Many patients are told by non-optometric doctors that double vision may resolve on its own within a vague timeline of months without mentioning vision therapy or prism. It is dismaying that people with stroke-related hemiplegia are recommended to have physical and occupational therapy, but patients with diplopia are given only an eye patch and not afforded a chance for binocular rehabilitation. I suggest prescribing prism glasses as a stopgap measure to help the patient feel more comfortable.
Simple vision therapy procedures using a Brock string or red-green tranaglyphs may help until vision therapy is initiated. Never prescribe a ground prism into glasses until a two- to three-month trial with a Fresnel prism has shown the angle of deviation to be steady and that the double vision has been resolved.
It is important to prescribe the total amount of Fresnel prism with the prismatic compensation broken up between the two eyes to allow the Fresnel-induced reduction in contrast to be distributed evenly between both eyes.
For example, if an esotropia-related diplopia is resolved with 20 D base-out prism, it may seem simple to prescribe a single 20 D base-out Fresnel prism before the deviating eye. However, the patient will usually complain of blur in the eye with the Fresnel prism. Two 10 D base-out prisms are a better choice because they equalize the 20 D Fresnel-induced poor contrast, which reduces patient complaints.
Furthermore, splitting the prism power between two eyes allows the freedom to fine tune the prism power when, or if, the patient's angle of deviation changes. Peel off one of the Fresnel prisms and replace it with another power as clinically indicated. Keep in mind that the angle of the paresis measured when viewing at distance may be very different than at near, so separate prismatic distance glasses and reading glasses are often required.
When prescribing compensating prism for vertical diplopias, remember that the angle of deviation usually varies depending on head position. Be sure to prescribe the vertical prism with the patient's head in a straight-ahead position and warn the patient that a chin-up or chin-down head position will likely cause him to see double in spite of the prism.
A patient whose diplopia is resolved with prism may begin to complain again of diplopia in a few months. Never assume that a renewed complaint of diplopia implies a worsening of the condition. It may mean that the strabismic angle is decreasing.
Do not be disheartened if a rare patient can't fuse binocularly with any amount of prism. A prism bar may seem to neutralize the diplopia while the patient is in the chair, but you may find that when you prescribe Fresnel prisms, the patient still complains of double vision. At times despite re-measuring, fine tuning, and changing prism power, the patient continues to not fuse the diplopic images.
Some neuro-related diplopias are difficult to resolve because of damage in the brain pathways responsible for the binocular vision reflex, and horror fusionalis, when it occurs, is difficult or impossible