FREQUENCY DOUBLING PERIMETRY(FDT) Dr.Jyoti Shetty Medical Director Bangalore West Lions Superspeciality Eye Hospital

Used for early diagnosis of functional loss in glaucoma. Based on the principle of Frequency Doubling Illusion. Tests the My cell subset of type III M cells(Magnocellular pathway)-These comprise 3-5%of the all retinal ganglion cells. These neurons are damaged early in glaucoma.

The target used here is a 5 degree square with alternate dark and light bands. The target has – Spatial contrast-Adjacent light and dark areas. Temporal contrast-One area that flickers between light and dark.

This low spatial frequency sinusoidal grating(0 This low spatial frequency sinusoidal grating(0.5 cycles /degree )undergoes high temporal frequency flicker (18Hz)-The target appears to have twice as many light and dark bands---Frequency doubling illusion.

METHOD OF TESTING Portable instrument Tested under normal room illumination No patching No pupillary dilatation required No spectacle correction required if refractive error ≤ ± 7D.

Newer software modifications are available- Tests Points Tested Matrix Perimetry-Tests available-Screening -Full Threshold Tests Points Tested Time Taken N-30F 17+2 < 3 min 30-2 FDT 69 6-7 min 24-2 FDT 55 5 min 10-2FDT 44 4 min Macula 16

In the 10-2 and macular programmes,target used is a 2° x 2° square with a spatial frequency of 0.5 cycles /degree and a temporal frequency of 12 Hz-No frequency doubling ,only flicker registered.

Statistical software analysis is also available with age matched controls MD,PSD,GHT Reliability Indices-False Negative,False Positive,Fluctuation

ADVANTAGES Easy to use Not affected by refractive blur,lenticular changes,room lighting Portable instrument Rapid screening test with good accuracy With SWAP as a gold standard for early detection of glaucoma,FDT has a sensitivity of 88.9% and a specificity of 96.2%.

Disadvantages Small scotomas <5° can be missed due to large stimulus. Though good statistical analysis is available it is not a good instrument for monitoring progress ,especially in advanced glaucoma.

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