Cholesterol accumulation in human cornea: evidence that extracellular cholesteryl ester-rich lipid particles deposit independently of foam cells

Prevalence of corneal arcus and associated factors in a German population-Results from the Gutenberg Health Study

Purpose

We aimed to determine the prevalence of corneal arcus and to identify associated factors in the general population of Germany.

Methods

The Gutenberg Health Study (GHS) is a population-based cohort study in Germany, which includes an ophthalmological assessment. Refraction, distance-corrected visual acuity, non-contact tonometry and anterior segment imaging were performed for the five-year follow-up examination. Anterior segment photographs were graded for the presence of corneal arcus. Prevalence estimates were computed, and multivariable logistic regression analysis was applied to determine associated factors for corneal arcus including sex, age, spherical equivalent, central corneal thickness, intraocular pressure (IOP), socio-economic status, smoking, BMI, systolic and diastolic arterial blood pressure, HbA1c, HDL-C, LDL-C, triglyceride, and lipid modifying agents.

Results

A total of 9,850 right and 9,745 left eyes of 9,858 subjects (59.2±10.8 years), 49.0% females were included in this cross-sectional analysis. 21.1% of men (95%-CI: 20.0%– 22.3%) had a corneal arcus in at least one eye, and 16.9% (95%-CI: 15.9%– 18.0%) of women. In multivariable analyses, the presence of corneal arcus was associated with male gender (OR = 0.54 for female, p<0.0001), higher age (OR = 2.54 per decade, p<0.0001), smoking (OR = 1.59, p<0.0001), hyperopia (OR = 1.05 per diopter, p<0.0001), thinner cornea (OR = 0.994 per μm, p<0.0001), higher IOP (OR = 1.02, p = 0.039), higher HDL-C-level (OR = 2.13, p<0.0001), higher LDL-C-level (OR = 1.21, p<0.0001), and intake of lipid modifying agents (OR = 1.26, p = 0.0001). Arcus was not associated with socio-economic status, BMI, arterial blood pressure, and HbA1c.

Conclusions

Corneal arcus is a frequent alteration of the cornea in Germany and is associated with ocular parameters and systemic parameters of dyslipidemia.

LCAT, ApoD, and ApoA1 Expression and Review of Cholesterol Deposition in the Cornea

Lecithin:cholesterol acyltransferase (LCAT) is an enzyme secreted by the liver and circulates with high-density lipoprotein (HDL) in the blood. The enzyme esterifies plasma cholesterol and increases the capacity of HDL to carry and potentially remove cholesterol from tissues. Cholesterol accumulates within the extracellular connective tissue matrix of the cornea stroma in individuals with genetic deficiency of LCAT. LCAT can be activated by apolipoproteins (Apo) including ApoD and ApoA1. ApoA1 also mediates cellular synthesis of HDL. This study examined the expression of LCAT by epithelial cells, keratocytes, and endothelial cells, the cell types that comprise from anterior to posterior the three layers of the cornea. LCAT and ApoD were immunolocalized to all three cell types within the cornea, while ApoA1 was immunolocalized to keratocytes and endothelium but not epithelium. In situ hybridization was used to detect LCAT, ApoD, and ApoA1 mRNA to learn what cell types within the cornea synthesize these proteins. No corneal cells showed mRNA for ApoA1. Keratocytes and endothelium both showed ApoD mRNA, but epithelium did not. Epithelium and endothelium both showed LCAT mRNA, but despite the presence of LCAT protein in keratocytes, keratocytes did not show LCAT mRNA. RNA sequencing analysis of serum-cultured dedifferentiated keratocytes (commonly referred to as corneal stromal fibroblasts) revealed the presence of both LCAT and ApoD (but not ApoA1) mRNA, which was accompanied by their respective proteins detected by immunolabeling of the cultured keratocytes and Western blot analysis of keratocyte lysates. The results indicate that keratocytes in vivo show both ApoA1 and LCAT proteins, but do not synthesize these proteins. Rather, keratocytes in vivo must take up ApoA1 and LCAT from the corneal interstitial tissue fluid.

Is Corneal Arcus Independently Associated With Incident Cardiovascular Disease in Asians?

PURPOSE

To examine the longitudinal relationship between baseline corneal arcus (CA) and incident cardiovascular disease (CVD) in ethnic Indian and Malay adults in Singapore.

DESIGN

Population-based cohort study.

METHODS

Indian and Malay adults aged 40-80 years were recruited for baseline and 6-year follow-up visits between 2004-2009 and 2010-2015, respectively (follow-up response rate 73.9%). CA was assessed by ophthalmologists using slit-lamp biomicroscopy. The main outcome was self-reported incident CVD, defined as new myocardial infarction, angina pectoris, or stroke, which developed between baseline and follow-up. Multivariable logistic regression models assessed independent associations between baseline CA and incident CVD, adjusting for traditional CVD risk factors including age, sex, serum cholesterol, hypertension, diabetes, and smoking. We further conducted sex-stratified analyses to identify possible effect modifications.

RESULTS

Of the total 3637 participants (overall mean [SD] age: 56 [9] years, 46% male) with available follow-up data, without history of CVD at baseline, 208 (5.7%) incident CVD cases were reported. Participants with CA were more likely to have incident CVD (7.5%) than those without (4.9%). After controlling for traditional CVD risk factors, CA was independently associated with incident CVD (odds ratio [95% confidence interval]: 1.52 [1.07-2.16]) in adjusted models. In sex-stratified models, associations between CA and incident CVD were seen in men (1.73 [1.12-2.67]) and not in women (1.05 [0.56-1.97]).

CONCLUSIONS

CA is associated with incident CVD, independent of serum lipids and traditional CVD risk factors, in ethnic Malay and Indian men. Our finding suggests that CA is an additional observable indicator of CVD in men.

Corneal Biomechanical Properties in Patients With Arcus Senilis

PURPOSE

To compare the corneal biomechanical properties and intraocular pressure (IOP) levels in patients with and without arcus senilis (AS).

METHODS

Ocular response analyzer measurements were performed on the right eyes of 37 patients with AS (group 1) and 37 control eyes (group 2). Corneal hysteresis, corneal resistance factor, Goldmann-correlated IOP, and corneal compensated IOP were recorded with Ocular response analyzer. Spherical equivalent value of the refractive errors, axial length, central corneal thickness, and IOP measured with Goldmann applanation tonometer were noted for each study eyes. Statistical analyses were performed with Student t, Kruskal-Wallis, and Pearson correlation tests.

RESULTS

Mean age was 67.6 ± 9.8 years in group 1 and 65.3 ± 8.1 years in group 2 (P = 0.308). Mean corneal hysteresis and corneal resistance factor readings were 9.8 ± 0.9 versus 10.6 ± 0.8 (P < 0.001) and 10.05 ± 1.07 versus 10.9 ± 0.9 (P < 0.001) in groups 1 and 2, respectively. Mean corneal compensated IOP and Goldmann-correlated IOP values were found as 16.1 ± 3.3 mm Hg versus 15.8 ± 2.6 mm Hg (P = 0.719) and 15.1 ± 3.3 mm Hg versus 15.0 ± 2.6 mm Hg (P = 0.912) in groups 1 and 2, respectively. There was no statistical difference in IOP measured with Goldmann applanation tonometer, central corneal thickness, spherical equivalent value of the refractive error, axial length measurements, and mean keratometry readings between the 2 groups (P = 0.983, P = 0.289, P = 0.938, P = 0.886, P = 0.07, respectively).

CONCLUSIONS

The mean corneal hysteresis and corneal resistance factor values of eyes with AS were lower when compared with the controls. This study demonstrated that AS may change the corneal biomechanical properties.

Visible aging signs as risk markers for ischemic heart disease: Epidemiology, pathogenesis and clinical implications

Association of common aging signs (i.e., male pattern baldness, hair graying, and facial wrinkles) as well as other age-related appearance factors (i.e., arcus corneae, xanthelasmata, and earlobe crease) with increased risk of ischemic heart disease was initially described in anecdotal reports from clinicians observing trends in the physical appearance of patients with ischemic heart disease. Following these early observations numerous epidemiological studies have reported these associations. Since the prevalences of both visible aging signs and ischemic heart disease have a strong correlation with increasing age, it has been extensively debated whether the observed associations could be entirely explained by a common association with age. Furthermore, the etiologies of the visible aging signs are rarely fully understood, and pathophysiological explanations for these associations remain controversial, and are mostly speculative. As a consequence of inconsistent findings and lack of mechanistic explanations for the observed associations with ischemic heart disease, consensus on the clinical importance of these visible aging signs has been lacking. The aim of this review is for each of the visible aging signs to (i) review the etiology, (ii) to discuss the current epidemiological evidence for an association with risk of ischemic heart disease, and (iii) to present possible pathophysiological explanations for these associations. Finally this review discusses the potential clinical implications of these findings.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

Corneal arcus is a sign of cardiovascular disease, even in low-risk persons

PURPOSE

To examine the association of corneal arcus to cardiovascular disease (CVD) in an adult, ethnic Indian population.

DESIGN

Population-based cross-sectional study.

METHODS

Population-based study of ethnic South Asian Indians 40 to 80 years of age in Singapore from June 2007 through March 2009. We obtained a 75.5% response rate (3397/4497). All participants underwent standardized interview and systemic and ocular examinations, followed by nonfasting blood sampling. Corneal arcus was detected using a standardized slit-lamp examination. The main outcome measure was CVD, defined from a self-reported history of previous myocardial infarction, angina, or stroke.

RESULTS

Corneal arcus, found in 1701 (50.1%) of 3397 participants, was associated with older age (odds ratio [OR], 3.07; 95% CI, 2.78 to 3.40; P < .001), male gender (OR, 2.17; 95% CI, 1.81 to 2.62; P < .001), higher levels of total cholesterol (OR, 1.14; 95% CI, 1.05 to 1.24; P = .002), hypertension (OR, 1.14; 95% CI, 1.05 to 1.24; P = .013), and cigarette smoking (OR, 1.59; 95% CI, 1.25 to 2.03; P < .001). Corneal arcus was associated with CVD (OR, 1.31; 95% CI, 1.02 to 1.7; P = .0038) independent of the above-named cardiovascular risk factors. Participants with low-risk Framingham scores were more likely to be associated with CVD if they had corneal arcus (men: OR, 2.02; 95% CI, 1.20 to 3.40; P = .008; women: OR, 2.78; 95% CI, 1.36 to 3.01; P < .001). Corneal arcus was associated with CVD independent of the Framingham score (men: Akaike information criterion, 1524.39 for Framingham Score and corneal arcus vs 1527.38 for Framingham Score alone; women: 1000.14 vs 1003.54, respectively).

CONCLUSIONS

Corneal arcus is associated with CVD, independent of risk factors in ethnic Indian adults, even in those at low risk for vascular disease.

Cardiovascular risk factors, inflammation, and corneal arcus: the singapore malay eye study

PURPOSE

To examine the relationship of corneal arcus with cardiovascular risk factors and inflammation in Malay adults living in Singapore.

DESIGN

Population-based cross-sectional study.

METHODS

A total of 3280 Malays aged 40-80 years (out of 4168 eligible participants; 78.7% response rate) had a standardized interview, systemic and ocular examinations, and laboratory investigations, including measurement of C-reactive protein (CRP), chronic kidney disease, and peripheral artery disease. Corneal arcus was defined from anterior segment images taken with a slit-lamp camera.

RESULTS

Corneal arcus was seen in 2345 out of 3260 participants who had anterior segment images (73.2%). After adjustment for age, gender, total cholesterol, serum glucose, and current smoking, many cardiovascular risk factors significantly associated with corneal arcus, including male gender (odds ratio [OR] 1.65, 95% confidence interval [CI] 1.27-2.03), older age (per 10 years, OR 4.49, 95% CI 3.91-5.15), higher body mass index (per kg/m(2), OR 1.02, 95% CI 1.00-1.04), higher levels of CRP (per 10 mg/L, OR 1.36, 95% CI 1.13-1.64), total cholesterol (per mmol/L, OR 1.21, 95% CI 1.11-1.32), low-density lipoprotein cholesterol (per mmol/L, OR 1.94, 95% CI 1.38-2.74), presence of peripheral artery disease (OR 3.85, 95% CI 1.29-11.5), chronic kidney disease (OR 1.14, 95% CI 1.03-1.38), and current smoking (OR 1.29, 95% CI 1.02-1.69).

CONCLUSIONS

This study confirms known associations of traditional cardiovascular risk factors with corneal arcus in an Asian population. Additionally, corneal arcus may be associated with systemic inflammatory markers, peripheral artery disease, and chronic kidney disease.

Significance of lipid mediators in corneal injury and repair

Corneal injury induces an inflammatory reaction and damages the sensory nerves that exert trophic influences in the corneal epithelium. Alterations in normal healing disrupt the integrity and function of the tissue with undesirable consequences, ranging from dry eye and loss of transparency to ulceration and perforation. Lipids play important roles in this complex process. Whereas lipid mediators such as platelet activating factor (PAF) and cyclooxygenease-2 metabolites contribute to tissue damage and neovascularization, other mediators, such as the lipoxygenase (LOX) derivatives from arachidonic acid, 12- and 15-hydroxy/hydroperoxyeicosatetraenoic acids, and lipoxin A4, act as second messengers for epidermal growth factor to promote proliferation and repair. Stimulation of the cornea with pigment epithelial derived factor in the presence of docosahexaenoic acid gives rise to the synthesis of neuroprotectin D1, a derivative of LOX activity, and increases regeneration of corneal nerves. More knowledge about the role that lipids play in corneal wound healing can provide insight into the development of new therapeutic approaches for treating corneal injuries. PAF antagonists, lipoxins, and neuroprotectins can be effective therapeutic tools for maintaining the integrity of the cornea.

Aging, age-related macular degeneration, and the response-to-retention of apolipoprotein B-containing lipoproteins

The largest risk factor for age-related macular degeneration (ARMD) is advanced age. A prominent age-related change in the human retina is the accumulation of histochemically detectable neutral lipid in normal Bruch's membrane (BrM) throughout adulthood. This change has the potential to have a major impact on physiology of the retinal pigment epithelium (RPE). It occurs in the same compartment as drusen and basal linear deposit, the pathognomonic extracellular, lipid-containing lesions of ARMD. Here we present evidence from light microscopic histochemistry, ultrastructure, lipid profiling of tissues and isolated lipoproteins, and gene expression analysis that this deposition can be accounted for by esterified cholesterol-rich, apolipoprotein B-containing lipoprotein particles constitutively produced by the RPE. This work collectively allows ARMD lesion formation and its aftermath to be conceptualized as a response to the retention of a sub-endothelial apolipoprotein B lipoprotein, similar to a widely accepted model of atherosclerotic coronary artery disease (CAD) (Tabas et al., 2007). This approach provides a wide knowledge base and sophisticated clinical armamentarium that can be readily exploited for the development of new model systems and the future benefit of ARMD patients.

Apolipoprotein B-containing lipoproteins in retinal aging and age-related macular degeneration

The largest risk factor for age-related macular degeneration (ARMD) is advanced age. With aging, there is a striking accumulation of neutral lipids in Bruch's membrane (BrM) of normal eye that continues through adulthood. This accumulation has the potential to significantly impact the physiology of the retinal pigment epithelium (RPE). It also ultimately leads to the creation of a lipid wall at the same locations where drusen and basal linear deposit, the pathognomonic extracellular, lipid-containing lesions of ARMD, subsequently form. Here, we summarize evidence obtained from light microscopy, ultrastructural studies, lipid histochemistry, assay of isolated lipoproteins, and gene expression analysis. These studies suggest that lipid deposition in BrM is at least partially due to accumulation of esterified cholesterol-rich, apolipoprotein B-containing lipoprotein particles produced by the RPE. Furthermore, we suggest that the formation of ARMD lesions and their aftermath may be a pathological response to the retention of a sub-endothelial apolipoprotein B lipoprotein, similar to a widely accepted model of atherosclerotic coronary artery disease (Tabas, I., K. J. Williams, and J. Borén. 2007. Subendothelial lipoprotein retention as the initiating process in atherosclerosis: update and therapeutic implications. Circulation. 116:1832-1844). This view provides a conceptual basis for the development of novel treatments that may benefit ARMD patients in the future.

Histochemistry and lipid profiling combine for insights into aging and age-related maculopathy

Aging is the major risk factor for age-related maculopathy (ARM), the biggest cause of vision loss among the elderly in industrialized societies, and a major change in the affected tissues is the age-related accumulation of neutral lipid in Bruch's membrane (BrM) of the eye throughout adulthood. Here we show that esterified cholesterol (EC) is the major neutral lipid species in this tissue, which has implications for potential sources of this material. The combination of filipin histochemistry and comprehensive lipid profiling made possible this insight on a complex tissue.

Relation of corneal arcus to cardiovascular disease (from the Framingham Heart Study data set)

Corneal arcus is a lipid-rich deposit at the corneoscleral limbus that shares some similarities with the lipid deposition of atherosclerosis. Epidemiologic studies examining the association between corneal arcus and coronary artery disease (CAD) have yielded mixed results. This study was conducted to determine if corneal arcus is an independent risk factor for cardiovascular disease (CVD) and CAD. A prospective analysis was performed using Cox proportional-hazards regression models in the Framingham Heart Study Original Cohort and Offspring Cohort database. This cohort included 23,376 patient-examinations, during 3,890 (17%) of which corneal arcus was identified. Corneal arcus was a predictor of CVD and CAD at 4 years (hazard ratios [HRs] 2.28 and 1.99, respectively) and 8 years (HRs 2.52 and 2.35, respectively) of follow-up (p <0.0001 for all). Corneal arcus was no longer predictive of either CVD or CAD, however, after adjustment for age and gender at 4 years (HRs 1.07 and 1.01, respectively) and 8 years (HRs 1.18 and 1.17, respectively) of follow-up (p >0.05 for all). In conclusion, corneal arcus predicted CVD and CAD in the community-based Framingham Heart Study cohort because of the strong association of corneal arcus with increasing age. To date, this is the largest and lengthiest population-based cohort study examining the direct association between corneal arcus and CVD and CAD.

Accumulation of cholesterol in the lesions of focal segmental glomerulosclerosis

Intraglomerular deposition of low-density lipoprotein (LDL) and oxidized LDL has been described in various human glomerular diseases. Yet it is not clear whether esterified cholesterol (EC) and unesterified cholesterol (UC) carried in LDL are mobilized from deposited LDL particles or accumulate in the diseased human glomeruli, particularly in the segmentally sclerotic lesions. To address this issue, frozen sections of renal biopsies were first immunostained to localize apolipoprotein B (apo B) and then oil red O (ORO) stained to colocalize neutral lipids. By using 124 ORO-positive biopsies and nine ORO-negative ones, UC was visualized directly with filipin staining, and EC was visualized after its enzymatic hydrolysis and staining with filipin. Seventy-seven biopsies (58%) showed filipin staining of accumulated EC and/or UC in the glomeruli. Of these, 11 showed heavy filipin staining for both EC and UC in the segmentally sclerotic lesions. In a group with UC deposits in the sclerotic segments, the percentage of the glomeruli affected by sclerosis and the intensity of filipin fluorescence for UC were significantly higher than biopsies with only mesangial UC deposits. Most filipin-positive biopsies showed apo B staining mainly in the mesangium. Yet in the sclerotic segments, apo B staining was rarely noted. Accumulated apo B-stained lipoprotein was not coincident with ORO-stained lipid in the diseased glomeruli. These results suggest that both EC and UC accumulate in the sclerotic glomerular segments as the glomerular lesions are advanced, and that these EC and UC appear to be derived from altered LDL with progressive loss of apo B.

Correlating corneal arcus with atherosclerosis in familial hypercholesterolemia

BACKGROUND

A relationship between corneal arcus and atherosclerosis has long been suspected but is controversial. The homozygous familial hypercholesterolemia patients in this study present a unique opportunity to assess this issue. They have both advanced atherosclerosis and corneal arcus.

METHODS

This is a cross-sectional study of 17 patients homozygous for familial hypercholesterolemia presenting to the Clinical Center of the National Institutes of Health. Plasma lipoproteins, circumferential extent of arcus, thoracic aorta and coronary calcific atherosclerosis score, and Achilles tendon width were measured at the National Institutes of Health.

RESULTS

Patients with corneal arcus had higher scores for calcific atherosclerosis (mean 2865 compared to 412), cholesterol-year score (mean 11830 mg-yr/dl compared to 5707 mg-yr/dl), and Achilles tendon width (mean 2.54 cm compared to 1.41 cm) than those without. Corneal arcus and Achilles tendon width were strongly correlated and predictive of each other. Although corneal arcus was correlated with calcific atherosclerosis (r = 0.67; p = 0.004), it was not as highly correlated as was the Achilles tendon width (r = 0.855; p < 0.001).

CONCLUSION

Corneal arcus reflects widespread tissue lipid deposition and is correlated with both calcific atherosclerosis and xanthomatosis in these patients. Patients with more severe arcus tend to have more severe calcific atherosclerosis. Corneal arcus is not as good an indicator of calcific atherosclerosis as Achilles tendon thickness, but its presence suggests increased atherosclerosis in these hypercholesterolemic patients.

Distribution and composition of esterified and unesterified cholesterol in extra-macular drusen

More details about the distribution of esterified and unesterified cholesterol (EC, UC), abundant druse components, would inform models of druse biogenesis and new technologies for ocular imaging. From donors with grossly normal maculas (n=10, 66-86years), whose eyes were preserved in paraformaldehyde within 6h of death, extra-macular drusen encased with retinal pigment epithelium (RPE) were isolated manually. Cryosections of pelleted drusen, stained with filipin for UC and EC, were used to investigate filipin staining patterns within single drusen (n=193) and to quantify fluorescence (n=146). From lipid extracts of other drusen/RPE and RPE samples, total cholesterol (TC) and UC were determined by enzymatic fluorimetry. Drusen contained cores, basally located regions that were intensely bright when stained for UC or deeply dark when stained for EC; many were surrounded by concentric lamellae. Within the same cores, the EC-poor regions were significantly smaller (13.0mum) than UC-rich regions (17.1mum). Drusen with highly fluorescent EC-rich shells lacked UC-rich shells. Small spots representing lakes were visible only in drusen stained for EC. Some drusen had small, refractive spherical inclusions lacking both UC and EC. Of drusen examined, 32% had a UC-rich core, 35% had an EC-poor core, 31% had an EC-rich shell, 25% had EC-rich lakes, and 4-5% had UC-, EC-poor inclusions. Shells and cores occurred in significantly non-overlapping druse populations. The percentage of TC that was esterified ranged from 32-66% for drusen/RPE and 5-21% for RPE. The disposition of cholesterol in cores may reflect the activity of invading cellular process. The greater size of UC-rich cores relative to EC-poor cores may reflect a declining gradient of enzymatic activity with increased radial distance from the putative invaders. The relative sizes of sub-domains defined by cholesterol composition are compared to sub-domains detected in drusen by in vivo imaging methods.

Corneal arcus as coronary artery disease risk factor

Corneal arcus is a lipid-rich and predominantly extracellular deposit that forms at the corneoscleral limbus. It represents the most common peripheral corneal opacity and is not associated with tissue breakdown but rather with the deposition of lipids. The deposition of cholesterol in the peripheral cornea and arterial wall are similar in that both are accelerated by elevated serum levels of atherogenic lipoproteins, such as low-density lipoproteins (LDL). Corneal arcus is more prevalent in men than in women and in Blacks than in Whites. Its prevalence increases with advancing age. It has been associated with hypercholesterolemia, xanthelasmas, alcohol, blood pressure, cigarette smoking, diabetes, age, and coronary heart disease. Nevertheless, it is not clear whether or not corneal arcus is an independent risk factor for coronary heart disease (CHD). The present systematic review examines the relationship of corneal arcus and CHD to determine if corneal arcus is an independent CHD risk factor. We conclude that there is no consensus that corneal arcus is an independent risk factor. The presence of corneal arcus in a young person should prompt a search for lipid abnormalities. Also, because corneal arcus represents physical evidence of early lipid deposition, its presence suggests the need for aggressive lipid therapy.

Morphologic Characteristics of Lamellar Channel Deposits in the Human Eye

OBJECTIVE

To report the histologic characteristics of lamellar channel deposits after polymethyl methacrylate intrastromal corneal ring segments in a human eye.

METHODS

A 34-year-old man previously diagnosed with keratoconus received photorefractive keratectomy in his left eye that exacerbated his corneal ectasia. To delay or possibly prevent corneal transplant, the patient elected to have intrastromal corneal segments implanted in the left eye. One year later the patient underwent corneal transplant. Before transplant, lamellar channel deposits were noted on clinical microscopic examination of the left eye. We performed histologic evaluation to assess changes to the patient's excised corneal tissue and to further characterize the observed lamellar channel deposits.

RESULTS

Lamellar channel deposits in humans are histologically similar to our previous descriptions of deposits in rabbits. These deposits primarily consist of intracellular lipid accumulations that stain positively with oil red O and filipin but not periodic acid Schiff reaction. Immunohistochemistry of cells located in the deposit region stained positively with vimentin, suggesting that these cells were keratocytes. Stains for cytokeratins 3 and 12 and for CD68 were both negative, indicating that epithelial cells and macrophages were not present in the deposit region.

CONCLUSION

Lamellar channel deposits are a common clinical finding after intrastromal corneal implants. This case provides further evidence to support our hypothesis that lamellar channel deposits in humans are a tissue response to corneal implants that consist of intracellular lipid accumulations. We use this case to propose a theoretical construct for the observation of stromal lipid accumulation in response to corneal injury.

Histologic evaluation of corneal stroma in rabbits after intrastromal corneal ring implantation

PURPOSE

Previous refractive corneal implants have produced histologic changes characteristic of nonspecific lipid keratopathy. Intacs intrastromal corneal ring segments are polymethylmethacrylate (PMMA) corneal inserts approved for the correction of low myopia by the U.S. Food and Drug Administration. The purpose of this study was to describe histologic changes associated with these corneal implants in rabbit eyes.

METHODS

Two 150-degree PMMA arc segments, 0.30 mm thick, were surgically implanted through a single radial incision at two-thirds stromal depth in the mid-peripheral cornea of five New Zealand white rabbits. We collected corneas 6 months after implantation that were prepared for light and electron microscopy. We analyzed tissue with oil red O, filipin, and periodic acid-Schiff (PAS) staining. Additional studies with energy-dispersive x-ray analysis and autofluorescent microscopy were conducted.

RESULTS

In all samples, we observed new collagen formation with lamellar organization adjacent to the implant and increased keratocyte density. Intracellular accumulations of osmophilic and saturated lipid material corresponded to stromal opacification visible by slit-lamp microscopy. These same regions were autofluorescent and stained positively with oil red O, and filipin but negatively with PAS. Granular lipid inclusions characteristic of lipofuscinosis were absent on transmission electron microscopy. Energy-dispersive x-ray analysis demonstrated elevated levels of calcium compared with adjacent tissue.

CONCLUSION

Tissue response to these corneal inserts includes keratocyte activation, intracellular lipid accumulation, and new collagen formation. Our histologic findings suggest that these deposit formations are not lipofuscin accumulations.

Ocular lipid deposition and hyperlipoproteinaemia

In all species there are potential ocular manifestations when circulating lipoproteins are raised and these may be transient or permanent Many factors, both systemic and local, influence lipid influx and accumulation (progression) and lipid mobilisation and efflux (regression). In both humans and animals some types of lipid deposition will regress if the local and systemic factors involved in pathogenesis can be modified. There are inescapable parallels with the same phenomena in other tissues.Three types of corneal lipid deposition have been linked with hyperlipoproteinaemia. In corneal arcus, lipid is deposited preferentially in the warmest part of the cornea initially and, in people, the lipid remains almost exclusively extracellular. In animals, corneal arcus is associated with initial extracellular lipid deposition followed by the appearance of intracellular lipid and vascularisation, so that established corneal arcus tends to become more typical of lipid keratopathy. In humans, hyperlipoproteinaemia may be an associated systemic factor and early onset corneal arcus is a recognised feature of certain primary hyperlipoproteinaemias and their secondary phenotypes. In dogs, corneal arcus is always associated with hyperlipoproteinaemia. Corneal vascularisation is a ubiquitous feature of lipid keratopathy in all species and both necrotic fibroblasts and foam cells are common in progressive lesions. The extent and position of lipid deposition and the evolution of lipid keratopathy can be related to local ocular disease and circulating lipids and lipoproteins. Many aspects of the pathogenesis of lipid keratopathy are similar to those of atherogenesis. Hyperlipoproteinaemia, especially hypercholesterolaemia is the commonest systemic abnormality. In crystalline stromal dystrophy (Schnyder's crystalline stromal dystrophy) of the cornea there is no inflammatory element and no vascularisation. The dystrophy is associated with accumulation of lipid within the corneal fibroblasts, but typical foam cells are absent, the crystalline opacity involves the coolest part of the cornea, correlates with local fibroblast death, and is always bilateral. Hyperlipoproteinaemia, may be present, but this is not universally so.The objective of this paper is to evaluate the factors that may influence ocular involvement in hyperlipoproteinaemia. A comparative approach, utilising information available from studies of both ocular and non-ocular tissues, aids elucidation of the complex pathogenesis.

Free cholesterol deposition in the cornea of human apolipoprotein A-II transgenic mice with functional lecithin: cholesterol acyltransferase deficiency

We have developed several lines of transgenic animals that overexpress different levels of human apolipoprotein A-II (apoA-II). The 11.1 transgenic line has human apoA-II in plasma at threefold the level in normolipidemic humans and a functional lecithin:cholesterol acyltransferase (LCAT) deficiency. The latter is a biochemical phenotype similar to that of fish-eye disease (FED), which is characterized by free cholesterol (FC) and phospholipid accumulation in the cornea, leading to opacity and impaired vision. To assess whether the metabolic alterations in these mice also lead to lipid accumulation in the cornea, we fed them on a long-term regular chow or high-fat/high-cholesterol (HF/HC) diet. The 11.1 transgenic mice showed a moderate accumulation of FC in the cornea, but only when fed the regular chow diet. This FC accumulation was less severe than the accumulation described in FED, which may explain the lack of corneal opacity in these mice. Electron microscopy and immunoblotting analysis of the cornea of 11.1 transgenic mice in comparison to control mice showed (1) a mild but nevertheless more intense intracytoplasmatic lipid particle deposition in the epithelial cells and (2) a decrease of immunoreactive apoA-I in the area of Bowman's layer and at the superficial stroma. The serum capacity to cause cholesterol efflux from rat fibroblasts was decreased in 11.1 transgenic mice, but only in those fed a regular chow diet. We conclude that 11.1 human apoA-II transgenic mice may be a useful model for studies of early lipid deposition in the cornea and its possible prevention.

Corneal arcus, case finding and definition of individual clinical risk in heterozygous familial hypercholesterolaemia

Premature corneal arcus may identify individuals with hyperlipidaemia and increased cardiovascular risk. We have attempted to quantitate relationships through determination of graded prevalence of corneal arcus with age for 81 males and 73 females suffering from heterozygous familial hypercholesterolaemia (HFH) at presentation, and for 280 male and 353 female unselected patients (age range 16-76 years) attending a country general practice. Some degree of arcus affected 50% of HFH patients by age 31-35 years, and 50% of practice patients by age 41-45 years. Complete full ring arcus affected 50% of the familial hypercholesterolaemia (FH) group by age 50 years, with only 5% similarly affected in the non-FH group. Arcus grade with age was advanced by some 5 years in males versus females. Premature arcus potentially alerting to HFH can be broadly defined for males and females combined, as heavy full ring by age 50 years, or any degree of arcus by age 30-35 years. Arcus grade was not related to the presence of coronary disease. Accelerated development of corneal arcus with age is an indicator of HFH, but premature arcus is not an additional marker of premature coronary disease for individual cases of HFH.