This chapter uses a case-based approach to discuss the electrographic patterns associated with focal cortical lesions in critically ill patients. Focal amplitude attenuation and/or slowing may suggest an underlying physiological dysfunction or a structural lesion. Additionally, epileptiform abnormalities such as sharp waves within the region may suggest increased seizure risk. A focal pattern of higher amplitudes, sharper morphologies, and faster activities is characteristic of breach effect from a craniotomy. Lateralized rhythmic delta activity (LRDA) is a pattern of rhythmic focal slowing that is associated with increased seizure risk. Epilepsia partialis continua (EPC) is an unusual form of focal motor status epilepticus that is often refractory to antiseizure medications. This pattern may be seen in a rare form of focal epilepsy called Rasmussen syndrome among other causes.

SE is defined as 5 minutes or longer of continuous clinical and/or electrographic seizure activity or recurrent seizures without interval recovery; t1 refers to the time point beyond which there is failure of mechanisms responsible for seizure termination or initiation of mechanisms which lead to abnormally prolonged seizures, and t2 refers to the time point beyond which there are long term consequences due to neuronal injury, death, and alteration of neuronal networks. Semiologically, SE can be classified as with or without prominent motor features. Convulsive SE may evolve into NCSE in a significant minority after convulsive activity ceases. NCSE may be diagnosed on EEG in a significant minority of critically ill patients. EPC may not be associated with ictal activity on surface EEG. De novo absence SE may be seen in older individuals in the setting of abrupt benzodiazepine withdrawal. They may have a previous or family history of IGE.