Most guidelines indicate neuroimaging following a first-time unprovoked seizure.
In this scenario CT is usually performed in the emergency setting, to exclude acute intracranial events (e.g. hemorrhage) which may have caused the seizure.
This is usually followed up with brain MRI in stable patients without urgent neuroimaging indications, often on an outpatient basis. MRI offers excellent soft tissue detail to facilitate detection of subtle lesions. Brain abnormalities are found in about 1 in 3 adults with new-onset seizures1 and localization of a seizure causing focus can be seen in 15-20% of children. MRI is also the investigation of choice in the neurodiagnostic workup of recurrent unprovoked seizures, known as epilepsy, for both adults and children.
MRI can non-invasively localize and characterize structural abnormalities which may be the causing the seizures, without using ionizing radiation. For patients with medically refractory epilepsy the potential is greatest for permanent surgical cure and improved quality of life when a focal structural brain abnormality identified on MRI can be anatomically correlated as the epileptogenic focus.
MRI vs CT
Compared with brain CT, MRI has a higher sensitivity to detect epileptogenic lesions which is useful for surgical planning. MRI’s yield for detecting these underlying lesions has increased over time and the sensitivity of brain MRI at 1.5 T for patients with medically refractory partial epilepsy approaches 902.
Conventional MRI protocols can often be completed within ~35 minutes of scan time, reducing patient anxiety.
With the application of more refined imaging protocols, specialized image processing techniques (such as diffusion tensor imaging, proton pectroscopy and thickness and volume measurements of cortical gray matter) there are even greater advancements in detecting epileptogenic lesions. These newer techniques are especially useful in cases when conventional MRI does not clearly depict a lesion3,4.
Several studies indicate that MRI scans at 3 Tesla detect more lesions (due to greater signal : noise ratio) and research is ongoing regarding clinical utility of 7T and higher magnetic field strengths3, 5, 6. However, there are well-known limitations of high-field-strength imaging, including their propensity to susceptibility and other imaging artifacts.
Epilepsy etiology: some MRI features7,8
Hippocampal sclerosis (HS), which is the most common cause of medically refractory epilepsy. When the abnormality involves extra-hippocampal tissues in the temporal lobe, the condition is referred to as mesial temporal sclerosis (MTS). It is characterized by gliois and neuronal loss and may be bilat-eral in 10-20% of cases.
Malformations of cortical development account for 10-50% of all pediatric epilepsy cases and 4-25% of all adult cases. The most common group overall is focal cortical dysplasia. Other cortical development malformations include the agyria-pachygyria spectrum, gray matter heterotopia, polymicrogyria and schizencephaly.
Open-lip and closed-lip (red arrow) schizencephaly on the upper far left (A)
Tuberous sclerosis– an inherited (autosomal dominant) condition. Intracranial manifestations include cortical tubers and subependy-mal hamartomas. (B1 & B2)
Neoplasms – Cortically based tumors, typically in the temporal lobe e.g glial tumors, developmental tumors (eg. DNET, ganglioglioma). (C)
Vascular malformations, e.g. cavernomas
Dual pathology” occurs when HS is associated with another epileptogenic abnormality.
It occurs in 8-22% of patients with epilepsy.
OTHER CAUSES OF GLIOSIS
Post- chronic CNS infection e.g. neurocysticercosis
- T. Hakami, et al. MRI-identified pathology in adults with new-onset seizures. Neurology (2013).
- Bronen RA, Fulbright RK, Spencer DD, Spencer SS, Kim JH, Lange RC, et al. Refractory epilepsy: comparison of MR imaging, CT, and histopathologic findings in 117 patients. Radiology (1996).
- https://www.ajronline.org/doi/10.2214/AJR.07.3933 4. https://www.epilepsysociety.org.uk/closer-look-mri#.XJE9QtF7kfF 5. https://consultqd.clevelandclinic.org/negative-3t-mri-in-an-epilepsy-patient-heres-what-7t-can-bring-to-the-table/ 6. Duncan JS, Sander JW, Sisodiya SM, Walker MC. Adult epilepsy. Lancet (2006)
Contributor: Dr. Avia Forbes-Chang – Consultant Radiologist