EEG is usually abnormal in all of the following, except:

Cerebral neurons are fully dependent on cerebral blood flow (CBF) and the delivery of oxygen and glucose. CBF is 75 mL per 100g/min in gray matter and 30 mL per 100 g/min in white matter (mean 55 mL per 100 g/min);Oxygen consumption is 3.5 mL per 100 g/min, and glucose utilization is 5 mg per 100 g/min. Brain stores of glucose provide energy for 2 minutes after blood flow is interrupted, and oxygen stores last 8-10 seconds after the cessation of blood flow. Simultaneous hypoxia and ischemia exhaust glucose more rapidly. The electroencephalogram (EEG) rhythm in these circumstances becomes diffusely slowed, typical of metabolic encephalopathies, and as conditions of substrate delivery worsen, eventually brain electrical activity ceases.The presence of complexes occurring with a regular repetition rate (so-called periodic complexes) suppos a diagnosis of Creutzfeldt-Jakob disease or subacute sclerosing panencephalitisThe EEG generally slows in metabolic encephalopathies, and triphasic waves may be present. The findings do not permit differentiation of the underlying metabolic disturbance but help to exclude other encephalopathic processes by indicating the diffuse extent of cerebral dysfunction. Ref: Harrisons 18/e p2251-2252, e-45 chapter