Association of keratoconus and Avellino corneal dystrophy

Changes in Stress-Strain Index and Corneal Biomechanics in Granular Corneal Dystrophy

Background: The aim of this study was to assess stress-strain index (SSI) and corneal biomechanical parameters in eyes with granular corneal dystrophy (GCD). Methods: This case-control study included 12 eyes of 12 patients with GCD (mean age 45.2 ± 18.7 years) and 20 eyes of 20 healthy individuals (mean age 54.4 ± 3.8 years). In addition to SSI, dynamic corneal response (DCR) parameters were assessed at the first and second applanation, including length (AL1, AL2), velocity (AV1, AV2), time (AT1, AT2), and deformation amplitude (DA A1, DA A2), and at the highest concavity (HC) phase, including DA, peak distance (PD), radius (HCR), and DA ratio (DAR 1 and 2 mm), by Corvis ST. Central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered covariates in comparing DCR parameters between the two groups. Results: SSI was statistically significantly lower in eyes with GCD than in normal eyes (p = 0.04). The corneal velocity towards the first applanation was 0.02 m/s faster in the GCD eyes AV1 (0.15 ± 0.02 vs. 0.13 ± 0.02 m/s, p < 0.001) and IR (7.48 ± 1.01 vs. 6.80 ± 1.22 mm, p = 0.003) parameters were significantly higher in the GDC group, while AT1 (7.33 ± 0.66 vs. 7.47 ± 0.36 ms, p = 0.002) and HCR (7.42 ± 0.76 vs. 8.20 ± 1.08 mm, p = 0.014) were significantly lower in the normal group. Conclusions: GCD led to a change in biomechanical properties of the cornea. SSI refers to fewer stiff corneas in GDC than normal.

Detailed phenotypic description of stromal corneal dystrophy in a large pedigree carrying the uncommon TGFBI p.Ala546Asp pathogenic variant

PURPOSE

The purpose of this study is to describe the corneal clinical spectrum and the intrafamilial phenotypic differences in an extended pedigree suffering from stromal corneal dystrophy due to the rare p.Ala546Asp mutation in TGFBI.

METHODS

A total of 15 members from a four-generation Mexican family were ascertained for clinical and genetic assessment. All individuals underwent slit-lamp biomicroscopic examination and an extensive ophthalmological examination including corneal topography (OCULUS Pentacam® AXL), corneal biomechanics (OCULUS Corvis ST), and corneal confocal biomicroscopy (Heidelberg Engineering®). A total of 10 individuals carried the heterozygous c.1637C>A (p. Ala546Asp) mutation at TGFBI exon 12.

RESULTS

Nine out of 10 mutation positive patients were available for clinical characterization. The mean age was 35.5 years, with the youngest and the eldest ones being 3 years old and 62 years old, respectively. The median age of onset of the symptoms was 19.7 years. Five (55.6%) patients presented with a predominantly granular corneal dystrophy type 2 (GCD2) phenotype, one presented with a lattice corneal dystrophy (LCD) phenotype, and one with a granular corneal dystrophy type 1 (GCD1) phenotype. Interestingly, two mutation positive subjects had no clinical deposits in the cornea, demonstrating incomplete penetrance of the disorder in this family.

CONCLUSIONS

Clinical differences in corneal phenotypes within this CD family and with other pedigrees carrying the same TGFBI genetic defect could be explained by the age of clinical examination of individual patients and/or by the presence of genetic and/or environmental modifiers.

The keratoconus enigma: A review with emphasis on pathogenesis

PURPOSE

To review the literature on the etiopathogenesis of keratoconus (KC).

METHODS

A literature search was conducted using PUBMED and Google Scholar for keratoconus. The authors analyzed epidemiology studies, reviews, and case reports.

RESULTS

Atopy and ocular surface inflammation are a common features of KC and should lead to a reconsideration of the traditional definition of KC as a "non inflammatory" condition. Co-morbidities suggest that KC may be the ocular manifestation of a systemic disease. Finally, KC shows higher prevalence in certain ethnicities, which calls into question the status of KC as a rare disease, at least in these communities.

CONCLUSION

We believe that future studies should test whether selected, high prevalence populations exhibit specific genetic background and/or ethno-specific environmental risk factors.

Familial association of keratoconus and granular corneal dystrophy: The familial case series

OBJECTIVE:

The aim of the present study was to evaluate the coexistence of bilateral keratoconus and granular corneal dystrophy (GCD) in the members of a family.

METHODS:

A total of 22 patients were examined in four generations of the family tree in this family screening study. Visual acuity test, biomicroscopic examination, and fundus examination were performed in all patients. The diagnosis of granular dystrophy was based on biomicroscopic examination findings. Corneal topography was performed on the patients diagnosed with granular dystrophy and other family members aged >5 years with normal examination findings. Corneal photographs were obtained from all patients with granular dystrophy except one case.

RESULTS:

Keratoconus or subclinical keratoconus was detected in seven cases. In addition, GCD type 1 was found in six of the seven cases. All patients diagnosed with keratoconus and granular dystrophy were females. On the other hand, there was no ophthalmologic problem in the men of the family tree. Although an autosomal dominant inheritance was found, the onset of the disease only in women suggests that there may be a variant expression.

CONCLUSION:

The present study showed an association of GCD and keratoconus in four generations of a family. More research is required to further explain this association.

Genetic Aspects of Keratoconus: A Literature Review Exploring Potential Genetic Contributions and Possible Genetic Relationships with Comorbidities

Introduction

Keratoconus (KC) is a complex, genetically heterogeneous, multifactorial degenerative disorder that is accompanied by corneal ectasia which usually progresses asymmetrically. With an incidence of approximately 1 per 2000 and 2 cases per 100,000 population presenting annually, KC follows an autosomal recessive or dominant pattern of inheritance and is, apparently, associated with genes that interact with environmental, genetic, and/or other factors. This is an important consideration in refractive surgery in the case of familial KC, given the association of KC with other genetic disorders and the imbalance between dizygotic twins. The present review attempts to identify the genetic loci contributing to the different KC clinical presentations and relate them to the common genetically determined comorbidities associated with KC.

Methods

The PubMed, MEDLINE, Google Scholar, and GeneCards databases were screened for KC-related articles published in English between January 2006 and November 2017. Keyword combinations of “keratoconus,” “risk factor(s),” “genetics,” “genes,” “genetic association(s),” and “cornea” were used. In total, 217 articles were retrieved and analyzed, with greater weight placed on the more recent literature. Further bibliographic research based on the 217 articles revealed another 124 relevant articles that were included in this review. Using the reviewed literature, an attempt was made to correlate genes and genetic risk factors with KC characteristics and genetically related comorbidities associated with KC based on genome-wide association studies, family-based linkage analysis, and candidate-gene approaches.

Results

An association matrix between known KC-related genes and KC symptoms and/or clinical signs together with an association matrix between identified KC genes and genetically related KC comorbidities/syndromes were constructed.

Conclusion

Twenty-four genes were identified as potential contributors to KC and 49 KC-related comorbidities/syndromes were found. More than 85% of the known KC-related genes are involved in glaucoma, Down syndrome, connective tissue disorders, endothelial dystrophy, posterior polymorphous corneal dystrophy, and cataract.

Genomic strategies to understand causes of keratoconus

Keratoconus (KTCN) is a degenerative disorder of the eye characterized by the conical shape and thinning of the cornea. The abnormal structure of KTCN-affected cornea results in loss of visual acuity. While many studies examine how environmental factors influence disease development, finding the genetic triggers has been a major emphasis of KTCN research. This paper focuses on genomic strategies that were implemented for finding candidate genes, including linkage and association studies, and presents different approaches of mutation screening. The advantages and limitations of particular tools are discussed based on literature and personal experience. Since etiology underlying KTCN is complex, numerous findings indicating heterogeneity of genetic factors involved KTCN etiology are presented.

Late occurrence of granular dystrophy in bilateral keratoconus: penetrating keratoplasty and long-term follow-up

We report a rare case of keratoconus with granular dystrophy with a follow-up of two decades, documenting the sequential presentation of two diseases confirmed by histology and genetic studies. A 13-year-old boy was diagnosed in 1988 with keratoconus in both eyes (BE) based on slit-lamp biomicroscopy findings of corneal ectasia in BE accompanied by Fleischer's ring, Vogt's striae, a small, old, healed hydrops. The left eye (LE) had central corneal thinning and scar in the central area involving the mid and posterior stroma secondary to healed hydrops. Penetrating keratoplasty (PKP) was advised. The boy was lost to follow-up till 1991 and presented with white, dot-like opacities in the central cornea in the RE only, suggestive of granular corneal dystrophy. Similar findings of white, dot-like opacities were noted in the LE in 1995 and the patient subsequently underwent PKP in BE. Histopathology of corneal buttons confirmed the presence of patchy, crystal-like orange deposits, which stained bright red with Masson's trichrome. Mutational analysis of the TGFBI gene in patient's DNA revealed a heterozygous mutation corresponding to a change in Arg555Trp in the keratoepithelin protein. Granular dystrophy recurred after 8 years in the RE.

Keratoconus associated with other corneal dystrophies

OBJECTIVE

To report the concomitant presentation of keratoconus and corneal dystrophies at Wills Eye Hospital for the 10-year period from January 1, 1997, to December 31, 2006.

METHODS

Patients with concomitant keratoconus and corneal dystrophies were identified using a computer database. Complete ophthalmologic examination, keratometry, pachymetry, and computerized videokeratography were performed in all patients. When present, cornea guttata were confirmed by clinical examination and specular microscopy. Histopathologic examination with special stains of excised corneal buttons was performed.

RESULTS

Fifty-one patients manifested typical signs and topographic evidence of keratoconus associated with another corneal dystrophy. Fuchs dystrophy was the most common association accounting for 27 cases (52.9%), followed by anterior basement membrane dystrophy with 13 cases (25.5%) and posterior polymorphous dystrophy with 7 cases (13.8%). A bilateral combination of Fuchs dystrophy and anterior basement membrane dystrophy with keratoconus was seen in 3 cases (5.8%). Finally, there was 1 bilateral case (2%) of granular dystrophy. Histopathologic studies in cases that underwent penetrating keratoplasty confirmed the clinical diagnoses.

CONCLUSION

To our knowledge, this is the largest report of such a concurrence in the English literature and could lead to further studies on the possible pathophysiologic or genetic links between these entities, although a chance association cannot be excluded.

A New Method for Grading the Severity of Keratoconus

PURPOSE

To define a new method for grading severity of keratoconus, the Keratoconus Severity Score (KSS).

METHODS

A rationale for grading keratoconus severity was developed using common clinical markers plus 2 corneal topographic indices, creating a 0 to 5 severity score. An initial test set of 1012 eyes, including normal eyes, eyes with abnormal corneal and topographic findings but not keratoconus, and eyes with keratoconus having a wide range of severity, was used to determine cutpoints for the KSS. Validation set 1, comprising data from 128 eyes, was assigned a KSS and compared with a clinician's ranking of severity termed the "gold standard" to determine if the scale fairly represented how a clinician would grade disease severity. kappa statistics, sensitivity, and specificity were calculated. A program was developed to automate the determination of the score. This was tested against a manual assignment of KSS in 2121 (validation set 2) eyes from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study, as well as normal eyes and abnormal eyes without keratoconus. Ten percent of eyes underwent repeat manual assignment of KSS to determine the variability of manual assignment of a score.

RESULTS

From initial assessments, the KSS used 2 corneal topography indices: average corneal power and root mean square (RMS) error for higher-order Zernike terms derived from the first corneal surface wavefront. Clinical signs including Vogt striae, Fleischer rings, and corneal scarring were also included. Last, a manual interpretation of the map pattern was included. Validation set 1 yielded a kappa statistic of 0.904, with sensitivities ranging from 0.64 to 1.00 and specificities ranging from 0.93 to 0.98. The sensitivity and specificity for determining nonkeratoconus from keratoconus were both 1.00. Validation set 2 showed kappa statistics of 0.94 and 0.95 for right and left eyes, respectively. Test-retest analysis yielded kappa statistics of 0.84 and 0.83 for right and left eyes, respectively.

CONCLUSION

A simple and reliable grading system for keratoconus was developed that can be largely automated. Such a grading scheme could be useful in genetic studies for a complex trait such as keratoconus requiring a quantitative measure of disease presence and severity.