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Hashimoto’s thyroiditis has heterogeneous clinical and electrographic manifestations the EEG can show generalised or frontal slow activity, triphasic waves, periodic sharp waves, and lateralised temporal slowing. Endocrine disordersĮEG changes parallel the severity of hormonal disturbance, but it may be difficult to distinguish primary endocrine from secondary electrolyte effect. Induction of clinical seizures and EEG abnormalities are more likely at high dosages or with rapid dose escalation. Clozapine is quite commonly associated with EEG change, often pronounced, and manifest by both generalised slow activity and spike wave discharge in non-epileptic patients. Generalised slow activity can alert to hyperammonaemia in a confused or obtunded patient who has epilepsy treated with sodium valproate. Some agents, particularly benzodiazepines and barbiturates, induce fast or beta (β) rhythms, and the EEG may be a useful pointer to drug intoxication when this is clinically unsuspected. Most drugs or toxins have diffuse effects on the EEG. In hypoglycaemia there may be generalised slow activity or focal/lateralised δ rhythms, and these may be associated with focal neurological deficits.ĮEG can be useful for early detection of dialysis dementia or encephalopathy (occurring in about 1% of dialysis patients), as abnormalities can precede clinical symptoms by several months in this disorder. Spike wave epileptiform discharges (and seizures) may occur in any severe encephalopathy, but are said to be more common than in uraemic encephalopathy and in insulin coma. Thus, EEG changes are usually more severe in uraemic encephalopathy if there is acute deterioration of renal function, with broad correlation between degree of EEG slowing and increase in serum creatinine.
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The rate of change rather than the absolute level of a given metabolite, toxin or electrolyte is more important in determining the degree of EEG abnormality. They have been considered as specific to severe hepatic encephalopathy, but TW are seen in encephalopathies associated with renal failure or electrolyte imbalance, as well as anoxia and intoxications (such as lithium, metrizamide, and levodopa). Triphasic waves (TW) consist of moderate to high amplitude complexes with three (but sometimes two or four) negative-positive-negative phases, usually occurring in runs at 1.5–3 per second. Metabolic encephalopathiesĪ patient with acute change in awareness whose EEG shows triphasic waves and diffuse slow activity will usually have a metabolic encephalopathy. None of these patterns is specific to a particular pathophysiological process or diagnosis, but PEDs are most likely to occur in acute or subacute focal destructive pathologies or focal epileptogenic lesions triphasic waves are typically found in metabolic encephalopathies and some patients with mesial fronto-parietal lesions or third ventricle tumours show FIRDA in their EEG. A variety of additional EEG patterns can be seen, such as frontal intermittent rhythmic delta (FIRDA), periodic lateralised or bilateral epileptiform discharges (PEDs, BIPEDs), and triphasic waves. In mild encephalopathic states, slowing of normal alpha (α) rhythms occurs, and with more severe encephalopathy, the appearance of theta (θ) and continuous or non-continuous delta (δ) activities. There is a progressive increase in slow wave activities, the degree of which parallels the severity of brain dysfunction. EEG changes in encephalopathies are similar, whether the cause is septic, metabolic, toxic, or structural.