Astigmatism Fan and block test

The Fan and block chart is similar to an astigmatism fan chart but additionally includes a block which contains an arrow at the center and two blocks of the line adjacent to the arrow which are perpendicular to one another. This additional block makes this test easier for the patient.

Before reading procedure read optics behind Astigmatism Fan and block test

Astigmatism charts

Astigmatism Fan and block test Procedure:

• Step 1: Determine the patient’s Best vision sphere and record visual acuity.
• Step 2: Add plus power (equal to half of approximated cylinder power depending upon patient visual acuity shown in Table 1). At this point, the patient should have one set of lines in the chart more clearly compared to others, if not add more plus power. Or fog patient using plus power until all lines in the astigmatic fan chart appear to blur and slowly reduce plus power in 0.25 D step till one set of lines becomes clear compared to the other.
• Step 3: Read the correcting cylindrical axis from the chart and rotate the arrow so it points towards a clear line. Ask the patient to only focus on two blocks containing lines. Both block line’s orientations will be perpendicular to one another.
• Step 4: Add a minus cylinder lens with it axis similar to that mentioned in the chart and simultaneously keep questioning about equal clarity of the two blocks. The examiner continues adding a minus cylinder lens until the patient reports both blocks are equally clear.
• Step 5: To determine whether you found the correct cylindrical power or not. Add +0.50 DS and ask the patient for the appearance of the two blocks. If both are equally blur means astigmatism is fully corrected. If one block is clearer than another it means astigmatism is still present.
• Step 6: Remove additional +0.50 DS. Note down the final power of the patient.
• Step 7: Repeat a similar procedure for the left eye.

Vision with COLC on retina	Astigmatism (D)
6/6	Less than 1
6/9	1.00
6/12	1.50
6/18	2.00
6/24	3.00
6/36	4.00
6/60	High