Neuroimaging in the Evaluation of Epilepsy

Naymee J. Velez-Ruiz, MD; Joshua P. Klein, MD, PhD2

Disclosures

Semin Neurol. 2012;32(4):361-373. 

In This Article

Traumatic Brain Injury

Head trauma is responsible for more than 20% of symptomatic cases of epilepsy and 5% of all epilepsy.[110] Seizures may begin anytime after the injury. Early seizures, which occur within a week of the injury, are acute symptomatic attacks often accompanied by neurologic or systemic abnormalities. Although not considered epilepsy, these seizures increase the risk for posttraumatic epilepsy (PTE).[110] Seizures that occur after a week of the head trauma are thought to reflect permanent changes in the brain, tend to recur, and therefore often represent onset of PTE.[110] The most important risk factor for the development of either early or late seizures is the severity of the injury. Greater severity, brain contusion, intracranial hematoma, depressed skull fracture, and dural penetration are associated with increased seizure risk.[110,111] Although sometimes the severity of the injury is greater than appreciated on imaging, the presence of some of these factors may aid in the identification of individuals that are at increased risk of developing PTE and that may require closer monitoring.

Imaging of head trauma has a primary role both in assessing the extent of the trauma and in determining the appropriate therapy. The most efficient method to triage for acute head trauma remains CT. It is readily available, fast, and usually very accurate at detecting acute hemorrhage (high density on unenhanced scan) and depressed skull fractures. Rarely, there are isodense or low-density hemorrhages in patients with severe anemia or disseminated intravascular coagulopathy, and these may be more difficult to detect. Another limitation of this technique is the relatively limited visualization of the infratemporal, subfrontal, and posterior fossa regions. When head trauma is evaluated with CT, analysis should include review of brain, subdural, and bone windows. The wide window setting aids in separating high-density blood from the high density of bone, and is particularly useful in acute subdural and epidural hematomas, which can be small and difficult to differentiate from the calvarium.[112] Bone windowing is essential to identify fractures.

Overall, MRI reveals the extent of brain damage after head trauma with greater sensitivity than CT.[113] However, few studies have assessed MRI findings in patients with PTE. Angeleri et al obtained MRI scans from 104 TBI patients one year after injury, and found that the presence of cortical or subcortical hyperintense lesions visible on T2WI with accompanying hemosiderin on GRE sequences was associated with an increased risk of PTE.[114] Susceptibility-weighted imaging (SWI), which is related to the GRE imaging, takes advantage of susceptibility differences between tissues. This results in enhanced contrast sensitive to paramagnetic properties of intravascular deoxyhemoglobin, i.e., sensitive to venous blood, hemorrhage, and iron in the brain. SWI is frequently utilized for the detection of blood products in the patient with PTE and no apparent abnormalities in other MRI sequences. Diaz-Arrastia et al analyzed high-resolution MRIs in patients with intractable PTE and found that 35% of these patients had HS.[115] Often diffuse abnormalities, such as global cerebral atrophy, are seen along with HS.[116] Less commonly, dual pathology is noted with HS present in addition to neocortical lesions, usually in the temporal or frontal lobes.[116]

The exact implications of other more subtle abnormalities, such as diffuse axonal injury (DAI), for the subsequent development of seizures are not completely understood. One study suggests that patients may have isolated DAI on MRI scans yet achieve neurologic recovery without the development of seizures.[117] Further investigations are required to corroborate this finding. The radiologic characteristics of DAI are discussed elsewhere.

More recently, novel MRI techniques such as DTI have been used in an attempt to increase the sensitivity of MRI, as well as to gain new insights into the pathogenesis of PTE. DTI permits the examination of white matter integrity in vivo by measuring the diffusivity of water in biologic tissues, as well as an index of the directionality of diffusion. This allows the identification and quantification of microstructural changes that extend beyond the obvious lesions seen on the T2 and FLAIR images. Gupta et al compared DTI findings in patients with chronic traumatic brain injury with and without epilepsy with those of age-matched controls.[118] They found that the mean regional fractional anisotropy (FA) was significantly lower whereas the mean regional mean diffusivity (MD) was higher in patients with TBI compared with controls, and that the mean regional FA ratio was significantly lower in TBI patients with epilepsy than in those without epilepsy.[118] This suggests that the severity of injury as predicted by the DTI-derived increased volume of microstructure damage may be associated with PTE. However, their sample was small and further studies are needed to make definite conclusions.

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