Accurately Evaluate Visual Function
Accurately Evaluate Visual Function
Using this testing method may improve patient satisfaction with vision correction.
NORMAN YOUNG, O.D., Sarasota, Fla.
A complete or comprehensive eye exam includes many things, such as the evaluation of the external and internal ocular structure, ocular motility, pupil function and tonometry. The evaluation of visual function is also integral to a complete eye exam and entails refractive testing and the evaluation of convergence/binocularity and accommodation.
Following is a testing procedure regarding the diagnosis of near-point accommodative dysfunction and the advisability and magnitude of acceptable near-point and distance correction needed if the initial evaluation suggests accommodative/convergence systems dysfunction.
You can perform the entire testing sequence in just a couple minutes at the refractor.
Test Positive Relative Accommodation (PRA or #20). Then, measure the phoria at the patient's point of PRA (modified 15B) and perform a Dynamic Binocular Cross Cylinder (modified 14B). The procedure's second half is similar, but measures Negative Relative Accommodation (NRA or #21), followed by measuring the phoria (modified 15B) and then the modified Dynamic Cross Cylinder (14B).
How to do it
The first step, measuring PRA, shows the limit of accommodative effort that the visual system can withstand before sacrificing clarity.
Starting from the distance-subjective finding (7A) in the refractor, at 16 inches, gradually provide binocularly minus-lens power in quarter-diopter steps, having the patient retain J1 clarity. When he reaches the point of sustained blur, have the patient continue to try to maintain clarity as you introduce Risley prism and test the phoria. This shows the posture of the convergence system when accommodation is maximally stimulated.
The last step in this half of the testing procedure is to conduct the 14B from that over-minussed and stressed posture by gradually reducing minus power (or increasing plus) until the patient reaches neutrality. This test shows the desired posture of the accommodative system after maximum stimulation.
The second half of the testing procedure is similar to the first, except that you now relax accommodation by starting from the distance subjective correction and gradually provide binocularly plus-lens power in quarter-diopter steps to first sustained blur (NRA). This indicates how much accommodation can be relaxed. The following phoria from that position shows how the convergence system postures when accommodation is maximally relaxed.
The last test, the Cross Cylinder test (14B), starting from the relaxed, over-plussed posture, shows how accommodation wants to position coming from that relaxed state.
You can analyze the accommodative blur tests by first comparing the magnitude of the findings. Then, compare the NRA and PRA to each other. The quality of the response is also important. If the patient reports pulsating clarity (suggesting spasm of accommodation), discomfort or diplopia, this provides further confirmation of binocular distress. If the magnitude and quality of the response of the NRA is high and the PRA is low, you should consider using a plus-lens prescription, or prescribe more on the plus side. If you find the opposite, the patient is likely to reject the plus-lens prescription, and you should use a fuller minus prescription for the myopia or a minimum-plus prescription for the hyperopia.
|Examples of Findings|
|Here are three examples of how this testing method works.|
PRA (#20): -3.00, Phoria: ortho, XCyl (14B): -0.25D
NRA (#21): +2.25, Phoria: 10 exo, XCyl (14B) +1.00D
This patient shows 3.00D of minus power (or reduced plus) to first blur. You could calculate accommodative amplitude to 3.00D plus 2.50D (for the 16" working distance) for a total of 5.50D.
The patient's relaxed accommodation is 2.25D (#21). The calculated maximum relaxation is 2.50D (for 16" working distance). When NRA (#21) is significantly higher than 2.50D, suspect latent hyperopia or spastic myopia.
This patient has a relatively normal pattern showing no specific need to modify his basic prescription.
#20 -3.50, 2exo, -1.50D
#21 +1.00, 16exo, -0.50D
This patient shows a lower NRA compared with PRA. You cannot relax accommodation to the endpoint, but you can significantly stimulate it. The convergence system doesn't move much toward eso with additional minus but does significantly move to exo with plus. And, the 14B, showing where accommodation wants to posture, strongly confirms the contraindication of additional plus at near.
This pattern may describe a patient who has symptoms of difficulty with a maximally corrected-plus (or minimum-minus) prescription or who has a near-add or induced-Base Out (BO) prism. Thefindings suggest a need for a full-minus power prescription, no add and no induced BO prism.
#20 -1.00, 4eso, +0.50D
#21 +2.75, 8exo, +2.25D
This patient shows a higher NRA compared with PRA. His accommodation wants to relax, but it isn't comfortably stressed, suggesting the need for an added-plus prescription. The phorias move more significantly to eso with minus lenses and less to exo with plus lenses, again suggesting near need for a plus prescription. The cross-cylinder findings show that accommodation, even coming from the stressed, overminussed position, still wants to posture in plus. Need forplus prescription seems evident. This example might describe a patient with asthenopic symptoms, one who has difficulty with a maximally corrected in-minus (or minimal plus) prescription orwho has a base in prism component.
We normally expect the conventional #14B Cross Cylinder test to be +0.62D over the subjective. I would expect the modified Cross Cylinder findings to be about a half diopter from that, with the finding coming from the PRA more minus and from the NRA more plus, due to the direction of accommodative change. When the Cross Cylinder and PRA findings are much more in the plus range, this, again, is a sign that the visual system is in need of a more plus prescription.
When the phorias don't move much in an exophoric (exo) direction with accommodative relaxation, but does move more significantly in an esophoric (eso) direction with accommodative stimulation, this indicates the need to prescribe more plus for near or at least to undercorrect the full-minus prescription. (See "Examples of Findings," above.)
The testing and recording of a single procedure provides some information, but tells nothing about how the visual systems are interrelated. Prescribing based on such limited and isolated information can be correct, but is more of a guess.
The procedure described above shows the two systems — accommodation and convergence. When we maximally relax one (accommodation), we observe its effect on the convergence system and its effect on the equilibrium of accommodation. When we maximally stimulate accommodation, we again observe its effect on convergence and accommodative equilibrium.
With this tool, we can more confidently prescribe what the patient needs. OM
Dr. Young resides in Sarasota, Fla. You can e-mail him at email@example.com.
Optometric Management, Issue: June 2008