July 15, 2017
The role of the posterior corneal surface in the optics of the eye has been poorly understood until recently, when Scheimpflug imaging and anterior segment optical coherence tomography instruments enhanced researchers' understanding of the effect of posterior corneal shape on visual outcomes.
"The relationship between the anterior and posterior corneal surface is important; normally the shape of the posterior cornea 'parallels' that of the anterior cornea, compensating for a small but variable percentage of anterior corneal power, astigmatism and high-order aberrations," says Sanjay V. Patel, M.D., Ophthalmology chair at Mayo Clinic's campus in Rochester, Minnesota.
Dr. Patel and a research team used Scheimpflug imaging to assess changes in the profiles of the anterior and posterior corneal surfaces over a range of severity of corneas with Fuchs' endothelial corneal dystrophy (FECD) and normal corneas. The study, which examines the relationship between corneal surface profiles, corneal power and corneal thickness, was published in Investigative Ophthalmology & Visual Science in 2015.
"FECD is characterized by progressive central corneal edema because of endothelial dysfunction, with relative bulging of the posterior corneal plane toward the anterior chamber when edema is clinically detectable," notes Dr. Patel. "A change in the relationship between anterior and posterior corneal surfaces may have implications on corneal power, and has been suggested to cause the hyperopic shift after Descemet membrane endothelial keratoplasty. Because corneal changes are known to start earlier in the course of FECD than when keratoplasty is clinically indicated, changes in posterior corneal power and toricity may also be important when considering refractive cataract surgery in these eyes."
The research team acquired Scheimpflug images for 112 corneas from 64 participants with FECD and 101 normal corneas from 54 age-matched participants. Eyes with FECD were categorized as mild, moderate or advanced according to the area and confluence of guttae and the presence of clinical edema. Normal corneas were devoid of guttae. Findings include:
- Normal corneas were 8.6 ± 4.8 μm thicker vertically than horizontally (P = 0.001); the steep posterior meridian was vertical in 91 percent of corneas.
- The difference between vertical and horizontal thicknesses decreased to 4.7 ± 7.3 μm in eyes with advanced FECD (P = 0.008); only 46 percent had a steep vertical posterior meridian (P = 0.001).
- Vertical radius of posterior curvature was flatter than normal in eyes with moderate (by 0.2 millimeters, or mm, P = 0.011) and advanced (by 0.4 mm, P < 0.001) FECD.
- Mean posterior corneal power was less negative in eyes with moderate (by 0.2 diopters, or D, P = 0.009) and advanced (by 0.4 D, P < 0.001) FECD compared with normal.
"The normal posterior cornea has an ellipsoid shape that manifests as against-the-rule astigmatism because the cornea is thicker in the vertical than horizontal meridian. Yuta Ueno, M.D., with the University of Tsukuba Hospital, Japan, and others reported the same findings in Ophthalmology in 2015.
"In contrast, in eyes with FECD, the posterior cornea is flatter and more spherical than normal, resulting in less negative power and loss of normal posterior surface toricity," says Dr. Patel. "These changes might indeed contribute to the hyperopic shift that occurs after Descemet membrane endothelial keratoplasty (DMEK) and have implications for planning refractive outcomes after cataract surgery in eyes with FECD.
"In eyes with FECD, the directional change in peripheral corneal thickness can result in loss of normal posterior corneal toricity, and the axes of anterior and posterior corneal astigmatism can change significantly as FECD progresses. Therefore, toric intraocular lenses and use of limbal relaxing incisions in FECD should be selected cautiously, if at all, considering that both corneal surfaces could change after DMEK.
"As noted by Katrin Wacker, M.D., with Ophthalmology at Mayo Clinic's campus in Minnesota, and others in Ophthalmology in 2015, surgeons should also recognize that corneal high-order aberrations can increase early in the course of FECD, and these can degrade optical quality after cataract surgery or endothelial keratoplasty. Eyes with moderate and advanced FECD should be rendered slightly myopic if DMEK may be required in the future."
For more information
Wacker K, et al. Directional posterior corneal profile changes in Fuchs' endothelial corneal dystrophy. Investigative Ophthalmology & Visual Science. 2015;56:5904.
Ueno Y, et al. Corneal thickness profile and posterior corneal astigmatism in normal corneas. Ophthalmology. 2015;122:1072.
Wacker K, et al. Corneal high-order aberrations and backscatter in Fuchs' endothelial corneal dystrophy. Ophthalmology. 2015;122:1645.