Pearls for Prism

One of the most interesting aspects of my specialty contact lens practice came about by accident. It started when I saw a patient who survived the Oklahoma City bombing. They had an orbital fracture with muscle entrapment that was causing double vision, which became worse when they looked down. The patient wanted to get rid of their double vision so they could play golf again.

I hadn’t really managed double vision since I was in school 10 years ago, and I had to look up how to test phorias and tropias. It seems funny that patients now come from all over the country to see me for contact lenses with prism.

There is this myth out there that contact lenses cannot be made with prism, but they can. A common reason this option is not always offered is that doctors, and certainly patients, do not know it exists.

WORKING WITH PRISM

To begin using prism with contact lenses, you must first have an account set up with SpecialEyes for custom soft contact lenses and with Valley Contax or EyePrintProsthetics for scleral lenses.

The limitation for incorporating prism into custom soft contact lenses is 4.00 D base down. You can only use base down due to gravity. Anything over 4.00 D will cause the lens to slide down. You cannot put base up, out, or in because if you do, the lens will just move down anyway.

To create custom soft lenses, you need the patient’s manifest refraction, prism needs, keratometry values, and horizontal visible iris diameter. I usually choose the hioxyfilcon 49% material because the lower water content material tends to be more durable. I also frequently increase the contact lens center thickness for handling purposes.

If the patient has any horizontal double vision and needs base in or base out prism or more than 4.00 D base down, you will need to prescribe a scleral lens because it will provide a more stable fit on the eye. The prism limitations for scleral lenses are 4.00 D in any direction per eye, which means you can effectively correct 8.00 D total in any direction.

Scleral lenses must be well fit and completely stable, as the prism creates a hefty thickness in the direction of the base. When I first started fitting scleral lenses, I thought patients would notice thickness of the lens and find it uncomfortable, but that has never been the case in my experience. In fact, these lenses tend to be comfortable and work well most of the time.

I test the prism myself in-office over trial scleral lenses. (I never rely on previous glasses prescriptions because many times, the prism I measure is different.) I do Von Graefe phorias and put trial frames in front of the scleral lenses. I recently had a patient who had been to many neurologists, neuro-ophthalmologists, and other specialists with unresolved double vision. Her glasses had vertical and horizontal prism. I found only 3.00 D of base out prism, which I then tested on multiple visits to make sure it was stable. She was able to remain singular and wear scleral lenses with prism.

I do not start with a smaller prism and work my way up. I need each patient’s fusion to be consistent and without eyestrain. I find that when the prism is just right, the patient’s vision feels sharper.

For example, working with a patient with a 9.00 D esophoria who wears glasses with 3.00 D base out in each eye, I would insert scleral lenses in the office and test 3.50 D base out and 4.00 D base out in each eye to see which is better. I push a bit more prism because I don’t want my patients to have eyestrain or decompensation at the end of the day. Scleral lenses with prism are a big investment of time and resources, and I want my patients to be able to use the same pair of lenses for as long as they possibly can.

CASE EXAMPLE

A 78-year-old male presented with a history of LASIK and radial keratotomy scars OU, IOLs, and a corneal scar OD from a prior injury. He wore progressive spectacle lenses with 2.50 D base out due to decompensated esophoria. His habitual spectacle acuity at baseline was 20/40- OD and 20/25 OS. Binocular vision testing yielded a horizontal phoria of 9.00 D and a 1.50 D vertical phoria.

I took impressions of both eyes using the EyePrint Impression Process (EyePrint Prosthetics) and designed single-vision distance scleral lenses with 3.00 D of base out prism OU and 1.50 D of base up prism OD. After I finalized the prescription and fit of the lenses, I used the Ovitz XWave Aberrometer (Ovitz) to take pupillary measurements, which were sent to the lab to offset the optics and apply the extended depth-of-focus (EDOF) multifocal with +1.50 D add (Figure 1). The current add limitation of the Ovitz EDOF multifocal is +1.50 D. I thought that offset optics would help this patient achieve better intermediate vision and that readers would help with smaller type.

At the final visit, the patient achieved 20/20 VA at distance and intermediate with no complaints of diplopia or discomfort (Figure 2). He was able to reach a level of vision he had not been previously able to with progressive prism spectacle lens wear.

OPTIONS ON THE TABLE

Most patients wearing glasses with prism come to my office having been told that contact lenses with prism don’t exist. Offering these patients contact lens options with prism can not only set your practice apart, but also provide a lot of happiness to patients who thought they were bound to glasses forever.