MRI (magnetic resonance imaging) is the diagnostic tool that identifies structural abnormalities in the brain that may be associated with the cause of seizures. 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 to classify both 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 seizures. For example, it can identify mesial temporal sclerosis or cavernous angioma, which carry a favorable surgical prognosis, and malformations of cortical development, which have a less favorable prognosis for postoperative seizure control.
- Before surgery, MRI helps to identify the surgical approach and the volume of tissue to be removed.
What seizure-causing abnormalities can MRI show?
Overall, brain tumors are a relatively rare cause of new seizures. Tumors are found in about 4% of all such patients but they may be found in 10% or more of middle-aged people. Seizures are the first symptom of 33% to 38% of brain tumors and occur 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, the "gray matter"). They may affect the inner layer of the bones of the cranium. 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.
Malformations of cortical development
The term "malformations of cortical development" refers to a number of disorders of brain development that are present at birth. They originate during the first trimester of pregnancy, when the neurons (brain cells) are moving into the final organizational structure of the baby's brain. 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. For instance, seizures occur in 24% to 69% of patients with a type called arteriovenous malformation, and in 34% to 51% of those with another kind called cavernous hemangioma. The suggested ways in which these malformations may cause epilepsy include direct effects on brain cells, altered neurotransmitter levels, and other physiological changes. Some kinds of vascular malformations, such as venous angiomas, do not produce seizures. On MRI, vascular malformations may be seen directly. 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 identify the location of the sclerosis, especially which side of the brain is affected. The MRI shows atrophy (shrinking) of the hippocampus and changes in the cortex of the mesial 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 exceed 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. Regardless of its cause, the gliosis will have the same appearance on MRI: atrophy and increased free water in the tissues. Seizures often arise from this area of gliosis, but their precise cause is unknown.
Topic Editor: Ruben Kuzniecky, M.D. Last Reviewed: 3/19/04