Hypercapnic alteration of visual evoked responses in acute cerebral infarction

Visual Evoked Potential Using Head-Mounted Display Versus Cathode Ray Tube: A Pilot Study

Objective

To present a new stimulation method based on the use of a head-mounted display (HMD) during pattern reversal visual evoked potential (PR-VEP) testing and to compare variables of HMD to those of conventional cathode ray tube (CRT).

Methods

Twenty-three normal subjects without visual problems were recruited. PR-VEPs were generated using CRT or HMD stimuli. VEP outcome measures included latencies (N75, P100, and N145) and peak-to-peak amplitudes (N75–P100 and P100–N145). Subjective discomfort associated with HMD was determined using a self-administered questionnaire.

Results

PR-VEPs generated by HMD stimuli showed typical triphasic waveforms, the components of which were found to be correlated with those obtained using conventional CRT stimuli. Self-administered discomfort questionnaires revealed that HMD was more comfortable in some aspects. It allowed subjects to concentrate better than CRT.

Conclusion

The described HMD stimulation can be used as an alternative to the standard CRT stimulation for PR-VEPs. PR-VEP testing using HMD has potential applications in clinical practice and visual system research because HMD can be used on a wider range of subjects compared to CRT.

Hyperglycemia with occipital seizures: images and visual evoked potentials

PURPOSE

Hyperglycemia may rarely be seen with visual seizures. Observation of both visual evoked potentials (VEPs) and magnetic resonance imaging (MRI) in visual status epilepticus (SE) has not been reported. We describe acute and follow-up VEP and MRI findings of a patient with hyperglycemia-related visual SE of occipital origin.

METHODS

In a 59-year-old diabetic woman, complex visual hallucinations and illusions developed with < or =10 seizures per hour as an initial manifestation of nonketotic hyperglycemia.

RESULTS

Neurologic examination revealed ictal nystagmus to the right and continuous right hemianopsia. Ictal electroencephalography (EEG) and Tc-99m hexamethylpropylene amine oxime (HMPAO) single-photon emission computed tomography (SPECT) revealed an epileptogenic focus in the left occipital lobe. MRI with fluid-attenuated inversion recovery showed focal subcortical hypointensity and gyral hyperintensity. Follow-up MRI showed only minimal gyral hyperintensity at 6 months. The P100 amplitude of VEP was significantly higher at the right occipital area during SE, but slightly higher on the left after the patient had been seizure free for 6 months.

CONCLUSIONS

Occipital seizures and hemianopsia can be caused by hyperglycemia and may be accompanied by special MRI and VEP findings.

Effect of brain damage and source location on left-right asymmetry of visual evoked potentials in a realistic model of the head

The left-right asymmetry of visual evoked potentials (VEP) was studied using a realistic three-dimensional model of the head, constructed from CT scans. The integral equation describing the potential distribution due to an inner current-dipole (simulating the VEP source), inside the biological volume conductor (the head) was solved numerically using the finite volume method. The effect of several structural parameters, such as the natural head geometry, the location and orientation of the current source, and conductivity changes (simulating a brain damage) on the VEP asymmetry was examined. The results revealed that the natural anatomical asymmetry presented in a normal head induces some degree of VEP asymmetry (up to 0.42% at the occipital electrodes), yet the major cause of left-right asymmetry is due to asymmetrical location of the source: up to 6.53% in the O(2)-O(1) electrodes for an angular shift of 3 degrees to the left. It was also found that conductivity changes inside the head have a smaller effect on the VEP asymmetry (up to 3.35% in hemorrhaged brain compared to 1.96% in the normal brain at the C(4)-C(3) electrodes). These findings may help in better understanding VEP sources of asymmetry, originating not only due to cerebral functionality, but from structural parameters as well.

Functional correlates with left-right asymmetry of visual evoked potentials in stroke patients: modeling and experimental results

OBJECTIVE

To determine the correlation between a clinical measure of function in patients after first stroke and left-right scalp amplitude of visual evoked potentials using a theoretical model of the head.

DESIGN

A random sample of first-stroke patients underwent routine function measurement and investigation of left-right scalp potential asymmetry. Results of the encephalographic tests were compared with those of a healthy subject. To examine the effect of the conductivity in the damaged area on the potential asymmetry, numerical calculations were performed on a model, with four concentric circular compartments representing the brain, cerebrospinal fluid layer, skull, and scalp. The damaged region was modeled as a circular section.

SETTING

Neurologic rehabilitation ward of a major rehabilitation hospital and university-affiliated biomedical engineering laboratory.

PATIENTS

Four men aged 58 to 71 years, 3 with brain thrombosis and 1 with hemorrhagic stroke. The patients were admitted for rehabilitation an average of 3 weeks after the stroke and stayed for an average of 137 days. Damage was confined to the right brain in all cases; three of the patients had neglect syndrome and/or sensory disturbances. A healthy subject without stroke was also examined.

MEASURES

Function was measured with the Functional Independence Measure (FIM) at 48 to 72 hours from admission and during the last week before discharge. Functional gain was calculated by subtracting the FIM admission score from the discharge score. Left-right scalp visual evoked potential amplitude was studied with flash stimuli according to the 10-20 international system and a theoretical model of the head based on two-dimensional computed tomography images; the volume conductor equation was solved numerically using the finite volume method. Left-right potential asymmetry and the damaged-region-to-brain-area ratio were calculated and correlated with the FIM values by linear regression analysis. Negative asymmetry indicates that the activity in the right damaged hemisphere is lower than in the undamaged one.

RESULTS

A negative correlation was noted between the FIM score on admission and the left-right scalp potential amplitude asymmetry, and between the FIM gain and the damaged-region-to-brain-area ratio obtained from the computed tomography image. Asymmetry was negative in the thrombotic patients and positive in the hemorrhagic one. The healthy subject showed nonsignificant asymmetry.

CONCLUSION

A relationship might exist between the left-right asymmetry of the scalp visual evoked potential and both the damaged-region-to-brain-area ratio and the functional outcome of rehabilitation in poststroke patients. The modeling study shows that the left-right asymmetry is most likely the result of changes in the conductivity at the damaged area, which, in turn, are probably associated with patient functional status and evolution. Further validation in larger groups of patients and normal subjects is needed before these parameters can serve as useful indices for clinical purposes.

Left-right asymmetry of visual evoked potentials in brain-damaged patients: a mathematical model and experimental results

The left-right asymmetry in the potential amplitude on the scalp was studied in poststroke patients by using flash visual evoked potential (VEP) and a numerical two-dimensional model of the head. The left-right asymmetry of the VEP was measured in three patients after thrombosis, in one after hemorrhage, and in one healthy subject. The numerical model used computed tomography images to define the different compartments of the head. The volume conductor equation for the potential distribution created by a dipole source in the occipital region was solved numerically with use of a finite volume method. Left-right asymmetry was calculated with several values of conductivity of the damaged region. The experimental results revealed a negative asymmetry in the three patients after thrombosis (i.e., the potential amplitude over the ischemic hemisphere was smaller than that over the intact hemisphere), whereas, in the patient after hemorrhage, a positive asymmetry was found. Nonsignificant left-right asymmetry was found in the healthy subject. The numerical model revealed that the electrical conductivity of the damaged tissue has a major effect on the left-right asymmetry. Negative asymmetry, such as that found for patients after thrombosis, was obtained when the conductivity of the damaged region was greater than that of the brain, whereas positive asymmetry (hemorrhage patient) was obtained when that conductivity was smaller than that of the brain. This finding indicates that the left-right asymmetry in the scalp VEP of patients after brain damage may be a result of changes in the conductivity of the volume conductor (the ischemic region) between the source and the electrodes.

Pattern reversal visual evoked potentials (PVEPs) in transient ischemic attacks (TIAs) and prolonged reversible ischemic neurological deficits (PRINDs) of anterior circulation with normal EEGs and normal cranial CTs

Out of 75 patients with TIA or PRIND we selected 9 TIAs and 6 PRINDs with normal EEGs and CCTs, full recovery of neurological function, no history of amaurosis fugax and no findings of visual impairments. PVEPs were derived from 01-02 to Fz and Cz as ground following binocular pattern reversal visual stimuli of 1.9 Hz. Interhemispheric differences of the latencies of P60, N80, P100 and of the amplitudes N80/P100 and P100/N140 were compared with the corresponding parameters of 22 age matched controls. In contrary to the latency differences the interhemispheric difference of the amplitude N80/P100 was highly significantly larger (33.5 +/- 16.0%) in patients than in the control group (12.8 +/- 9.8%) (p < or = .0005). The amplitude P100/N140 behaved the same way (p < or = .025); the amplitude of the affected side being smaller. There were no statistical differences between TIAs and PRINDs and a tendency was seen for normalization of the differences with increasing time distances between the onset of the ischemic attack and the point of time of the recordings.

The relationship between visual seizures and visual evoked potentials

We have studied a patient (pt) in status epilepticus with visual seizures (szs) arising focally from the right occipital area and have recorded the visual evoked potential (VEP) to three different stimuli under three different conditions (during, between and with no szs). The pt experienced "sparkling" in the contralateral visual field as the sz and the intensity of the "sparkling" was directly related to the frequency of the ictal activity recorded on the EEG. During the szs the VEPs could still be recorded, but the amplitude of the P100 was higher on the contralateral side with pattern reversal (PR) stimuli, and with flashes (FL) the positive peak after the P100 was less evident on the ipsilateral side. Latencies to these latter two positivities were generally shorter than in normals, with much greater standard deviations ipsilaterally with FL and contralaterally with PR, especially during the actual szs. The relationship between VEP generation and visual sz phenomena is discussed.

Hemispheric asymmetry of visual evoked potentials in patients with well-defined occipital lesions

Visual evoked potentials were examined in 3 patients who had well defined unilateral occipital lobectomy. In an attempt to resolve discrepancies in the interpretation of hemispheric asymmetries in evoked potentials, recordings were obtained from a wide array of electrodes with multiple montages during full-field and hemi-field stimulation with checkerboard pattern reversal and during full-field flash stimulation. The discrepancies appear to be due primarily to differences in electrode montages used in recording. The use of a midfrontal reference electrode reliably produces a paradoxical EP lateralization: abnormal EPs are recorded over the intact occipital lobe and normal EPs are recorded over the ablated occipital lobe. These findings are fully consistent with the interpretation of Barrett et al. (1976) that the generator of the P100 has an equivalent dipole located in the calcarine cortex on the posteromedial surface of the contralateral hemisphere. Our findings support the use of the Queen Square montage (Halliday et al. 1977). The use of electrodes at O1 and O2 referred to the ipsilateral earlobe yields inconsistent lateralization and should be discouraged. The lateralization of EP abnormalities did not differ for flash and pattern reversal, although the asymmetries were sometimes more prominent with pattern reversal.

The electroencephalogram of elderly subjects revisited

The EEGs of 98 elderly volunteers were compared with those of 84 patients with a recent cerebral infarction who had achieved a stable clinical course. All subjects were uniformly evaluated according to a special protocol. The elderly volunteers were accepted for the study if they had no history, signs or symptoms of central nervous system disease. The EEGs were found to be significantly different between the two groups of subjects in several aspects. These included not only possible abnormalities, focal or diffuse, but also some normal features, such as alpha frequency and responses to photic stimulation and to hyperventilation. Groups of these differentiating features were analyzed. Using the single variable of ER (evoked response), discrimination of 80% was achieved. The variables that distinguish the volunteers from the patients may be used in the future to determine whether they are helpful in differentiating normals from patients with conditions other than stroke.

Evoked potentials in the clinical neurosciences

The use of evoked potentials for the evaluation of disorders of the nervous system has become a most valuable aid to the neurosurgeon and neurologist, often providing information of critical value without recourse to invasive techniques. In order to employ these techniques, it is helpful to understand the principles of evoked potential electrogenesis and the methodology used for analysis of evoked potential clinical data. This article is aimed at providing the clinical neurosurgeon with this type of information and with a review of current clinical applications in this rapidly developing field.