I had to prescribe a 70-year-old patient a +6.00DS reading add in single vision reading spectacles because she has bilateral dry age-related macular degeneration with nearly equal near visual acuity in each eye. I counselled her about the reduced working distance (around 16 cm) and she accepts this and is coping with it. However, she complains of frontal headaches and eye ache after about 15 minutes of use. What has gone wrong?
High add for reading allows people to hold things closer. The closer something is held the bigger it is. With high add spectacle lenses, most of the magnification comes from the reduced working distance with a little from the magnification properties of plus lenses. making things bigger helps people with age-related macular degeneration.
Of course when something is held close by a person with presbyopia that something is blurred. The high add lenses provide some clarity at the close working distance. Focus is the main effect of high add lenses.
The downside to all of this is that as the person is not accommodating there is no drive for accommodative convergence. In this case, the person ends up in a situation where they are holding close things at 16 cm in order to achieve maximum clarity but can only summon up enough convergence for a point further away than that. The symptoms of frontal headaches and eye ache after about 15 minutes of high add lens use are because of convergence insufficiency. This is only a problem for people with reasonable near visual acuity in each eye. With dry age-related macular degeneration, there is usually one eye that has much poorer near visual acuity than the other. When the person is monocular, convergence does not need to be considered as only the better-seeing eye will be directed to the close item.
There are several rules of thumb when it comes to determining the amount of prism to include in high add lenses. The one that I use is to add 1 prism dioptre base-in in front of each eye for each dioptre of the sphere above +4.00 DS. Base-in is always the prism base direction used for convergence problems.
So, in the example above where a +6.00 DS high add has been prescribed, I would incorporate 2 prism dioptres in front of each eye. It depends on the underlying refractive error but this prism incorporation can sometimes be achieved by lens decentration at the manufacturing stage. Sometimes prism needs to be ground onto the lens. People at the lens lab will be able to advise on which gives the best lens appearance.