Behavioral toxicity associated with antiepileptic drugs: concepts and methods of assessment

Quantitative EEG effects of carbamazepine, oxcarbazepine, valproate, lamotrigine, and possible clinical relevance of the findings

Quantitative EEG (QEEG) effects of therapeutic doses of carbamazepine (CBZ), oxcarbazepine (OXC), valproate (VA) and lamotrigine (LA) monotherapy were investigated in patients with beginning epilepsy. Baseline waking EEG (EEG1) was recorded in the untreated state, the second EEG (EEG2) was done after 8 weeks of reaching the therapeutic dose. Left occipital data were used for analysis. QEEG target parameters were absolute band-power (delta: AD, theta: AT, alpha: AA, beta: AB), and alpha mean frequency (AMF). Group effects (untreated versus treated condition in the CBZ, VA, OXC, LA groups) were computed for each target parameter. One group with benign rolandic epilepsy remained untreated for clinical reasons and served to estimate the QEEG test-retest differences. In addition, the individual QEEG response to each drug was calculated as (EEG2-EEG1).

RESULTS

statistically significant (p<0.05) group differences indicated the QEEG domain systematically affected by the drugs. CBZ caused AT increase and AMF decrease. OXC caused AMF decrease. VA and LA did not decrease AMF (LA even increased it), but reduced broad-band power. Individual power and AMF changes showed considerable variability in each group. >0.5 Hz AMF decrease (that was reported to predict cognitive impairment in prior studies) occurred in 10/41 patients in the CBZ group but never in the OXC, VA, LA groups. The results may be utilized in planning further studies addressing the relationship between antiepileptic drugs and their CNS effects. In addition, the relationship of AED-related cognitive impairment and AMF changes was discussed.

Objective assessment of neurotoxicity while shifting from carbamazepine to oxcarbazepine

OBJECTIVES

Objective assessment of non-overt neurotoxicity of carbamazepine (CBZ) vs oxcarbazepine (OXC) in patients with difficult-to-treat partial epilepsy, who were resistant to CBZ treatment and were converted from CBZ monotherapy to OXC monotherapy.

MATERIAL AND METHODS

Therapeutically equivalent doses (150 mg OXC for every 100 mg CBZ) were compared in 20 adult patients. Neurological investigation, conventional and spectral EEG analysis, brainstem auditory evoked responses (BAER) were carried out in both treatment conditions. EEG and BAER data of 20 age-matched healthy controls helped interpretation. Primary target variables (electrophysiological parameters) were evaluated blindly.

RESULTS

There were no significant differences between treatment conditions concerning the neurological condition, lack of clinically evident neurotoxicity, seizure frequency and EEG spike frequency. OXC treatment was characterized by less delta, theta, and alpha power, more beta power, and significantly greater mean alpha frequency (P = 0.03 and 0.05 for the left and right occipital leads, respectively), than CBZ treatment. Interpeak latencies were prolonged in the CBZ condition as compared with normals (P = 0.01) and OXC (P = 0.02).

CONCLUSION

In this cohort of patients substitution of OXC for CBZ was associated with significant normalization of electrophysiological parameters, indicating decreasing neurotoxicity while shifting from CBZ to OXC monotherapy.

Complex partial and secondarily generalized seizure patients: cognitive functioning prior to treatment with antiepileptic medication. VA Epilepsy Cooperative Study 264 Group

This investigation of cognitive functioning in patients with symptomatic localization-related (partial) epilepsy prior to administration of antiepileptic medication is part of a nationwide prospective, double-blind study of drug efficacy and longitudinal changes in cognition associated with seizure disorders. Recently-diagnosed patients with complex partial or secondarily generalized tonic clonic seizures, equated for age, education and IQ, were compared with normal controls on a battery of neuropsychological tests: verbal and figural memory measures (Rey auditory verbal learning test, Rey Osterrieth complex figure), and a brief behavioral toxicity battery comprising measures of motor function, concentration and mental flexibility (Lafayette grooved pegboard, controlled word association test, Stroop, paced auditory serial addition test [PASAT]). Control subjects perform significantly better than both groups of seizure patients on several measures of motor speed/integration and memory, specifically the pegboard and the RAVLT learning (3-5) and recall trials. Secondarily generalized seizure patients show greatest impairment. They perform significantly worse than patients with complex partial seizures and control subjects on the controlled word association test and the most demanding Stroop color word trial, both measures of concentration and mental flexibility. These findings document deficits in memory as well as concentration and motor function in complex partial and in secondarily generalized seizure patients prior to treatment with antiepileptic medication.

Cognitive side-effects of chronic antiepileptic drug treatment: a review of 25 years of research

Over 90 investigations have been conducted over the past 25 years to determine what effect AEDs have on cognition. No satisfactory answer to this problem can be given, however, chiefly because there is a paucity of studies that pass fairly basic standards of methodology, design and analysis that apply to the evaluation of any clinical research. This severely limits the precision of statements regarding cognitive AED effects. More particularly, there is little reason to recommend any of the first-line AEDs as the AED of choice from the standpoint of cognitive side-effects. On the basis of the present review we are not in a position to provide a straightforward answer to the most pertinent question, i.e., whether AEDs in therapeutic doses have any cognitive effects at all, good or bad. If we reduce the available database to monotherapy studies in epilepsy that use control group data for comparison, employ an appropriate form of repeated measures analysis, and provide sufficient information, very few studies remain that are directly relevant to this issue. This in itself precludes definitive conclusions. As can be seen from Table 9, absolute effects of CBZ and VPA have been examined in epilepsy patients three times each (in four studies), PB has been examined two times, PHT only once. In addition to the paucity of relevant data, there are miscellaneous validity concerns in all of these studies, one recurring theme being that of inconclusive 'no effect' findings with small samples. Without firm knowledge about absolute effects, relative effects, and particularly their absence, are difficult to interpret. Employing the above criteria (except that concerning controls), ten epilepsy studies that address this issue remain (Table 10). It is instructive to look at the number of times particular AEDs have been compared against each other (Table 11). CBZ has been compared to PHT five times, other comparisons occur only once or twice. Again, this is hardly a basis for definitive statements, particularly because validity concerns occur here as well. Recurring concerns here are scattered significant findings that tend to disappear if adjustment of the significance level for multiple comparisons is done, and inconclusive 'no difference' findings with small samples. Even if there were no conclusion validity concerns in individual studies, comparison between studies would be complicated by considerable variation in the subjects studied. Five of the studies summarized in Tables 8 and 9 use children as subject, nine use adults; results obtained in one group may not be generalizable to the other. Also, subjects may be newly diagnosed cases, or patients already on chronic treatment. The latter choice of subjects may be a factor working against detecting cognitive side-effects, as the damage (if any) may already have been done before the beginning of the trial. In addition, a wide variety of assessment tools have been used to search for cognitive effects of AEDs, ranging from measurements of reaction time and motor speed to intelligence tests. Some of these may be more sensitive to drug induced changes in cognition than others. Still, the tentative overall picture emerging from the creme de la creme of research on cognitive AED effects is that differences in cognitive profiles may not be very large. An important point here, of course, is the magnitude of the difference one considers worth detecting. Very few studies have attempted to answer this question. In the majority of studies we examined, a large treatment effect was anticipated implicitly, judging from the generally limited sample sizes. The choice of a study design based on a large treatment effect size may not always be appropriate, though. Of course, one could argue that it is only large effects that may be of practical or clinical significance anyways [30] and that effects of lesser magnitude are of no consequence. However, there are many examples where even a small benefit of one treatme

Phenytoin and carbamazepine: differential effects on cognitive function

This review summarizes the studies on the cognitive side-effects of two important antiepileptic drugs: phenytoin and carbamazepine. A large literature database was compiled through the DIMDI computer database and the inspection of recent reviews. Only scientific articles published in peer-reviewed journals during the last 25 years were selected. Of the 358 potentially relevant papers on cognitive effects of AEDs, a total of 16 studies have been found that have studied both carbamazepine and phenytoin. After excluding studies with designs that do not permit valid inferences regarding the cognitive effects of AEDs, only five studies remained. The evaluation of these studies reveal that our current knowledge allows us to draw conclusions about the cognitive side-effects of phenytoin and carbamazepine only with great caution. The claim in reviews that 'both drugs have an impact on cognitive function, PHT to a larger degree than CBZ' is simply not supported by valid 'high quality' data. The same is true for the overall conclusion in more recent reviews that 'drug-induced cognitive effects of these AEDs on cognitive function are probably mild or even negligible'. Apparently, the only information that we have is that the differential impact of PHT and CBZ on cognitive function is not extremely different. No conclusive and reconfirmed data are available on the absolute effects of CBZ and PHT (differences between the two drugs and a no-treatment condition). Our review summarizes some recommendations for future studies.

Advances in the assessment of refractory epilepsy

The assessment of patients with refractory epilepsy presents both a challenge and an opportunity. The use of appropriate instruments to carefully and consistently measure a patient's seizure type and frequency, hormonal function, and medication side effects and the impact of those variables on quality of life ensures consistent, standardized assessment. A patient's epilepsy then can be classified as refractory based on any combination of factors that preclude a reasonably normal life-style by current practice standards, given the person's capabilities. Diagnostic studies and treatment strategies should address all significant adverse findings resulting from the assessment process. Accurate diagnosis of seizure type usually requires electroencephalographic (EEG) monitoring. Postictal hormonal assays and periodic measurements of estrogen and progesterone may be useful diagnostically in a selected group of patients. Awareness of a patient's compliance history, environmental stressors, and seizure triggers may reduce seizure frequency without a change in medication. For patients with medication intolerance, the usual strategy is to modify the medication regimen or treatment schedule to minimize side effects while maximizing seizure control and compliance. For most patients, monotherapy provides a greater therapeutic window than combination therapy. Devising an effective and safe therapeutic regimen is an ongoing process and can be lengthy depending on seizure type(s), frequency, and temporal pattern; anticonvulsant pharmacokinetics; drug interactions and side effects; patient's life-style; and other factors. Serial measurements of quality-of-life variables may assist in the process. Rapid strides have been recently made, and as the pace of new antiepileptic drug development quickens and surgical centers become more widely available, clinicians will need to redefine refractoriness as the expectations improve for patients with epilepsy.

Differential effects of first antiepileptic drug application on cognition in lesional and non-lesional patients with epilepsy

It has been shown that carbamazepine (CBZ) and valproic acid (VPA) have no significant negative or even positive side effects upon cognition in patients with epilepsy. The present study evaluated cognitive effects of CBZ or VPA in 16 patients with newly diagnosed epilepsy. Twelve patients had complex partial seizures, four patients primarily generalized seizures. Special attention was directed towards seizure type, presence of structural lesions and seizure control. Neuropsychological testing was carried out prior to medication and when drugs were within therapeutic ranges. Nineteen untreated healthy subjects served as controls. The following results were obtained: first of all, no general effect of antiepileptic drugs upon cognition could be detected. Only a subgroup with lesion (n = 8) and pre-existing cognitive deficits showed a significant decrease in memory performance during medication. This group additionally obtained poor seizure control. In conclusion, adverse cognitive changes under medication cannot be explained by antiepileptic drugs alone. The presence of cerebral lesions and obtained seizure control have to be considered as essential risk factors.

Effect of acute doses of controlled-release carbamazepine on clinical, psychomotor, electrophysiological, and cognitive parameters of brain function

The neurotoxic effect of acute doses of carbamazepine controlled-release (CBZ-CR) divitabs (800, 1,200, and 1,600 mg) was assessed on clinical, psychomotor, electrophysiological, and cognitive parameters of brain function in 10 healthy volunteers in a double-blind, randomised, placebo-controlled, phase I study. Significant changes compared to placebo were demonstrated for the clinical scales, ataxia (AT), convergence of the near-point (CNP), peak saccadic velocity (PSV), critical flicker fusion (CFF), spectral analysis of the EEG, and brainstem auditory evoked potential (BAEP) tests. Digit repetition, digit symbol substitution, Sternberg memory scanning time, Sternberg choice reaction time, saccadic latency, and saccadic accuracy showed important negative findings. Significant clinical tolerance to side effects developed within 20 to 33 h after CBZ-CR dosage during a period in which the mean CBZ blood levels remained virtually unchanged. CBZ-CR, 800, 1,200, and 1,600 mg yielded low, medium, and high therapeutic blood levels, respectively, for +10 to +33 h after dosage without the development of severe clinical side effects.

Intracranial, intraaxial, space-occupying lesions in patients with intractable partial seizures: an anatomoclinical, neuropsychological, and surgical correlation

Fifty of approximately 250 patients evaluated for intractable partial seizures were shown to have a space-occupying lesion detected with radiographs and/or neuroimaging. Twenty-eight males and 22 females had a mean age at seizure onset of 13 years and a mean duration of seizures of 11 years. All patients had closed-circuit television with EEG monitoring and complete neurologic and neuropsychological assessment. Findings were correlated with lesion location and surgical data. Twenty-seven lesions (54%) were located in the temporal lobe. Thirty-five lesions (70%) were neoplastic. All patients with temporal lobe lesions had complex partial seizures, as did 74% of patients with extratemporal lesions. A good correlation between clinical seizure characteristics and lesion localization was found with the temporal, occipital, and frontal lesions but not with the parietal lesions. Sixty-six percent of patients had focal interictal EEG findings. Lateralization corresponded to the side of the lesion in 64% and was localized to the region of the lesion in 30%. Lateralized ictal EEGs occurred in 58% of patients, corresponding with the side of the lesion in all but one patient. Abnormal findings on neuropsychological testing were congruent with lesion lateralization in 56% of patients and were localized to the region in 26%. Thirty-nine of 47 patients who underwent a subtotal lobectomy to include the lesion are seizure-free after greater than or equal to 1 year of follow-up, and five others are markedly improved.

Developmental neurotoxicity of anticonvulsants: human and animal evidence on phenytoin

Most epileptic women delivering children each year take anticonvulsants throughout pregnancy. The teratogenic potential of anticonvulsants is most notable for phenytoin, trimethadione, valproic acid, and carbamazepine. This review focuses on the human and animal evidence for the teratogenicity of phenytoin, with emphasis on neurobehavioral end points. The Fetal Hydantoin Syndrome (FHS) consists of craniofacial defects and any two of the following: pre/postnatal growth deficiency, limb defects, major malformations, and mental deficiency. Available data suggest a prevalence of FHS of 10-30% in infants of women ingesting 100-800 mg/kg of phenytoin during the first trimester or beyond. Unfortunately, data on neurobehavioral development in FHS children is limited. Animal models of FHS have been developed and those focusing on neurobehavioral effects are reviewed. Phenytoin produces multiple behavioral dysfunctions in rat offspring at subteratogenic and nongrowth retarding doses. These behaviorally teratogenic doses produce maternal serum phenytoin concentrations in rats comparable to those found in humans. The dysfunctions in rats are dose-dependent and exposure-period-dependent, but independent of nutritional, maternal rearing, or seizure disorder confounds. Effects include vestibular dysfunction, hyperactivity and deficits in learning and memory. General comparability between the human and animal findings for phenytoin are apparent, however, difficulties with existing studies prevent precise comparisons. Animal studies have not dealt satisfactorily with the potential contribution of epileptic disease state to the FHS, with fetal brain drug concentration determinations, a complete dose-effect range, effects in multiple species (although limited nonhuman primate data exist), site of CNS injury, and the comparability of end points assessed. Human studies have not dealt satisfactorily with issues of the need for prospective study designs, separation of the effects of different anticonvulsants, or adequate long-term follow-up of cases, especially with attention to neuropsychological assessment.

Concentration-effect relationships with carbamazepine and its epoxide on psychomotor and cognitive function in epileptic patients

A battery of psychometric tests was administered to 85 patients with epilepsy, of whom 26 were untreated, 40 received carbamazepine monotherapy and 19 took carbamazepine with another anticonvulsant. Carbamazepine alone had little effect on performance, but carbamazepine polypharmacy produced significant impairment. Increasing concentrations of carbamazepine (four tests) and its active metabolite, carbamazepine 10,11 epoxide (seven tests), correlated with decreasing performance in the monotherapy patients.

Results of a nationwide Veterans Administration Cooperative Study comparing the efficacy and toxicity of carbamazepine, phenobarbital, phenytoin, and primidone

In 1985 a 5-year multicenter Veterans Administration Cooperative Study was completed that compared the efficacy and toxicity of phenobarbital, carbamazepine, phenytoin, and primidone in a double-blind prospective study design. A total of 622 patients, either previously untreated or undertreated, were entered into the study. Strict exclusion criteria limited confounding factors such as drug or alcohol abuse. Results showed that each of the four drugs used as monotherapy were similarly effective in the treatment of generalized tonic clonic seizures, but carbamazepine was significantly more effective in the treatment of complex partial seizures as measured by 100% control. When the results for all four drugs were combined, the data showed that approximately 80% of the patients were adequately managed on monotherapy. Differences in toxicity were the most significant factor that discriminated between these four drugs. Both carbamazepine and phenytoin were associated with significantly lower incidences of intolerable side effects than were primidone or phenobarbital. A behavioral toxicity battery was performed whenever possible prior to administration of any antiepileptic drug and at 1, 3, 6, and 12 months after initiation of monotherapy. Significant differences in performance on all subtests of the battery were found between patients with epilepsy and a control group matched by age, sex, and education. When the differential effects of all four drugs on behavioral toxicity were compared, few statistically significant differences emerged. However, carbamazepine consistently produced fewer adverse effects on tests of attention/concentration and motor performance than did the other three antiepileptic drugs.

Behavioral characteristics of epilepsy patients compared with normal controls

Using neuropsychological testing procedures, we measured the behavioral characteristics of patients with epilepsy unaffected by anticonvulsant drugs and compared these results with control subjects matched for age, sex, and education. In a nationwide VA Cooperative study, 622 patients with well-defined seizure types were given a selected battery of neuropsychological tests prior to and at 1 month after initiation of drug therapy. Seventy-four control subjects were administered the same test battery under standardized conditions at the same VA testing centers. All tests of controls were also repeated 1 month later. The patients with epilepsy scored significantly and consistently below the level of the control subjects on all but three behavioral measures. Differences reaching statistical significance were found on tests of motor function (Finger Tapping, Pegboard, Color Naming), cognitive-attention (Digit Symbol, Discriminative Reaction Time, Word Fluency), and subtests of the Profile of Mood Status (tension, depression, vigor, and confusion). These data provide a profile of behavioral characteristics of unmedicated patients with epilepsy.