Selective frontal, parietal, and temporal networks in generalized seizures

Are "generalized" seizures truly generalized? Generalized tonic-clonic seizures are classified as either secondarily generalized with local onset or primarily generalized, without known focal onset. In both types of generalized seizures widespread regions of the nervous system engage in abnormally synchronous and high-frequency neuronal firing. However, emerging evidence suggests that all neurons are not homogeneously involved; specific nodes within the network may be crucial for the propagation and behavioral manifestations of generalized tonic-clonic seizures. Study of human tonic-clonic seizures has been limited by problems with patient movement and variable seizure types. To circumvent these problems, we imaged generalized tonic-clonic seizures during electroconvulsive therapy, in which seizure type and timing are well controlled. (99m)Tc-hexamethylpropylene amine oxime injections during seizures provide a "snapshot" of cerebral blood flow that can be imaged by single photon emission computed tomography (SPECT) after seizure termination. Here we show that focal regions of frontal and parietal association cortex show the greatest relative signal increases. Involvement of the higher-order association cortex may explain the profound impairment of consciousness seen in generalized seizures. In addition, focal involvement of the dominant temporal lobe was associated with impaired retrograde verbal memory. Similar focal increases were also seen in imaging of spontaneous secondarily generalized tonic-clonic seizures. Relative sparing of many brain regions during both spontaneous and induced seizures suggests that specific networks may be more important than others in so-called generalized seizures.