FREQUENCY DOUBLING PERIMETRY(FDT)

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

2. 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.

3. 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.

4. 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.

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

6. Newer software modifications are available- • Matrix Perimetry-Tests available-Screening • -Full Threshold

8. 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.

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

10. 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%.

11. 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.

12. THANK YOU