Do I need reading glasses, or maybe just longer arms?
Nearly every day, people come in to the office for their eye exams and ask: “Why am I starting to have trouble reading small print?” Some patients who are nearsighted may start to notice that they cannot see well up close unless they remove their prescription glasses. Other people who have never needed to wear glasses before may start to notice this problem – and also that the problem can be solved by wearing either reading glasses or bifocal lenses.
What is this strange phenomenon that affects seemingly every adult over the age of 40 to 50 years old?
The medical name for this change is “presbyopia.” Presbyopia is due to an age-related change that occurs within the natural lens of the eye.
First we need to review the function of the natural lens:
The lens is a small, nearly spherical part that that sits inside the eye and helps to focus the light coming in. When we are younger, the lens has the ability to change shape easily – the shape is not rigid, but very flexible. There are even tiny muscles (called ciliary muscles) that contract and relax to control changes in the shape of the young lens. This flexibility allows our eyes to focus over a broad range of distances – from distant to near. One example of this would be a young person with perfect distance vision who can also read up close without glasses. Another example is someone who wears glasses to correct their nearsightedness – giving them good distance vision – but who can also easily read with the same pair of glasses on. The action of changing the shape of the lens to focus on near objects is called accommodation. When we are young, due to the flexibility of the natural lens, we are able to accommodate to see up close for reading, writing, or looking at near objects.
The examples above are so common – encompassing nearly all young people – that they may seem obvious. But if every young person has the ability to change the shape of their lens in order to focus on both near and far objects, this begs the question:
What changes when we get older?
The natural lens is made of layers of cells, similar to the layers of an onion. With time, more and more layers of cells develop, causing the lens to become thicker. Even though the ciliary muscles still function, the thickened lens of someone who is 50 years old is much less flexible than that of someone who is 20 years old. The older we get, the less flexible the lens becomes, and the less it can change shape, which limits our capacity to accommodate. Now, the 45 year-old person with perfect distance vision is starting to have difficulty reading for more than few minutes at a time, because they are straining to focus on the small print that is close to the eye. The 50 year-old nearsighted person who is wearing their prescription glasses may also have more difficulty seeing up close while wearing their usual glasses, because they have lost some of their capacity to accommodate. This person will have to start taking their glasses off to read or will need bifocal lenses (which are distance prescription glasses with the reading glasses built in to the bottom of the lens). As we age, this limitation of accommodative ability continues to progress.
Presbyopia is often misunderstood by patients and doctors, alike. In my next blog, we will revisit presbyopia, some of the common misconceptions about it, and even treatments for presbyopia in certain situations.
If you want to understand presbyopia better or have any questions, feel free to leave comments or contact me, Dr. Geoffrey Hill, at firstname.lastname@example.org.