Quantitative MR relaxometry study of effects of vigabatrin on the brains of patients with epilepsy

Altered regional brain T2 relaxation times in individuals with chronic orofacial neuropathic pain

The neural mechanisms underlying the development and maintenance of chronic pain following nerve injury remain unclear. There is growing evidence that chronic neuropathic pain is associated with altered thalamic firing patterns, thalamocortical dysrhythmia and altered infra-slow oscillations in ascending pain pathways. Preclinical and post-mortem human studies have revealed that neuropathic pain is associated with prolonged astrocyte activation in the dorsal horn and we have suggested that this may result in altered gliotransmission, which results in altered resting neural rhythm in the ascending pain pathway. Evidence of astrocyte activation above the level of the dorsal horn in living humans is lacking and direct measurement of astrocyte activation in living humans is not possible, however, there is evidence that regional alterations in T2 relaxation times are indicative of astrogliosis. The aim of this study was to use T2 relaxometry to explore regional brain anatomy of the ascending pain pathway in individuals with chronic orofacial neuropathic pain. We found that in individuals with trigeminal neuropathic pain, decreases in T2 relaxation times occurred in the region of the spinal trigeminal nucleus and primary somatosensory cortex, as well as in higher order processing regions such as the dorsolateral prefrontal, cingulate and hippocampal/parahippocampal cortices. We speculate that these regional changes in T2 relaxation times reflect prolonged astrocyte activation, which results in altered brain rhythm and ultimately the constant perception of pain. Blocking prolonged astrocyte activation may be effective in preventing and even reversing the development of chronic pain following neural injury.

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Reduced T2 relaxation time in the ascending pain pathway in chronic orofacial pain.

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These reductions may be associated with astrogliosis.

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Increase astrocyte activity associated with chronic orofacial pain.

Cerebral MRI abnormalities associated with vigabatrin therapy

PURPOSE

Investigate whether patients on vigabatrin demonstrated new-onset and reversible T(2)-weighted magnetic resonance imaging (MRI) abnormalities.

METHODS

MRI of patients treated during vigabatrin therapy was reviewed, following detection of new basal ganglia, thalamus, and corpus callosum hyperintensities in an infant treated for infantile spasms. Patients were assessed for age at time of MRI, diagnosis, duration, and dose, MRI findings pre-, on, and postvigabatrin, concomitant medications, and clinical correlation. These findings were compared to MRI in patients with infantile spasms who did not receive vigabatrin.

RESULTS

Twenty-three patients were identified as having MRI during the course of vigabatrin therapy. After excluding the index case, we detected new and reversible basal ganglia, thalamic, brainstem, or dentate nucleus abnormalities in 7 of 22 (32%) patients treated with vigabatrin. All findings were reversible following discontinuation of therapy. Diffusion-weighted imaging (DWI) was positive with apparent diffusion coefficient (ADC) maps demonstrating restricted diffusion. Affected versus unaffected patients, respectively, had a median age of 11 months versus 5 years, therapy duration 3 months versus 12 months, and dosage 170 mg/kg/day versus 87 mg/kg/day. All affected patients were treated for infantile spasms; none of 56 patients with infantile spasms who were not treated with vigabatrin showed the same abnormalities.

DISCUSSION

MRI abnormalities attributable to vigabatrin, characterized by new-onset and reversible T(2)-weighted hyperintensities and restricted diffusion in thalami, globus pallidus, dentate nuclei, brainstem, or corpus callosum were identified in 8 of 23 patients. Young age and relatively high dose appear to be risk factors.

Development of vigabatrin-induced lesions in the rat brain studied by magnetic resonance imaging, histology, and immunocytochemistry

Vigabatrin, the gamma-aminobutyric acid transaminase (GABA-T)-inhibiting anticonvulsant drug, was given orally at a dose of 275 mg/kg/day to rats (n = 6) in their feed for a period of 12 weeks, during which T2-weighted magnetic resonance images (MRIs) and diffusion-weighted MRIs (DWIs) were collected at weeks 1, 3, 6, 9, and 12. Half the rats (n = 3; and half their age-matched littermate controls; n = 3) were then killed for histopathological confirmation of the observed VGB-induced cerebellar and cortical white-matter lesions. VGB was removed from the diet and additional MRIs of the remaining rats taken at weeks 14, 17, 20, and 24, at which time they (n = 3), along with remaining controls (n = 3), were also killed for histopathology. The T2-weighted MRIs acquired were used to compute T2 relaxation time maps. Statistically significant VGB-induced T2 increases were observed in the frontal and occipital cortices and in the cerebellar white matter (CWM). The cerebellar lesions were more clearly discerned by eye in the DWIs than by T2-contrast alone. During the recovery period the VGB-treatment group CWM-T2 and CWM-DWI hyperintensity greatly decreased as the reversible lesion disappeared. As expected, histological and immunocytochemical examinations demonstrated the presence of intra-myelinic edema, microvacuolation, and reactive astrocytosis in the CWM and cortex after 12 weeks VGB-treatment. In the remaining animals microvacuolation of the white matter had not completely resolved during the 12-week recovery phase. The data show that quantitative MRI T2-relaxometry can be used to detect VGB-induced CNS pathology, and also suggest that DWI is particularly sensitive to the cerebellar lesion. The reversible neurotoxicity of global GABA-elevation in experimental animals is discussed.

Vigabatrin visual toxicity: evolution and dose dependence

PURPOSE

To investigate the prevalence and prognosis of visual field defects (VFDs) in epilepsy patients with and without vigabatrin (VGB) treatment; to investigate the possible relationship between VFDs and cumulative VGB dose, and to characterise the evolution of VFDs.

METHODS

A cohort of 155 presurgical candidates who had undergone full-field Goldmann perimetry (GP) was studied, 99 (64%) of whom had been treated with VGB. All GPs were reevaluated in 1998 by one experienced examiner, blinded to medication. Duration of treatment and total VGB dose were related to perimetric results.

RESULTS

Twenty-five (16%) of the 155 patients had VFDs: Nineteen (19%) of the 99 VGB-treated patients, and six (11%) of the 56 patients unexposed to VGB. VGB-treated patients with VFDs had been treated significantly longer than those without VFDs. Cumulative VGB dose could be calculated for 84 patients. The prevalence of VFDs increased significantly with increasing total VGB-dose, from 4% in the 51 patients who had been exposed to <or=1 kg VGB, to 75% in the eight patients with a total dose of 3-5 kg of VGB. Sixteen VGB-treated patients were reexamined 2-10 years later. In the 12 where evaluation was possible, all still had VFDs, which had worsened in five cases (42%) and improved in none.

CONCLUSIONS

This study indicates a strong relationship between VFDs and duration and total dose of VGB. VFDs were irreversible and in a substantial percentage progressive. Similar VFDs may, however, also be found in patients unexposed to VGB. A model of the evolution of the VFDs in VGB toxicity is introduced, and a new simple visual field test is proposed.

The potential for vigabatrin-induced intramyelinic edema in humans

PURPOSE

Vigabatrin (Sabril, Hoechst Marion Roussel) is an antiepilepsy drug (AED) presently marketed in 64 countries for the treatment of partial and secondarily generalized seizures. Vigabatrin (VGB) is marketed in a subset of these countries for the treatment of infantile spasms. Clinical experience in humans has shown that VGB provides effective seizure control with a wide margin of safety. However, animal toxicity studies raised concern when prolonged administration of VGB was shown to induce intramyelinic edema (IME) in some laboratory animal species.

METHODS

Animal and human data were reviewed with respect to the potential for VGB-induced IME. Surveillance of patients receiving VGB in clinical trials or by prescription has been conducted for >15 years to identify patients developing clinical abnormalities that might be IME related.

RESULTS

The histologic lesions of VGB-induced IME in animals are reliably reproduced and correlate with changes in multimodality evoked potentials (EPs) and magnetic resonance imaging (MRI). Numerous studies of the effects of VGB on EP and MRI in epilepsy patients have demonstrated no clear-cut IME-related changes in these modalities. Additionally, autopsy and surgical brain samples from VGB-treated patients have been scrutinized for potential IME histopathology. In an estimated 350,000 patient-years of VGB exposure (approximately 175,000 patients exposed for 2 years at an average dose of 2 g/day), no definite case of VGB-induced IME has been identified.

CONCLUSIONS

Comprehensive review of a variety of sources of data failed to identify any definite case of IME in humans treated with VGB.

Safety and efficacy of vigabatrin and carbamazepine in newly diagnosed epilepsy: a multicentre randomised double-blind study. Vigabatrin European Monotherapy Study Group

BACKGROUND

Vigabatrin is a newly licensed drug for use in patients with epilepsy. We investigated whether this drug was comparable to standard first-line monotherapy in efficacy and incidence of adverse events.

METHODS

We enrolled 459 patients with newly diagnosed, previously untreated partial epileptic seizures from 44 European centres and randomly assigned them carbamazepine 600 mg daily (n=230) or vigabatrin 2 g daily (n=229). After initial maintenance doses were reached, doses were adjusted downwards (in the case of adverse events) or upwards (in the case of seizures) by the clinician. The primary outcome was time to withdrawal because of lack of efficacy or adverse events. Secondary outcomes included efficacy (time to 6-month remission of seizures, time to first seizure after initial dose stabilisation), and adverse events (incidence and severity). Analysis was by intention to treat.

FINDINGS

Time to withdrawal for lack of efficacy or adverse events did not differ between groups (p=0.318). Vigabatrin was better tolerated than carbamazepine with fewer withdrawals, but was more frequently associated with psychiatric symptoms (58 [25%] vs 34 [15%]) and weight gain (25 [11%] vs 12 [5%]). Carbamazepine was associated with rash (22 [10%] vs seven [3%]). All efficacy outcomes favoured carbamazepine and failed to show equivalence between the two drugs. No significant difference was found for time to achieve 6 months of remission from seizures (p=0.058), but the most powerful outcome, time to first seizure after the first 6 weeks from randomisation, showed carbamazepine to be significantly more effective than vigabatrin (p=0.0001).

INTERPRETATION

Vigabatrin seems less effective but better tolerated than carbamazepine, which is the first-choice drug for the treatment of partial epilepsies. Vigabatrin cannot therefore be recommended as a first-line drug for monotherapy in this group of patients.

Treatment of infantile spasms: results of a population-based study with vigabatrin as the first drug for spasms

PURPOSE

The efficacy of a protocol consisting of vigabatrin (VGB) as the first and adrenocorticotropic hormone (ACTH) or valproate (VPA) as the second drug was studied in the treatment of newly diagnosed infantile spasms (IS) during 1994 to 1997 in a population-based design.

METHODS

Only total disappearance of the spasms with a minimal duration of 1 month was accepted as a response. The treatment response was confirmed by video-EEG study. All infants were studied by magnetic resonance imaging (MRI) or computed tomography (CT) for etiology.

RESULTS

Altogether 42 infants, 10 with cryptogenic and 32 with symptomatic etiology, were treated. Eleven (26%) responded to VGB, five (50%) with cryptogenic, and six (19%) with symptomatic etiology; 91% of infants responded to a dose of 50-100 mg/kg/day, and 82% of them within 1 week. ACTH was offered in combination with VGB to 22 and VPA to four infants for whom VGB failed. Eleven responded to ACTH and one to VPA. In total, 26 (62%) infants responded to the treatment protocol; all (100%) with cryptogenic etiology and 16 (50%) with symptomatic etiology. ACTH treatment was associated with more severe side effects than VGB or VPA. Only one infant relapsed after a spasm-free period with VGB of >4 months, but none after ACTH was combined with VGB.

CONCLUSIONS

We suggest VGB as a first drug to all infants with IS. After a treatment trial of 10-14 days with increasing dose from 50 to 150 mg/kg, ACTH should be considered.

Neuroimaging in epilepsy

Neuroimaging techniques have improved the understanding, diagnosis, and management of epilepsy. By providing excellent structural information, MRI is the technique of choice in evaluating patients with epilepsy. Functional imaging techniques, including MR spectroscopy, functional MRI, positron emission tomography, and single photon emission CT, permit noninvasive assessment of the epileptic substrate, its functional status, and neuroreceptors. The MRI-based techniques will potentially assume a greater role in the cost-effective workup of the patient. Currently, newer techniques such as magnetoencephalography, magnetic source imaging, and optical imaging are research tools.

A comparison of the neuropathological effects of vigabatrin and carbamazepine in patients with newly diagnosed localization-related epilepsy using MR-based cerebral T2 relaxation time measurements

BACKGROUND

Magnetic resonance (MR)-based T2 relaxation time measurement is a sensitive technique to detect neuropathological changes such as intramyelinic edema in vivo.

OBJECTIVE

To determine whether vigabatrin (VGB) causes an increase in T2 relaxation time in patients with newly diagnosed localization-related epilepsy over 1 year.

METHODS

Patients with newly diagnosed localization-related epilepsy who participated in a VGB-carbamazepine (CBZ) monotherapy trial were included. All were scanned on a 1.5 T Siemens SP63 Magnetom scanner. T2 maps of the brain were obtained at baseline and at follow-up 1 year later. Nine control subjects had repeated hippocampal T2 maps with a median interval of approximately 2 years.

RESULTS

23 patients (12 on VGB and 11 on CBZ) were included. There were no increased T2 relaxation times in the VGB treated group at follow-up and no significant differences between the two antiepileptic drug groups. There was a trend for the temporal and frontal white matter T2 relaxation times to be lower on follow-up in the patients compared to the control subjects.

CONCLUSION

The findings do not suggest that intramyelinic edema occurs in patients taking monotherapy VGB for 1 year.

Vigabatrin

gamma-Aminobutyric acid (GABA) was first proposed as a putative inhibitory neurotransmitter by Elliot and van Gelder in 1958. Since then, numerous efforts have been made to find ways to increase GABA at its receptor sites, based on the findings that decreased GABA results in convulsions in animals and that agents enhancing GABA-mediated functions can have antiepileptic effects. However, the relationship between GABA levels and seizures is not simple. Seizures can occur even in the presence of elevated GABA levels. Indeed, it is possible that regional biochemical differences in the brain can be important. The antiepileptic effects of GABA depend on the mechanism whereby GABA-mediated inhibition is enhanced. Since the 1970s, several compounds have been developed that are designed to act in some manner on the GABA system. These compounds affect GABA-mediated inhibition at different levels and appear to have varied effects, depending on their mechanism of action. To date, specific antiepileptic drugs (AEDs) with potential GABA-inhibitory effects have been designed either to have GABA agonist properties, to inhibit GABA catabolism, to inhibit GABA uptake, or to facilitate GABA release or facilitate GABAA receptor activity. Vigabatrin (VGB) was designed specifically to inhibit GABA transaminase and thereby increase the availability of GABA in the brain. Study data and clinical experience over the past 14 years have demonstrated VGB to be an effective AED.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.