Examining visual field defects in the paediatric population exposed to vigabatrin.

The antiepileptic drug, vigabatrin, has been linked to a specific pattern of visual field loss. The majority of studies have not included the paediatric population due to difficulties assessing visual field function. This is a particular problem as vigabatrin is effective against infantile spasms. A field-specific visual evoked potential was developed which consisted of a central stimulus (0-5 degrees radius) and a peripheral stimulus (30-60 degrees radius). Both stimuli consist of black and white checks which increase in size with eccentricity. Responses are recorded from occipital electrodes O2 and O1 referred to frontal electrode Fz. Electroretinograms and perimetry was performed were possible as a comparison. Thirty-nine children with epilepsy treated with vigabatrin aged from 3 to 15 years were included in the study; 35/39 children complied with the field-specific VEP, 26/39 complied with the ERG and 11/39 performed perimetry. Of these results, 18 children had normal ERG responses and eight had abnormal response. Visual field testing revealed four children had abnormal and seven had abnormal visual field results. The Field-specific VEP identified three of four abnormal perimetry results and six of seven normal perimetry results, giving a sensitivity of 75% and a specificity of 85.7%. When comparing perimetry results with the ERG parameters only the 30-Hz flicker amplitude, with a cut-off amplitude below 70 microV, gave a useful sensitivity of 75% and a specificity of 71%. The field-specific VEP is a useful alternative method that is both well tolerated by young children and gives a reliable indication of likely peripheral visual field loss associated with vigabatrin. The defect appears to have a similar prevalence in children as it does in adults.

Vigabatrin; its effect on the electrophysiology of vision.

Vigabatrin is known to induce visual field defects in approximately one third of patients treated with the drug. It is apparent from electrophysiological studies that the cause of this defect is at retinal level probably as a result of the build up of GABA. Studies of electrophysiological retinal parameters such as the EOG and photopic, scotopic and 30-Hz flicker ERG have revealed changes in Arden Index, photopic a and b wave latency and amplitude, changes in oscillatory potentials, and changes in latency and amplitude of the 30Hz response. However, many of these changes such as the Arden Index, oscillatory potentials, latency and amplitude of photopic b wave appear to be related to current anti-epileptic drug treatment rather than visual field defects. Certain parameters, particularly the amplitude of the 30-Hz flicker response, do appear to correlate with the severity of the field defect. Paediatric patients treated with the drug at age 9 years or below cannot reliably perform visual field perimetry. To identify these patients a special VEP H-Stimulus has been developed to produce separate responses from central and peripheral field stimulation by alternating at slightly separate rates. Forty-five healthy children between ages 3 and 10 years have been used to develop a normal database. This technique has a sensitivity of 75% and a specificity of 87.5% in identifying the field defect and may be used in children with epilepsy from age 3 upwards.

Field-specific visual-evoked potentials: identifying field defects in vigabatrin-treated children.

OBJECTIVE: To derive a visual-evoked potential (VEP) technique for identifying visual field defects in children with epilepsy treated with vigabatrin and unable to perform perimetry. BACKGROUND: Studies have linked vigabatrin to a specific pattern of visual field loss. Few studies have included the pediatric population because of difficulties in assessing the visual field by perimetry below a developmental age of 9 years. METHODS: A field-specific VEP was developed with a central (0 degrees to 5 degrees radius) and peripheral stimulus (30 degrees to 60 degrees radius). Stimuli consisted of black and white checks that increased in size with eccentricity. Checks reversed at different rates, allowing separate central and peripheral responses to be recorded. Five vigabatrin-treated young adults with field defects were identified using this stimulus. Electroretinograms (ERG) were recorded to examine the effects of vigabatrin on retinal function. Thirty-nine children aged 3 to 15 years were included in the study. Twelve patients were examined by both the field-specific stimulus test and perimetry. The diagnostic performance of the field-specific stimulus test was compared with that of perimetry. RESULTS: Thirty-five of 39 children complied with the field-specific stimulus, 26 of 39 complied with the ERG, and 12 of 39 complied with perimetry. Using the summed amplitude of the peripheral response from O(2) and O(1), responses below 10 microV were deemed abnormal. The field-specific stimulus identified 3 of 4 abnormal perimetry results and 7 of 8 normal perimetry results, giving a sensitivity of 75% and a specificity of 87.5%. When comparing perimetry results with the ERG parameters, only the 30-Hz flicker amplitude, with a cutoff below 70 microV, gave a useful indication of visual field loss. CONCLUSION: Field-specific VEP are well tolerated by children older than 2 years of age and are sensitive and specific in identifying vigabatrin-associated peripheral field defects.