In a previous blog I discussed how reading is frequently disrupted by head injury. Often, reading can be disrupted by direct damage to the brain, but sometimes there are more specific markers for eye focusing imbalance which can be assessed during the exam of a traumatic brain injury (TBI) patient. I will try to keep my explanation straightforward.
Accommodation is the eyes’ ability to focus up close. Have you ever wondered why a person can focus on a plane in the sky and then focus on a word on a page? If you’ve ever operated a camera you know that you must refocus the camera to take clear pictures at different distances. Many smart phones have an auto focus which performs this task for you.
Your brain and eyes have a coordinated auto focus and it can be measured. The lens inside the eye can move and change shape to adjust focus. This ability gradually diminishes with age which is why most people over the age of 45 require reading glasses.
But as an example, let us take a typical 35 year old, he/she can read without reading glasses. During an eye exam, the doctor can hold up a small letter (not too small) and have the patient hold it at arm’s length. The letter will be in sharp focus. Now, you have the patient gradually move the letter forward (closer to patient’s nose) and at some point the letter becomes blurred. If you measure the distance between that spot and the patient’s eyes, you are measuring accommodative amplitude. That distance can be directly translated into focusing power, the closer to the nose the greater the power.
Large studies have been done to measure the accommodative amplitude in people of various ages. So there are standard tables that an individual can be compared to.
What is interesting is that people with significant head injuries or severe whiplash often have documented loss of accommodative amplitude. Estimates vary, but approximately 20% of such patients can be found to have deficits when compared to normal patients of the same age.
Most ophthalmologists don’t measure accommodative amplitude because a person can still have 20/20 vision and decreased accommodative amplitude. I measure accommodative amplitude in most routine eye exams and in ALL my patients with TBI.
Why is it important? Patients with decreased accommodative amplitude often have difficulty reading or working on a computer screen- this can impair work productivity and normal daily function. Many of these patients are forced into using reading glasses at an earlier age. If the eye doctor fails to measure accommodative amplitude, these patients are often ignored or are told “it is all in your head.”
We may not know exactly why accommodative amplitude may be diminished by head injury- the relationship between the brain and the lens in the eye is complex. But the disruption is real and well-documented in vision research.
Besides reading glasses, can anything be done? Over time, some of these younger TBI patients may slowly recover accommodative amplitude as they recover from their concussion, but many are left with permanent deficits. In my previous blog I discussed the role of Vision Therapy in treating TBI patients. Diminished accommodative amplitude is one place where Vision Therapy is likely to be of value. Eye exercises can often improve accommodative amplitude and help restore normal function. Even simple exercises like a “pencil push-up” can help (a patient is asked to focus on the tip of a pencil and it is moved closer and further from the eye in repetitive fashion).
Near vision is a complex task and should not be ignored in evaluating any TBI patient. Only a trained eye care professional who specializes in the care of head injury patients will bother to make these measurements. The good news is treatment can sometimes help. They key again is to talk to the patient about up-close vision and make the necessary measurements.
