FREQUENCY DOUBLING TECHNOLOGY

 

 

INTRODUCTION:

• FDT is also known by the moniker FDP (Frequency Doubling Perimetry).

• FDT is a rapid method to analyze the visual field.

• The machine uses a large (10°x 10°), low-spatial-frequency sinusoidal grating (< 1 cycle/degree).

• The grating has black and white bars that undergo a rapid counter-phase flicker (>15 Hz), so that the black bars become white and the white bars become black.

• The use of low-spatial-frequency target undergoing rapid flicker leads to the frequency doubling illusion, so that at a certain level of contrast the number of visible lines appear to double.

• When this state is achieved the patient starts seeing twice as many bars as are actually present in the grating.

• The ability to perceive this optical illusion is the function of the Magnocellular pathway. This pathway is apparently damaged early in glaucoma.

• About 15% of the optic nerve fibers are made up of M cells and 15 to 25% of these M cells (called My cells) have non-linear properties capable of eliciting the frequency doubling illusion.

• Thus, only about 5% of all retinal ganglion cells are tested during FDT, allowing loss in any of these cells, to be more readily identified due to reduced redundancy.

• On the other hand, standard automated perimetry (SAP) assesses a larger number of visual cells, which has the disadvantage of overlapping during the testing procedure. Therefore, FDT is supposed to pick up glaucomatous field defects earlier than SAP.

• During FDT testing, the counter-phase flicker rate (25 times per second) and dimension of grating pattern are kept constant and contrast between black and white bars are modified in bracketing phenomena like perimetry till the patient starts seeing shimmering bars in his field of vision.

• This thresholding is done several times within 3 db and the threshold is obtained.

• An advantage of FDT is that refractive errors, even upto 6 Diopters, do not affect the procedure.

 

COMMON TESTING STRATEGIES:

1. Suprathreshold – (used as a screening test)

2. Threshold

 

TESTS AVAILABLE: (IN BOTH STRATEGIES)

1. C-20: 17 targets in central 20°, evaluating four 10° test locations per quadrant, in addition to a central circular target.

                                                               i.      C-20-1 suprathreshold test begins with stimuli with a contrast that 99% of normal population is able to see (1% probability level); best for screening large population; therefore 1% of normal population will miss it. Thus, the false +ve will be less but mild glaucoma field defect can be missed.

                                                             ii.      C-20-5: stimulus used is at a contrast level that 95% of the normal population can detect (5% probability level). Therefore, the rate of false +ve will be more but it will be more sensitive to shallow early field loss. It is useful for patients with risk factor/ symptoms.

    2.    N-30: It adds two test points out to 30 degrees, in the nasal field to improve sensitivity to nasal step defects. In the same way as C-20 suprathreshold testing, the N-30 suprathreshold testing also can be done at 1% and 5% probability level.

 

PARTS OF PRINTOUT:

1. General data- i.e. the program (Full threshold N-30) used, name of the patient, identity number, date and time of testing, which eye is tested, test duration.
2. Below that is the threshold map. (contrast level converted to threshold) in dB in all 19 squares of the right eye.
3. Total deviation probability map is below the threshold map and is given for all 19 squares.
4. Pattern deviation probability map is below the total deviation.
5. The legends of the symbols are given at the bottom, below the left eye, i.e. the second eye.

 

 Matrix FDT- Reliability indices

 

RELIABILITY INDICES:
                                                                 1. Fixation loss- Heijl-Krakau fixation loss method- two or more are significant.
                                                                 2. False +ve and
                                                                3. False –ve

INDICES:

                                                               i.      Mean deviation (MD): Average difference from expected value for patient’s eye +ve value is better than expected, however here it is –ve.

                                                             ii.      PSD (pattern standard deviation) measure for entire field of localized loss relative to expected hill of vision.

       +ve PSD means that it is more than expected. This indicates that localized glaucoma will have higher PSD and in diffuse or general VF loss, PSD will be closer to zero. However, in such cases the MD will be high. In glaucoma associated with cataract or cases of advanced glaucoma, the MD as well as the PSD will be abnormal. Significant abnormal MD or PSD is printed with a p-value besides it. The report of the other eye (i.e. left eye) is printed below the report of the right eye, in same manner. 

WHAT TO LOOK FOR:

                                                               i.      No. of points depressed

                                                             ii.      Location of point involved

                                                           iii.      Pattern of involved points

                                                           iv.      Depth of depression

                                                             v.      Comparison between two eyes

                                                           vi.      MD and PSD

                                                          vii.      In repeat test, the change from previous test should be noted

                                                        viii.      Correlation with ocular examination

ADDITIONAL POINTS:

1. Unlike SAP, even one depressed FDT area is significant because fewer and larger regions are tested and more weightage is given if consistent and repeated defects are seen in following test.
2. Total deviation map and pattern deviation map depict the expected location of physiological blind spot.
3. Diffuse field changes seen with an elevated PSD may suggest underlying glaucomatous loss.

 FURTHER READING: http://webeye.ophth.uiowa.edu/ips/PerimetryHistory/FDP/