Neural mechanism underlying impaired visual judgement in the dysmetabolic brain: an fMRI study

Brain default-mode network abnormalities in hepatic encephalopathy: a resting-state functional MRI study

Many neuroimaging investigations focus on hepatic encephalopathy (HE); however, few investigate default-mode network (DMN) in the patients with HE and its underlying physiological relevance using resting-state fMRI. In this study, independent component analysis was used to retrieve components representing the DMN of patients with HE (n = 14) and healthy volunteers (n = 14). Four patients were excluded because of head motion (n = 3) and the artifact from the artificial tooth (n = 1). Comparison results between the two groups revealed significantly reduced functional connectivity in the right middle frontal gyrus and left posterior cingulate cortex in the HE patients. A statistical t-map from the comparison of venous blood ammonia levels and the z-scores of the DMN obtained from independent component analysis was computed in the HE group, which showed negative correlation with the changes in left angular gyrus. In conclusions, resting-state fMRI can be used to examine DMN changes in HE patients. Reduced functional connectivity in the right middle frontal gyrus and left posterior cingulate cortex consisting of the DMN and negative correlation between the functional connectivity changes in left AG and the venous blood ammonia levels support the notion of damages in functional organization of the central nervous system in HE patients.

Critical flicker frequency fails to disclose brain dysfunction in patients with primary biliary cirrhosis

BACKGROUND

Recent studies suggest that stage-independent symptoms of primary biliary cirrhosis (PBC) such as chronic fatigue are a consequence of structural and functional abnormalities of the brain. Critical flicker frequency (CFF) is a psychophysiological modality analysing function of cerebral cortex.

AIM

To analyse the usefulness of CFF in detection of brain dysfunction in patients with PBC.

METHODS

Fifty-one (37 non-cirrhotic/14 cirrhotic) patients with PBC were included. Control group consisted of 31 matched healthy individuals. Fatigue and health-related quality of life (HRQoL) were assessed using Fatigue Impact Scale (FIS) and questionnaire PBC-40. CFF was analysed with HEPAtonorm Analyzer(®).

RESULTS

When compared to healthy controls all patients with PBC showed significantly impaired HRQoL in majority of PBC-40 domains and increased fatigue level in physical domain of FIS. No differences in HRQoL and PBC-40 domains were seen, when patients with and without cirrhosis where compared. CFF analysis showed no difference between healthy controls and patients with PBC. CFF did not correlate with PBC-40 and FIS domains.

CONCLUSION

CFF fails to determine brain dysfunction in non-encephalopatic patients with PBC, suggesting that functional efficiency of their cerebral cortex remains unaffected and other central mechanisms are responsible for chronic fatigue in these patients.

Persistence of cognitive impairment after resolution of overt hepatic encephalopathy

BACKGROUND & AIMS

In patients with cirrhosis, hepatic encephalopathy (HE) has acute but reversible as well as chronic components. We investigated the extent of residual cognitive impairment following clinical resolution of overt HE (OHE).

METHODS

Cognitive function of cirrhotic patients was evaluated using psychometric tests (digit symbol, block design, and number connection [NCT-A and B]) and the inhibitory control test (ICT). Improvement (reduction) in ICT lures and first minus second halves (DeltaL(1-2)) were used to determine learning of response inhibition. Two cross-sectional studies (A and B) compared data from stable cirrhotic patients with or without prior OHE. We then prospectively assessed cognitive performance, before and after the first episode of OHE.

RESULTS

In study A (226 cirrhotic patients), 54 had experienced OHE, 120 had minimal HE, and 52 with no minimal HE. Despite normal mental status on lactulose after OHE, cirrhotic patients were cognitively impaired, based on results from all tests. Learning of response inhibition (DeltaL(1-2) > or =1) was evident in patients with minimal HE and no minimal HE but was lost after OHE. In study B (50 additional patients who developed > or =1 documented OHE episode during follow-up), the number of OHE hospitalizations correlated with severity of residual impairment, indicated by ICT lures (r = 0.5, P = .0001), digit symbol test (r = -0.39, P = .002), and number connection test-B (r = 0.33, P = .04). In the prospective study (59 cirrhotic patients without OHE), 15 developed OHE; ICT lure response worsened significantly after OHE (12 before vs 18 after, P = .0003), and learning of response inhibition was lost. The 44 patients who did not experience OHE did not have deteriorations in cognitive function in serial testing.

CONCLUSIONS

In cirrhosis, episodes of OHE are associated with persistent and cumulative deficits in working memory, response inhibition, and learning.

Persistence of cognitive impairment after resolution of overt hepatic encephalopathy

BACKGROUND & AIMS

In patients with cirrhosis, hepatic encephalopathy (HE) has acute but reversible as well as chronic components. We investigated the extent of residual cognitive impairment following clinical resolution of overt HE (OHE).

METHODS

Cognitive function of cirrhotic patients was evaluated using psychometric tests (digit symbol, block design, and number connection [NCT-A and B]) and the inhibitory control test (ICT). Improvement (reduction) in ICT lures and first minus second halves (DeltaL(1-2)) were used to determine learning of response inhibition. Two cross-sectional studies (A and B) compared data from stable cirrhotic patients with or without prior OHE. We then prospectively assessed cognitive performance, before and after the first episode of OHE.

RESULTS

In study A (226 cirrhotic patients), 54 had experienced OHE, 120 had minimal HE, and 52 with no minimal HE. Despite normal mental status on lactulose after OHE, cirrhotic patients were cognitively impaired, based on results from all tests. Learning of response inhibition (DeltaL(1-2) > or =1) was evident in patients with minimal HE and no minimal HE but was lost after OHE. In study B (50 additional patients who developed > or =1 documented OHE episode during follow-up), the number of OHE hospitalizations correlated with severity of residual impairment, indicated by ICT lures (r = 0.5, P = .0001), digit symbol test (r = -0.39, P = .002), and number connection test-B (r = 0.33, P = .04). In the prospective study (59 cirrhotic patients without OHE), 15 developed OHE; ICT lure response worsened significantly after OHE (12 before vs 18 after, P = .0003), and learning of response inhibition was lost. The 44 patients who did not experience OHE did not have deteriorations in cognitive function in serial testing.

CONCLUSIONS

In cirrhosis, episodes of OHE are associated with persistent and cumulative deficits in working memory, response inhibition, and learning.

The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy

Hepatic encephalopathy (HE) is a diverse manifestation of acute and chronic liver failure, ranging from cognitive impairment, only detectable on psychometric evaluation through to confusion, coma and death from cerebral oedema. While there is widespread acceptance of its importance, there is little consensus on how best to diagnose and monitor HE. Clinical descriptions, psychometric testing, electroencephalography and magnetic resonance (MR) imaging (and lately, MR spectroscopy) have all been proposed. MR techniques, in contrast to other modalities, have the benefit of objectivity and of being able to interrogate the brain directly with respect to changes in brain size, function and the metabolic disturbances thought to underlie HE, particularly in the context of astrocyte swelling. Modern clinical MRI scanners with multinuclear MR spectroscopy capabilities and brain mapping software can demonstrate structural and functional cellular changes using volumetric MRI, magnetization transfer MRI, diffusion-weighting MRI, functional MRI with oxygenation measurements and in vivo and in vitro (1)H and (31)P MR spectroscopy. This review describes the relative merits of these techniques and provides guidance on the directions for future research and translation into clinical practice.

Diagnostic and prognostic values of critical flicker frequency determination as new diagnostic tool for objective HE evaluation in patients undergoing TIPS implantation

OBJECTIVES

The diagnostic and prognostic value of critical flicker frequency (CFF) analysis for assessment of severity and dynamics of hepatic encephalopathy (HE) was studied before and after implantation of a transjugular intrahepatic portosystemic shunt (TIPS).

BASIC METHODS

Sixty-three cirrhotic patients were retrospectively analyzed for the consequences of TIPS implantation. Thirty-one cirrhotic patients without TIPS implantation served as age-matched, sex-matched, Child-Pugh-matched controls. CFF and computer psychometric tests as objective test parameters of HE-severity were evaluated for analysis of visual discrimination ability, general arousal and cognitive function. Kaplan-Meier method and Cox proportional hazards regression model were used for analysis of prognostic significances.

MAIN RESULTS

In the control group, HE-severity was stable during the observation period (442+/-428 days) with minimal changes in CFF (-0.1+/-1.9 Hz). In the intervention group, TIPS implantation had no effect on HE-severity in 44% of the patients and CFF shifted by only 0.01+/-1.5 Hz. Thirty-five and 21% of the patients experienced an aggravation or improvement of HE after TIPS implantation, respectively. In HE improvers CFF increased by 3.3+/-2.3 Hz and decreased by 3.5+/-1.5 Hz in those experiencing an aggravation of HE-severity. Univariate analysis showed that overall survival in the study population inversely correlated with HE-severity and serum alkaline phosphatase activity and positively correlated with serum sodium, albumin and CFF. Serum albumin, alkaline phosphatase levels and CFF were independent predictors of survival in a multivariate Cox regression analysis.

CONCLUSION

The data show that pre-TIPS HE does not predict post-TIPS encephalopathy. Otherwise, CFF can reliably pick up the evolution of HE-severity after TIPS implantation. Low pre-TIPS CFF is indicative for a poor prognosis and may help to identify transplant candidates without delay.

Voxel-based analyses of magnetization transfer imaging of the brain in hepatic encephalopathy

AIM: To evaluate the spatial distribution of cerebral abnormalities in cirrhotic subjects with and without hepatic encephalopathy (HE) found with magnetization transfer imaging (MTI).

METHODS: Nineteen cirrhotic patients graded from neurologically normal to HE grade 2 and 18 healthy control subjects underwent magnetic resonance imaging. They gave institutional-review-board-approved written consent. Magnetization transfer ratio (MTR) maps were generated from MTI. We tested for significant differences compared to the control group using statistical non-parametric mapping (SnPM) for a voxel-based evaluation.

RESULTS: The MTR of grey and white matter was lower in subjects with more severe HE. Changes were found in patients with cirrhosis without neurological deficits in the basal ganglia and bilateral white matter. The loss in magnetization transfer increased in severity and spatial extent in patients with overt HE. Patients with HE grade 2 showed an MTR decrease in white and grey matter: the maximum loss of magnetization transfer effect was located in the basal ganglia [SnPM (pseudo-)t = 17.98, P = 0.0001].

CONCLUSION: The distribution of MTR changes in HE points to an early involvement of basal ganglia and white matter in HE.

Hepatic encephalopathy and fitness to drive

BACKGROUND & AIMS

Low-grade hepatic encephalopathy (HE) may impair fitness to drive. Driving deficits have not yet been characterized, and their relation to psychometric test results is unclear.

METHODS

Fifty-one cirrhotic patients and 48 age-matched controls underwent real driving in a multiple sensor and camera-equipped car, laboratory and "in-car" computer psychometry, and driving instructor's assessment.

RESULTS

Ten cirrhotic patients had no hepatic encephalopathy (HE0); 27 and 14 patients suffered from minimal HE (mHE) and overt HE grade I (oHE), respectively. During real driving, mHE and oHE patients showed significantly more violations of in-lane keeping, reduced break use, prolonged reaction times, and diminished stress tolerance compared with control or cirrhotic HE0 patients. In a self-evaluation questionnaire, mHE and oHE, but not the HE0, patients strongly overestimated their driving abilities. According to the driving instructor's assessment, 75%, 48%, and 39% of the patients with HE0, mHE, and oHE, respectively, were fit to drive, compared with 87% in the control group. Driving deficits in oHE patients were largely due to cognitive defects and prolonged reaction times, whereas, in mHE patients, mistakes and attention deficits predominated. Computer psychometric test results worsened with HE severity and age, whereas real driving was age independent. In 25 out of 94 patients, discordant results for driving fitness were obtained (driving instructor's assessment vs computer psychometry); in mHE and oHE patients, the concordance rates were only 62% and 64%, respectively.

CONCLUSIONS

Despite significant driving deficits, HE patients overestimate their driving abilities. The presence of mHE does not necessarily predict driving unfitness, and computer-based testings cannot reliably predict driving fitness.

Voxel-based diffusion tensor magnetic resonance imaging evaluation of low-grade hepatic encephalopathy

PURPOSE

To quantify the changes in brain water diffusivity in hepatic encephalopathy (HE) associated with cirrhosis using diffusion tensor imaging (DTI) and to correlate with neuropsychological (NP) scores.

MATERIALS AND METHODS

DTI was performed in 14 patients with low-grade HE and age/gender-comparable 16 healthy controls. Whole brain mean diffusivity (MD) and fractional anisotropy (FA) maps were calculated, normalized to common space, smoothed, and compared voxel-by-voxel between groups using analysis of covariance with age included as a covariate. The average MD and FA values were also calculated from individual subjects for selected brain regions and correlated with the neuropsychological scores.

RESULTS

Patients with HE showed increased MD in the cortical gray and white matter and the internal capsule. Less extensive brain regions with decreased FA were observed in the bilateral frontal and occipital white matter. MD values from the corpus callosum correlated inversely with several NP scores among HE patients and controls. Positive correlations were observed with FA values and cognitive scores.

CONCLUSION

Voxel-based DTI analysis showed widespread brain regions with increased MD values, indicating enhanced water content and decreased FA in cirrhotic patients with HE. The MD and FA values from selected regions correlated with the NP scores.

Quantitative cerebral water content mapping in hepatic encephalopathy

There is increasing evidence that the pathophysiology of hepatic encephalopathy is tightly associated with low-grade cerebral oedema; however, no method has yet specifically and unambiguously confirmed this hypothesis in vivo. The current study describes the quantitative measurement of localised water content using MRI in a cohort of 38 patients suffering from hepatic encephalopathy. A significant global increase in cerebral water content was observed in white matter whereas water content in grey matter was globally unaffected. However, significant spatial variations in the water content distribution, especially in grey matter, were observed and were correlated with disease grade and critical flicker frequency. In addition, regions-of-interest were defined and a significant change in water content with disease grade was found in the frontal and occipital white matter, the globus pallidus, the anterior limb of the internal capsule and the putamen. No association of water content and HE grade was established for the occipital visual and frontal cortices, the thalamus, the posterior limb of the internal capsule, the caudate nucleus and the coronal white matter. In conclusion, the measurements presented here are the first direct and quantitative demonstration of the presence of low-grade cerebral oedema in patients with hepatic encephalopathy. Further, absolute changes in tissue water content were quantified for various brain regions.

Navigation skill impairment: Another dimension of the driving difficulties in minimal hepatic encephalopathy

Patients with minimal hepatic encephalopathy (MHE) have attention, response inhibition, and working memory difficulties that are associated with driving impairment and high motor vehicle accident risk. Navigation is a complex system needed for safe driving that requires functioning working memory and other domains adversely affected by MHE. The aim of this study was to determine the effect of MHE on navigation skills and correlate them with psychometric impairment. Forty-nine nonalcoholic patients with cirrhosis (34 MHE+, 15 MHE-; divided on the basis of a battery of block design, digit symbol, and number connection test A) and 48 age/education-matched controls were included. All patients underwent the psychometric battery and inhibitory control test (ICT) (a test of response inhibition) and driving simulation. Driving simulation consisted of 4 parts: (1) training; (2) driving (outcome being accidents); (3) divided attention (outcome being missed tasks); and (4) navigation, driving along a marked path on a map in a "virtual city" (outcome being illegal turns). Illegal turns were significantly higher in MHE+ (median 1; P = 0.007) compared with MHE-/controls (median 0). Patients who were MHE+ missed more divided attention tasks compared with others (median MHE+ 1, MHE-/controls 0; P = 0.001). Similarly, accidents were higher in patients who were MHE+ (median 2.5; P = 0.004) compared with MHE- (median 1) or controls (median 2). Accidents and illegal turns were significantly correlated (P = 0.001, r = 0.51). ICT impairment was the test most correlated with illegal turns (r = 0.6) and accidents (r = 0.44), although impairment on the other tests were also correlated with illegal turns.

CONCLUSION

Patients positive for MHE have impaired navigation skills on a driving simulator, which is correlated with impairment in response inhibition (ICT) and attention. This navigation difficulty may pose additional driving problems, compounding the pre-existing deleterious effect of attention deficits.

Therapeutic studies in hepatic encephalopathy

This article has arisen from presentations made at the 4th International Hannover Conference on Hepatic Encephalopathy held in Dresden, 2006. Each author as listed describes their presentation given as part of a section entitled "Therapeutic Studies in Hepatic Encephalopathy." The first section deals with the justification for placebo-controlled trials in hepatic encephalopathy. The other two sections discuss, in detail, outcome parameters for therapeutic studies in the clinical and research setting, respectively.

Activation of soluble guanylate cyclase by nitric oxide in lymphocytes correlates with minimal hepatic encephalopathy in cirrhotic patients

Patients with liver cirrhosis with normal neurological and mental status examination may present minimal forms of hepatic encephalopathy, showing intellectual function impairment that cannot be detected through general clinical examination but can be unveiled using specific neuropsychological or neurophysiological examination. Evaluation of minimal hepatic encephalopathy (MHE) in cirrhotic patients would have prognostic value. The psychometric hepatic encephalopathy score (PHES) has been recommended as the "gold standard" in the diagnosis of MHE. Altered modulation of cyclic GMP (cGMP) levels in the brain seems to be responsible for the impairment of some types of cognitive function in liver disease. In animal models of liver disease, some of the alterations in modulation of cGMP levels in the brain are reproduced in lymphocytes. The aim of the present work was to assess whether there is a correlation between the alterations in different parameters involved in modulation of cGMP levels and the presence of MHE in patients with liver disease. We studied in 46 patients with liver cirrhosis and 26 controls the performance in the PHES battery of psychometric tests and the critical flicker frequency (CFF), the concentration of cGMP in plasma and lymphocytes, activation of guanylate cyclase by nitric oxide (NO) in lymphocytes, and several parameters likely involved in altered cGMP homeostasis in liver disease such as ammonia, NO metabolites, and atrial natriuretic peptide (ANP). Activation of guanylate cyclase by NO in lymphocytes and cGMP in plasma were higher and CFF lower in patients with MHE than in patients without MHE. Ammonia, ANP, and metabolites of NO were higher in patients than in controls but were no different in patients with or without MHE. Alteration in activation of guanylate cyclase by NO in lymphocytes correlates with PHES performance, CFF, and ammonia levels. This suggests that altered modulation of guanylate cyclase by NO in lymphocytes would reflect a parallel alteration in the brain occurring in patients with MHE that would be involved in their cognitive impairment.

Top-down and bottom-up processes in the extrastriate cortex of cirrhotic patients: An ERP study

OBJECTIVE

This study aims to evaluate the efficiency of top-down and bottom-up processes in the extrastriate cortex of cirrhotic patients without overt hepatic encephalopathy (HE).

METHODS

Reaction times (RTs), accuracy and event-related potentials (ERPs) were recorded during the execution of a visual Simon task in 17 cirrhotic patients and 10 healthy controls. Amplitude and latency of the P1 and N1 (indexes of bottom-up processes) and of the N2pc (index of top-down processes) were measured.

RESULTS

Patients were slower than controls, and patients with minimal HE (MHE) were slower than patients without MHE. The distribution analysis of RTs showed that the Simon effect decays with slower RTs in all the groups and that the shape of the distribution was different in MHE patients. No differences were found between cirrhotic patients and controls for P1 and N1 amplitude and latency. In contrast, N2pc latency was delayed in cirrhotic patients compared to controls independently of MHE.

CONCLUSIONS

In the extrastriate cortex of cirrhotic patients without HE, top-down processes are altered whereas bottom-up processes are preserved.

SIGNIFICANCE

The analysis of exogenous and endogenous visual components of ERPs provides a model to study the functional dissociation between top-down and bottom-up processes inside the extrastriate cortex.

Conscious awareness of flicker in humans involves frontal and parietal cortex

Even when confined to the same spatial location, flickering and steady light evoke very different conscious experiences because of their distinct temporal patterns. The neural basis of such differences in subjective experience remains uncertain . Here, we used functional MRI in humans to examine the neural structures involved in awareness of flicker. Participants viewed a single point source of light that flickered at the critical flicker fusion (CFF) threshold, where the same stimulus is sometimes perceived as flickering and sometimes as steady (fused) . We were thus able to compare brain activity for conscious percepts that differed qualitatively (flickering or fused) but were evoked by identical physical stimuli. Greater brain activation was observed on flicker (versus fused) trials in regions of frontal and parietal cortex previously associated with visual awareness in tasks that did not require detection of temporal patterns . In contrast, greater activation was observed on fused (versus flicker) trials in occipital extrastriate cortex. Our findings indicate that activity of higher-level cortical areas is important for awareness of temporally distinct visual events in the context of a nonspatial task, and they thus suggest that frontal and parietal regions may play a general role in visual awareness.

Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neuro-psychiatric abnormality, which complicates the course of patients with liver disease and results from hepatocellular failure and/or portosystemic shunting. The manifestations of HE are widely variable and involve a spectrum from mild subclinical disturbance to deep coma. Research interest has focused on the role of circulating gut-derived toxins, particularly ammonia, the development of brain swelling and changes in cerebral neurotransmitter systems that lead to global CNS depression and disordered function. Until recently the direct investigation of cerebral function has been difficult in man. However, new magnetic resonance imaging (MRI) techniques provide a non-invasive means of assessment of changes in brain volume (coregistered MRI) and impaired brain function (fMRI), while proton magnetic resonance spectroscopy (¹H MRS) detects changes in brain biochemistry, including direct measurement of cerebral osmolytes, such as myoinositol, glutamate and glutamine which govern processes intrinsic to cellular homeostasis, including the accumulation of intracellular water. The concentrations of these intracellular osmolytes alter with hyperammonaemia. MRS-detected metabolite abnormalities correlate with the severity of neuropsychiatric impairment and since MR spectra return towards normal after treatment, the technique may be of use in objective patient monitoring and in assessing the effectiveness of various treatment regimens.

Neural synchronization in hepatic encephalopathy

Hepatic encephalopathy (HE) is clinically characterized by a large variety of symptoms including motor symptoms, cognitive deficits, as well as changes in the level of alertness up to hepatic coma. A number of pathological processes affecting glial and neuronal function have been identified, including hyper-ammonia, changes within the excitatory and inhibitory transmitter systems, as well as osmolytic changes with consecutive cell swelling. One explanation how these pathological processes result in neurological deficits in HE is the concept of pathologically synchronized oscillations within and between relevant brain regions. A number of studies suggest that the cognitive deficits and the reduced level of alertness in patients with HE can be attributed to a significantly slowed and pathologically synchronized spontaneous oscillatory brain activity, depending on the grade of HE. Moreover, HE motor symptoms, like postural tremor called"mini asterixis," have recently been shown to be associated with abnormal thalamo-cortical and cortico-muscular synchronization. Indirect evidence exists from studies of processing and recognition of flicker stimuli that in HE slowing of oscillations also occurs in the visual system. Taken together, pathological synchronization of neuronal activity may turn out to be a promising pathophysiological concept for linking neuronal dysfunction to the diversity of clinical deficits in HE.

Gender differences in the functional neuroanatomy of emotional episodic autobiographical memory

Autobiographical memory is based on interactions between episodic memory contents, associated emotions, and a sense of self-continuity along the time axis of one's life. The functional neuroanatomy subserving autobiographical memory is known to include prefrontal, medial and lateral temporal, as well as retrosplenial brain areas; however, whether gender differences exist in neural correlates of autobiographical memory remains to be clarified. We reanalyzed data from a previous functional magnetic resonance imaging (fMRI) experiment to investigate gender-related differences in the neural bases of autobiographical memories with differential remoteness and emotional valence. On the behavioral level, there were no significant gender differences in memory performance or emotional intensity of memories. Activations common to males and females during autobiographical memory retrieval were observed in a bilateral network of brain areas comprising medial and lateral temporal regions, including hippocampal and parahippocampal structures, posterior cingulate, as well as prefrontal cortex. In males (relative to females), all types of autobiographical memories investigated were associated with differential activation of the left parahippocampal gyrus. By contrast, right dorsolateral prefrontal cortex was activated differentially by females. In addition, the right insula was activated differentially in females during remote and negative memory retrieval. The data show gender-related differential neural activations within the network subserving autobiographical memory in both genders. We suggest that the differential activations may reflect gender-specific cognitive strategies during access to autobiographical memories that do not necessarily affect the behavioral level of memory performance and emotionality.