Todays discussion will be about the correction of astigmatism by glasses
The general rules for correction of astigmatism are:
1)you can fully correct astigmatism in children.
2)if not worn before do not fully correct above 10 yrs.
3)avoid introducing cylinder for the first time above 40.
4)generally do not make big cylinder changes in power all the time (0.5-1.0 max).
5)even for axes do not make more than 15 degree changes if wrong axis worn for a long time.
6)you can always use the rule of spherical equivalent to put the circle of least confusion on the retina with acceptable vision.
7)simple astigmatics see better than the corresponding errors because one focal line is on the retina.
8)you can have meridional amblyopia when testing optotypes in astigmatics.
9)sometimes the correction of astigmatism is much more tiring to the patient than the astigmatism itself.
10)if your patient sees 6/6-6/12 use the cross cylinder to refine the cylinder power and axis for those wearing sharp glasses before.
Lets work out some examples
*An 18 year female has cyclorefraction of
-3.0×90 OU and needs best possible vision what will you prescribe?
1)i will under correct cylinder and there are 2 ways.
-change cylinder by 1.0 D at a time.
-give 50% of the cylinder.
2)in this example it is acceptable to give -1.0/-1.5 C (according to patient personality and style of life).
3)if you give -1.0 Cx90, the remaining 2.0 cyl have a spherical eq of -1.0, so her prescription for best vision
-1.0/-1.0×90 or -0.5/-1.5×90 if you use the 50% rule.
* a 40 yr male never worn glasses with refraction +1.0/+2.0/180
1)we will not prescribe cylinder or max 0.5.
2)SE is +2.0 then apply rules of hypermetropia
*a 5 yr old with ref
+2.0/+3.0×180 OD
+1.0/+4.0×180 OS
1)fully correct the cylinder
2)OD now +2.0 S, he may not be able to relax accommodation so you can give -1.0
Presc -1.0/+3.0×180
If u prefer minus C transpose
+2.0/-3.0×90.
3)OS -2.0/+4.0×180
Transpose
+2.0/-4.0×90.