Focal cortical dysplasia (FCD) is a developmental abnormality of the cerebral cortex in which a region of brain tissue forms with disorganised architecture, abnormal cells or both. It is one of the most important structural causes of focal epilepsy and is particularly associated with drug-resistant seizures in children and young adults.
FCD can be obvious on MRI, extremely subtle, or apparently invisible on routine imaging. In some people it is discovered only after epilepsy surgery, when removed tissue is examined by a neuropathologist.
Because of this, focal cortical dysplasia sits at the intersection of epilepsy, neurodevelopment, MRI, genetics, neuropathology and epilepsy surgery.
What focal cortical dysplasia actually is
The cerebral cortex is normally organised into highly structured layers containing different types of neurons and supporting cells. During fetal brain development, neurons must be generated, migrate to the correct location and organise into functional cortical networks.
In focal cortical dysplasia, that process is locally disrupted. The abnormal region may contain disordered cortical layering, unusually large or malformed neurons, balloon cells or more subtle microscopic abnormalities.
The result is a patch of cortex that is structurally different from surrounding brain and can become highly epileptogenic.
FCD is a developmental lesion, not a scar acquired later in life
Unlike post-stroke epilepsy or post-traumatic epilepsy, focal cortical dysplasia develops because cortical formation was abnormal during brain development.
The lesion may remain clinically silent for years before seizures begin. Some people present in infancy or childhood, while others are not diagnosed until adolescence or adulthood.
The age at which epilepsy starts therefore does not tell us when the dysplasia formed.
Why dysplastic cortex can generate seizures
FCD can disrupt the normal balance between excitation and inhibition. Abnormal neurons may have altered membrane properties, abnormal receptor expression and atypical connectivity.
Local networks can become excessively synchronised and capable of generating recurrent epileptic discharges.
In some FCD type II lesions, molecular abnormalities involving the mTOR pathway appear to contribute directly to altered cell growth and network excitability.
The ILAE classification
The International League Against Epilepsy introduced a consensus classification of focal cortical dysplasia in 2011 and updated it in 2022.
The system is based mainly on neuropathology, while the updated framework increasingly integrates imaging and molecular-genetic information.
The major categories remain FCD type I, type II and type III, with additional recognition of mild malformations of cortical development and related entities.
FCD type I
FCD type I is characterised mainly by abnormal cortical architecture rather than the striking abnormal cell types seen in type II.
The normal six-layer organisation of the cortex may be disturbed, with abnormal radial or tangential arrangement of neurons.
Type I lesions can be difficult to detect on MRI and can also be challenging for neuropathologists because changes may be subtle.
The biological basis is less well understood than for FCD type II.
FCD type II
FCD type II is usually more distinctive histologically and radiologically. It includes dysmorphic neurons and, in subtype IIb, balloon cells.
These lesions are strongly associated with drug-resistant focal epilepsy and are among the best-characterised malformations of cortical development.
Type II lesions frequently involve somatic mutations affecting the mTOR signalling pathway.
FCD type IIa
FCD IIa contains dysmorphic neurons but no balloon cells.
Dysmorphic neurons are abnormally enlarged neurons with altered internal structure and abnormal orientation.
The lesion can be highly epileptogenic even when the abnormal region is relatively small.
FCD type IIb
FCD IIb contains dysmorphic neurons together with balloon cells.
Balloon cells are very large abnormal cells with pale cytoplasm and unusual molecular characteristics. They resemble some abnormal cells seen in tuberous sclerosis complex, which is one reason researchers have studied shared mTOR-related biology between these conditions.
FCD IIb often has more recognisable MRI features than type I, although it can still be missed.
FCD type III
FCD type III refers to cortical architectural abnormalities occurring next to another principal brain lesion.
The associated lesion determines the subtype. Examples include dysplasia associated with hippocampal sclerosis, epilepsy-associated tumours, vascular malformations or early acquired brain injury.
This category is important because the abnormal cortex and the associated lesion may form part of the same epileptogenic network.
Why the 2022 ILAE update matters
The 2022 ILAE update recognised that histology alone does not capture the full biology of focal cortical dysplasia.
The modern approach is increasingly multilayered: neuropathology, MRI phenotype and molecular-genetic findings can all contribute to an integrated diagnosis.
This is particularly important in type II FCD, where somatic mTOR-pathway mutations are now well established.
Somatic mutations can be confined to the brain
Many FCD-associated genetic changes are somatic rather than inherited. This means the mutation may arise after conception in a subset of developing brain cells instead of being present in every cell of the body.
A blood genetic test can therefore be normal even when abnormal brain tissue contains a pathogenic mutation.
In some surgical cases, genetic testing of resected brain tissue can identify low-level mosaic variants that would not have been detected in blood.
The mTOR pathway is central to many FCD II lesions
The mechanistic target of rapamycin, or mTOR, pathway regulates cell growth, metabolism and development.
Activating variants in genes within this pathway can cause abnormal cell growth and cortical development. Genes implicated in FCD include members of the PI3K-AKT-mTOR pathway and related regulators.
This molecular overlap connects FCD biologically with disorders such as tuberous sclerosis complex and some forms of megalencephaly.
FCD does not always form a sharply defined lump
The word ‘lesion’ can suggest a discrete object with clear borders. FCD is often more complicated.
The abnormality may blend gradually into surrounding cortex, extend along the depth of a sulcus or involve a wider microscopic region than MRI suggests.
This becomes important during surgery because incomplete removal of the epileptogenic dysplastic tissue is associated with poorer seizure outcomes.
The bottom of a sulcus can hide the lesion
Some type II dysplasias cluster around the deepest part of a cortical sulcus. These ‘bottom-of-sulcus dysplasias’ can be tiny but highly epileptogenic.
They may be overlooked on routine MRI unless thin-slice epilepsy-protocol imaging and careful multiplanar review are performed.
This is one reason a supposedly MRI-negative focal epilepsy can later prove to have a structural cause.
What MRI can show
Typical MRI features can include focal cortical thickening, blurring of the boundary between grey and white matter, abnormal cortical signal, abnormal gyral or sulcal shape and increased T2 or FLAIR signal.
In FCD IIb, a tapering band of abnormal signal extending from the cortex toward the lateral ventricle may produce the so-called transmantle sign.
No single MRI feature is present in every case.
The transmantle sign
The transmantle sign is classically associated with FCD type II, particularly IIb.
On MRI, abnormal signal appears to extend through the white matter from the cortex toward the ventricle.
It can be diagnostically helpful, but its absence does not exclude FCD.
Why routine MRI can miss FCD
Subtle dysplasia can be obscured by thick image slices, motion, non-optimised imaging planes or insufficient contrast between grey and white matter.
An epilepsy-protocol MRI uses high-resolution three-dimensional sequences and carefully selected T1, T2 and FLAIR imaging designed to reveal subtle structural abnormalities.
Expert neuroradiology review can be as important as the scanner itself.
Repeat MRI can reveal a lesion that was previously missed
A person labelled ‘MRI-negative’ years ago may benefit from repeat imaging when technology, protocols or specialist interpretation have improved.
Higher-field scanners, improved sequences and post-processing techniques can reveal abnormalities that were not visible on earlier studies.
In children, brain maturation can also make previously subtle abnormalities easier to recognise over time.
MRI-negative does not mean lesion-negative
Some people with pathologically confirmed FCD have no visible lesion on preoperative MRI.
In these cases, seizure semiology, scalp EEG, PET, SPECT, MEG, high-density EEG or intracranial recordings may provide evidence of where seizures begin.
Modern post-processing can also identify subtle cortical abnormalities that are difficult to appreciate visually.
PET can help when MRI is unrevealing
Interictal FDG-PET may show reduced glucose metabolism in the region surrounding an FCD.
The area of hypometabolism can be larger than the actual structural lesion, reflecting disruption of a wider functional network.
PET is therefore used as part of multimodal presurgical assessment rather than as a standalone test.
SPECT can capture seizure-related blood-flow changes
Ictal SPECT performed during or immediately after seizure onset can show increased perfusion in regions involved early in the seizure network.
When combined with MRI, subtraction techniques may help localise otherwise subtle lesions.
Its usefulness depends heavily on injection timing and local expertise.
MEG and high-density EEG can provide additional localisation
Magnetoencephalography and advanced EEG source imaging can help estimate the cortical origin of interictal epileptic activity.
These methods may be especially valuable in MRI-negative cases or where non-invasive tests give partly conflicting results.
They do not replace pathology or intracranial EEG when those are needed.
Scalp EEG may underestimate how focal the lesion is
A small dysplasia can produce widespread scalp EEG abnormalities because electrical activity spreads through connected networks before it reaches the scalp.
Conversely, very deep or small lesions may produce little visible scalp EEG change.
The relationship between lesion size and scalp EEG appearance is therefore not straightforward.
Seizure symptoms depend on location
FCD can occur almost anywhere in the cerebral cortex. Symptoms reflect the function of the involved region and the network through which seizure activity spreads.
Frontal lesions may cause brief motor or hypermotor seizures. Temporal lesions can produce impaired awareness, fear, déjà vu or automatisms. Parietal and occipital lesions can cause sensory or visual symptoms.
Because FCD is an underlying cause rather than a seizure type, there is no single ‘FCD seizure’ pattern.
Seizures can be very frequent
Some people with FCD experience many seizures each day, including clusters or seizures arising from sleep.
Others have much less frequent events.
Seizure burden depends on lesion biology, location, network organisation and treatment response.
FCD is strongly associated with drug-resistant epilepsy
Focal cortical dysplasia is one of the most common pathologies found in children and young adults undergoing epilepsy surgery for drug-resistant focal epilepsy.
Many people do respond partially or completely to antiseizure medicines, but a substantial proportion continue to have seizures despite appropriate treatment.
This is why early recognition matters: prolonged repeated medication failure should prompt specialist evaluation rather than indefinite cycling through drugs.
Children can be affected particularly severely
When seizures begin in infancy or early childhood and occur frequently, they can interfere with development, learning and behaviour.
In some children, widespread epileptic activity may affect cognitive development beyond the immediate effects of individual seizures.
For appropriate surgical candidates, earlier seizure control may therefore be important not only for seizure reduction but also for developmental trajectory.
FCD can occur outside the visibly abnormal cortex
The epileptogenic zone is the tissue necessary and sufficient for seizure generation. It is not always identical to the MRI lesion.
Some dysplasias produce epileptic activity from tissue extending beyond the visible border. In other cases, the most epileptogenic cortex lies within only part of the structural abnormality.
Presurgical assessment tries to define this relationship.
SEEG can be important when localisation is uncertain
Stereoelectroencephalography uses depth electrodes to record electrical activity directly from selected brain regions.
It can be useful when the suspected FCD is deep, close to eloquent cortex, MRI-negative or part of a complex network.
SEEG is not needed for every person with FCD. Some lesions are sufficiently concordant across MRI, EEG and clinical features to allow surgery without invasive recording.
Complete removal is a major predictor of seizure freedom
Across surgical studies, complete resection of the epileptogenic dysplastic tissue is one of the strongest predictors of good seizure outcome.
Outcome also depends on lesion type, location, whether MRI identifies the lesion, the extent of the seizure network and proximity to functionally critical cortex.
Incomplete removal may be necessary when tissue responsible for essential language, movement or sensory function overlaps the epileptogenic region.
Surgery is not simply ‘cutting out the MRI abnormality’
Epilepsy surgery for FCD is based on an integrated hypothesis using seizure semiology, MRI, scalp EEG, functional imaging, neuropsychology and sometimes intracranial recordings.
The aim is to remove or disconnect enough epileptogenic tissue to control seizures while preserving neurological function.
The visible dysplasia is an important part of that map, but it is not the whole decision.
Lesionectomy, lobar resection and disconnection
The surgical procedure depends on the location and extent of the dysplasia.
Some people undergo focused lesionectomy. Others require a larger cortical resection when the epileptogenic zone extends beyond the visible lesion. In extensive unilateral developmental abnormalities, more complex disconnective procedures may sometimes be considered.
There is no single standard operation for FCD.
Eloquent cortex can limit surgery
FCD may involve motor, sensory or language cortex. Removing all abnormal tissue in these regions could cause serious permanent deficits.
Functional MRI, neuropsychology, cortical stimulation mapping and intracranial recordings can help define safe surgical boundaries.
In some cases, the balance between seizure control and neurological risk means complete resection is not possible.
Neuromodulation may be considered when resection is unsafe
If the epileptogenic network cannot be removed safely, neuromodulation such as vagus nerve stimulation, deep brain stimulation or responsive neurostimulation may be considered in suitable patients.
These treatments generally aim to reduce seizures rather than remove the dysplasia itself.
Availability varies substantially between health systems.
Laser ablation is being used in selected cases
Laser interstitial thermal therapy can destroy a targeted lesion through a minimally invasive trajectory in carefully selected cases.
Its role in FCD continues to evolve and depends on lesion geometry, location and local expertise.
Open surgery remains necessary for many dysplasias, especially when the abnormal tissue is broad or poorly defined.
Pathology after surgery can change the diagnosis
Preoperative imaging may suggest FCD, but final classification depends on examination of resected tissue when surgery occurs.
Neuropathologists assess cortical architecture, neuronal morphology, balloon cells, immunohistochemical markers and associated lesions.
The pathological diagnosis can therefore be more specific than the MRI diagnosis.
Not every suspected FCD is confirmed histologically
The 2022 ILAE update recognises that some surgical specimens do not show definite FCD despite convincing clinical and imaging evidence of a focal developmental abnormality.
Sampling limitations, subtle pathology and heterogeneous lesions can contribute.
This is one reason the modern framework emphasises integrated diagnosis instead of relying on a single layer of evidence.
MOGHE and mild malformations
The updated classification also recognises mild malformations of cortical development and cortical malformation with oligodendroglial hyperplasia, often abbreviated MOGHE.
MOGHE has been associated with somatic variants involving SLC35A2 and can be linked to drug-resistant epilepsy, particularly in children.
These entities illustrate how molecular findings are reshaping the boundaries of what was once grouped under FCD.
FCD can coexist with another lesion
Dual pathology is possible. Dysplasia may occur alongside hippocampal sclerosis, tumours, vascular malformations or other structural abnormalities.
When two lesions are present, determining which tissue participates in seizure generation can be difficult.
This is especially important during surgical planning because treating only one part of a broader epileptogenic network may not control seizures.
FCD and hippocampal sclerosis can coexist
Some temporal lobe epilepsy cases contain both hippocampal sclerosis and cortical developmental abnormalities.
Under the ILAE framework, cortical architectural abnormality associated with hippocampal sclerosis can fall within FCD type IIIa.
The clinical significance varies and has to be interpreted alongside the full presurgical dataset.
FCD and epilepsy-associated tumours can coexist
Developmental tumours such as ganglioglioma and dysembryoplastic neuroepithelial tumour can be associated with abnormal surrounding cortex.
These cases are classified differently from isolated FCD because the tumour is the principal lesion.
The boundary between tumour-related developmental pathology and cortical dysplasia can be biologically complex.
Can FCD grow?
FCD itself is generally considered a developmental malformation rather than a tumour and does not behave like a progressively enlarging neoplasm.
However, MRI appearance can seem to change as imaging improves, as the brain matures or as associated signal abnormalities become more apparent.
A changing scan therefore requires expert interpretation rather than assuming the dysplasia itself is expanding.
Can FCD appear suddenly?
No. The malformation arises during brain development.
What can appear suddenly is the epilepsy, the recognition of the lesion or the ability of improved imaging to detect it.
This distinction is important when someone receives an FCD diagnosis years after earlier scans were reported as normal.
Why epilepsy can begin years after birth
A developmental lesion can exist long before the first seizure. The brain may compensate for abnormal tissue for years before network changes reach a threshold at which spontaneous seizures begin.
Age-related changes in connectivity, hormones, sleep patterns and other biological factors may influence when epilepsy becomes clinically apparent.
The exact mechanisms are still being studied.
Does every FCD cause epilepsy?
FCD is strongly associated with epilepsy because many lesions are identified during epilepsy investigation, but developmental cortical abnormalities can occasionally be found incidentally.
The presence of an MRI abnormality alone does not prove that it is epileptogenic.
Clinical concordance remains essential.
Why pathology can be more extensive than MRI
MRI detects differences in tissue structure and signal, not individual abnormal cells.
Microscopic dysplasia can therefore extend beyond the region visible on imaging.
This helps explain why apparently complete removal of the MRI lesion does not always produce seizure freedom.
Post-processing and artificial intelligence
Computer-assisted MRI analysis can measure cortical thickness, grey-white blurring and abnormal signal patterns that may be too subtle for visual inspection alone.
Machine-learning systems are increasingly being studied for detection of MRI-negative FCD.
These tools can highlight suspicious regions but do not replace expert neuroradiology or multidisciplinary epilepsy assessment.
Research is moving toward targeted molecular therapy
The discovery of mTOR-pathway abnormalities raises the possibility of treatments aimed at the molecular mechanisms that drive some FCDs.
mTOR inhibitors already have established roles in tuberous sclerosis complex, but their role in isolated FCD remains an active research area rather than standard treatment for most patients.
Future treatment may depend more on the specific molecular abnormality present in the dysplastic tissue.
The most important clinical question
For an individual with FCD, the diagnosis matters less than the complete clinical picture: where seizures begin, how often they occur, whether medicines have failed, what functions the affected cortex performs and whether the epileptogenic network can be treated safely.
Two people with the same pathological subtype can therefore have very different treatment options and outcomes.
What this page does — and does not — explain
This page focuses on focal cortical dysplasia as a structural cause of epilepsy. The broader mechanisms of brain development, MRI protocols, SEEG, drug-resistant epilepsy and epilepsy surgery are covered on their own pages. The Structural & Acquired Epilepsies hub connects FCD with other lesions such as hippocampal sclerosis, stroke, trauma, tumours and vascular malformations.
Sources
ILAE — The consensus classification of focal cortical dysplasia: 2022 update
ILAE — The clinicopathologic spectrum of focal cortical dysplasias
Updates from the ILAE classifications: imaging phenotype and genetic characterisation of focal cortical dysplasia
International League Against Epilepsy — Epilepsy Surgery resources
Evidence checked: September 2026.