MRI (magnetic resonance imaging) is the diagnostic tool that identifies structural changes in the brain that may cause seizures or be associated with epilepsy. Being able to look at brain structures is important for the treatment of patients with epilepsy in several distinct ways:
- After the first seizure, MRI can be used to identify any serious disorder that may have provoked the seizure, such as a brain tumor or arteriovenous malformation (a blood vessel abnormality).
- It can help determine the proper seizure type and syndrome. For example, partial onset seizures that secondarily generalize rapidly can be misinterpreted as primary generalized seizures. If MRI shows a structural lesion that is the likely source of the seizures, then they can be correctly classified as partial in nature. The classification often is crucial for the best treatment.
- Information from MRI is used to evaluate the likelihood that surgery would be effective in controlling a person's seizures. For example, it can identify mesial temporal sclerosis or cavernous angioma (which usually gives a favorable surgical prognosis), and malformations of cortical development (which have a less favorable prognosis for seizure control after surgery).
- Before surgery, MRI helps to identify the surgical approach and the amount of brain tissue to remove.
What types of changes common in epilepsy can be found on an MRI?
Overall, brain tumors are a relatively rare cause of new seizures.
- Tumors are found in about 4% of people with new onset seizures, but more often (up to 10%) in people who are middle-aged and have newly diagnosed seizures.
- Seizures are the first symptom of a brain tumor in about 33% to 38% of people and are seen in over half of patients by the time the tumor is found.
- Many different types of tumors commonly cause seizures. They are labeled astrocytomas, oligodendrogliomas, mixed tumors (tumors with both astrocytic and oligodendroglial elements), gangliogliomas, harmartomas, and dysembryoplastic neuroepithelial tumors.
- On MRI, these tumors tend to be small and clearly limited, with little or no swelling or fluid around them.
- They are most often found in the frontal or temporal lobes, usually in the cortex (the outer layer of the brain or the "gray matter"). They may affect the inner layer of the bones of the skull.
- Their appearance on MRI varies, and it is difficult to determine the precise type of tumor from an MRI scan alone.
- A biopsy or other surgical procedure may be needed to find out what type of tumor it is.
Malformations of cortical development
The term "malformations of cortical development" refers to a number of disorders of brain development that are present at birth. These refer to problems in the way certain brain areas develop.
- They originate during the first trimester of pregnancy, when the neurons (brain cells) are moving into the final step of how the baby's brain is organized and will look like.
- They have many causes, including lack of oxygen, toxins, metabolic abnormalities, or genetic defects, but the type of disorder often depends less on the cause than on the timing of the injury.
- Structural abnormalities of this kind are commonly associated with epilepsy and are usually seen on routine MRI scans, although in some cases high-resolution scans may be necessary.
Some kinds of blood vessel (vascular) malformations in the brain commonly produce seizures.
- Seizures occur in 24% to 69% of patients with a condition called arteriovenous malformation or AVM. Seizures can also occur in 34% to 51% of those with a cavernous hemangioma.
- These malformations may cause epilepsy in a variety of ways such as a direct effects on brain cells, a change in levels of important neurotransmitters or substance in the brain, and other physiological changes.
- Some kinds of vascular malformations, such as venous angiomas, do not produce seizures.
- Vascular malformations may be seen directly on MRI scans. Sometimes they also produce characteristic effects on surrounding tissues.
Mesial temporal sclerosis
Sclerosis (hardening and related tissue changes) in the mesial (inner) part of the temporal lobe is common in temporal lobe epilepsy.
- MRI can find the location of the sclerosis, especially which side of the brain is affected.
- The MRI shows atrophy (shrinking) of the hippocampus (an important structure in the temporal lobe). It can also show changes in the cortex or outer layer of the mesial (inner) part of the temporal lobe.
- If the EEG shows that the seizures arise from the temporal lobe on one side of the brain and the MRI shows these characteristic changes on the same side, the possibility of an excellent outcome from surgery may be greater than 90%.
Neocortical gliosis due to brain injury
Brain injuries of all types, whether from trauma, infection, inflammation, or infarction, commonly lead to the development of an area of gliosis in the neocortex (a layer of cerebral cortex, called "neo" [new] because it is the most recent to evolve). This area of gliosis may surround an area of brain necrosis, in which all the cells have died. When this happens, the brain cells don't work properly.
- Regardless of its cause, the gliosis will look the same on MRI: atrophy and increased free water in the tissues.
- Seizures often arise from this area of gliosis, but their exact cause is unknown.