Central nervous system functional levels

Numerous factors, many of them poorly understood, are involved in the development of HE. In severe hepatic disease, systemic circulation bypasses the liver, so many of the substances normally metabolized by the liver remain in the systemic circulation and accumulate to toxic levels. In excess, these metabolic by-products, especially nitrogenous waste, cause alterations in central nervous system functioning.20... 

Acute high-level exposures to methylmercury may result in impaired central nervous system function, kidney damage and failure, gastrointestinal damage, cardiovascular collapse, shock, and death. The estimated lethal dose is 10-60 mg kg... 

The absolute level of ammonia and its metabolites, such as glutamine, in the blood or cerebrospinal fluid in patients with hepatic encephalopathy correlates only roughly with the presence or severity of the neurologic signs and symptoms. y-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in the brain, is also produced in the gut lumen and is shunted into the systemic circulation in increased amounts in patients with hepatic failure. In addition, other compounds (such as aromatic amino acids, false neurotransmitters, and certain short-chain fatty acids) bypass liver metabolism and accumulate in the systemic circulation, adversely affecting central nervous system function. Their relative importance in the pathogenesis of hepatic encephalopathy remains to be determined. 

In elderly patients, especially those with multisystemic diseases, hypothyroidism may become severe and fife threatening. Myxedema coma represents the most extreme form of severe hypothyroidism. This medical emergency may occur when severe hypothyroidism is complicated by trauma, infection, myocardial infarction, cold exposure, or administration of hypnotics or opiates, medications that suppress central nervous system function, particularly ventilatory drive. It typically presents in older women in winter. The main clinical features are hypothermia and a variable degree of altered consciousness (Iglesias et ai, 1999). Serum T4, TSH and cortisol should be measured to confirm the diagnosis and evaluate adrenal reserve. When there is a reasonably high level of suspicion, treatment should not be delayed to await laboratory results. 

In this study, central nervous system function was evaluated in multiple tests including perceptual speed, simple reaction time, short-term memory, numerical ability, and manual dexterity. A lowest-observed- adverse-effect level (LOAEL) was seen in the same study when humans were exposed to 4,000 mg/m for 50 minutes. The subjects had a prolonged simple reaction time compared to control results in the same volunteers during a non-exposure period. It should be noted that because of practical constraints, these tests were conducted on volunteers at rest and that parallel pharmacokinetic studies have demonstrated greater uptake during exercise (Astrand et al. 1975). 

If the patient with kidney failure also has cirrhosis or some other form of hver failure, this additional ammonia load may present a stress that cannot be adequately handled by the diseased liver. The result may be increased blood and central nervous system ammonia levels with development of encephalopathy (Fraser Arieff, 1985). Thus, patients with cirrhosis and end-stage kidney disease are at particular risk for developing encephalopathy since both conditions act synergistically to increase both blood and central nervous system ammonia. It should also be noted that plasma urea and serum creatinine do not always adequately reflect renal function in patients with severe liver disease. Recent studies suggest that many patients who have cirrhosis, ascites, and normal plasma urea and creatinine may in fact have severe renal functional impairment (Gines et al., 1988 Papadakis Arieff, 1987 Takabatake et al., 1988). In such individuals, differentiation of hepatic from uremic encephalopathy on clinical grounds may be difficult. 

The ammonia produced by enteric bacteria and absorbed into portal venous blood and the ammonia produced by tissues are rapidly removed from circulation by the liver and converted to urea. Only traces (10—20 Ig/dL) thus normally are present in peripheral blood. This is essential, since ammonia is toxic to the central nervous system. Should portal blood bypass the liver, systemic blood ammonia levels may rise to toxic levels. This occurs in severely impaired hepatic function or the development of collateral links between the portal and systemic veins in cirrhosis. Symptoms of ammonia intoxication include tremor, slurred speech, blurred vision, coma, and ultimately death. Ammonia may be toxic to the brain in part because it reacts with a-ketoglutarate to form glutamate. The resulting depleted levels of a-ketoglutarate then impair function of the tricarboxylic acid (TCA) cycle in neurons. 

There is a definite trend of bidirectional cross-talk between opioid and chemokine receptors in the central nervous system. In vitro, as well as in vivo studies, have shown desensitization of CXCR4 by MOR and thus prevent the neuroprotective action of this chemokine. Although the precise molecular mechanism underlying this cross-talk is still under investigation, based on the evidences in literature several possible pathways can be expected to act independently or in concert and lead to the deficit of CXCR4 function. Our studies have shown that p opioids can increase the brain levels of FHC which can subsequently block CXCR4 signaling. Eurther studies... 

Attempts to diminish the overall metabolism of trichloroethylene might be useful (e.g., hypothermia, mixed-function oxidase inhibitors, competitive inhibitors of trichloroethylene metabolism [i.e., P-450 substrates]), if instituted soon enough after trichloroethylene exposure. Catecholamines (especially beta agonists) act in concert with trichloroethylene, increasing the risk of cardiac arrhythmias. Hence, catecholamines should be administered to patients only in the lowest efficacious doses and for certain limited presentations of trichloroethylene poisoning. Ethanol should also be avoided because concurrent exposure to trichloroethylene and ethanol can cause vasodilation and malaise and may potentiate central nervous system depression at high dosage levels of either compound. 

Ammonia (NH3) is just one of the toxins implicated in HE. It is a metabolic by-product of protein catabolism and is also generated by bacteria in the GI tract. In a normally functioning liver, hepatocytes take up ammonia and degrade it to form urea, which is then renally excreted. In patients with cirrhosis, the conversion of ammonia to urea is retarded and ammonia accumulates, resulting in encephalopathy. This decrease in urea formation is manifest on laboratory assessment as decreased blood urea nitrogen (BUN), but BUN levels do not correlate with degree of HE. Patients with HE commonly have elevated serum ammonia concentrations, but the levels do not correlate well with the degree of central nervous system impairment.20... 

Zinc is important to the normal functioning of the central nervous system (CNS). At low concentrations, zinc protects mammalian brain neurons by blocking N-methyl-D-aspartate receptor-mediated toxicity. At high concentrations, zinc is a potent, rapidly acting neurotoxicant in the mammalian brain, as judged by zinc-induced neuronal injury of in vitro mature cortical cell cultures (Choi et al. 1988). Increased brain levels of zinc are associated with Pick s disease in certain strains of rodents with inherited epileptic seizures. Intravenous injection of zinc in rats with genetically inherited epilepsy produces seizures a similar response occurs with intracranial injection of zinc in rabbits with inherited audiogenic seizures (Choi et al. 1988). 

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