April 13, 2019
A study by Sanjay V. Patel, M.D., Ophthalmology chair, and fellow researchers at Mayo Clinic's campus in Rochester, Minnesota, supports a new method of classifying Fuchs' endothelial corneal dystrophy (FECD) severity that is independent of central corneal thickness and employs Scheimpflug tomography for preoperative assessment of corneas for the presence of subclinical edema.
The severity of FECD is currently assessed based on the distribution of guttae and the presence of edema using a method documented by Jay H. Krachmer, M.D., and others in The Archives of Ophthalmology in 1978. Modified versions of this morphological and subjective method of assessment have also been described.
"Although current grading scales incorporate the presence of edema as indicative of worse disease severity, the scales are confusing," says Dr. Patel. "Krachmer grading suggests that corneal edema can only be present with extensive guttae, whereas modified versions suggest that clinically apparent edema can be present with lower grades of guttae and that these eyes should be described as the lower grade, with edema noted separately. In reality, the onset of edema is gradual and early edema is not easily detected by using slit-lamp examination. Corneal edema warrants a classification that reflects this subtlety." The study by Dr. Patel and others was published in Ophthalmology in 2019.
In the cross-sectional study (with follow-up of outcomes) of 93 eyes from 57 patients with a range of severity of FECD and 74 eyes from 40 patients with normal corneas, the Mayo researchers clinically assessed corneas for FECD and corneal edema via slit-lamp biomicroscopy. They categorized the eyes as:
- Having clinically definite edema (obvious visible edema)
- Being suspicious for subclinical edema (possible corneal thickening without obvious edema on slit-lamp examination)
- Not having edema (no clinical suspicion of edema)
Tomographic pachymetry and posterior elevation maps derived from Scheimpflug images were evaluated for specific features believed to be consistent with corneal edema. FECD clinical disease course and outcomes were reviewed from the time of Scheimpflug image acquisition to the last available follow-up. Specific information collected included:
- Clinical management recommendation at the consultation when Scheimpflug imaging was performed
- Clinical outcomes if endothelial keratoplasty was performed
- Clinical progression of FECD if endothelial keratoplasty was not performed
Clinical progression was defined as a subsequent decision to proceed to endothelial keratoplasty based on an increase in central corneal thickness or the presence of visible edema associated with subjective impairment of vision by the patient.
Changes in central corneal thickness throughout the clinical course were measured by using ultrasonic pachymetry, and subsequent Scheimpflug imaging and measurements were reviewed if available.
"We initially proposed a fourth feature of corneal edema on tomography, defined as focal elevation of the anterior corneal surface corresponding to the same location as focal posterior depression," says Dr. Patel. "We found only three eyes that had focal anterior elevation, and all three had clinically definite edema by slit-lamp examination. Because our interests lie in detecting subclinical edema, we elected not to include this fourth feature in our analysis."
3 main outcome measures
"In this study, careful evaluation of tomographic pachymetry and posterior corneal elevation maps successfully identified corneas with subclinical edema because subtle corneal thickening manifested as loss of the normal parallel isopachs, displacement of the thinnest point of the cornea and posterior corneal surface depression," says Dr. Patel. "Clinical outcomes included the changes in central corneal thickness and vision after endothelial keratoplasty."
The three specific tomographic features were all present in all FECD corneas with clinically definite edema, in 81 percent or more of FECD corneas suspicious for subclinical edema, and in 5 percent or less of normal corneas. The tomographic features of edema were present in 42 percent or less of FECD corneas deemed not to have edema clinically, suggesting that tomographic analysis might be more sensitive than clinical suspicion.
Of corneas suspicious for subclinical edema that subsequently underwent endothelial keratoplasty, all had at least two of the tomographic features present before endothelial keratoplasty. Improvements in vision, central corneal thickness and tomographic features after endothelial keratoplasty confirmed the presence of subclinical edema preoperatively.
"We recommend classifying FECD corneas as having clinically definite edema, subclinical edema or no edema, independent of central corneal thickness," says Dr. Patel. "Tomography should be performed only when clinically definite edema is not visible and when assessing patients with visual complaints or prior to surgery in the setting of FECD."
For more information
Krachmer JH, et al. Corneal endothelial dystrophy: A study of 64 families. The Archives of Ophthalmology. 1978;96:2036.
Sun SY, et al. Determining subclinical edema in Fuchs endothelial corneal dystrophy: Revised classification using Scheimpflug tomography for preoperative assessment. Ophthalmology. 2019;126:195.