Multistage polar switching in bent-core mesogens.

We report a novel type of electro-optical switching in a tilted smectic phase of bent-shaped mesogens. The switching consists of a continuous stage and two bistable transitions. Detailed optical and electro-optical measurements using high-speed imaging are given and possible interpretations of the experimental results are discussed.

Field-induced texture transitions in a bent-core nematic liquid crystal.

It is demonstrated in electro-optic experiments that an external electric field of the order of 10;{5} V/m induces persisting texture transitions in a nematic phase formed by bent-core mesogens. The field-induced metastable state is identified by its optical and electric properties. After the field is switched off, the original and induced states can coexist in domains for about one hour in planar sandwich cells. During this time, the induced domains gradually shrink but they can be stabilized in moderate electric fields. The occurrence of similar domains in homeotropic cells suggests that the transition into a metastable biaxial state is observed. In the field-free planar ground state, the formation of inversion walls is observed inside the metastable domains.

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens.

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle approximately 45 degrees). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35 V microm(-1)) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, 13C- and 1H-NMR investigations, dielectric and electro-optical measurements.

Optically isotropic liquid-crystal phase of bent-core molecules with polar nanostructure.

We found that the optically isotropic (I(M)) mesophase observed recently below the nematic phase of the bent-core liquid crystal 4-chlororesorcinol bis[4-(4-n-dodecyloxybenzoyloxy)benzoate] shows ferroelectric-type switching. Polarizing microscopic, electric current, dielectric, and dynamic light scattering studies lead us to propose that the I(M) phase is composed of interconnected orthoconic racemic smectic (Sm-Ca P(F)) nanodomains with random layer orientations. Near the nematic phase, where the polarization can be saturated by electric fields, the system responds in a fashion analogous to the granular structure of a magnetic spin glass--in particular, we observed that the relaxation back to the nonpoled structure follows a similar, inverse logarithmic rule.

Polarization-modulated smectic liquid crystal phases.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Field-induced switching between states of opposite chirality in a liquid-crystalline phase.

A tilted smectic phase of a new achiral banana-shaped mesogen is presented. It possesses liquidlike order within the layers and appears with a fan-shaped texture. At sufficiently high electric fields, this texture can be transformed into a texture that displays a complete extinction between crossed polarizers and it forms randomly distributed chiral domains. Above a threshold these domains can be reversibly switched into a state of opposite handedness.

Experimental evidence for Sm-CG-->Sm-CP polymorphism in fluorinated bent-shaped mesogens.

We report the experimental results on pure fluorinated bent-shaped mesogens showing very unusual textures and electro-optical behavior. In the view of recent publications such behavior can be explained by triclinic symmetry of the mesophase (Sm-C(G)) formed by these compounds. Based on the results of the x-ray diffraction and electro-optical investigations, we give evidence for the Sm-C(G) -->Sm-CP polymorphism.

Electroconvection in nematic liquid crystals with positive dielectric and negative conductivity anisotropy.

Electroconvection in an unusual nematic compound with strongly positive dielectric anisotropy and negative anisotropy of the conductivity is investigated. For homeotropic alignment, where one has a direct transition to rolls or squares depending on the frequency of the applied voltage, we present a quantitative theory. From the comparison we infer values for some viscosities, which are rather unusual, but not unreasonable in view of the vicinity of the nematic-smectic transition. For planar alignment, electroconvection sets in above a splay Freedericksz transition with "parallel rolls," which is also captured by the theory.

Experimental evidence for an achiral orthogonal biaxial smectic phase without in-plane order exhibiting antiferroelectric switching behavior.

We report unambiguous experimental evidence for an achiral orthogonal biaxial smectic-A phase which exhibits antiferroelectric switching behavior. The evidence is based on x-ray-diffraction measurements, texture observation, and the results of dielectric and electro-optical measurements.

Severe, isolated thrombocytopenia under polytherapy with carbamazepine and valproate.

It is not known whether polytherapy with carbamazepine (CBZ) and valproate (VPA) increases the risk of thrombocytopenia. A 67-year-old woman with symptomatic epilepsy since age 6 years, classified as Lennox-Gasteau syndrome, developed severe, isolated thrombocytopenia of 5 GIGA/L (normal: 150-360 GIGA/L) after being on CBZ (1200 mg/day) for 15 days and additional VPA (300 mg/day) for 5 days in combination. After discontinuation of CBZ and VPA and two thrombocyte transfusions, the thrombocyte count normalized within 3 days. Because she had been taking VPA for only 5 days in addition to CBZ, it could not be confirmed whether it was CBZ alone or the combination of CBZ and VPA that was responsible for the severe thrombocytopenia.

Video-EEG monitoring in respiratory chain disorders.

Subclinical epileptiform activity in patients with respiratory chain disorders (RCDs) has not been previously investigated by video-EEG monitoring. The purpose of this study was to look for the type and frequency of epileptiform activity during a 24 h-video-EEG recording in RCD patients. Eleven patients with RCD, 7 women and 4 men, aged 24-72 years, underwent a clinical neurologic examination, blood tests, CT/MRI scans of the brain, routine scalp EEG and continuous video-EEG monitoring over 24 consecutive hours. Ten patients had normal 24 h-video-EEG recordings. One of these patients had no CNS involvement; her resting EEGs showed diffuse background slowing. In one patient, automatic spike detection revealed 12 spikes during a 6 hour period. Resting scalp EEG in this patient showed extensive, bilaterally synchronous, pseudoperiodic polyspike-waves. In conclusion, subclinical epileptiform activity could be recorded by 24 h-video-EEG monitoring in only 1 of 11 RCD patients. The single patient with subclinical epileptiform activity presented with intermittent myoclonic jerks.

[Current problems in epilepsy]. / Aktuelle Fragen der Epilepsie.

Three new aspects of epilepsy are discussed: the mesiotemporal syndrome, vagus nerve stimulation, and epilepsy and driving fitness. In recent years mesiotemporal epilepsy has been recognised as the most frequent epileptic syndrome in adults. The main clinical features are febrile convulsions during childhood, followed by characteristic focal seizures in the second decade of life. The typical seizure is characterised by an aura, followed by loss of consciousness, with motor phenomena and automatisms followed by longer periods of postictal confusion. Atrophy of the hippocampus and sclerosis are observed in MRI. The syndrome is frequently drug resistant, however, 80% of the patients are free of seizure after surgical treatment. Vagus nerve stimulation is a new option in the treatment of patients with drug resistant epilepsy (partial seizures with or without secondary generalization, Lennox-Gastaut syndrome), especially when surgical intervention is not indicated. Worldwide a total of more than 4000 patients have been treated. More than 50% reduction in the frequency of seizures can be obtained in 35-40% of drug resistant patients. Complications are rare. Finally, the issue of driving fitness and epilepsy as well as provoked seizures are discussed. The current regulations and laws are taken into consideration and revised regulations for Austria are suggested.

Reliability of automatic and visual analysis of interictal spikes in lateralising an epileptic focus during video-EEG monitoring.

We evaluated 2 h of two night recordings of surface EEG of 10 patients with drug-resistant focal epilepsy using video-EEG monitoring, giving 40 h of EEG. The raw data of the automatic spike analysis according to the Gotman algorithm was visually corrected by rejecting false detections. Furthermore, the complete EEG recordings were analysed independently visually by two experienced electroencephalographers. For each method we analysed the total count of detections and the topographical distribution (left-right) of spikes. The total number of detections was significantly higher (243%) in the raw data and significantly lower after elimination of false detections (57%) in comparison to conventional analysis (100%). Lateralisation was concordant between the methods in 9/10 patients. The extent (< 75%, 75-90%, > or = 90%) was concordant in 80% between the two human raters. The automatic analysis with elimination of false detections was concordant with each of the human raters in 60% of patients. Extent of concordance was dependent of the total number of spikes with patients having more spikes being more reliably lateralised. Our results suggest that visually corrected automatic spike analysis is an economical method to use interictal epileptogenic activity as an independent indicator of the side of the epileptogenic focus in the setting of non-invasive presurgical evaluation. This is especially true in patients with many spikes.

[A single daily dose with valproic acid. A pharmacodynamic and clinical study]. / Einmaltagesdosis mit Valproinsäure. Eine pharmakodynamische und klinische Studie.

A prospective clinical pharmacokinetic study was carried out in 10 adult patients with primary or secondary generalized tonic-clonic seizures on the efficacy of valproic acid (VPA) administered as a single daily dose in comparison with divided doses thrice daily. The observation period covered 28 weeks. Without changing the total daily dosage the once-daily treatment proved to be of at least equal clinical efficacy. The single daily dose was well tolerated without any increase in adverse effects. Following a once-daily evening dose the VPA plasma profiled displayed--as expected--a steep increase during the early evening hours, reaching a maximal value around 2 a.m.; afterwards the plasma levels decrease continuously until the next evening. By comparison, the thrice-daily regimen gave maximal VPA levels at 2 p.m. With respect to the pharmacokinetic parameters, the minimum VPA plasma concentrations proved to be significantly lower, and the fluctuations significantly higher during the once-daily administration period the mean maximum VPA plasma level was higher, but not significantly so. The bioavailability was not affected by the change in dosage regimen. Since once-daily evening administration results in better patient compliance it seems appropriate to initiate VPA therapy with this simplified dosage regimen. For patients with inadequate seizure control on multiple daily doses of VPA it seems feasible--from a theoretical point of view--that the rate of attacks might be reduced after a changeover to a once-daily evening dosage regimen, but this has not yet been proven in clinical practice.

Hexa-methyl-propylene-amine-oxime (HMPAO) single photon emission computed tomography (SPECT) in epilepsy.

Twenty-eight normal volunteers and 74 seizure patients were investigated with hexa-methyl-propylene-amine-oxime (HMPAO) brain single photon emission computed tomography (SPECT). Fifty-four patients suffered from partial seizures and 20 patients had generalized seizures. Indices describing regional tracer distribution (RIs) were calculated in all investigated subjects. Regions whose RI exceeded the mean normal RI +/- 3 SD were defined as abnormally perfused. In normals a significant interhemispheric asymmetry of HMPAO deposition was found, with higher RI values in the right frontal, parietal, temporal and occipital regions. In 59.3% of partial seizure patients abnormal RIs were found. Low RIs were detected predominantly in the frontal and temporal cortex, while elevated RIs were observed predominantly in the anterior basal ganglia. Only in 20% of the cases with generalized seizures, abnormal RIs were found. In one patient an ictal and 3 follow-up SPECT studies were obtained. Here SPECT results indicated transient rCBF changes between the ictal and seizure free state. EEG and SPECT foci were ipsilaterally located in 69.2% of the partial seizure cases. The results indicate that HMPAO brain SPECT is valuable for the detection of rCBF abnormalities in seizure patients and that patients with partial seizures have mostly several abnormally perfused areas in their brains.