Intrinsic severity as a determinant of antiepileptic drug refractoriness

Polyspike ictal onset absence seizures

Polyspike ictal-onset absence seizures may predict an unfavorable response to treatment in patients with epilepsy. The authors assessed absences in adults with idiopathic generalized epilepsy (IGE) associated with polyspike onset before classic 3-Hz generalized spike-and-waves (GSW). Seven adults with absences undergoing video EEG were accompanied by 3-Hz GSW, preceded by polyspike-ictal onset by >1 second in two or more seizures. The clinical profile, interictal, and ictal EEG noted had at least 1-year follow-up. A total of 1,164 patients underwent video EEG, with 58 (5%) receiving a diagnosis of IGE or subsyndrome. All seven patients with absence seizures had an ictal EEG pattern of polyspike onset prior to 3-Hz GSW. Four female and three male adults (mean age, 28.9 years) had absence as their primary seizure type. All but one had seizure onset between 4 and 20 years and failed at least two broad-spectrum antiepileptic drug trials. Typical features of IGE were normal neurologic examination, brain MRI, and 3- to 3.5-Hz GSW and generalized polyspike-and-waves on interictal EEG. Atypical features included learning disability (2 of 7), absence status (1 of 7), and paroxysmal fast activity during nonrapid eye movement sleep (3 of 7).

CONCLUSIONS

Polyspikes are common during video EEG in adults with IGE. Absence seizures with polyspike onset 3-Hz GSW suggest an intermediary form of IGE that may be drug resistant.

Can slow breathing exercises improve seizure control in people with refractory epilepsy? A hypothesis

Studies on various medical conditions have shown that poor health is associated with lower parasympathetic tone. People with epilepsy appear to have decreased parasympathetic tone, with a greater decrease in those with intractable seizures than in those with well-controlled epilepsy. Slow breathing exercises have been shown to increase parasympathetic tone in healthy volunteers. Slow breathing exercises have been shown to improve a number of medical conditions including asthma, hypertension, anxiety states, and posttraumatic stress disorder. We hypothesize that slow breathing exercises in people with epilepsy can lead to an increase in parasympathetic tone and an accompanying reduction in seizure frequency. The slow breathing exercises, probably through baroreceptors, chemoreceptors, and pulmonary stretch receptors, affect cortical activity and hence seizure thresholds. It is also possible that slow breathing exercises might reduce seizure frequency by reducing anxiety. The hypothesis can be tested by employing devices and protocols that have been used to reduce breathing rates and have been shown to improve health outcomes in other medical conditions.

New developments in antiepileptic drug resistance: an integrative view

Current theories on drug resistance in epilepsy include the drug transporter hypothesis, the drug target hypothesis, and a novel approach called the inherent severity model of epilepsy, which posits that the severity of the disease determines its relative response to medication. Valuable as each of these hypotheses is, none is currently a stand-alone theory that is able to convincingly explain drug resistance in human epilepsy. As a consequence, it may be of interest to update and integrate the various hypotheses of drug resistance and to explore possible links to the severity of epilepsy. The observation that a high frequency of seizures prior to onset of treatment is a prognostic signal of increased severity and future drug failure suggests that common neurobiological factors may underlie both disease severity and pharmacoresistance. Such a link has been proposed for depression; however, the evidence for a direct mechanistic link, genetic or otherwise, between drug response and disease severity of human epilepsy is still elusive. Although emerging data from experimental studies suggest that alterations in GABA(A) receptors may present one example of a mechanistic link, clearly more work is needed to explore whether common neurobiological factors may underlie both epilepsy severity and drug failure.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

ECoG studies of valproate, carbamazepine and halothane in frontal-lobe epilepsy induced by head injury in the rat

The use of electrocorticography (ECoG) with etiologically realistic epilepsy models promises to facilitate the discovery of better anti-epileptic drugs (AEDs). However, this novel approach is labor intensive, and must be optimized. To this end, we employed rostral parasagittal fluid percussion injury (rpFPI) in the adolescent rat, which closely replicates human contusive closed head injury and results in posttraumatic epilepsy (PTE). We systematically examined variables affecting the power to detect anti-epileptic effects by ECoG and used a non-parametric bootstrap strategy to test several different statistics, study designs, statistical tests, and impact of non-responders. We found that logarithmically transformed data acquired in repeated-measures experiments provided the greatest statistical power to detect decreases in seizure frequencies of preclinical interest with just 8 subjects and with up to approximately 40% non-responders. We then used this optimized design to study the anti-epileptic effects of acute exposure to halothane, and chronic (1 week) exposures to carbamazepine (CBZ) and valproate (VPA) 1 month post-injury. While CBZ was ineffective in all animals, VPA induced, during treatment, a progressive decrease in seizure frequency in animals primarily suffering from non-spreading neocortical seizures, but was ineffective in animals with a high frequency of spreading seizures. Halothane powerfully blocked all seizure activity. The data show that rpFPI and chronic ECoG can conveniently be employed for the evaluation of AEDs, suggest that VPA may be more effective than CBZ to treat some forms of PTE, and support the theory that pharmacoresistance may depend on the severity of epilepsy. The data also demonstrate the utility of chronic exposures to experimental drugs in preclinical studies and highlight the need for greater attention to etiology in clinical studies of AEDs.

[Drug-resistant epilepsy]

BACKGROUND

Despite the introduction of many new antiepileptic drugs during recent years, about one third of the epilepsy population continue to have drug-resistant seizures. We present two hypotheses on mechanisms of drug-resistant epileptic seizures.

MATERIAL AND METHODS

The article is based on our own clinical experience and literature identified through a PubMed search using the words: refractory, intractable, pharmacoresistant, drug resistant epilepsy/seizures.

RESULTS

There is no international consensus on the definition of drug-resistant epilepsy. Some epilepsy types appear to respond to drug treatment, while others are more resistant. Animal models and refractory patients have been studied to find mechanisms that can explain the lack of drug response in some types of epilepsy. Currently, two main hypotheses prevail. The transporter hypothesis implies an over-expression of the proteins (e.g. P-glycoprotein) capable of transporting drugs out of the epileptic focus. The target hypothesis implies changes in receptors or ion channels in the epileptic cellular network, which render this area less susceptible to antiepileptic drugs.

INTERPRETATION

Both hypotheses have weaknesses, and alternative theories have been presented. Much remains before the causes of drug-resistant epilepsy are understood. If patients do not achieve seizure control after having tried 2-3 antiepileptic drugs, they should be referred to the national epilepsy centre for a renewed diagnostic evaluation and for consideration of non-pharmacological treatment.

High seizure frequency prior to antiepileptic treatment is a predictor of pharmacoresistant epilepsy in a rat model of temporal lobe epilepsy

PURPOSE

Progress in the management of patients with medically intractable epilepsy is impeded because we do not fully understand why pharmacoresistance happens and how it can be predicted. The presence of multiple seizures prior to medical treatment has been suggested as a potential predictor of poor outcome. In the present study, we used an animal model of temporal lobe epilepsy to investigate whether pharmacoresistant rats differ in seizure frequency from pharmacoresponsive animals.

METHODS

Epilepsy with spontaneous recurrent seizures (SRS) was induced by status epilepticus. Frequency of SRS was determined by video/EEG (electroencephalography) monitoring in a total of 33 epileptic rats before onset of treatment with phenobarbital (PB).

RESULTS

Thirteen (39%) rats did not respond to treatment with PB. Before treatment with PB, average seizure frequency in PB nonresponders was significantly higher than seizure frequency in responders, which, however, was due to six nonresponders that exhibited > 3 seizures per day. Such high seizure frequency was not observed in responders, demonstrating that high seizure frequency predicts pharmacoresistance in this model, but does not occur in all nonresponders.

DISCUSSION

The data from this study are in line with clinical experience that the frequency of seizures in the early phase of epilepsy is a dominant risk factor that predicts refractoriness. However, resistance to treatment also occurred in rats that did not differ in seizure frequency from responders, indicating that disease severity alone is not sufficient to explain antiepileptic drug (AED) resistance. These data provide further evidence that epilepsy models are useful in the search for predictors and mechanisms of pharmacoresistance.