Cerebral function analyzing monitor and visual evoked potentials as a noninvasive method of detecting cerebral dysfunction in patients with acute hepatic and renal failure treated with intermittent machine hemofiltration

Clinical neurophysiology of hepatic encephalopathy

BACKGROUND/OBJECTIVES

Hepatic encephalopathy (HE) has relevant impact on the quality of life of patients and their caregivers and causes relevant costs because of hospitalizations and work days lost. Its quantification is important to perform adequate clinical trials on this relevant complication of cirrhosis and portal-systemic shunting. Clinical neurophysiology, which detects functional alterations of the nervous system, has been applied to the study of HE for over 60 years. This review aims at summarizing and clarifying the role of neurophysiologic techniques in the study of HE.

METHODS

A narrative review was performed aiming at interpreting the cited papers and the techniques on the basis of their physiological and pathophysiological meaning.

RESULTS

The potential role of EEG, quantified EEG, evoked potentials-both exogenous, endogenous and motor-have been clarified to the reader that may be unfamiliar with neurophysiology.

CONCLUSIONS

The EEG, reflecting the oscillatory changes of neural network is the preferable tool to detect and monitor HE, with the exception of its most severe stage, when EEG flattens. SSEP and MEP have indication to detect and monitor transmission alterations that are likely related to myelin changes and microedema.

Flash visual evoked potentials are unreliable as markers of ICP due to high variability in normal subjects

BACKGROUND

Previous publications have suggested a high correlation between flash visual evoked potential (F-VEP) N2 peak latency and intracranial pressure. This would enable F-VEP to be used as a non-invasive and inexpensive method to estimate ICP in a number of settings. However, basic knowledge about variability across subjects and test-retest properties of the F-VEP is lacking.

METHODS

Fifteen healthy adult subjects were tested on three different occasions. F-VEP responses were recorded using international standards.

FINDINGS

For the tested population, mean N2 latency was 65.7 ms (SD 10.7 ms) and the range was 48-110 ms. Intra-individual variability was high, in four of the 15 subjects more than 15 ms between testing sessions. The same was found for P2 latency and for N2 and P2 amplitudes. The response waveform was very variable and unambiguous marking of peaks was often difficult. One out of the 15 subjects had a very poorly developed F-VEP response, but a normal pattern-reversal VEP response.

CONCLUSIONS

F-VEP has a wide range of latency, amplitude and waveform across normal subjects. A large proportion of subjects also had a high intra-individual variability over time. This variability makes F-VEPs unreliable as a marker for intracranial pressure, and caution in interpreting F-VEP changes in clinical work is advised.

Neurophysiological investigations of hepatic encephalopathy: ISHEN practice guidelines

By studying neuronal activity through neuronal electrogenesis, neurophysiological investigations provide a functional assessment of the nervous system and, therefore, has been used for quantitative assessment and follow-up of hepatic encephalopathy (HE). The different clinical neurophysiological approaches can be classified depending on the function to explore and their sensitivity to HE. The reliable techniques are those that reflect cortical function, i.e., cognitive-evoked potentials (EPs) (P300 paradigm), electroencephalogram (EEG), visual EPs (latency>100 ms) and somatosensory EPs (SEPs) (latency between 25 and 100 ms). Short-latency EPs (brainstem acoustic EPs, SEPs of a latency<25 ms) are in principle insensitive to HE, but can disclose brainstem conduction deficits due to oedema. SEPs and motor EPs can disclose myelopathies. Because of its parallelism to the clinical examination, clinical neurophysiology can complement the neurological examination: (i) to provide evidence of HE in patients who have normal consciousness; (ii) to rule out, at least under some conditions, disturbances of consciousness due to other causes (e.g. drug-induced disturbances, non-convulsive status epilepticus) with the reservation that the mildest degrees of encephalopathy might be associated with an EEG pattern similar to that induced by drugs; and (iii) to demonstrate the worsening or, conversely improvement, of HE in the follow-up period.

Clinical experience with the noninvasive ICP monitoring system

The Noninvasive ICP (Intracranial Pressure) Monitoring System NIP-200/210 has been used in several hospitals with more than 2000 patients since March 2002. It is based on the N2 wave response to flash visual evoked potentials (FVEP). According to our data, the mean latency period for the FVEP-induced N2 wave in healthy controls was 126.61 +/- 14.64 ms, in which that of females was shorter than that of males (123.95 +/- 10.345 ms vs. 130.75 +/- 14.632 ms; p < 0.05). There was no significant difference between the left or right side response (126.71 +/- 14.91 ms vs. 124.468 +/- 15.043 ms, p > 0.05). No significant difference in latency was found across age groups in our patient pool. In general, the N2 wave was stable and easily identified in most of the patients or healthy controls. When the data obtained with the NIP-200/210 Noninvasive ICP Monitoring System was compared with that from invasive techniques, the results were quite consistent (correlation index 0.651-0.97, standard error 8-15%). From our clinical trial results, we conclude that the latency periods for the FVEP-induced N2 wave reflected ICP values. However this technique is not suitable in patients with bifrontal hematoma, retinal concussion, or contusion of the optical nerve, because an FVEP value cannot be measured accurately in these cases. In our clinical trials, we used the FVEP technique to determine the effectiveness of mannitol in decreasing the ICP. The data revealed that ICP values decreased significantly within 20 minutes after a mannitol injection, and reached a minimum level at 40 minutes. For a single bolus of mannitol, the duration of the ICP decrease ranged from 30-210 minutes. Elevated ICP is one of the most important clinical issues in neurosurgery and neurology. The present noninvasive technique is safe and easy to perform, with a minimal risk of complications.

Neurophysiological investigation of hepatic encephalopathy

The neurophysiological tools used to study hepatic encephalopathy (HE) are the electroencephalogram (EEG) and the evoked potentials (EPs), both exogenous and endogenous. These tools are used (1) to diagnose HE in patients with severe liver disease and mental alteration, (2) to grade overt HE and monitor the effect of treatment for HE, (3) to diagnose minimal HE (4) to predict the occurrence of episodes of overt HE or liver-related death. The rationale for the use of each of these tools together with their theoretical and practical role is reviewed.

Renal replacement therapy in the patient with acute brain injury

The patient with an acute brain injury requiring renal replacement therapy presents a major problem in that conventional intermittent hemodialysis may exacerbate the injury by compromising cerebral perfusion pressure, either after a reduction in cerebral perfusion or because of increased cerebral edema. Compared with standard intermittent hemodialysis, the continuous forms of renal replacement therapy (CRRT) provide an effective therapy in terms of solute clearance, coupled with improved cardiovascular and intracranial stability. The disadvantage of CRRT is that anticoagulation may be required, and anticoagulants with systemic effects may provoke intracerebral hemorrhage, either at the site of damage or around the intracranial pressure monitoring device. Although peritoneal dialysis does not require anticoagulation, the clearances achieved are often less than those of CRRT, and sudden changes in intraperitoneal volume may provoke cardiovascular and thus intracranial instability.

Electrocortical brain activity, cerebral haemodynamics and oxygenation during progressive hypotension in newborn piglets

OBJECTIVES

To investigate the relationships between systemic and cerebral haemodynamics and oxygenation, and electroencephalogram (EEG) amplitude and frequency analysis studied by the cerebral function analyzing monitor (CFAM) during progressive hypovolemic hypotension.

METHODS

Six piglets of 1 week of age, weighing 1.9-3.4 kg were mechanically ventilated under 1-1.5% halothane anaesthesia. After 1 h stabilization, blood was withdrawn in aliquots of 10 ml/kg over 15 min up to a total of 40-60 ml/kg. Arterial oxygenation was maintained at normal levels. Thereafter, the total blood volume previously withdrawn, was reinfused. Changes in near infrared spectroscopy (NIRS) parameters [cerebral oxidized cytochrome aa3 (Cytaa3), cerebral blood volume (CBV) or total haemoglobin (tHb: oxy- + deoxyhaemoglobin)], carotid blood flow (Q(car)), maximal EEG amplitude and EEG frequency percentages were analyzed continuously.

RESULTS

The EEG amplitude remained stable until the mean arterial blood pressure (MAP), Q(car) and tHb dropped below 30 mmHg (41% of baseline), 20 ml/min (33% of baseline) and 82% of baseline, respectively. Delta (delta) wave frequency percentage of the CFAM increased significantly at MAP below 30 mm Hg. EEG amplitude remained depressed after blood reinfusion and haemodynamic recovery. Cytaa3 changes were not statistically significant, reflecting sufficient neuronal oxygenation.

CONCLUSION

Our results show that electrocortical function is affected only by profound systemic hypotension. This occurred at a higher level of cerebral oxygen delivery than the level associated with neuronal hypoxia and secondary cell damage.