Some years back I had a chance to interact with a gentleman who happened to visit a doctor around the same time I was visiting with a prior appointment. As we both waited in the OPD, I picked up a discussion with the gentleman. I came to know that he had been frequently visiting the doctor and had astigmatism. Knowing I was from an eye care company, the man asked more about astigmatism and how his glass actually corrected for the same. Having worked in a surgical company , I had a feeling that I knew everything about astigmatism. As I returned back to my hotel that evening, I understood that it was time to have my concepts clear beyond understanding astigmatism as a Rugby ball or an egg shaped cornea.
Today I will try to reboot understanding on astigmatism and try to answer the questions that many a times peak in our mind.
Imagine our eyes. You have not developed cataract. But you visit your doctor as you are not comfortable in seeing things around you. The doctor examines under a phoropter/Snellen’s Chart and finds out that you have only astigmatism. This is Manifest Refraction (MR), the sum total of the corneal and lenticular refractions (total eye refraction). It shows that you have astigmatism without spherical power, that is , you do not have myopia or hyperopia.
To keep matters simple, let us for now forget the Human Crystalline Lens (HCL), and assume that the astigmatism is solely due to cornea. So without any myopia or hyperopia, your cornea is now a cylinder in shape ( if you are thinking of rugby shape here, no you are wrong).
What does a cylinder look like ? A cylinder as depicted in figure 1 ( see image in title ) has one side curved, and the other side flat. The side which is flat has no
Power and is called axis. The side which has power is called Power Meridian. Here the horizontal side has power and the vertical side is the
Axis, or a place of no power. The difference between the Principal Meridian (power meridian) and the axis (no power) is astigmatism.
How to correct ? A glass given to you ( remember you have no cataract ) would have a cylinder shape, but would be placed in the opposite meridian.
So here the glass cylinder will be aligned to the cornea horizontally ( Fig 2 in the title image ). Thus the power meridian aligned to 90 deg, will negate the corneal Power meridian at 180 deg. The result, you have a spherical refraction depicted in green. ( Figure 3). Such a patient would have a prescription written like this : +.75@90/180 or -.1.5@180/90 or plano- 2@ 90/180
Now let us consider, you have both astigmatism as well as myopia (or hyperopia) found by your doctor. How will your cornea look like ? It would look like a (bus tryre) tube ( see Fig 4 in the title ). Unlike the cylinder, a tube would not have any flat surface. All surfaces will be curved. However, there will be one surface more curved than the other. In Fig4 you can see the tube representing the cornea. Such cornea can be described like a rugby ball or an egg shaped cornea. Here the horizontal meridian is more curved than the vertical, but both sides are curved. Since both sides have power, they will be called power meridians, or Principal Meridians. Astigmatism here is just the difference between the two principal meridians. However, the concept of correction of astigmatism with myopia (or hyperopia) is the same as that of the patient with astigmatism only (in the earlier example). The patient with a difference between two principal meridians ( astigmatism ) will have to be given a similar difference of two meridian power in the glass, albeit, on the opposite meridians. Such lenses, whether glass or an IOL ( if a cataract patient ) are also called sphero- cylindrical lenses. The TORIC IOLs of different surgical companies implanted intraoperatively, is a sphero-cylindrical lens where you have a difference of power in the two opposite meridian.
Fig 4 showing a combination of spherical and astigmatic cornea
Such a patient will have a manifest refraction typically like this –
-1-2@90, whereby the -1 denotes the myopia the patient is having, the -2 denotes the difference in principal meridians (astigmatism), and the axis 90 degree is where you are placing the spherical -1. In the world of physicians and optometrists, this is also referred to as an optical cross.
If you do a transposition then you get the second principal meridian ( remember the difference between the two is the astigmatism, which in this case is 2)
So, when you transpose -1-2@90 would be -3+2@180. ( To do transposition add the spherical and the cylinder, then change the sign before the cylinder , keep the magnitude of astigmatism unchanged, and change the axis by 90 deg.
Therefore the two spherical powers are -1 and -3 and the difference between the two spherical (principal) meridians is the astigmatism, which in this case is 2D.
Now that we have a deeper understanding, explaining astigmatism beyond labelling it just as a rugby ball is easy, right ?