**1. Introduction**

Periodic electroencephalographic (EEG) patterns are frequently recorded during ICU EEG monitoring in patients with Altered Mental Status (AMS) [1]; these controversial EEG patterns consist of discharges usually epileptiform in appearance, which occur at regular intervals, in critically ill patients. They are commonly classified as periodic lateralized discharges (PLDs), bilateral independent PLDs or BIPLDs, generalized periodic discharges (GPDs) and triphasic waves. Stimulus-induced rhythmic,

periodic or ictal discharges (SIRPIDs) are peculiar EEG patterns, which may be present as periodic discharges. Other periodic, rhythmic patterns are Occasional frontally predominant brief 2/s GRDA (FIRDA previously), Lateralized rhythmic delta activity (LRDA) and Brief potentially ictal rhythmic discharges or B (I)RDs.

There is still no consensus on which specific EEG features are associated with ongoing neuronal injury, which ones should be treated and how aggressively they should be treated [2]. In critically ill patients, a high index of suspicion of ongoing status epilepticus (SE), particularly non-convulsive epileptic status (NCSE) should alert the intensive care team and the neurologist, in the presence of these EEG periodic discharges, especially in patients with impaired consciousness, prompting the use of antiseizure medications [3, 4]. In addition, the occurrence of such EEG features may be in favor of cerebral impairment, acute or subacute [4, 5].

The aim of this study is to make a review of these periodic EEG features, emphasizing the importance of their recognition and clinical significance. Their clinical significance is uncertain, it is related to a variety of etiologies, and many authors suggest that these patterns are unequivocally epileptogenic in some cases. Their recognition and classification are important to establish a correlation between clinical, neurological, neuroimaging data with the EEG results.
