You might have heard the terms “aberrations” or “high-order aberrations”, but what are they, and do you need to worry about them? Simply put, “aberrations” are the different types of refractive error that an individual might have. When you go to have your glasses checked, your eye doctor generates a prescription that includes numbers for the “sphere” and “cylinder”; these are “low-order aberrations”, dominate most people’s refractive error, and are correctable with glasses and soft contact lenses. “High-order aberrations”, often abbreviated as “HOAs”, are frequently referred to as “irregular astigmatism”; everybody (every eye) has them, but usually to a very small degree and they can often confer a subconscious benefit in vision. However, when HOAs are higher than normal, vision becomes compromised by not being sharp or being degraded by starbursts or halos, especially around lights. HOAs, or irregular astigmatism, cannot be corrected by glasses or soft contact lenses, but can be corrected by rigid contact lenses. Larger degrees of HOAs can occur from diseases, including dystrophies, corneal scarring after infection or injury, and after corneal transplantation (especially penetrating keratoplasty [PK] and deep lamellar anterior keratoplasty [DALK], though also to a lesser degree after Descemet stripping endothelial keratoplasty [DSEK/DSAEK] or Descemet membrane endothelial keratoplasty [DMEK]).
When light passes through the cornea, it bends according to the laws of refraction. Most corneas are predominantly spherical (like a soccer ball) or toric (like an American football) in shape, and these shapes generate low-order aberrations. However, when corneas assume an irregular shape (i.e. it has multiple different curves without a regular pattern), they induce HOAs. High-order aberrations can originate from any irregular surface in the eye, but especially from the front surface of the cornea because of the large change in refractive index at that surface (between air and cornea) compared to any other surface in the eye. Now that we are able to measure the regularity of the back (inside) surface of the cornea, we have learned that this surface can also induce HOAs that affect vision, but typically to a lesser extent than irregularities on the front surface of the cornea. Note that the natural lens of the eye (which eventually develops into a cataract) can also generate HOAs, but after lens removal and replacement at cataract surgery, most of these HOAs disappear. So if you have already had your cataract removed but you have irregular astigmatism, then the source of the irregular astigmatism is almost certainly your cornea.
So how do corneal dystrophies cause HOAs and affect vision? This is best thought about by dividing dystrophies into those that affect the front and back surfaces of the cornea. The two most common corneal dystrophies are “anterior/epithelial basement membrane dystrophy” (ABMD or EBMD), which affects the front surface of the cornea, and Fuchs endothelial corneal dystrophy (FECD), which affects the back surface of the cornea. Other less common dystrophies can affect either corneal surface and induce HOAs in a similar manner to that described below.
Most of the time, ABMD is present but does not cause any symptoms. When ABMD does affect vision, it is because the front (epithelial) layer of the cornea loses its smooth and regular contour. Irregularity of the front corneal (epithelial) surface is caused by deposition of abnormal tissue a few cell layers deep in the front (epithelial) layer. The abnormal tissue contour transmits through to the very front surface of the cornea, and because of the large change in refractive index at the front corneal surface, it does not take much irregularity in that surface to upset vision. Patients with this condition experience fluctuations in vision, lack of clarity that cannot be improved with glasses, double vision or ghosting, and starbursts or streaks through lights. Vision can sometimes be degraded as much as cataracts degrade vision, and therefore it is important for surgeons to carefully examine the cornea to rule out such conditions because they often coexist with cataracts. Depending on the type of dystrophy, treatment involves removal of the abnormal epithelial cell layer (which then regenerates normally) alone or in conjunction with laser removal of abnormal tissue (phototherapeutic keratectomy), both of which take just a few minutes to do. Improvement in vision can be dramatic with a significant improvement in quality of life. The alternative is to be fit with a rigid contact lens, though most patients opt for surgical debridement as contact lens wear is usually not necessary after that.
How do dystrophies on the back surface of the cornea affect HOAs and vision? FECD is by far the most common dystrophy in this location, and a topic we have studied extensively with respect to optical performance and beyond. In FECD, HOAs can originate from the front and/or back surface of the cornea. On the back surface (endothelium), abnormal tissue deposition (guttae) can result in loss of a smooth surface; it is unclear if the guttae alone impair vision through HOAs or not. The reason for this is that guttae are often accompanied by corneal swelling (edema), and swelling affects the contour of the back surface of the cornea much more than guttae. Swelling can be present in FECD even when not seen during routine examination in the doctor’s office; tomographic imaging (which is available in most cornea clinics) can detect early (or subclinical) swelling in FECD and should be considered as a test when evaluating the disease. When swelling develops in FECD, the back surface of the cornea bulges towards the inside of the eye, whereas the front surface does not change much if at all. This leads to an increase in corneal thickness, but more importantly, the bulge on the back surface induces HOAs sufficient enough to affect vision. Furthermore, when corneas are swollen over a long period of time in FECD (even if the swelling is mild or subclinical), there is secondary deposition of abnormal tissue at the front of the cornea (subepithelial fibrosis), which creates a similar situation to that described above for ABMD, i.e. vision can sometimes be affected by front surface HOAs in FECD. The treatment for FECD is endothelial keratoplasty, whether by DMEK or DSEK/DSAEK. DMEK tends to result in better vision because the back surface contour of the cornea approaches that of the normal host cornea, whereas after DSEK/DSAEK, the back surface contour is often abnormal (depending on whether the graft cut was uniform or not, and because the curve of the donor is not typically a perfect match for the host) and induces HOAs. Many DSEK/DSAEK grafts are of uniform thickness, and these eyes can have excellent vision afterwards. Although endothelial keratoplasty treats the back surface of the cornea in FECD, any front surface abnormalities and HOAs from the disease can persist, and rarely some patients need a rigid contact lens to enjoy their best vision.
Recognizing that HOAs may be induced by the front and/or back surfaces of the cornea is important not just in the management of the underlying condition, but also when considering cataract surgery in the setting of these conditions. It is important for cataract surgeons to detect and understand how corneal dystrophies might affect visual outcomes after cataract surgery. This is critical when the cataract surgeon considers using intraocular lenses (IOLs) to correct astigmatism (toric IOL) or near vision (multifocal IOL). If front surface corneal diseases, including ABMD, Salzmann nodules, and pterygia, are not detected or considered, the surgeon can implant an IOL that may not give the best visual outcome; these conditions need to be managed before considering toric or multifocal IOLs. The same is true for FECD affecting the back surface of the cornea; when the back surface bulges towards the inside of the eye, cataract surgery with a toric or multifocal IOL (whether performed alone or in conjunction with DMEK/DSEK/DSAEK) can result in suboptimal visual outcomes compared to that expected. Ultimately, surgeons and patients should consider the realistic goal of surgery, which should be improvement in quality of vision and life, even if vision without glasses is not perfect.
Sanjay V. Patel, MD FRCOphth
Fellow of the Royal College of Ophthalmologists (UK)
Professor & Chair of Ophthalmology
Mayo Clinic, Rochester, MN
