|Year : 2010 | Volume
| Issue : 3 | Page : 263-268
Keratoconus: A review of presentation patterns
Rajesh Sinha, Nupur Gupta, Namrata Sharma, Raghav Gupta, Jeewan S Titiyal
S7, R. P. Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, Ansari Nagar, New Delhi, India
|Date of Web Publication||21-Apr-2010|
S7, R. P. Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, Ansari Nagar, New Delhi
|How to cite this article:|
Sinha R, Gupta N, Sharma N, Gupta R, Titiyal JS. Keratoconus: A review of presentation patterns. Indian J Ophthalmol 2010;58:263-8
|How to cite this URL:|
Sinha R, Gupta N, Sharma N, Gupta R, Titiyal JS. Keratoconus: A review of presentation patterns. Indian J Ophthalmol [serial online] 2010 [cited 2013 May 25];58:263-8. Available from: http://www.ijo.in/text.asp?2010/58/3/263/62672
Keratoconus (KC) is a progressive, noninflammatory, bilateral (but usually asymmetrical) ectatic corneal disease, characterized by paraxial stromal thinning and weakening that leads to corneal surface distortion. Visual loss occurs primarily from irregular astigmatism and myopia, and secondarily from corneal scarring.
Fatima et al. (Cont Lens Anterior Eye2010;33(1):19-22) studied the demographic profile and functional outcomes of 77 patients (142 eyes) with keratoconus attending a contact lens clinic at a tertiary eye care centre. Forty-nine (63%) were males and 28 (37%) were females; their median age was 24 years (15-36 years). Twenty eyes (14.4%) were diagnosed to have mild keratoconus, 51 eyes (36.7%) had moderate, 45 (32.4%) had advanced and 23 eyes (16.6%) had severe keratoconus. Visual rehabilitation was provided with RGP lenses in 113 eyes (79.5%) while 29 eyes (20.4%) fitted best with Rose-K lenses and in one patient (0.1%) Boston scleral lens was given in both eyes. They observed that in India, KC presents at an earlier age as compared to the Western population.
Yildiz et al. (Eye Contact Lens. 2009;35(6):309-11) studied demographic features of patients aged 50 years or more, diagnosed with KC. Out of a total of 697 patients with KC, 279 (40.0%) patients were 50 years of age or above. The disease was bilateral in 266 (95.3%) patients. Of the 279 patients, 167 had surgery (59.8%), 25 (9.0%) were treated with glasses in both eyes, 85 (30.5%) with contact lenses in both eyes, and two (0.7%) with glasses in one eye and contact lenses in the other eye.
Jonas et al. (Am J Ophthalmol.2009;148(5):760-5) evaluated the prevalence and associated factors of KC in the adult Indian population. KC defined as corneal refractive power of 48D or more, was detected in 212 eyes (2.3%) of 128 subjects. In multivariate analysis, the presence of KC was significantly associated with lower body height (P < .001), lower level of education (P=.03), higher myopic refractive error (P=.004), and thinner central corneal thickness (P=.006). It was not significantly associated with alcohol consumption (P =.99) or smoking (P=.08) nor with questions relating to the psychiatric status.
Sharma et al. (Cornea 2009;28(4):367-70) reviewed the profile of 120 Indian patients (76 males; 44 females) with KC. The mean age at presentation was 20.07±6.4 years with 5% revealing a positive family history. Earlier age at onset (P=0.002) and eye rubbing (P=0.02) were found significantly associated with increased risk of surgery. Patients with younger age at onset, history of eye rubbing, and atopy had higher risk of developing corneal hydrops.
Ertan et al. (Cornea 2008;27(10):1109-13) evaluated topographic findings and manifest refraction in 182 eyes of 248 patients and demonstrated an inverse correlation between age and severity of KC.
Tatematsu-Ogawa et al . (Eye Contact Lens 2008;34(1):13-6) explored the vision-related quality of life (VR-QOL) parameters in 45 patients with KC and 36 age-matched controls. All NEI-VFQ-25 subscale scores were significantly lower (P<0.05) in patients with KC than in control subjects. Subscales evaluating general health, ocular pain, and vision-specific mental health showed particularly low values.
Weed et al. (Eye (Lond).2008;22(4):534-41) provided an overview of a large population with KC highlighting presenting features and clinical and topographic progression over a four-year period. This Dundee University Scottish Keratoconus study enrolled 200 consecutive patients presenting with KC. Mean age at enrolment was 30.9±10.4 (range, 12.2-72) years with a 5% family history of KC, atopic diseases included asthma (23%), eczema (14%), and hay fever (30%). Only 9% wore contact lenses before referral. The mean simulated K1 corneal power at enrolment was 51.74±5.36 (range, 42.59-67.32) D and 88.5% exhibited bilateral KC. Forty-eight per cent of subjects reported significant eye rubbing (P=0.018).
Nielsen et al. (Acta Ophthalmol Scand. 2007;85(8):890-2) reported a prevalence of 86 KC patients per 100,000 residents and an incidence of 1.3 per 100,000 per year in Denmark.
GrAnauer-Kloevekorn et al . (Klin Monbl Augenheilkd.2006;223(6):493-502) in their review on KC reported an incidence of approximately one per 2,000 in the general population. The most common presentation of KC was as a sporadic disorder; however, patients exhibited a family history as an autosomal dominant mode of inheritance.
Barr et al. (Cornea. 2006;25(1):16-25) published their results of the multicenter Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study including 1209 KC patients. Of the 1,209 patients, 878 patients with at least one unscarred cornea at baseline were included in this study. The five-year incidence of corneal scarring was 13.7% (120 of 878) overall, 16.7% (102 of 609) for contact lens-wearing eyes, and 38.0% (46 of 121) for contact lens-wearing eyes with corneal curvature greater than 52 D. Baseline factors predictive of incident scarring included corneal curvature greater than 52 D (P <0.001), contact lens wear (P = 0.003), marked corneal staining (P = 0.0002), and age less than 20 years (P < 0.0001).Contact lens wear increased the risk of incident scarring more than twofold. These findings suggested a causal contribution of contact lens wear to corneal scarring in KC and imply that corneal scarring might be reduced by modifying the contact lens fit.
Georgiou et al . (Eye (Lond).2004;18(4):379-83) reported an incidence of 25 per 100,000 (1 in 4000) per year for Asians, compared with 3.3 per 100,000 (1 in 30,000) per year for white people (P <0.001). Asians presented at a younger age than white patients. The incidence of atopic disease was found to be significantly higher in whites.
Saini et al . (Clin Exp Optom.2004;87(2):97-101) studied age at diagnosis, keratometry and slit-lamp signs in 61 keratoconic eyes. The age at presentation was 20.2±6.4 years. Based on average keratometry, 67.2% of eyes had severe and 32.8% had moderate KC. Eyes with severe KC presented at a younger age (18.8±5.35 years) than moderate KC (23.69±8.07 years). Twenty eyes (32.7%) had apical sub-epithelial scarring of which 95% had severe KC.
Olivares et al . (Optom Vis Sci.1997;74(3):147-51) reported an average age of 15.39±3.95 years (second decade) for the onset of KC, with earlier onset in females.
Clinical features and Etiopathogenesis
Yeniad et al . (Cornea2009;28(4):477-9) reported a case of recurrent KC after penetrating keratoplasty because of allergic conjunctivitis and eye rubbing. The diagnosis of recurrent KC was confirmed on clinical findings and topography.
McMonnies et al. (Clin Exp Optom. 2003;86(6):376-84) reported association of teenage allergy, ocular itch and associated eye-rubbing with bilateral KC in 53 subjects. McMonnies (Cornea2009;28(6):607-15) in his review examined known and putative mechanisms for rubbing-related corneal trauma and cone formation in KC. Cone formation appears to depend on a loss of shear strength and may be a consequence of a reduction in ground substance viscosity and glue function, which could allow the cornea to bend and yield to intraocular pressure.
Gatzioufas et al. (J Endocrinol Invest. 2008;31(3):262-6) suggested that thyroid gland dysfunction may be associated with KC. They reported development of acute KC at the time during pregnancy which coincided with the lowest plasma T4 level.
Li et al. (Zhonghua Yan Ke Za Zhi. 2005;41(7):610-3) described the clinical features of 233 cases of KC (216 males and 17 females). The clinical natural history of these patients followed the model of "myopia-astigmatism-poor spectacle correction-acute corneal hydrops or stromal scar". The presenting age of myopia ranged from six to 29 years (mean: 13.8 years); 10.1% of the patients were younger than 10 years, 67.1% were between 11 and 15 years. The time interval between emerging myopia and poor spectacle correction (best corrected visual acuity less than 20/200) ranged from 1 to 20 years, of which 85.2% were shorter than 6 years . Twenty per cent of the patients had acute corneal hydrops in one eye at the age of 12 to 31 years, and 4.3% of them resulted in corneal perforation. Thirty-four patients had corneal stromal scars one to 13 years after onset, and 94.1% of them were within eight years. Topography examination demonstrated a tendency of the cone base position progress: first, paracentral-enlargement following dominant astigmatism axis, then, enlargement along smaller astigmatism axis, and finally, the cone base moving centrally. These eyes tend to have a deeper anterior chamber with higher incidence of longer axial length.
Totan et al. (Ophthalmology2001;108(4):824-7) reported the incidence of KC by videokeratography in patients with vernal keratoconjunctivitis (VKC). The distribution of clinical forms in 82 consecutive subjects with the diagnosis of VKC was as follows: 46.34% mixed, 43.90% palpebral, and 9.76% limbal types. Twenty-six (31.7%) of 82 subjects had complications with keratopathy such as pseudo-genontoxon, punctate keratitis, and shield ulcer. Forty-four eyes (26.8%) were detected as KC by quantitative evaluation of videokeratography maps; 14 eyes (8.5%) by biomicroscopy, and 30 eyes (18.3%) by keratometry. The increased incidence of KC was associated with male gender, longstanding disease, mixed and palpebral forms, and advanced corneal lesions.
Bawazeer et al. (Br J Ophthalmol. 2000;84(8):834-6) in their study reported an association between KC and atopy as well as eye rubbing and family history of KC. However, in the multivariate analysis, only eye rubbing was still a significant predictor of KC (odds ratio=6.31; P=0.001). This study supports the hypothesis that the most significant cause of KC is eye rubbing. Atopy may contribute to KC but most probably via eye rubbing.
KoAsak Altintas et al . (Eur J Ophthalmol. 1999;9(2):130-3) reported the possible association of KC with a multisystem autoimmune disease like Hashimoto thyroiditis.
Grewal et al. (Trans Am Ophthalmol Soc.1999;97:187-98) identified factors associated with the development of hydrops in 22 eyes. Twenty-one (95%) eyes had seasonal allergies and 20 of 22 (91%) eyes had allergy-associated eye-rubbing behavior. Six of 22 (27%) eyes had a diagnosis of Down's syndrome. Six patients were able to identify a traumatic inciting event: vigorous eye rubbing in four and traumatic contact lens insertion in two patients.
Zadnik et al. (Cornea1996;15(2):139-46) in their study reported the average age as 37 years, with 84% between 20 and 49 years. Thirteen per cent of patients had unilateral KC, defined as unilateral corneal irregularity. Penetrating keratoplasty was reported in 12.3% of eyes.
Ezra et al. (Ophthalmology2010: 22 [Epub ahead of print]) reported significant association of KC with floppy eyelid syndrome (FES) (P<0.0001). KC grading was made using the Pentacam imaging system (Oculus Optikgerate GmbH, Wetzlar, Germany).
Lin et al. (Cont Lens Anterior Eye.2010;33(1):41-2) reported the association of Pierre Robin sequence with acute unilateral hydrops and bilateral KC.
Ozcan et al. (Ann Ophthalmol (Skokie). 2007;39(2):158-60) reported association of acute corneal hydrops and KC with Down's syndrome and vigorous eye rubbing.
Casta et al. (Rev Neurol. 2004;39(11):1017-21) examined 49 patients between 40 and 62 years of age with Down's syndrome; 6.2% of the patients had KC.
Maurino et al . (Am J Ophthalmol. 2002;133(2):266-8) reported the occurrence of fixed dilated pupil and iris ischemia (Urrets-Zavalia syndrome) after anterior chamber air/gas injection after deep lamellar keratoplasty for KC.
Mojon (Ther Umsch. 2001;58(1):57-60) reported the association of KC with sleep apnea syndrome due to floppy eyelids.
Macsai et al. (Cornea1997;16(5):534-6) reported the association of KC and Turner's syndrome in three patients.
Thalasselis (J Am Optom Assoc.1995;66(8):495-9) demonstrated a statistical and physiological relation between KC, magnesium deficiency, Type A behavior, and allergy, which constitute the Thalasselis Syndrome. This syndrome suggests that the genetic disturbance that causes these alterations could be influenced by metabolic factors in which magnesium deficiency could be involved.
Ascaso et al . (Eur J Ophthalmol. 1993;3(2):101-3) reported the association of Noonan's syndrome with KC.
Fernandes et al. (Pathology.2008;40(6):623-6) studied histopathological features of 49 cases of KC. Forty of the 49 specimens (82%) presented with epithelial thinning. Other common features included breaks in Bowman's layer (71%), compaction of stromal collagen fibers (63%), and folds in Descemet's membrane [DM] (63%) cases. Other less common histopathological findings were: presence of superficial iron deposits (29%), deep stromal scarring (24%), epithelial scarring (22%), endothelial cell loss (22%), and breaks in DM (18%).
Akhtar et al. (Acta Ophthalmol.2008;86(7):764-72) investigated the ultrastructural alterations in the distribution of collagen fibrils (CFs) and proteoglycans (PGs) in the KC cornea. In severe KC, stromal lamellae were seen to undulate in most regions, whereas in mild KC only the middle and posterior lamellae were affected. In KC corneas the mean diameter and interfibrillar spacing of CFs was reduced in all zones (P < 0.0001) and the CF and PG number density and area fractions were significantly increased (P < 0.0001) compared to normal corneas and were higher (P < 0.0001) in the corneas with severe KC than in those with mild KC. This suggests that as KC progresses, the PG content of the stroma increases, whereas fibril diameter is reduced.
BystrAm et al . (Histochem Cell Biol.2007;127(6):657-67) suggested that laminin chains participate in the process of corneal scarring in KC.
Matthews et al . (Exp Eye Res. 2007;84(6):1125-34) reported that significantly more apoptotic cells were identified in the anterior stroma of keratoconic corneas than normal corneas.
Samimi et al . (J Cataract Refract Surg. 2007;33(2):247-53) evaluated the histopathological changes induced in keratoconic corneas after implantation of Intacs intracorneal ring segments. Conventional histology showed hypoplasia of the epithelium immediately surrounding the channel. There was no evidence of an inflammatory response or foreign-body granuloma. Keratocyte density was decreased above and below the tunnel, and collagen IV synthesis was seen in the scar area.
Stabuc-Silih et al. (Mol Vis. 2009;15:2848-60) studied the alterations in collagen Type IV, alpha-3 (COL4A3) and collagen Type IV, alpha-4 (COL4A4) genes that may be responsible for a decrease in collagen Types I and III, a feature often detected in KC. They detected eight polymorphisms in the COL4A3 gene and six in the COL4A4 gene. Allele differences in D326Y in COL4A3 and M1237V and F1644F in COL4A4 are significantly distinctive of KC patients (Fisher's exact test, P<0.05). Their data indicates a probability that polymorphisms in COL4A3 and COL4A4 genes are associated with KC.
Lee et al. (Mol Vis. 2009;15:2480-7) identified the differentially expressed genes (DEGs) in the human keratocytes in KC. Eight genes (overexpression of bone morphogenetic protein 4 (BMP4), cofilin 1 (CFL1), and JAW1-related protein (MRVI1) and underexpression of actin, alpha 2 (ACTA2), gene rich cluster, and C10 gene (GRCC10), tissue inhibitor of metalloproteinase 3 (TIMP3), tissue inhibitor of metalloproteinase 1 (TIMP1), and somatostatin receptor 1 (SSTR1)) were identified to be differentially expressed in KC and related with apoptosis, the cytoskeleton, wound healing, and nerve fibers. The genes identified may be involved in the mechanism underlying stromal thinning in KC.
Mok et al. (J Hum Genet. 2008;53(9):842-9) investigated the possibility of visual system homeobox 1 (VSXI) as a candidate susceptibility gene for Korean patients with KC. They found two heterozygous novel missense mutations in exon 2: N151S and G160V. The G160V mutation was identified in 13 KC patients (5.3%), and the N151S mutation was found in only one KC patient (0.4%). VSX1 gene variants seem to be significant genetic variants for KC predisposition in unrelated Korean patients.
Olofsson et al. (Mol Vis. 2007;13:1285-90) studied regulation of corneal extracellular superoxide dismutase (SOD3) synthesis in KC. Interleukin-1alpha had an inhibitory effect on SOD3 synthesis exclusively in the KC cultures (P<0.01). Cultured keratoconic stromal cells respond with a reduced SOD3 synthesis to interleukin-1alpha, which is not the case in corresponding normal or bullous keratopathy cells.
Udar et al. (Cornea 2009;28(8):902-7) studied the role of SOD1 haplotypes in familial KC and reported the uniqueness of seven-base deletion in intron 2 of the SOD1 gene to the KC phenotype.
McMahon et al. (Invest Ophthalmol Vis Sci. 2009;50(7):3185-7) suggested that patients with Leber congenital amaurosis demonstrating a CRB1 mutation may have a particular susceptibility to developing KC.
Gajecka et al. (Invest Ophthalmol Vis Sci. 2009;50(4):1531-9) localized a novel gene for KC to a 5.6-Mb interval on 13q32 chromosome.
Eran et al. (Ophthalmic Genet. 2008;29(2):53-9) identified the genetic defect associated with KC in an Ashkenazi Jewish family in the form of mutational analysis of the VSX1 gene by direct sequencing of polymerase chain reaction (PCR)-amplified exons, and a BseR1 restriction assay. Sequencing revealed a previously described missense mutation (D144E).
Aldave et al. (Cornea 2007;26(8):963-5) evaluated the role of the COL8A1 and COL8A2 genes in the pathogenesis of KC through mutation screening in affected patients. Screening of COL8A1 in KC patients revealed a previously identified single nucleotide polymorphism (SNP; c.1850C>T; Pro535Pro), in one patient. Screening of COL8A2 in KC patients revealed seven previously described SNPs: c.14G>A (Gly3Arg); c.112G>A (Ala35Ala); c.1012C>GLeu335Leu); c.1308G>A (Arg434His); c.1492G>A (Gly495Gly); c.1512C>T (Thr502Met); and c.1765C>T (Pro586Pro). Four novel sequence variants were also identified, each in one affected patient: c.38_40dupCTG (Leu11dup), also identified in an unaffected relative of the affected proband, c.667G>A (Gly220Gly), c.1588G>A (Pro527Pro), and c.2026C>T (Val673Val). None of the three novel synonymous substitutions identified in COL8A2 was predicted to produce a splice acceptor site. The absence of pathogenic mutations in COL8A1 and COL8A2 in patients with KC indicates that other genetic factors are involved in the pathogenesis of this corneal ectatic disorder.
Tang et al. (Genet Med. 2005;7(6):397-405) reported a novel locus for KC on Chromosome 5q14.3-q21.1 through genome wide linkage scan in a multi-generation Caucasian family. Fine mapping by testing an additional 11 microsatellite markers at 1 to 3 cM intervals revealed a narrower and higher peak (99-119 cM) with LOD 3.53. By analysis of recombination of haplotypes, the putative locus was further narrowed to a 6 cM region.
Hutchings et al. (J Med Genet. 2005;42(1):88-94) identified a new locus for isolated familial KC at 2p24.
Brancati et al. (J Med Genet. 2004;41(3):188-92) reported a locus for autosomal dominant KC to human Chromosome 3p14-q13.
Tyynismaa et al . (Invest Ophthalmol Vis Sci. 2002;43(10):3160-4) mapped the disease locus in 20 Finnish families with autosomal dominant KC to Chromosome 16q, between the markers D16S2624 and D16S3090, with a maximum parametric multipoint LOD score of 4.10 and corresponding nonparametric score of 3.27 (NPL, P = 0.00006). The results suggest that the causative gene in KC is located within the 16q22.3-q23.1 chromosomal region.
Wang et al. (Am J Med Genet.2000;93(5):403-9) in their review on the genetic epidemiology of KC demonstrated an estimated KC prevalence in first-degree relatives of 3.34%.
Bisceglia et al. (Invest Ophthalmol Vis Sci. 2000;50(3):1081-6) observed that the chromosomal regions 5q32-q33, 5q21.2, 14q11.2, 15q2.32 exhibited the strongest evidence of linkage.
Tai et al. (Cornea2009;28(5):589-93) identified stromal thinning, epithelial basement membrane abnormalities, and focal disruption of Bowman's layer and stromal amyloid deposits in corneal buttons of patients of KC.
Mootha et al. (Mol Vis.2009;15:706-12) identified differentially expressed genes in KC corneal fibroblasts. Protein levels of selected differentially expressed genes were further examined by immunohistochemistry. Microarray analysis revealed up to a 212-fold reduction in the mRNA levels of alcohol dehydrogenase (Class 1) beta polypeptide (ADH1B) in KC fibroblasts (P=0.04). Decreased alcohol dehydrogenase in KC corneal fibroblasts represents a strong marker and possible mediator of KC.
Chwa et al. ( Biochem Biophys Res Commun. 1996;224(3):760-4) opined that Type VI collagen may be degraded or modified by the increased gelatinase A activity in KC.
Fukuchi et al. (Arch Ophthalmol. 1994;112(10):1368-74) observed elevated lysosomal enzyme levels in the epithelium of corneas with KC, implicating its role in the disease.
Fontes et al. (Ophthalmology2010 Feb 4. [Epub ahead of print]) compared the corneal hysteresis (CH), corneal resistance factor (CRF),spherical equivalent (SE), average central keratometry (K-Avg), corneal astigmatism (CA), corneal volume (CV), anterior chamber (AC) depth, and central corneal thickness (CCT) between patients with mild KC and healthy controls and to estimate the sensitivity and specificity of CH and CRF in discriminating mild KC from healthy corneas . The values for CH, CRF, CV, and CCT were statistically lower and those for SE, K-Avg, CA, and AC depth were statistically higher in patients with mild KC compared with controls. Corneal hysteresis and CRF were poor parameters for discriminating between mild KC and normal corneas.
MihAiltz et al. (Ophthalmology2010;117(1):41-8) evaluated the ocular wavefront aberrations owing to KC compared with normal controls and to describe the changes in the axis of line of sight (LoS) among keratoconic patients . Ocular aberrations over a 4.5 mm (undilated) pupil were measured with a Hartmann-Shack sensor. Corneal topographic measurement was performed with a Tomey corneal topographer. The axis of the displacement of LoS was calculated by vector analysis. In keratoconic patients there was a significant correlation between the axis of the shift in the LoS and the steepest keratometric axis on topography (r = 0.59; P<0.001), the distance of the LoS from pupil center and vertical coma (r = -0.39; P = 0.004), and spherical aberration (SA; r = 0.29; P<0.04). There was also a significant correlation between the average keratometry value measured by topography and SA (r = -0.49; P<0.001). A significant displacement of the LoS was observed in KC and that related to the position of the cone on topography and the induced vertical coma measured by aberrometry.
Tu et al. (J Refract Surg. 2009;25(8):715-22) analyzed anterior corneal elevation changes on Orbscan II following corneal collagen cross-linking (CXL) with riboflavin in eight patients (14 eyes) with KC. On preoperative maps, distances from maximum anterior elevation to pupil center and to topographic geometric center were compared between the two patterns identified: (1) paracentral steepening, no change, or flattening centrally; and (2) central steepening. Mean maximum topographic simulated keratometry decreased (P=.004) and mean irregularity indices at 3 mm (P=.03) and 5 mm (P=.04) were reduced postoperatively in Pattern 1 eyes; all increased in Pattern 2 eyes. Corneal shape change influenced by anisotropy of collagen distribution is a factor in the outcome of CXL treatment for KC.
Lema et al. (Eye Contact Lens. 2009;35(2):65-8) observed that corneal wavefront indices (root mean square and vertical coma) exhibited the best performance for discriminating between controls and the fellow eye of unilateral KC patients. Corneal curvature and thickness indices showed poor capability to differentiate between the groups.
Wolf et al. (Eur J Ophthalmol.2009;19(1):10-7) studied the mild topographic abnormalities on Scheimpflug imaging to rule out forme fruste keratoconus (FFKC). While in placido-based topography evaluation of corneal topography did not show a clear FFKC, the evaluation of corneal topography on Scheimpflug imaging together with the data of spatial corneal thickness revealed distinctive FFKC in all cases presented.
Nakagawa et al. (Invest Ophthalmol Vis Sci.2009;50(6):2660-5) investigated higher-order aberrations (HOAs) due to the posterior corneal surface in keratoconic eyes compared with normal eyes recorded with a rotating Scheimpflug camera. The mean total corneal HOAs (root mean square [RMS]) from the anterior/posterior surfaces were significantly (P < 0.001) higher in keratoconic (4.34/1.09, respectively) than in control eyes (0.46/0.15). Corneal HOAs on both corneal surfaces in keratoconic eyes were higher than in control eyes. Coma from the posterior surface compensated partly for that from the anterior surface. Residual irregular astigmatism in patients with KC wearing rigid gas-permeable contact lenses can be estimated by measuring the HOA from the posterior corneal surface.
Patel et al . (Eye (Lond). 2009;23(3):586-92) reported decreased corneal innervation, sensation, and basal epithelial density in KC.
PiAero et al. (Invest Ophthalmol Vis Sci. 2009) reported significantly lower values of CH and CRF as measured by the Ocular Response Analyzer (ORA) in severe KC in comparison to mild and moderate KC. A significant difference in the CRF was found between mild and moderate cases (P=0.04). Multiple regression analysis revealed significant correlation of CRF with keratometry and the corneal spherical-like RMS (R2=0.40, P<0.01).
PiAero et al. (Clin Exp Optom.2009;92(3):297-303) evaluated the anterior and posterior corneal aberrations provided by the Pentacam system in normal and early to moderate keratoconic eyes. Statistically significant differences were found in all anterior aberrometric parameters (all P < 0.02), except for horizontal primary and secondary coma Zernike terms (P = 0.61 and 0.72). Regarding posterior corneal surface, statistically significant differences among groups were found in primary spherical aberration, primary vertical coma, and coma RMS and coma-like RMS (all P < 0.01).
Holladay (J Refract Surg. 2009;25(10 Suppl):S958-62) reviewed the topographic patterns associated with KC suspects and provided criteria using maps from the NIDEK OPD-Scan II and OPD Station for KC screening. Five criteria are listed for the detection of KC: 1) apex of the cone is not centered at the 6-o'clock semi-meridian, 2) cone should appear round on the tangential map, 3) keratometry >45.00 diopters, 4) corneal thickness at the apex of the cone is approximately 30 micron thinner than the corresponding distance above the pupil center, and 5) topographic patterns are not symmetric.
Ertan et al. (J Refract Surg. 2009;25(11):1012-6) evaluated the topographic patterns in keratoconic cases by Pentacam and noted that the most frequent topographic patterns were vertical bowtie pattern (28.4%) in the younger group, inferior global cone pattern (23.7%) in the middle group (20-40 years), and inferotemporal global cone pattern (16.4%) in the older group (>40 years). Temporal global cone pattern was more frequently seen among younger (<20 years) KC patients. In KC, the steepest part of the cone can be located temporally, especially in younger patients, which is unusual.
Belin et al. (Clin Experiment Ophthalmol. 2009;37(1):14-29) in their review highlighted that elevation-based Scheimpflug imaging has advantages in that it allows for the measurement of both the anterior and posterior corneal surfaces. Posterior measurements are often the first indicators of future ectatic disease, in spite of completely normal anterior curvature.
Schweitzer et al. (Invest Ophthalmol Vis Sci. 2009 Nov 11) reported significantly lower CH (P<0.001) and CRF (P<0.001) values by ORA in FFKC.
Steele et al. (Clin Experiment Ophthalmol. 2008;36(9):824-30) assessed the prevalence of Orbscan II corneal abnormalities in relatives of patients with KC. Four Orbscan II-derived corneal parameters (average keratometry ≥47.2 D), Inferior-Superior [I-S] value (≥1.2 D), posterior float apex (≥42 μ) and thinnest pachymetry (≤463 μ) were examined in relations of individuals with KC and a control group of low myopes (<2.5 D). Of 178 eyes from relatives of patients with KC, 45 (25.3%) had one or more keratoconic traits.
Haque et al. (Optom Vis Sci.2008;85(10):E963-76) measured corneal and epithelial thickness across four meridians using Optical Coherence Tomography (OCT) and compared these measurements between normal non-lens wearers (NLW), rigid gas permeable (RGP) lens wearers, and RGP-wearing keratoconics (KC). Central corneal thickness (CCT) was thinnest in KC (447 ±68 μ) and similar between RGP (518 ±32 μ; pKC < 0.001) and NLW (517 ±21 μ) (p(KC) < 0.001 NLW pRGP > 0.05) . KC epithelium was thinnest in the inferior temporal meridian (42 ±5 μ). Thickness of the normal cornea and epithelium was greatest in the superior region. In all groups, the inferior cornea and epithelium was thinnest, and to a greater extent in the KC group.
Mollan et al. (Br J Ophthalmol. 2008;92(12):1661-5) compared various methods of intraocular pressure (IOP) measurement in eyes with KC and concluded that the Pascal dynamic contour tonometer (DCT) and Reichert ocular response analyzer (ORA) are currently the most appropriate tonometers for such eyes.
Chen et al. (Invest Ophthalmol Vis Sci. 2008;49(12):5645-52) characterized posterior corneal aberrations in keratoconic eyes. The central 6-mm diameter of both anterior and posterior corneal topographies was decomposed into Zernike polynomials. Significantly larger amounts of posterior corneal aberrations and stronger compensation effects were observed in KC eyes than in normal eyes.
Kim et al. (J Refract Surg. 2008;24(6):600-5) observed the progression of KC over three years using Orbscan II, and estimated risk correlations with rapid progressive changes. They showed that eye rubbing and inferior steepening compared to superior steepening were associated with progression after diagnosis (P<.05).
Shankar et al. (J Cataract Refract Surg. 2008;34(5):727-34) assessed the repeatability of corneal wavefront aberrations derived from Pentacam in 45 normal and 10 keratoconic participants. Wavefront aberrations calculated from Pentacam corneal topography were large in magnitude, and reliability was poor, largely due to variability in corneal elevation data.
Mahmoud et al. (Cornea2008;27(4):480-7) developed an index for the detection of keratoconic patterns in corneal topography maps from multiple devices. The strongest significant sole predictor in stepwise logistic regression was CLMI, which is cone location and magnitude index calculated from axial data.
de Sanctis et al. (Ophthalmology2008;115(9):1534-9) estimated the sensitivity and specificity of posterior corneal elevation in discriminating KC and subclinical KC from normal corneas. Mean posterior corneal elevation was statistically higher in KC, and subclinical KC versus normal corneas.
Koller et al. (Ophthalmology2006;113(12):2198-202) evaluated the efficacy of customized surface ablation in cases of FFKC. Eleven eyes of eight contact lens-intolerant patients with FFKC were treated with topography-guided ablation by means of a scanning spot excimer laser. Statistically significant reduction of manifest refractive error, corneal irregularity, and ghosting was noted.
Hollingsworth et al. (Cornea2005;24(4):397-405) compared the morphologic features of KC as observed in vivo with a slit scanning confocal microscope (CM) and in vitro using light microscopy. Slit scanning CM was used to evaluate the central cornea of 29 keratoconic subjects (mean age, 31 ±10 years; range, 16-49). Light microscopy (LM) was performed on two of the keratoconic corneas post keratoplasty. With CM, the epithelium appeared more abnormal with increasing severity of KC. In severe disease, the epithelium displayed the following characteristics: superficial cells were elongated and spindle-shaped, wing cell nuclei were larger and more irregularly spaced, and basal cells were flattened. These findings were confirmed by LM. Images obtained using CM revealed disruption to Bowman's layer and the occasional presence of epithelial cells and stromal keratocytes. This was shown with LM to be due to breaks in Bowman's layer. Stromal haze and hyperreflectivity observed with CM corresponded with apical scarring seen on slit-lamp biomicroscopy. Increasing levels of haze detected with CM were found with LM to be due to fibroblast accumulation and irregular collagen fibers. Descemet's membrane appeared normal with both CM and LM. Evidence of endothelial cell elongation was apparent in one subject with CM.
Levy et al. (Ophthalmology2004;111(5):867-74) analyzed the videokeratographic anomalies in familial KC. Qualitative (using a 0.5-diopter [D] increment scale) and quantitative analyses of videokeratographs demonstrated two corneal patterns: the J and inverted-J form patterns. Results of the quantitative analysis of these suspect patterns showed that the inferior-superior values (reflecting the inferior-superior dioptric asymmetry) were close to 0.8 D and the Srax (relative skewing of the steepest radial axes) was superior to 21 degrees.