsy service.
Describes SEEG within a comprehensive UK epilepsy-surgery programme alongside video EEG, high-field MRI, functional MRI, PET, SPECT, neuropsychology and neuropsychiatry.
Great Ormond Street Hospital — Invasive EEG monitoring.
Current paediatric NHS guidance describing invasive monitoring using surface or depth electrodes to identify seizure-generating regions and important functional brain areas.
Birmingham Women's and Children's NHS Foundation Trust — Invasive monitoring with SEEG.
Current paediatric information covering implantation, medication reduction and procedural risks, including intracranial bleeding.
International League Against Epilepsy — Epilepsy Surgery Commission.
Current ILAE work includes presurgical assessment, intracranial EEG and research into the use of intracranial network analysis to localise epileptogenic networks.
ILAE-associated international comparative study — SEEG versus subdural electrodes.
An international registry study found lower observed complication rates with SEEG than subdural electrode evaluations in the analysed cohort, while emphasising that invasive monitoring technique should be chosen according to the individual clinical situation.
Information reviewed: September 2026.
This page provides general educational information. SEEG is an invasive specialist investigation. Individual benefits, alternatives and complication risks should be discussed with the person's epilepsy neurologist and neurosurgeon before consent.
