Some ophthalmologists say they can make the diagnosis of keratoconus, forme fruste keratoconus, pellucid marginal degeneration, or keratectasia by topography analysis alone. I cannot; however, I agree that most experienced ophthalmologists can diagnose keratoconus, forme fruste keratoconus, pellucid marginal degeneration, or keratectasia-like “patterns” from a quarter-mile away with a pair of binoculars. Even then, one is only making a diagnosis from a piece of paper. As physicians and surgeons, we do not treat patients based solely upon a printout. In this article, I want to elucidate several nuances regarding the diagnosis of keratoconus/forme fruste keratoconus and false-positive diagnoses for both entities.
FOUR ILLUSTRATIVE CASES
Case No. 1
A 38-year-old white male presented with an irregular astigmatism pattern in his left eye. The right eye revealed a refraction of -4.00 +0.50 X 085 for 20/15 BSCVA. The left eye refracted to -2.75 +1.75 X 090 for 20/25 BSCVA acuity. Topography revealed inferior steepening that at first glance resembles an ectasia-like pattern in the left eye (Figure 1). The numerical map shows greater than 3.00D of inferior/superior discrepancy. The patient reports that his vision has been stable in the left eye for as long as he can remember. Is this patient at risk for ectasia? Should he receive laser vision correction? The history is key for this patient. He reports having had a corneal infection as an adolescent. Slit-lamp examination revealed an area of corneal thinning just superior to fixation with mild-to-moderate anterior stromal opacity. Central thickness just inferior to fixation was 530µm. Customized therapeutic/refractive ablation could be considered. I would prefer a surface treatment to remove the anterior opacification. To date, this patient has not undergone surgery. This case could be classified as a false-positive keratoconus-like topographic pattern.
Case No. 2
A 44-year-old white male presented with an irregular astigmatism pattern in his left eye. The right eye revealed a refraction of -2.50D of sphere for 20/15 BSCVA. The left eye refracted to -2.50 +0.75 X 100 for 20/15 BSCVA. Topography revealed inferior steepening (Figure 2). The numerical map showed greater than 3.50D of inferior/superior discrepancy. The patient reported stable vision in the left eye for the past 25 years. Is this patient at risk for ectasia? Should he receive laser vision correction, lamellar, or surface ablation? Again, the history is crucial in determining an operative course for this patient. He reports that at age 18 he sustained a perforating inferior corneal injury with a stiletto-style blade. Except for the corneal findings, there was no structural injury to the anterior segment. Slit-lamp examination did reveal evidence of injury inferiorly, but also a clear central cornea. This patient's central corneal thickness was 530µm. He underwent uneventful LASIK bilaterally and has had no ill effects for 5 years postoperatively. This case could be classified as a false-positive forme fruste keratoconus-like topographic pattern.
Case No. 3
A 22-year-old white male presented with an abnormal topographic pattern of the left eye that depicted an against-the-rule pattern with the ends of the “dumbbells” meeting at the 270º meridian. I term this occurrence the against-the-rule kissing at 270º pattern, which should be evaluated due to being a high-risk factor for ectasia. Note the inferior/superior discrepancy of 5.00D. The patient's BSCVA was 20/100. Is this keratoconus? It is not. The patient has a recent history of soft contact-lens-related corneal ulcer with tissue loss. Is he a laser vision correction candidate? Obviously, he is not at present. Note the lower right photokeratoscopy image with marked distortion (Figure 3). With several months of healing, this will likely improve dramatically. This patient may improve to the point that a spectacle or soft contact lens will give adequate vision, or he may require a rigid contact lens or even possibly a penetrating keratoplasty for visual improvement. There is an outside chance that a therapeutic surface ablative technique can be considered. At present, this patient has a false-positive keratoconus-like topographic pattern.
Case No. 4
A 43-year-old white female presented with an abnormal topography bilaterally. Each eye had a BSCVA of 20/20 with approximately -6.00 +1.00 x 90 refractions respectively. There is approximately 4.00D of inferior/superior discrepancy on the numerical map of the left eye (Figure 4). Her central pachymetry is 550µm. Is this a case of bilateral forme fruste keratoconus? Should this patient have LASIK, or is surface ablation a better option?
The topography of the patient's same eye 3 weeks after superficial keratectomy for removal of Salzmann's Nodular Degeneration is presented in Figure 5. On preoperative topography, photokeratoscopy view (Figure 4), the far peripheral mires are irregular. On the postoperative photokeratoscopy view (Figure 5) the mire irregularity is markedly reduced. Of note is that the inferior/superior curvature disparity is now zero. The forme fruste keratoconus pattern in Figure 4 was a remarkable false-positive example. The patient has had an uneventful postoperative course nearly 4 years after undergoing bilateral LASIK.
As the above cases illustrate, the paper diagnosis needs to correlate with the patient's history and a thorough examination, and these must be filtered through the mind of an experienced practitioner.
I would not recommend that true-positive diagnoses of forme fruste variants of keratoconus and pellucid marginal degeneration be treated with a lamellar corneal refractive surgical procedure. Can a surface ablative procedure be completed with safe and stable long-term results? Currently, I believe they can be stable for at least out to 10 years. With time, we will have an even a better idea of their stability.
I do believe that the prudent surgeon should leave at least 50% thickness of the central cornea untreated at a minimum and/or leave at least 250µm of the central posterior cornea untouched. Any percentage above these values would be admirable and desirable, all other variables being equal.
Preoperatively, in my experience, there are seven major areas of investigation if a forme fruste keratoconic or pellucid marginal degeneration pattern is seen during a screening examination. (1) A detailed medical history for the patient is required, including information of trauma occurrences, stability of correction, decreasing quality of vision with spectacles or soft contact lenses, etc. (2) The inferior/superior discrepancy on topography as an arbiter of increased risk of postoperative ectasia is important. If it is greater than 1.50D, I personally do not perform a lamellar procedure and convert to a surface laser technique. The inferior/superior discrepancy is, in my experience, a significantly more valid and sensitive indicator for risk of postoperative ectasia than central corneal power or central pachymetry measurements. (3) Retinoscopy, photokeratoscopy, or manual keratometry mire observation are useful adjuncts to confirm suspicion when a small yet diagnostically abnormal scissoring or fish-mouthing reflex is present. (4) Comparing the right with the left eye is useful. If one eye is significantly different than the other on topography preoperatively, this should raise concern for risk of postoperative ectasia. (5) If one compares curvature symmetry across the major meridians of an individual eye (an intra-eye comparison), this can be quite helpful in identifying a cornea at increased risk for potential ectasia. Again, asymmetry raises the specter of increased risk; notably, an against-the-rule pattern with kissing at 270º, as described earlier, can be of considerable interest. (6) If all is equal, yet the patient is 20 years old versus 50, I am much more suspicious of the younger patient that may not have fully exhibited his endpoint. (7) Last, looking at a series of data points from the habitual refraction, topography, and pachymetry can be helpful, although more often than not these are unavailable. It is more conceivable that one could obtain records of spectacle correction. A progressive increase in myopia and astigmatism should heighten concern for an increased risk of ectasia. For those using the Orbscan topographer (Bausch & Lomb, Rochester, NY), the posterior float data point can assist in making a final confirmation of diagnosis for operative planning. However, we do not know what the float data truly mean, let alone agree with what is elevated. It should be also noted that a large number of patients with high posterior floats do well postoperatively in the long term.
Finally, I would like to clarify the common use of the words ectasia and/or ectatic. I believe it can be a quite confusing if not an inappropriate word selection to term a patient's cornea ectatic when a singular topographic pattern shows an irregular pattern. In my opinion, the usage of the term ectasia should be reserved for a progressive condition, as shown by changing/worsening refractive error, worsening irregular astigmatism, and the progressive loss of BSCVA. Many patients with a stable and irregular topographic pattern are termed ectatic by some, yet the corneas, topographies, and refractions are stable through time, and the eyes exhibit good spectacle-corrected vision.
I am hopeful that this article is helpful in promoting a more in-depth evaluation when one appreciates a concerning topographic pattern. It may be that the ectasia/ectatic pattern is just that—a pattern and not an actively changing cornea with resultant visual morbidity. The appreciation of this occurrence is a subtle detail but of enormous benefit to the process of patient education and management.
John F. Doane, MD, FACS, is in private practice with Discover Vision Centers in Kansas City, Missouri, and is Clinical Assistant Professor for the Department of Ophthalmology, Kansas University Medical Center. He states that he holds no financial interest in any company, product, or concept mentioned herein. Dr. Doane may be reached at (816) 478-1230; firstname.lastname@example.org.