Microkeratome-assisted posterior keratoplasty.
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Corneal Diseases↗
Biomedical subjects
Dimitri T Azar
Publications and source records attributed to Dimitri T Azar.
Three-dimensional representation and qualitative comparisons of the amount of tissue ablation to treat mixed and compound astigmatism.
PURPOSE: To compare the shape and volume of the lenticules of corneal tissue ablated for the correction of spherical, cylindrical, and spherocylindrical refractive errors using Boolean operations of theoretical 3-dimensional (3-D) surfaces. SETTING: Department of Ophthalmology, Rothschild Foundation, Paris, France. METHODS: Digital modeling software was used to perform graphic representations of ablated lenticules on 3-D virtual surfaces. Various Boolean operations were performed between different preoperative and postoperative surfaces, and the additional and subtractive properties of ablated theoretical lenticules were analyzed to determine profiles of ablated lenticules for mixed and compound myopic and hyperopic astigmatism. RESULTS: Negative-cylindrical treatment, used to treat simple myopic astigmatism, was equivalent to the combination of a positive-cylindrical and a negative-spherical treatment of the same magnitude. Combining a pure negative-cylindrical and a positive-spherical treatment in a sequential strategy when treating compound astigmatism resulted in redundant ablation (plano lenticule), leading to an unnecessary increase in the amount of tissue ablation. CONCLUSIONS: Negative-cylindrical treatments result in greater tissue ablation than corresponding positive-cylindrical treatments. For any given compound astigmatic error, the strategy using the greater magnitude of positive cylinder incurs the minimal amount of tissue ablation.
Astigmatism↗
Corneal asphericity after hyperopic laser in situ keratomileusis.
PURPOSE: To analyze corneal asphericity after hyperopic laser in situ keratomileusis (LASIK) and its relationship to the clinical outcomes. SETTING: Corneal and Refractive Surgery Service, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA. METHODS: In a retrospective case series, 23 patients (33 eyes) with hyperopia or hyperopic astigmatism who had LASIK were evaluated. A computer program (Holladay Diagnostic Summary, EyeSys Laboratories) was used to analyze corneal asphericity (Q) before and after LASIK. Corneal asphericity was evaluated to determine the association with the postoperative refractive error, best spectacle-corrected visual acuity (BSCVA), uncorrected visual acuity (UCVA), achieved refractive correction, mean corneal power (K), refractive yield (achieved/attempted correction), and keratometric yield (change in keratometry/attempted correction). RESULTS: After hyperopic LASIK, all corneas exhibited increased negative central Q. The postoperative corneal radius of curvature, BSCVA, and refractive and keratometric yields were not significantly correlated with the preoperative Q values. The asphericity change, Delta Q, was highly correlated with the achieved correction (r = 0.747, P <.0001). The postoperative Q value correlated well with the preoperative value (r = 0.534, P <.05) and the achieved correction (r = 0.601, P <.05) but not with the Delta Q. Neither the postoperative Q nor the Delta Q was correlated with the spherical equivalent, K, BSCVA, or UCVA. CONCLUSIONS: Asphericity may be a useful quantitative descriptor of the corneal optical contour after hyperopic LASIK. Negative central Q increased after hyperopic LASIK, especially when greater degrees of refractive correction were attempted.
Adult↗
Myopia and models and mechanisms of refractive error control.
Myopia represents a failure of the normal process of emmetropization, which is essentially endogenous to the eye. Emmetropization involves defocus detection at the level of the amacrine and bipolar cells of the outer retina, diffusion of a signal or signals across the retinal pigment epithelium and choroid, and alteration of the scleral matrix, likely through modulation of proteoglycan synthesis. Elucidating and effectively bolstering the deficient steps in this regulatory pathway would mark a significant advance given myopia's tremendous impact. Clinical experience, longitudinal studies, epidemiological data and numerous animal experiments have enhanced our understanding of myopia. Interpretation of the epidemiological data is often complicated by the difficulties of distinguishing environmental from genetic influences, especially those pertaining to slow developmental changes. Likewise, it is important that the animal models be interpreted with an appreciation that the human eye varies structurally and developmentally from that of other species. Studies of the chick eye have formed the basis for several hypotheses of myopic development, but the chick does not possess a fovea or retinal blood supply. It is unclear whether these differences alter the pathways of emmetropization. Even closely related primate species can exhibit different responses to form deprivation conditions, suggesting differing mechanisms of eye growth control. Monocular occlusion of the rhesus macaque, for instance, results in myopia when the ciliary muscle is paralyzed or the optic nerve cut, but does not in the stumptailed macaque, suggesting a role of excessive accommodation in the development of myopia in the stumptail but not the rhesus [36]. Given such variability in the models a persisting element of continued myopia research must be an evaluation of the relevance of any given model to the human condition. In this regard, the study of changing patterns of gene expression within and among species during emmetropization and myopic progression may offer a productive avenue for future research.
Disease Models, Animal↗
Lamellar keratitis following laser-assisted in situ keratomileusis.
Distinguishing between an infectious or sterile lamellar keratitis is the most important first step in evaluating patients with interface infiltrates after LASIK. The mechanisms by which infectious keratitis develops are more straightforward than for DLK and deal with the introduction of microbial pathogens into the lamellar flap during LASIK. Prevention emphasizes reducing contamination risks and treating any pre-existing ocular infections. The mechanisms of the development of DLK are likely multifactorial, and factors such as microkeratome debris, eyelid secretions, other debris, epithelial defects, and bacterial endotoxin have been suggested. Overall, much remains to be elucidated in order to devise effective prevention measures.
Cornea↗
Mechanisms of disease: cataracts.
Studying the mechanisms that are responsible for the transparency of the lens we see that multiple factors are involved in the maintenance of lens clarity as well as in cataractogenesis. Is there a unifying mechanism? Since the function of the lens is the transmission of electromagnetic radiation in the visible spectrum, fundamental physical laws should apply. Attenuation of a light beam that passes through a medium happens by two major processes: absorption and scattering. In absorption, light energy is dissipated into heat as the result of energy-absorbing electronic transitions. In scattering, radiation is redirected away from the incident pathway, thereby reducing the transmitting power. Depending on the angular dependence of the scattered light, the wave front of the transmitted light is distorted. In the case of cataract, the primary factor is turbidity secondary to scattering. A perfectly uniform medium exhibits no light scattering; thus a continuous medium can scatter light only when it contains spatial fluctuations around the mean density governed by specific equations. These equations simply state that the amplitude of scattering is proportional to the Fourier amplitude of the density fluctuations in the medium [12]. Electron micrograph analysis using Fourier transformation has shows an increase in the fluctuation of spatial density of the opaque fiber [12,26,37,93]. Thus, all transparency mechanisms and all cataractogenic factors can be thought of as opposing effectors of spatial density fluctuation affecting scatter and antithetical producers of light absorbing moieties.
Cataract↗
Management of traumatic cataracts.
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Cataract↗
Trauma after refractive surgery.
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Eye Diseases↗
Management of astigmatism after corneal trauma.
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Astigmatism↗
Laser subepithelial keratomileusis: evolution of alcohol assisted flap surface ablation.
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Adult↗
Analysis of customized corneal ablations: theoretical limitations of increasing negative asphericity.
PURPOSE: To determine the ablation depths of customized myopic excimer laser photoablations performed to change corneal asphericity after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). METHODS: A mathematical model of aspheric myopic corneal laser surgery was generated. The initial corneal surface was modeled as a conic section of apical radius R(1) and asphericity Q(1). The final corneal surface was modeled as a conic section of apical R(2) and asphericity Q(2), where R(2) was calculated from the paraxial optical formula for a given treatment magnitude (D), and Q(2) was the intended final asphericity. The aspheric profile of ablation was defined as the difference between the initial and final corneal profiles for a given optical zone diameter (S), and the maximal depth of ablation was calculated from these equations. Using the Taylor series expansion, an equation was derived that allowed the approximation of the central depth of ablation (t(0)) for various magnitudes of treatment, optical zone diameters, and asphericity. In addition to the Munnerlyn term (M), incorporating Munnerlyn's approximation (-D small middle dot S(2)/3), the equation included an asphericity term (A) and a change of asphericity term (Delta). This formula (t(0) = M + A + Delta) was used to predict the maximal depth of ablation and the limits of customized asphericity treatments in several theoretical situations. RESULTS: When the initial and final asphericities were identical (no intended change in asphericity; Q(1) = Q(2); Delta = 0), the maximal depth of ablation (t(0) = M + A) increased linearly with the asphericity Q(1). To achieve a more prolate final asphericity (Q(2) < Q(1); dQ < 0; Delta > 0), the maximal depth of ablation (M + A + Delta) was increased. For treatments in which Q(2) was intended to be more oblate than Q(1) (Q(2) > Q(1); dQ > 0; Delta < 0), the maximal depth of ablation was reduced. These effects sharply increased with increasing diameters of the optical zone(s). Similarly, in the case of PRK, the differential increase in epithelial thickness in the center of the cornea compared with the periphery resulted in increased oblateness. CONCLUSIONS: Aspheric profiles of ablation result in varying central depths of ablation. Oblateness of the initial corneal surface, intentional increase in negative asphericity, and enlargement of the optical zone diameter result in deeper central ablations. This may be of clinical importance in planning aspheric profiles of ablation in LASIK procedures to correct spherical aberration without compromising the mechanical integrity of the cornea.
Cornea↗
Volume estimation of excimer laser tissue ablation for correction of spherical myopia and hyperopia.
PURPOSE: To determine the theoretical volumes of ablation for the laser treatment of spherical refractive errors in myopia and hyperopia. METHODS: The cornea was modeled as a spherical shell. The ablation profiles for myopia and hyperopia were based on an established paraxial formula. The theoretical volumes of the ablated corneal lenticules for the correction of myopia and hyperopia were calculated by two methods: (1) mathematical approximation based on a simplified geometric model and (2) finite integration. These results were then compared for optical zone diameters of 0.5 to 11.00 mm and for initial radii of curvature of 7.5, 7.8, and 8.1 mm. RESULTS: Referring to a simplified geometrical model, the volume of ablated corneal tissue was estimated to be proportional to the magnitude of treatment (D) and to the fourth power of the treatment diameter (S(4)). For refractive correction of myopia and hyperopia, volume estimations using our formula, V congruent with D. (S/9)(4), were similar to those obtained by finite integration for optical zone diameters of 0.5 to 8.5 mm and for corneal radii of curvature within the clinical range (7.5, 7.8, and 8.1 mm). CONCLUSIONS: The theoretical volume of corneal tissue ablated within the optical zone for spherical corrections can be accurately approximated by this simplified formula. This may be helpful in evaluating factors that contribute to corneal ectasia after LASIK for myopia and hyperopia. Treatment diameter (S) is the most important determinant of the volume of tissue ablation during excimer laser surgery.
Cell Size↗
Human corneal epithelial cell viability and morphology after dilute alcohol exposure.
PURPOSE: To determine the effect of dilute alcohol on human corneal epithelial cellular morphology and viability. Dilute alcohol is used for epithelial removal during photorefractive keratectomy (PRK) and laser subepithelial keratomileusis (LASEK). METHODS: Corneal epithelial sheets harvested from human eyes after alcohol application during PRK were examined by light and electron microscopy (specimens I-IV). In addition, tissue cultures of human epithelial sheets were monitored for epithelial migration and attachment (specimens V-VII). To determine the effect of dilute alcohol on epithelial cell viability, simian virus (SV)40-immortalized human corneal epithelial cells were exposed to dilute alcohol in distilled water (EtOH-H2O) or to keratinocyte serum-free medium (EtOH-KSFM) for incubation periods of 20 to 45 seconds and concentrations of 10% to 70%. Cell membrane permeability and intracellular esterase activity were analyzed by calcein-acetoxymethyl ester (AM)/ethidium homodimer assay. TdT-mediated dUTP nick-end labeling (TUNEL) assay was used to detect apoptotic cells at 0, 8,12, 24, and 72 hours. RESULTS: Electron microscopy showed varying degrees of basement membrane alterations after alcohol application, including disruptions, discontinuities, irregularities, and duplication (specimens I-IV). Cellular destruction and vacuolization of basal epithelial cells associated with absent basement membrane were also observed (specimen III). One of three cultured epithelial sheets showed attachment and outgrowth in the tissue culture until day 15 (specimen V). Twenty-second exposure of cultured immortalized human cells to various concentrations of EtOH-H2O showed significant reduction of viable cells when EtOH-H2O concentration exceeded 25% (P = 0.005). Increasing the duration of application of 20% EtOH-H2O beyond 30 seconds resulted in a significant reduction in viable cells (69.69% +/- 16.34% at 30 seconds compared with 2.14% +/- 2.29%, 10.45% +/- 7.11%, and 11.09% +/- 15.73% at 35, 40, and 45 seconds, respectively; P = 0.01). TUNEL assay of cultured human corneal epithelial cells exposed to 20% EtOH-H(2)O for 20 and 40 seconds showed maximal labeling at 24 hours (58.05% +/- 33.10%) and 8 hours (94.12% +/- 1.21%), respectively. Exposure to 20% EtOH-KSFM for 20 and 40 seconds resulted in substantially lower TUNEL positivity (3.51% +/- 0.20% at 24 hours and 7.11% +/- 0.49% at 8 hours). CONCLUSIONS: The viability and electron microscopic findings in the basement membrane zone showed significant variation after treatment of the epithelium in vivo with dilute alcohol. The application of dilute alcohol on the monolayer of cultured corneal epithelial cells resulted in increasing cell death in a dose- and time-dependent manner.
Actins↗
Infections following laser in situ keratomileusis: an integration of the published literature.
Infections occurring after laser in situ keratomileusis (LASIK) surgery are uncommon, but the number of reports have steadily increased in recent years. This systematic, comprehensive review and analysis of the published literature has been performed in order to develop an integrative perspective on these infections. We have stratified the data by potential associations, microbiology, treatment, and the degree of visual loss, using Fisher's exact tests and Student's t-tests for analysis. In this review, we found that Gram-positive bacteria and mycobacterium were the most common causative organisms. Type of postoperative antibiotic and steroid use was not associated with particular infecting organisms or severity of visual loss. Gram-positive infections were more likely to present less than 7 days after LASIK, and they were associated with pain, discharge, epithelial defects, and anterior chamber reactions. Fungal infections were associated with redness and tearing on presentation. Mycobacterial infections were more likely to present 10 or more days after LASIK surgery. Moderate or severe visual reductions in visual acuity occurred in 49.4% of eyes. Severe reductions in visual acuity were significantly more associated with fungal infections. Flap lift and repositioning preformed within 3 days of symptom onset may be associated with better visual outcome.
Adolescent↗
Evolution, techniques, clinical outcomes, and pathophysiology of LASEK: review of the literature.
Laser subepithelial keratomileusis (LASEK) is a relatively new laser surgical procedure that combines certain elements of both laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) to improve the risk/benefit ratio. Diluted alcohol is used to loosen the epithelial adhesion to the corneal stroma. The loosened epithelium is moved aside from the treatment zone as a hinged sheet. Laser ablation of the subepithelial stroma is performed before the epithelial sheet is returned to its original position. We reviewed the literature regarding modifications of the technique, indications, outcomes, and complications, as well as wound healing after LASEK surgery. This literature review of 1,421 LASEK-treated eyes provided many findings: 1) The long-term stable results in the absence of serious complications, such as infections, recurrent erosions, scars, or late-onset corneal haze formation in patients re-examined up to 5 years after LASEK; 2) Epithelial closure with recovery of functional vision was completed at days 4 to 7 in most cases; 3) A tendency toward overcorrection with PRK nomograms; 4) We hypothesize that this tendency may be due to the decreased wound healing response, which may lead to myopic regression in PRK; and 5) Postoperative discomfort and prolonged visual recovery until the epithelium closes remain the biggest disadvantages of LASEK compared to LASIK. LASEK surgery is especially valuable in patients with thin corneas who would not qualify for LASIK surgery. However, a potential superiority of LASEK to LASIK in wavefront guided ablations still remains speculative.
Humans↗
Sterile corneal melting and necrotizing scleritis after cataract surgery in patients with rheumatoid arthritis and collagen vascular disease.
The onset of post-operative corneal melting and necrotizing scleritis in patients with rheumatoid arthritis and collagen vascular disease who undergo cataract surgery can have devastating ocular and systemic consequences. Even though ocular surface factors such as sicca and surgical trauma are among the important variables that contribute to this entities, signs and symptoms of systemic disease need to be thoroughly investigated in order to prevent life-threatening complications associated with these ocular manifestations. The management of surgical induced corneal melting and necrotizing scleritis in these patients, include local therapy and in most instances, systemic immuno-modulation. Moreover, the development of corneal melting and necrotizing scleritis in an otherwise "healthy" patient after uncomplicated cataract surgery, can be the first manifestation of a serious occult systemic disease. Therefore, an aggressive approach regarding the diagnosis, workup and treatment should be initiated by the ophthalmologist in order to maximize a successful ophthalmic and medical outcome.
Arthritis, Rheumatoid↗
Anti-angiogenic therapy: Prospects for treatment of ocular tumors.
The growth of new blood vessels (angiogenesis) within tumors is essential for tumor growth, maintenance, and metastasis. Angiogenesis research has identified a host of pro- and anti-angiogenic factors that regulate an "angiogenic switch," which when turned on, allows tumors to assume a more aggressive form. Angiogensis inhibitors that target this switch are in clinical trials for a wide array of tumor types. Although angiogenesis inhibitors are already widely used to treat ocular disease, only limited case reports are currently available for the use of angiogenesis inhibitors to treat ocular tumors. Evidence for angiogenesis in the growth and spread of uveal melanoma, retinoblastoma, and von Hippel Lindau (VHL) disease exists. The very limited trials of angiogenesis inhibitors in the treatment of uveal melanoma and VHL are promising, although more extensive controlled trials will be needed to confirm their efficacy.
Angiogenesis Inhibitors↗