Which Glasses Are Best for Keratoconus?
- July 1, 2020
Several keratoconus patients have asked me if special eyeglasses might help them. This follows University of Liverpool announcing their engineers are developing spectacle lenses for keratoconus. While new treatment for keratoconus is welcome, the laws of physics will likely make these special eyeglasses impractical. Read on to learn why.
Keratoconus: the eye disease with elevated higher order aberrations
Keratoconus is an eye disease where the cornea (or front clear dome of the eye) is distorted. About 1 in every 375 have keratoconus. The human eye has two classes of optical aberrations: “lower order” aberrations and “higher order” aberrations. Keratoconus patients have an abnormally large amount of higher order aberrations compared to normal eyes. This is caused by the corneal distortion in keratoconus. Normal eyeglasses only correct lower order aberrations – not the higher order aberrations that generally dominate in keratoconus.
In order for eyeglasses to effectively help most with keratoconus, they must correct both lower and higher order aberrations.
Lower and higher order aberrations
Lower order aberrations (LOAs) cause symmetrical blurring. Traditional glasses and soft contact lenses can correct LOAs. LOAs go by names like hyperopia, myopia, and regular astigmatism. Normal patients, i.e. without keratoconus, have mostly LOAs.
Higher order aberrations (HOAs) are complex and asymmetrical. In the past, doctors name these “irregular astigmatism”. Traditional glasses and soft contact lenses do not correct HOAs. They go by names like spherical aberration, coma, trefoil, tetrafoil, secondary astigmatism, and so forth. Patients with keratoconus usually have lots of lower and higher order aberrations, caused by their distorted corneas. The kind of HOAs especially common to keratoconus is called coma, because it tends to smear a point of light into a comet shape. In keratoconus, rigid surface contact lenses are the mainstay to reduce the lower and higher order aberrations. Many patients with keratoconus can enjoy rigid surface scleral contact lenses for unsurpassed clear vision with the stability and comfort approaching a soft disposable contact lens.
Can eyeglasses correct higher order aberrations?
Yes, special eyeglass lenses can correct HOAs that characterize keratoconus and it has already been done! In the early 2000s, the excimer laser manufacturer VISX produced PreVue ophthalmic lenses to simulate the best-case scenario of their laser vision correction for patients without keratoconus when higher and lower order aberrations were fixed. PreVue lenses were fabricated by first using an instrument called a wavefront aberrometer, to measure both lower and higher order aberrations before the excimer laser would cut the PreVue lens. Eye doctors no longer used PreVue lenses since custom laser vision correction has advanced beyond needing to create these simulations. Additionally, eye doctors never used PreVue lenses to treat keratoconus. They were used for laser vision correction candidates.
In the mid-2000’s, the now defunct start-up, Ophthonix, developed wavefront-guided iZon eyeglass lenses intended to correct both lower and higher order aberrations. Their Z-View aberrometer measured these aberrations, and a special laser altered the light-bending characteristics of a special polymer within the iZon lenses on a point-to-point basis. Shortly after commercialization, I prescribed iZon wavefront-guided glasses for several of my mild keratoconus patients. The results underwhelmed with outcomes indistinguishable from normal eyeglasses. Despite nearly $80 million in venture capital funding, Ophthonix closed abruptly due to market failure.
If glasses can fix higher order aberrations in keratoconus, why don’t they work?
When the amount of HOAs is great, which pertains to most cases of keratoconus, an aberrometer often cannot obtain a measurement. Especially with corneal scarring, the aberrations are out of range of what the aberrometer can measure. Even the smallest eye movement and misalignment creates significant measurement noise. Without a precise and repeatable measurement of LOAs and HOAs, it is impossible to effectively correct them.
If HOAs in keratoconus are precisely and accurately measurable, a possibility in mild keratoconus, an eyeglass lens correcting them must perfectly align in front of the eye. Namely, the lens would need to be just the right distance from the eye, the proper rotation, the right direction up and down, and the proper tilt. Any slight deviation can hurt the vision more than help. The resulting eyeglass viewing area would be restrictively small, making it feel like looking through a keyhole. By comparison, a scleral rigid contact lens maintains a consistent position on the eye while providing clear vision, effectively correcting both LOAs and HOAs in all fields of gaze.
Will glasses ever work for keratoconus?
I expect the engineers at University of Liverpool will rediscover the previous limitations of spectacle lenses for correcting LOAs and HOAs. The £15k funding (~$19K USD) from Fight for Sight is certainly well intentioned. But unless the approach is completely different than prior efforts, it will fall flat. Keep in mind that Ophthonix received over $80 million in funding and failed miserably.
There is greater hope with electronic goggles that measure aberrations of the eye in real time and compensate for them even with eye movement. Just do not hold your breath on this happening soon, let alone for such a venture to turn out fashionably.
For the vast majority with keratoconus, eyeglasses are the “wrong medicine” to restore optimal vision. Nothing is as effective as special rigid surface contact lenses — not even corneal cross-linking or Intacs surgery — for bringing out maximal vision for keratoconus. The new contact lens technology including scleral lenses and impression-based EyePrintPRO, has revolutionized the restoration of vision for keratoconus.
Meanwhile, in milder cases of keratoconus, conventional eyeglasses can often help. In mild keratoconus, LOAs dwarf the HOAs so that glasses can provide reasonable vision. In these situations, there are appropriate lens materials and lens options to make glasses perform best, including thinner and lighter high-index resins and anti-reflective treatments.
To find out if eyeglasses can help your keratoconus, request your appointment at ReVision Optometry.