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 is 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 to resolve—an alternating eye patch may be the only treatment available.
Stroke-related hemianopsia is reasonably common. The field defect is obvious on a 24-2 threshold visual field test.
However, some stroke survivors have hemi-spatial inattention (also known as “neglect”), which is an inattention to or lack of sensory awareness of visual space to one side. It may or may not be associated with a hemianopsia.
Patients with hemi-spatial inattention will usually be unaware of their inability to perceive space on the affected side, may not be able to follow a moving target in the direction of the neglect, and may say that their physician or occupational therapist “said” that they have visual concerns to the side (although the patient is not cognitively aware of the hemianopic like loss of visual field). That is a difficult concern to address and should be referred to an optometrist skilled in neuro-optometric rehabilitation.
Hemianopsia usually leaves a person disoriented and struggling to make it through daily living. People with hemianopsia are often afraid to leave their homes and are concerned about their safety. They are confused in a busy visual environment—such as the mall where they may bump into people—or have the fear of falling off a curb.
Hemianopsia can cause a sense of loss of independence due to discontinuing driving. Others find that ambulatory activities are more difficult. People with hemianopsia (but without hemi-spatial inattention) can often be helped by an optometrist.
As a minimum, recommend two separate pairs of glasses: one for distance and one for near. Separate pairs are needed because with hemianopsia, bifocals or progressive lenses limit the width of the seeing area through the glasses. In my experience, hemianopsia patients usually have fewer field-related complaints with full-field single-vision glasses.