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Nikki van Gemeren, Megan Johnson, Roly Megaw, Stephen A. Madill; Are Some Non-Organic Field Defects Easier to Fabricate than Others?. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5572.
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The usefulness of manual kinetic perimetry is well recognised when defining the horizontal and vertical meridia of neurological field defects. However, the fabrication of hemianopic field defects should in fact be more challenging with automated static perimetry due to the randomisation of sequential stimuli locations. We investigated the ability of naive subjects to feign neurological field defects using a Humphrey perimeter in order to identify specific characteristics that could suggest a non-organic field defect.
Observational case series. 20 subjects without visual pathway disease (normal fields confirmed post-protocol) were randomly assigned to feign a specific field defect, assessed using a Humphrey perimeter running 24-2 protocol. Defects included homonymous hemianopia with and without macular sparing (HH, HH+MS), monocular hemianopia (MH), superior quadrantanopia (Q) and bitemporal hemianopia (BT). The field defects were graded into 2 categories, accurate and inaccurate. Accurate field defects respected the appropriate meridia for the full width of the field with the majority of points within the defect graded as 0 decibels. Inaccurate fields had a combination of breakthrough across the appropriate meridia, the majority of points graded as >0 decibels or inaccurate placement of a specific feature, e.g. macular sparing. The results were cross-tabulated into complete hemianopic defects (HH, BT and MH) and incomplete hemianopic defects (HH+MS, Q) for analysis.
All subjects’ had acuities of 6/12 or better (age range 24-39). 12 patients feigned complete hemianopic field defects (3 HH, 4 MH and 5 BT) with all graded as accurate; 8 patients feigned incomplete hemianopic field defects (4 HH+MS, 4 Q) with 5 graded as accurate, 3 as inaccurate (2 HH+MS, 1 Q); Fisher’s exact test, p=0.003.
Naïve subjects can feign complete hemianopic field defects accurately on static automated perimetry. With regard to incomplete hemianopic field defects, fulfilling the criteria for accurate fields as defined above is more use in providing evidence for the field defect being organic compared to complete hemianopic defects.
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