KL is an 8 year-old child with achromatopsia. He has a history of atypical infantile nystagmus syndrome and underwent eye muscle surgery at age 3 for exotropia. KL’s parents related that his younger sister also has achromatopisa but to a more severe extent, needing to read at extremely close distances even with significant magnification.
KL’s distance visual acuity hovers around 20/80 – 20/100 but is variable. He can see print as small as 20/30 at a viewing distance of 12 inches. His parents’ greatest concern is that he has significant difficulty navigating independently. Particularly outdoors, he functions as if he has severely constricted peripheral vision. Hemeralopia is a condition in which vision is considerably poorer durig daytime and outdoor conditions due to bright sunlight, as compared to nighttime and indoor conditions. Tinted lenses, often in the red to magenta family, are sometimes used to mitigate the effects of hemeralopia.
Now let’s take a look at KL’s spectacle Rx. His habitual Rx was:
OD: +0.75 -3.00 cx 180; OS: +1.75 – 2.75 cx 170.
Another practitioner refracted him and was able to improve distance acuity by two lines using isolated letters on the Snellen Chart with the following Rx:
OD: -0.75 -3.50 cx 005; OS: -0.50-3.25 cx 175.
His parents reported that when KL got the new Rx, his orientation and mobility seemed to be more tenuous than with his prior Rx. What’s the most likely explanation? Increasing the minus direction that much (shift in SE of -1.75 OD and -2.50 OS) essentially de-tuned the periphery by accentuating central clarity. In his world, and given his parents’ observations, maximum central acuity isn’t the issue. It’s his poor peripheral awareness. So we’re going to move his Rx back toward his habitual direction, and continue to work on improving his binocular vision without pushing the minus.