FP751: Scheimpflug Imaging Parameters in Forme Fruste Keratoconus and Normal Subjects: a Case Control Study

Dr. Rahul Maheshwari, Dr. Parmar Gautam Singh, Dr. Ashok Kumar Meena, Dr. Sachin Arya

Dr. Rahul Maheshwari, Dr. Gautam singh parmar, Dr. Ashok Kumar meena

                                           ABSTRACT

AIM:

To evaluate corneal parameters in normal and forme fruste keratoconus (FFK) using pentacam

DESIGN: Retrospective case control study

METHODS:

Study reviewed patients with Keratoconus in 1 eye and FFK (25 patients) in fellow eye and normal subjects (80 eyes). Subjects were evaluated with pentacam including keratometry, posterior elevation(PE), corneal thickness(CT), corneal volume, pachymetric progression index(PPI) and back difference elevation(BDE) values. The receiver operating characteristic (ROC) curves were analyzed to evaluate sensitivity and specificity.

RESULTS:

Mean keratometry, PE, PPI and BDE values were significantly high in eyes with FFK (45.16±2.36D, 11.72±7.05µm, 1.16±0.21, 16.40±5.86µm respectively) than normal subjects (43.19±1.32D, 6.67±3.78µm, 0.88±0.13, 10.05±4.86µm respectively). Mean CCT and volume was lower in FFK (510.6±21.7µm, 58.43±1.8µm³) than normal (543.51±32.14µm, 58.88±3.57µm³ ).

CONCLUSION:

These results suggest that rather relying on a single parameter, comprehensive analysis of topography, pachymetric data and PE seems to be important in diagnosing FFK.

 INTRODUCTION 

Keratoconus is a progressive corneal ectatic disorder that may have an extremely variable expression at earlier stages, with subtle signs and borderline abnormal features that are difficult to establish with certainity.^1-3

Various terms have been proposed to describe early keratoconus. The termforme fruste keratoconus was proposed for corneas that have subtle topographic characteristics that do not reach the threshold of keratoconus suspect.^1-3 Keratoconus is usually a bilateral condition but can be asymmetric.⁴ Previous studies^5-7 report that patients with unilateral keratoconus will eventually develop keratoconus in the fellow eye with the same genetic makeup. Thus, the fellow eye of patients with unilateral keratoconus has the earliest and mildest form of the disease, corresponding to the proposed definition of forme fruste keratoconus.^6-8 Long-term studies^9-10 found laser in situ keratomileusis (LASIK) to be a safe and effective procedure to correct refractive errors. However, iatrogenic corneal ectasia remains a rare but feared complication of this procedure, and undiagnosed forme fruste keratoconus is reported to be a main reason for corneal ectasia after

LASIK.^4,10-11 Although corneal topography has been used to rule out the possibility of forme fruste keratoconus before LASIK,^12,13 recent studies^14-17 suggest that screening the posterior elevation may also be important in the diagnosis of early ectatic diseases.

Differentiating between healthy and ectatic cornea is a prerequisite for several ophthalmic procedures such as refractive surgeries and orthokeratology. Patients with irregular or ectatic cornea may have poor outcome or develop progressive ectasia after refractive surgeries. It has been reported that 1–6 % of myopic candidates for corneal refractive surgeries are suspected to have or have keratoconus or other forms of corneal ectasia^18,19. To date, many studies have been conducted to determine accurate criteria for detecting early-stage keratoconus, and many indices such as posterior corneal elevation ²⁰, corneal thickness (CT) and corneal volume (CV)¹⁵, and anterior and posterior corneal aberration²¹ have been presented.

Modern instruments such as corneal topographers provide information concerning both corneal topography and pachymetry simultaneously, and therefore CV can be calculated. This is the case for the rotator slit

Scheimpflug camera system, i.e., Pentacam (Oculus Inc., Wetzlar, Germany), which provides measurements from analysis of multiple corneal sections. The aforementioned values could help clinicians to better follow up the disease and improve keratoconus staging.

The purpose of the study was to evaluate and compare corneal parameters in normal subjects and forme fruste keratoconus (FFK) using pentacam.

PATIENTS AND METHODS

This case series and study was approved by the Ethics Committee, sadguru netra chikitsalya, chitrakoot. Patients examined at the Hospital were retrospectively enrolled. All patients included in the study provided

informed consent.

Clinical keratoconus was defined as findings characteristic of keratoconus (eg, corneal topography with asymmetric bow-tie pattern with or without skewed axes) and at least 1 keratoconus sign (eg, stromal thinning, conical protrusion of the cornea at the apex, Fleischer ring, Vogt striae, or anterior stromal scar) on slitlamp examination. Patients in whom clinical keratoconus was diagnosed in 1 eye and whose fellow eye had no slitlamp or significant topography finding that would lead to a diagnosis of clinical keratoconus were included in the study. The Amsler-Krumeich classification was used to grade the keratoconus.

Control cases (normal subjects) were selected from a database of consecutive candidates for refractive surgery with normal corneas and myopia or myopic astigmatism (sphere ˂6.00 diopters [D]; cylinder ˂3.00 D). Eyes were considered normal if they had no ocular pathology, no previous ocular surgery, and no irregular corneal pattern. Of the consecutively numbered control cases, 1 eye of each patient (right eye for single numbers and left eye for even numbers) was evaluated.

Exclusion criteria were a history of corneal surgery or contact lens wear, significant corneal scarring, and significant ophthalmic disease that may potentially affect the outcomes.

All eyes had a comprehensive ocular examination. This included rotating Scheimpflug corneal tomography. During the Scheimpflug tomography examination, the patient was positioned at the instrument

with proper placement on the chinrest and forehead strap. The patient was asked to blink a few times and to open both eyes and stare at the fixation target. After proper alignment was obtained, the automatic-release mode started the scan; 25 single Scheimpflug images of each eye were captured within 2 seconds. Only cases with acceptable-quality images were included in the study. Each eye was required to have a corneal map with at least 9.0 mm of corneal coverage and no extrapolated data.

The sagittal curvature and tangential curvature maps were evaluated, and the map patterns were noted. The following anterior and posterior corneal surface parameters were evaluated with the Scheimpflug system: corneal dioptric power in the flattest meridian in the 3.0 mm central zone (flat keratometry [K] value), corneal dioptric power in the steepest meridian in the 3.0 mm central zone (steep K value), and mean corneal power in the 3.0 mm zone (mean K value). The central corneal thickness (CCT) at the apex (geometric center of the examination), the minimum corneal thickness at the thinnest point, the difference between the CCT and minimum corneal thickness, and the distance between the CCT and minimum corneal thickness were recorded. The average progression index is calculated as the progression value at the different rings, referenced to the mean curve. The minimum pachymetric progression index and maximum pachymetric progression index values and the axis were recorded with the pachymetric progression index average. Corneal volume was also determined.

The posterior elevation maps were evaluated and posterior corneal elevation values from the corneal apex analyzed. Elevation was measured in a standardized fashion relative to a reference best-fit sphere (BFS) calculated at a fixed optical zone of 9.0 mm, as previously described.

The back difference posterior elevation values were extrapolated from the difference maps of the Belin-Ambrosio enhanced ectasia display of Pentacam device. A Different than the standard elevation maps, the reference surface is the enhanced BFS rather than the standard BFS for the exclusion maps. In these maps, the BFS is calculated using all raw

elevation data located outside a 4.0mmcircle centered on the thinnest point of the cornea. This area of excluded data is called the exclusion zone, and the map is an exclusion map. The location of the exclusion zone is indicated by a 4.0 mm red circle. The difference maps (posterior and back) show the relative change in elevation from the baseline elevation map to the exclusion map, in which each point corresponds to the amount of elevation change that occurs between the baseline elevation map and the exclusion map.

A Statistical analysis was performed using SPSS software (version 11.0, SPSS, Inc.). The data were not normally distributed; therefore, the nonparametric Mann-Whitney U test (Wilcoxon rank-sum test) was used to compare each parameter between 2 groups. Receiver operating characteristic (ROC) curves were used to determine the overall predictive accuracy of the test parameters as described by the area under the curve (AUC) and to calculate the sensitivity and specificity of the parameters. A P value less than 0.05 was considered statistically significant.      

  RESULTS

Forme fruste KC group comprised of 25 patients and control group comprised of 80 eyes of 80 patients. The mean age was 26.7 years ± 7.8 (SD) (range 13.0 to 51.0 years) in FFKC patients and 29.8 ± 7.2 years (range 19.0 to 53.0 years) in controls. There was no significant difference in age between the 2 groups (P═.086).

TABLE1. DEMOGRAPHICS OF PATIENTS

PARAMETERS	FFKC GROUP	CONTROL GROUP
NO. OF CASES	25 EYES	80 EYES
MEAN AGE	26.7 years ± 7.8 years	29.8 ± 7.2  years
RANGE	13.0 to 51.0 years	19.0 to 53.0 years

   Rotating Scheimpflug Imaging

 Table 2 shows the mean keratometric, pachymetric, and posterior elevation parameters in both groups. All parameters were significantly different between  both Group  (P˂.05). Also, all parameters except the flat K value and mean K value were significantly different between Groups (P˂.05).

Table2. Comparison of Scheimpflug parameters between forme fruste keratoconus and control groups

parameter	NORMAL	FFKC	KC
Ks	42.26±1.41	44.41±1.56	48.68±3.48
Kf	43.87±1.38	43.37±1.27	45.47±4.06
Km	43.19±1.31	45.16±2.36	47.45±2.89
CCT	543.51±32.14	510.6±21.78	499.72±39.40
CTmin	539±35.08	498.8±24.33	484.21±40.09
CV	58.88±3.57	58.43±1.85	61.07±3.54
PE	6.67±3.78	11.72±7.05	46.82±21.41
BDE	10.05±4.86	16.40±5.86	42.06±20.01
PPI(av)	0.88±0.13	1.16±0.22	1.97±0.51

Mean keratometry, PE, PPI and BDE values were significantly high in eyes with FFK (45.16±2.36D, 11.72±7.05µm, 1.16±0.21, 16.40±5.86µm respectively) than normal subjects (43.19±1.32D, 6.67±3.78µm, 0.88±0.13, 10.05±4.86µm respectively). Mean CCT and volume was lower in FFK (510.6±21.7µm, 58.43±1.8µm³) than normal (543.51±32.14µm, 58.88±3.57µm³ ).                        

  DISCUSSION

The rotating Scheimpflug device we used measures posterior elevation by fitting the best-possible sphere to the posterior cornea, which could miss small protrusions on the posterior cornea, such as the ones that can be seen with early keratoconus.

To overcome this problem, a new screening tool of the rotating Scheimpflug deviced the back difference Elevation was introduced. This software creates a difference map that shows the relative change in elevation from the baseline elevation map to the exclusion map (formed by an exclusion zone of 4.0 mm centered on the thinnest point of the cornea) to identify the posterior elevation.

The manufacturer claims that a back difference elevation greater than

20 mm is suggestive of ectatic disease and a change between 10 mm and 20 mm is in the suspect zone.In our study, we found a best cutoff level of 17.2 mm for the back difference elevation to discriminate between keratoconus eyes and normal eyes and 13.2 mm to discriminate between forme fruste keratoconus eyes and normal eyes.

In our study, we found that back difference elevation (0.75 mm) was better than posterior elevation (0.68 mm) in discriminating forme fruste keratoconus eyes from normal control eyes. This may imply that back difference elevation can be a better screening parameter than posterior elevation for early detection of ectatic diseases.

In summary, detection of preclinical and early stage of keratoconus is of paramount importance in preoperative examination for refractive surgeries. To date, most of the criteria for detection and grouping of

keratoconus have been based on anterior corneal curvature. Our study indicated significant differences in PE, BDE, CT distribution and CV between normal and forme frusta keratoconus. The findings showed that these new parameters obtained from the Pentacam Scheimpflug machine could be helpful in better discriminating between normal and ectatic

cornea.

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