• The initial phase of impairment of consciousness with metabolic encephalopathies is often delirium, with impairment of attention and fluctuations in alertness, clouding of consciousness, disturbances in the wake-sleep cycle, and, sometimes, agitation and restlessness.
• In conscious patients with metabolic encephalopathies, cognitive function typically fluctuates considerably, in contrast to dementia.
• In comatose patients with metabolic or diffuse or bihemispheric disturbances in cerebral function, the pupillary reactions are typically spared, even if other cranial nerve reflexes are lost and the patient is exhibiting decerebrate or decorticate posturing.
• Respiratory patterns, along with blood gas (acid-base balance) testing can narrow the differential diagnosis considerably.
Historical note and terminology
The term “metabolic encephalopathy” was coined by Kinnier Wilson in 1912 to describe a clinical state due to various causes in which the brain's integrated activity is impaired in the absence of structural abnormalities. Metabolic encephalopathy is, thus, not a diagnosis but a syndrome of global cerebral dysfunction induced by systemic stress, and can vary in clinical presentation from mild executive dysfunction or agitated delirium, to deep coma with decerebrate posturing. There are also some differences in presentation depending on the metabolic disorder.
In this discussion we shall refer to those disorders due to vital organ failure, nutritional deficiencies, electrolyte imbalances, hypoglycemia, hyperglycemia, endocrine disorders, and systemic sepsis; the following are excluded: cardiac arrest and anoxic-ischemic encephalopathy, direct CNS infections, exogenous toxins (including drugs, alcohol, and poisons), hematological conditions, immune-mediated CNS diseases, and direct and indirect effects of cancer on the nervous system.
It should be understood that various metabolic disorders causing encephalopathy are combined, rather than occur in isolation, especially in critically ill patients. This reflects the interaction among various organ systems, causing multiple metabolic derangements.
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