The qualitative comparative analysis of the visual field using computer assisted, semi-automated and manual instrumentation: II. Statistical analysis

Does the Swedish Interactive Threshold Algorithm (SITA) accurately map visual field loss attributed to vigabatrin?

Background

Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss.

Methods

The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years ± 14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm.

Results

SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots , defect area and defect severity.

Conclusions

Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2415-14-166) contains supplementary material, which is available to authorized users.

Vigabatrin and Epilepsy: Lessons Learned

PURPOSE

The risk factors for visual field loss attributable to vigabatrin (VAVFL) are equivocal. This multinational, prospective, observational study aimed to clarify the principal/major factors for VAVFL.

METHODS

Interim analysis of three groups with refractory partial epilepsy, stratified by age (8-12 years; >12 years) and exposure to vigabatrin (VGB). Group I comprised participants treated with VGB for >or=6 months, Group II participants previously treated with VGB for >or=6 months who had discontinued the drug for >or=6 months and Group III those never treated with VGB. Perimetry was undertaken at least every six months, for up to 36 months; results were evaluated masked to drug exposure.

RESULTS

Based upon 563 participants in the locked data set, 432 yielded one or more Conclusive visual field examinations. For Group I, the frequency of VAVFL at the last Conclusive examination was 10/32 (31.2%) for those aged 8-12 years and 52/125 (41.6%) for those aged >12 years. For Group II, the proportions were 4/39 (10.3%) and 31/129 (24.0%). No cases resembling VAVFL manifested in Group III. VAVFL was associated with duration of VGB therapy (Odds ratio [OR] 14.2; 95% CI 5.0 to 40.5); mean dose of VGB (OR 8.5; 95% CI 2.2 to 33.2); and male gender (OR 2.1; 95% CI 1.2 to 3.7). VAVFL was more common with static than kinetic perimetry (OR 2.3, 95% CI 1.3 to 4.2).

CONCLUSIONS

The therapeutic benefit of VGB is counteracted by the progressive accrual of the risk of VAVFL with continued exposure and with increase in mean dose.

Comparison of quantitative testing with the Octopus, Humphrey, and Tübingen perimeters

Meridional threshold testing was used to compare the quantitative abilities of the Octopus 201, Humphrey 620, and Tübingen perimeters. In 40 patients with mild to moderate glaucomatous visual field defects, the perimeters provided comparable results, and only small differences could be identified. Manual Tübingen testing identified slightly fewer defects, and scotomas were plotted as narrower and shallower than on the two automated devices, which were nearly equivalent in performance.

Esterman disability rating in severe glaucoma

Forty-two patients with severe visual loss from glaucoma were assigned an Esterman visual function score according to their performance on a binocular visual field test using an automated perimeter. (The Esterman score has been adopted by the AMA as a new standard for rating visual field disability.) The visual function score was correlated with patient responses to questions about perceived visual disability. Monocular visual field results from both eyes using automated suprathreshold static and manual kinetic perimetry were also compared to the binocular Esterman test results. The binocular Esterman test rapidly provided information concerning visual field disability which correlated well with combined monocular results, and with an objectivity lacking in patient responses to a disability questionnaire.

The interpretation of the differential threshold in the central visual field

Computer assisted perimetry has revolutionised the investigation of the visual field. Experience of central field assessment with the Octopus Automated Perimeter shows that sensitivity recorded with target size 3 across all age groups can frequently be greater than the published normative values. Use of the latter values can therefore provide a serious underestimation of field loss. Inter-individual variation in sensitivity is found within and between age groups. The limitations associated with the use of the measurement error to define abnormality and the additional problems of hypernormal thresholds and resolution of the blind spot are discussed. It is suggested that methods should be developed to evaluate sensitivity on an intra-individual basis.

Manipulation of sensitivity in visual field investigation

Sophistication of computer software and flexibility of stimulus and background luminance parameters in static perimeter design offer the opportunity for increased information in the assessment of the visual field. Relationships between dynamic ranges and resulting sensitivity gradients in a group of 10 normal 21 year old subjects using the Octopus automated perimeter, the Dicon Autoperimeter 2000 and the Friedmann VFA II are assessed. Inadequacies in the use of normative data to assess the visual field are noted. Implications for the use of dynamic range/sensitivity gradient relationships in visual field assessment without reliance on normative data are discussed.

The qualitative comparative analysis of the visual field using computer assisted, semi-automated and manual instrumentation: III. Clinical analysis

A comparative evaluation of the Octopus automated perimeter (Programmes 21 and 31), the Goldmann Bowl perimeter, the Bjerrum Screen and the Friedmann VFAs Mk I and Mk II was carried out on a heterogeneous sample of 75 patients. Field loss was categorized using a modification of the classifications proposed by Greve (1982). The results were analysed using the Level 4 analysis developed by Flanagan, Wild, Barnes, Gilmartin, Good and Crews (1984a). The performance of the various test logics was found to differ between the categories of field defect.