|Table 2: Presbyopic Addition Required|
|*From median line of Duane accomodative amplitude chart.|
Each issue, tech tips will explore some interesting aspect of optical technology. This month we look at accommodation, add power, and age.
Three A’s that go hand in hand.
The human eye’s ability to focus at varying distances is a wonder of nature. The crystalline lens inside the eye is Mother Nature’s variable focusing mechanism. Within a split second, a person can focus from an object at their reading distance to one located 5 ft. away, for example, then to another object 1 ft. away, all within a split second and all without the person ever noticing the eye’s refocusing between each position change. This refocusing ability is known as accommodation. Unfortunately, as the eye ages, it slowly and continuously loses some of its focusing ability until the loss progresses to the point where the person needs lenses to focus clearly on objects close up, followed by trouble focusing on intermediate distances. How much focusing ability does someone have at a particular age and what’s the clinical significance of it?
An American ophthalmologist named Alexander Duane researched the amount of focusing ability humans had as they aged (known as accommodative amplitude). Duane published a paper in 1922 containing this data and Table 1, at right, illustrates his results.
Known as the Duane chart, Table 1 shows three curves: a minimum, a maximum, and an average 2,000 testing results. The average (B line) is the one usually used for analysis.
Children are born with a substantial amount of accommodation (about 20 diopters). Duane’s chart tells us that at age 8, the average child will have 14 diopters of accommodation, by age 24 they’ll have about 10 diopters, at age 40 they’ll have about six diopters, and at age 52, they’ll have about 1.50 diopters. There is a slow and nearly straight line accommodative loss until age 52, where the decline lessens substantially. Accommodative loss is due primarily to the crystalline lens aging and becoming increasingly less elastic, which inhibits its ability to reshape and add additional plus power to the eye when needed.
Using Duane’s data, Russell Stimson created a chart that shows the add needed at various presbyopic ages (see Table 2, at right). Since accommodative ability drops off during a person’s lifetime, there comes a point where the eye will need near vision help. Table 2 indicates that this occurs in the early 40s (between 40 and 44), depending on the person’s near point focusing distance and how much accommodation they reserve in order to remain comfortable during close viewing tasks. If they don’t, they’ll develop discomforting symptoms.
Here’s an example. A 52-year-old person will statistically have 1.50D of accommodative amplitude. Remember this is their maximum accommodative effort and everyone knows, if you use your maximum effort for very long, you’ll feel the strain of it pretty quickly. That’s why Stimson’s chart has a “reserve” built into it. People use some accommodation for near viewing but they reserve some of it for comfort. Their maximum accommodative effort will focus them at 67cm (about 26 in.). At a fixation distance of 33cm (about 12 in.) and using a 50% reserve, they’ll need a 2.25D add. If their fixation distance is 40cm (about 16 in.), they’ll need a 1.75D add.
While there surely is a lot of variation for many reasons, it’s surprising how predictive Duane’s data remains. Duane’s data are the basis on which ECPs tell patients that presbyopia usually begins in their early 40s, as Stimson’s table depicts. Duane’s data also help doctors become suspicious if a patient’s accommodative ability is subnormal because it may indicate a ciliary muscle issue. Duane’s research also mentioned that there was no difference in the degree of accommodative loss between men and women.
Accommodative loss, adds, and aging…they all go hand in hand.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.