Metabolic encephalopathies: opportunities and challenges

Metabolic Encephalopathy: Behind the Name

Metabolic encephalopathy may be the most common diagnosis in consultative acute neurology. The origin of this term is not generally known but can be traced back. The term replaced more commonly used designations such as organic or functional. The term metabolic encephalopathy was originally linked to organ dysfunction but subsequently became more imprecise. When it expanded to include a large number of diseases, it evolved to "metabolic neuronal dysfunction" and soon could not be distinguished from "quiet delirium" and other designations. This vignette summarizes why the terminology has confused more than clarified but also why it will likely stay in the neurologist's vernacular.

The scope of neurology of critical illness

Critical illness increases the probability of a neurologic complication. There are many reasons to consult a neurologist in a critically ill patient and most often it is altered alertness with no intuitive plausible explanation. Other common clinical neurologic problems facing the intensive care specialist and consulting neurologist in everyday decisions are coma following prolonged cardiovascular surgery, newly perceived motor asymmetry, seizures or other abnormal movements, and generalized muscle weakness. Assessment of long-term neurologic prognosis is another frequent reason for consultation and often to seek additional information about the patient's critical condition by the attending intensivist. Generally speaking, consultations in medical or surgical ICU's may have a varying catalog of complexity and may involve close management of major acute brain injury. This chapter introduces the main principles and scope of this field. Being able to do these consults effectively-often urgent and at any hour of the day-requires a good knowledge of general intensive care and surgical procedures. An argument can be made to involve neurointensivists or neurohospitalists in these complicated consults.

Extracorporeal treatment in fulminant hepatic failure: pathophysiologic considerations

Fulminant hepatic failure is a life-threatening clinical syndrome following severe hepatic injury leading to cerebral edema and brainstem herniation. Excessive mortality can be currently reduced only by timely orthotopic liver transplantation. Due to the shortage of donor organs, a considerable proportion of patients develop irreversible neurological damage, multiorgan failure or death while waiting for transplantation. Consequently, alternatives to orthotopic liver transplantation and methods of stabilizing patients on the waiting list including extracorporeal detoxification treatment are currently investigated. Recent advances in the pathophysiology of cerebral edema have challenged some of the traditional assumptions on which many blood detoxification systems are based. This article aims to integrate pathophysiology of hepatic encephalopathy and cerebral edema into a proposed future concept of liver support.

Auditory P300 event-related potentials and number connection test for evaluation of subclinical hepatic encephalopathy in patients with cirrhosis of the liver: a follow-up study

BACKGROUND AND AIMS

The P300 event-related potentials (P3ERP) have been recently advocated for detection of cognitive disturbances in early encephalopathy. However, no systematic follow-up study has been conducted to understand the clinical significance of subclinical hepatic encephalopathy (SHE) detected by this or other methods. The present study was therefore undertaken to examine the diagnostic usefulness of auditory P3ERP in the detection of SHE, to compare it with that of the number connection test (NCT), and to investigate the clinical outcome of patients with SHE in terms of progression to overt encephalopathy.

METHODS

P300 event-related potential latencies were measured and the NCT time was recorded in 81 non-encephalopathic cirrhotic patients (Aged 43.8 +/- 11 years, 23 alcoholic and 58 non-alcoholics) attending the outpatient department at our tertiary care hospital (All India Institute of Medical Sciences Hospital). Cut-off values for abnormality in the tests were developed from age-, sex- and education-matched controls. Patients were followed up at regular intervals for the development of overt encephalopathy, and the identifiable precipitating factors were noted. The P3ERP latencies (363 +/- 34 msec vs 349 +/- 23 msec), as well as NCT time (54.6 +/- 30.6 s vs 39.5 +/- 15.8 s) were significantly prolonged (P< 0.01) in patients with liver cirrhosis when compared with the non-cirrhotic controls.

RESULTS

The P3ERP defects were seen in 24.6% of cirrhotic patients, while NCT time was prolonged in 19.7% of the patients. Nearly 43% of the patients with SHE progressed to overt encephalopathy within a mean duration of 5 months, while only 3.9% of the non-SHE patients did so. Of the patients who developed overt encephalopathy, 64.2% had P3ERP latency prolongations while 35.7% had abnormal NCT.

CONCLUSIONS

The results of the present study suggest that P3ERP and NCT are valid tools for the screening of SHE in cirrhotic patients as there is a greater likelihood of overt encephalopathy development in patients with an abnormality detected by these tests than in patients with no such abnormality.

Partial pressure of ammonia versus ammonia in hepatic encephalopathy

Ammonia is considered the major pathogenetic factor of cerebral dysfunction in hepatic failure. The correlation between total plasma ammonia and the severity of hepatic encephalopathy (HE), however, is variable. Because ammonia that is present in gaseous form readily enters the brain, the correlation with the grade of HE of the pH-dependent partial pressure of gaseous ammonia (pNH(3)) could be better than that of total arterial ammonia levels. To test this hypothesis, 56 cirrhotic patients with acute episodes of clinical HE (median age, 54 years; range, 21-75) were studied by clinical examination and by long-latency median-nerve sensory-evoked potentials (SEPs) N70 peak, an objective and sensitive electrophysiological measure of HE. pNH(3) was calculated from arterial blood according to published methods. The clinical grade of HE correlated (P <.001) with both pNH(3) and total ammonia, but correlation was stronger with pNH(3) (r =.79 vs.69, P =.01). A similar correlation was found for N70 peak latency (r =.71 with pNH(3) vs.64 with total ammonia, respectively, P =.08). In summary, arterial pNH(3) correlates more closely than total ammonia with the degree of clinical and electrophysiological abnormalities in HE. These findings support the ammonia hypothesis of HE and suggest that pNH(3) might be superior to total ammonia in the pathophysiological evaluation of HE.

Clinical correlation of neuropsychological tests with 1H magnetic resonance spectroscopy in hepatic encephalopathy

OBJECTIVE

The purpose of this project was to correlate neuropsychological test results with in vivo measures of regional cerebral biochemistry determined by 1H MRS in patients with subclinical and mild hepatic encephalopathy.

METHODS

Baseline 1H MRS scans and neuropsychological testing of patients occurred at entry into the study. The primary localized volume chosen for the 1H MRS study was the posteromedial parietal cortex, which consisted predominantly of white matter. Some of these patients were scanned again if they received a liver transplantation. In a subset of patients, the effect on cerebral biochemistry and neuropsychological test performance due to a dietary intervention of reduced protein intake was monitored. These patients underwent a baseline examination and a repeat examination after 2 weeks of dietary intervention. Measures were made of the correlation between the dietary intervention and 1H MRS determined biochemistry and the results of neuropsychological tests. Results in both patient groups (dietary intervention and no dietary intervention) were compared with healthy control subjects.

RESULTS

Subclinical and low grade HE patients showed a significant reduction in mI/Cr and Cho/Cr ratio when compared with healthy control subjects. These patients also showed impairment in frontal lobe mediated cognitive tasks and in motor ability that were not appreciated in a bedside examination. The patients did not return to normal cerebral metabolic states within 30 to 60 days of liver transplantation. In fact, reductions remained in mI/Cr. Cho/Cr values increased after transplantation compared with healthy control subjects.

CONCLUSIONS

1H MRS studies showed changes in regional cerebral biochemistry associated with all grades of HE. There was a reduction in mI/Cr and a reduction in Cho/Cr in patients with low grade and subclinical forms of HE compared with normal subjects. The reduction in mI correlated well with abnormalities observed in neuropsychological tests. Liver transplantation was not associated with significant improvement in these variables.

Cerebral abnormalities in patients with cirrhosis detected by proton magnetic resonance spectroscopy and magnetic resonance imaging

Hepatic encephalopathy is a common problem in cirrhosis. The pathogenesis of this complication of advanced liver disease still remains unclear. Magnetic resonance spectroscopy was used to assess prospectively cerebral metabolism in 51 patients with histologically proven cirrhosis (Child-Pugh classes A, B, and C, 18, 18, and 15, respectively) and 36 healthy volunteers. According to the results of psychometric tests, overt hepatic encephalopathy, subclinical encephalopathy, and no encephalopathy were found in 14, 21, and 16 patients, respectively. Myoinositol/creatine ratios in gray (.36 +/- .17) and white (.35 +/- .22) matter voxel were reduced significantly (P < .0001) in cirrhotic patients compared with healthy volunteers (gray matter, .51 +/- .11; white matter, .64 +/- .16). In addition, patients showed a significant reduction (P = .024) in white matter choline/creatine ratio (.77 +/- .27) compared with controls (.92 +/- .25), and glutamine/glutamate level was elevated in cirrhotic patients compared with controls (gray matter, P < .0001; white matter, P = .036). Changes in cerebral myoinositol and glutamine/glutamate levels correlated significantly with the severity of hepatic encephalopathy (P < .0001). However, these metabolic alterations were also detected in patients without hepatic encephalopathy (normal psychometric test results). N-acetyl aspartate/creatine ratios did not differ between patients and controls. Magnetic resonance imaging detected bright basal ganglia in 37 patients, which correlated significantly with portal-systemic shunting and elevation of glutamine/glutamate, but not with the degree of hepatic encephalopathy. In conclusion, magnetic resonance imaging and spectroscopy showed that alterations of cerebral metabolism are common in patients with cirrhosis, even without evidence of clinical or subclinical hepatic encephalopathy.

Effect of reducing brain glutamine synthesis on metabolic symptoms of hepatic encephalopathy

Liver failure, or shunting of intestinal blood around the liver, results in hyperammonemia and cerebral dysfunction. Recently it was shown that ammonia caused some of the metabolic signs of hepatic encephalopathy only after it was metabolized by glutamine synthetase in the brain. In the present study, small doses of methionine sulfoximine, an inhibitor of cerebral glutamine synthetase, were given to rats either at the time of portacaval shunting or 3-4 weeks later. The effects on several characteristic cerebral metabolic abnormalities produced by portacaval shunting were measured 1-3 days after injection of the inhibitor. All untreated portacaval-shunted rats had elevated plasma and brain ammonia concentrations, increased brain glutamine and tryptophan content, decreased brain glucose consumption, and increased permeability of the blood-brain barrier to tryptophan. All treated rats had high ammonia concentrations, but the brain glutamine content was normal, indicating inhibition of glutamine synthesis. One day after shunting and methionine sulfoximine administration, glucose consumption, tryptophan transport, and tryptophan brain content remained near control values. In the 3-4-week-shunted rats, which were studied 1-3 days after methionine sulfoximine administration, the effect was less pronounced. Brain glucose consumption and tryptophan content were partially normalized, but tryptophan transport was unaffected. The results agree with our earlier conclusion that glutamine synthesis is an essential step in the development of cerebral metabolic abnormalities in hyperammonemic states.

Effects of moderate hypoxemia and unilateral carotid ligation on cerebral glucose metabolism and acid-base balance in the rat

We used our recently developed method for the simultaneous measurement of the local CMRglc (LCMRglc) and composite tissue pH to evaluate the response to unilateral carotid ligation and moderate hypoxia [40.1 +/- 4.8 (SD) mm Hg]. The LCMRglc and tissue pH were measured simultaneously in brain slices using [14C]2-deoxy-D-glucose and [14C]5,5-dimethyl-2,4-oxazolidinedione. The ipsilateral LCMRglc was increased significantly in the caudate-putamen and medical thalamus and was surrounded by a much more extensive zone of acidosis, as shown by significant reductions in the tissue pH, which was affected in parietal cortex, caudate-putamen, lateral septal nucleus, medial geniculate, Ammon's horn, and nucleus reticularis of substantia nigra. In regions with an elevated LCMRglc and acidosis, anaerobic glycolysis combined with ATP hydrolysis are likely to co-exist. In regions characterized by normal glucose metabolism and acidosis, we hypothesize that a direct effect of hypoxia on the sodium/hydrogen ion antiporter may lead to secondary acidosis. Disturbed acid-base balance during hypoxia may have an adverse effect on cerebral function and cause clinical symptoms.