Improving the sensitivity of the OKP visual field screening test with the use of neutral density filters

Oculokinetic perimetry (OKP) has been developed to screen for glaucomatous field loss but has relatively poor sensitivity when compared with threshold perimetry. Forty-two eyes from 42 patients with glaucomatous field loss on Humphrey threshold perimetry and 32 normals performed hand-held OKP under controlled conditions of refraction and lighting. Those who passed the standard test had their OKP cutoff determined with increasing neutral density filters (NDFs) at a new point 15 degrees from fixation in the inferotemporal field (where first glaucomatous defects are rarest). OKP was then repeated with a NDF that increased the ambient light by 0.3 log units from cut off. Of the 16 glaucomatous eyes to pass standard OKP, 9 failed the NDF test, improving the sensitivity from 62% to 83% (p < 0.05). These 9 eyes had field defects that were significantly less severe (mean defect 4.85 vs 7.91 (p < 0.05) and corrected pattern standard deviation 4.12 vs 7.00 (p < 0.05) and were from younger patients (mean age 56 vs 66 years; p < 0.05) than those who failed standard OKP. None of the 32 normals failed standard OKP and only 1 of 32 failed the NDF test. The use of NDFs to customise OKP, producing essentially a staged suprathreshold contrast sensitivity test, appears to increase the sensitivity of the OKP screener without degrading its specificity, particularly in younger subjects.