Focal cortical dysplasia (FCD) is a brain malformation where a small area of the cerebral cortex develops abnormally before birth, creating a patch of disorganized neurons that can trigger seizures. It is one of the most common causes of drug-resistant epilepsy, particularly in children. About 74% of children with FCD-related epilepsy do not respond to standard seizure medications, making it a condition where surgery often becomes the primary path to seizure control.
How FCD Develops
During fetal brain development, neurons are supposed to migrate to specific layers of the cortex and organize themselves into a precise architecture. In FCD, something disrupts that process in a localized area. The result is a patch of cortex where the normal six-layer structure is scrambled: neurons may be the wrong size, in the wrong position, or missing their usual layered arrangement entirely.
The underlying cause in many cases is a genetic mutation that occurs spontaneously in a small number of brain cells during development. These are somatic mutations, meaning they aren’t inherited from parents and don’t appear in blood tests. They affect a cellular growth pathway called mTOR, which acts like a master switch controlling how cells grow and divide. When genes in this pathway (including MTOR, AKT3, and DEPDC5) are mutated, affected cells grow too large or fail to migrate properly, creating the dysplastic patch. In some patients, these mutations are detectable in as few as 2 to 3% of cells in the affected brain tissue, which explains why the malformation stays focal rather than affecting the whole brain.
Types of FCD
The International League Against Epilepsy classifies FCD into three main types based on what the tissue looks like under a microscope.
Type I involves abnormal layering of the cortex without dramatically abnormal-looking neurons. Type Ia, most often found in the occipital lobe, features columns of small neurons stacked vertically like strings of pearls. Type Ib, typically in the temporal lobe, shows a complete loss of the normal layered cortical organization. Type I tends to cause seizures that begin later in life, often in adulthood, and the affected area is usually small and located in the temporal lobe.
Type II is the more severe form, characterized by large, misshapen neurons scattered randomly through the cortex with no recognizable layering. Type IIb also contains distinctive “balloon cells,” abnormally swollen cells that are a hallmark of this subtype. Type II lesions appear more often in the frontal lobe and tend to cause seizures earlier, sometimes beginning in the newborn period or infancy. These lesions are often larger and may span multiple lobes or an entire hemisphere.
Type III refers to cortical dysplasia that occurs alongside another brain condition, such as scarring in the hippocampus, a vascular malformation like Sturge-Weber syndrome, or damage from a perinatal stroke. In these cases, the dysplasia is considered secondary to or associated with the other pathology.
Symptoms and Age of Onset
Seizures are the defining symptom. The specific type of seizure depends on where in the brain the dysplastic patch is located and how large it is. A lesion in the motor cortex might cause rhythmic jerking of one arm or leg. One in the temporal lobe might produce episodes of staring, confusion, or unusual sensations. Some patients experience multiple seizure types.
Symptoms can appear at any age, but most commonly start in childhood. Children with Type II FCD generally develop seizures earlier than those with Type I. Beyond seizures, cognitive difficulties affect roughly half of patients who eventually undergo surgery, including lower IQ, slower processing speed, problems with memory, and difficulty with attention and executive function. Intellectual disability occurs in up to 25% of patients. Psychiatric conditions like ADHD, autism spectrum disorder, anxiety, and depression show up in 5 to 30% of cases. Developmental delays are especially common in younger children, particularly when seizures begin early and are frequent.
How FCD Is Diagnosed
MRI is the first-line imaging tool, and it can detect FCD in many but not all cases. Radiologists look for several characteristic signs: thickening of the cortex (seen in 60 to 91% of cases), blurring at the boundary between gray and white matter (74 to 96%), and an abnormal pattern of brain folds. In Type IIb, the most distinctive finding is the “transmantle sign,” a cone-shaped streak of abnormal signal extending from the cortex deep into the white matter toward the ventricles, visible in 94% of patients with this subtype.
MRI sensitivity varies by type. It detects Type II lesions in 65 to 90% of cases but finds Type I lesions only 55 to 80% of the time. When MRI comes back normal but clinical suspicion remains high, metabolic brain imaging using PET scans can help. PET measures glucose activity in the brain and can identify areas of reduced metabolism that correspond to the dysplastic tissue. Its sensitivity reaches 70 to 90%, and in one study, PET detected abnormalities in 89% of patients compared to 74% for MRI. In four patients, neither test found anything, underscoring that some lesions remain invisible to current imaging.
Why Medications Often Fail
FCD-related epilepsy is notoriously resistant to anti-seizure medications. In one large study of children with FCD and epilepsy, 74% met criteria for drug resistance. A study from Great Ormond Street Hospital in London found an even higher rate of 94%. One striking finding is how early the pattern emerges: children who fail even a single medication face dramatically increased odds of becoming fully drug-resistant. This doesn’t mean medications are useless. About a quarter of children do achieve seizure control with medication alone. But for the majority, drugs reduce seizure frequency without eliminating seizures entirely.
Surgery and Seizure Outcomes
For patients whose seizures don’t respond to medication, surgical removal of the dysplastic tissue is the most effective treatment. The goal is to resect the entire abnormal area while preserving as much healthy brain as possible. In pediatric studies, about 66% of patients with FCD become seizure-free after surgery, with outcomes tracked over an average of two years. That rate is slightly lower than for brain tumors causing epilepsy (79%), likely because the borders of dysplastic tissue are harder to define precisely than tumor margins.
Complete removal of the lesion is the strongest predictor of seizure freedom. When surgeons can clearly see the abnormality on MRI and remove it entirely, outcomes improve significantly. This is one reason why accurate preoperative imaging matters so much. For patients with MRI-negative FCD, the surgical planning process is more complex and may involve invasive monitoring with electrodes placed directly on or within the brain to map the seizure-generating zone before resection.
Even among the 34% who aren’t seizure-free after surgery, many experience a meaningful reduction in seizure frequency and severity. Some are able to reduce their medications or see improvements in cognition and development, particularly children whose frequent seizures had been interfering with brain maturation.