What characterizes the tonic phase of a generalized tonic-clonic seizure on the EEG?

The tonic phase of a generalized tonic-clonic seizure is characterized by generalized voltage suppression and muscle artifact on the EEG. During this phase, the muscles become rigid, resulting in significant muscle artifact on the recording. The generalized voltage suppression reflects the electrical activity associated with the intense muscle contraction and the loss of normal brain rhythm.

In this phase, individuals often exhibit a decrease in background activities, leading to an overall attenuation of the EEG signal. The muscle artifact can obscure underlying brain activity but is still a significant feature since it shows the muscular involvement during the tonic phase. This is crucial for differentiating between seizure types and understanding seizure manifestations on an EEG.

The other options represent different features not typical of the tonic phase. Generalized voltage suppression with muscle artifact is unique to this stage, while the presence of intermittent spikes and slow waves or focal slowing indicates different seizure states or types of activity. Continuous high-frequency activity might suggest different pathological processes, typically seen in certain types of seizures or status epilepticus, rather than specifically characterizing the tonic phase of a generalized tonic-clonic seizure.