Morphological variants in keratoconus: anatomical observation or aetiologically significant?

A correlation analysis of cone characteristics and central keratometric readings for the different stages of keratoconus

Background:

Knowledge of the cone characteristics for the different stages of keratoconus may potentially assist practitioners in diagnosing and managing keratoconic patients.

Aim:

This study aims to determine if any correlation exists between the central keratometric readings and the cone characteristics for the different stages of keratoconus.

Setting:

A university eye clinic.

Materials and Methods:

In this retrospective study, a saturated sample of 190 eyes from 106 cases of previously diagnosed keratoconic patient files was analyzed. The stage of keratoconus and cone characteristics, namely, cone location, cone decentration, topographical patterns, and morphology were analyzed using an Oculus 3M corneal topographer.

Results:

Analysis revealed a correlation between cone decentration and stage of keratoconus (P = 0.007). The association was found to exist when central K-readings were between 45D and 52D and with an apical cone decentration of 3–4 mm. No correlations were obtained for the stage of keratoconus and the cone location; topography and morphology.

Conclusion:

It can be concluded that cone apices are not central in all stages. Practitioners should consider the peripheral cornea when diagnosing and managing keratoconic patients. No correlation between stage, morphology or topography was respectively revealed.

A frequency analysis of cone characteristics for the different stages of keratoconus

Background: The stages of keratoconus can be classified according to the degree of corneal conicity as either early or advanced, or morphologically by the shape of the cone. Knowledge of the different cone characteristics for the different stages of keratoconus may assist practitioners in diagnosing and managing these patients.Aim: To describe the cone characteristics for the different stages of keratoconus.Methods: In this retrospective study, a sample of 190 eyes from 106 cases of previously diagnosed keratoconic patients was analysed. The stage of keratoconus and cone characteristics, namely: cone location, cone decentration, morphology, and topographical patterns were analysed using an Oculus 3M corneal topographer.Results: Our study revealed that for all stages of keratoconus centrally located cones were the most frequent, with corneal apical decentration between 0 mm and 2 mm. Frequency analysis revealed that nipple cones were most frequent in all stages. Topographical pattern frequencies revealed a wide distribution amongst the different corneal patterns in all stages of keratoconus.Conclusion: Cone analysis should be of consideration to the contact lens practitioner when deciding on management of the condition. It additionally serves to compliment the clinical signs such as nerve visibility, Fleischer’s ring, Vogt’s striae and corneal scarring.

The variable expression of keratoconus within monozygotic twins: Dundee University Scottish Keratoconus Study (DUSKS)

INTRODUCTION

As part of a longitudinal observational prospective study (Dundee University Scottish Keratoconus Study, DUSKS) a caucasian female subject was identified with keratoconus affecting both corneas. Of interest was that the subject's mother was a monozygotic twin, however, the mother was reportedly unaffected by keratoconus, although her sister (the subject's aunt) had been diagnosed with keratoconus.

METHOD

Computer video-keratoscopy (CVK) was undertaken was on this family to clarify this apparent discordance.

RESULTS

CVK demonstrated the variable expression of the keratoconic phenotype, by identifying evidence of subclinical keratoconus in the 'unaffected' twin.

CONCLUSION

So far only one set of monozygotic twins with keratoconus have been reported within the UK, this paper reports a second. Since CVK has been available the majority agree on concordance of keratoconus expression but not all, including this study. If monozygotic twin studies demonstrate heterogeneity of keratoconus this may at least explain the contradictory data that exists within the literature and why the inheritance pattern of keratoconus is still unclear in 2005.

The genetics of keratoconus

Keratoconus is a relatively common, bilateral, non-inflammatory corneal ectasia. The aetiology of this condition is probably multifactorial, or it represents the final common pathway for a variety of different pathological processes. Although a familial history is present only in a minority of cases, one of the major aetiological factors is certainly genetic. This is evidenced by: the condition's familial inheritance; its discordance between monozygotic and dizygotic twins; and its association with other known genetic disorders such as Down's and Marfan's syndromes. In the keratoconic cornea, a possible genetic predisposition to increased sensitivity to apoptotic mediators by keratocytes has also been hypothesized. Differences in prevalence between ethnic groups have been identified. Recent advances in computerized topographic diagnostic techniques for keratoconus, including forme fruste keratoconus, enables higher accuracy in delineating abnormal from normal, and helps define study populations for genetic linkage studies. However, genetic heterogeneity and the phenotypic diversity of keratoconus means that genetic analysis continues to be a complex process. None the less, it is foreseeable that over the next decade, improved diagnostic techniques, in combination with molecular genetics, may reveal conclusive data on the precise nature of the genetic inheritance of keratoconus in specific populations. This review considers the evidence that suggests keratoconus is primarily an inherited condition, and examines research strategies aimed at unveiling the genetic predisposition, and the enigma of environmental influences on its phenotypic expression.

RGP fitting philosophies for keratoconus

Rigid gas permeable (RGP) lenses are the choice of vision correction for most keratoconic patients. Various fitting philosophies and different lens designs are available and the question of which technique is the most appropriate is a controversial issue in the field of contact lens practice. This article reviews the different philosophies with respect to their rationale, optical performance and physiological influence on the cornea. The 'three-point-touch' approach is now the most widely-accepted corneal lens fitting philosophy in clinical practice. It provides a well-distributed pressure between the cone apex and the relatively normal peripheral cornea but other philosophies such as apical bearing and apical clearance also have advantages. Having considered the advantages and disadvantages of each philosophy, a variant of the three-point-touch appears to be a practical solution. Practitioners can fit keratoconic patients with a three-point-touch tending towards apical clearance to allow for disease progression and unexpected lack of follow-up. Wearing time, visual acuity and comfort are also important issues to be considered as all of these are essential to constitute a successful contact lens fitting.