Nationwide epidemiological approach to identify associations between keratoconus and immune-mediated diseases.

BACKGROUND: The aetiology of keratoconus (KC) remains poorly understood. KC has typically been described as a non-inflammatory disorder of the cornea. Nonetheless, there is increasing presumptive evidence for the role of the immune system in the pathogenesis of KC. AIM: To evaluate the association between KC and immune-mediated diseases on a population level. We hypothesise that KC is immune-mediated rather than a predominantly degenerative disease. METHODS: Data were obtained from the largest health insurance provider in the Netherlands. Dutch residents are obligatorily insured. The data contained all medical claims and sociodemographic characteristics from all KC patients plus all those data from a 1:6 age-matched and sex-matched control group. The primary outcome was the association between KC and immune-mediated diseases, as assessed by conditional logistic regression. RESULTS: Based on our analysis of 2051 KC cases and 12 306 matched controls, we identified novel associations between KC and Hashimoto's thyroiditis (OR=2.89; 95% CI: 1.41 to 5.94) and inflammatory skin conditions (OR=2.20; 95% CI: 1.37 to 3.53). We confirmed known associations between KC and atopic conditions, including allergic rash (OR=3.00; 95% CI: 1.03 to 8.79), asthma and bronchial hyperresponsiveness (OR=2.51; 95% CI: 1.63 to 3.84), and allergic rhinitis (OR=2.20; 95% CI: 1.39 to 3.49). CONCLUSION: Keratoconus appears positively associated with multiple immune-mediated diseases, which provides a population-based argument that systemic inflammatory responses may influence its onset. The identification of these particular diseases might shed light on potential comparable pathways through which this proinflammatory state is achieved, paving the way for pharmacological treatment strategies.

Parameters Associated With Endothelial Cell Density Variability After Descemet Membrane Endothelial Keratoplasty.

PURPOSE: To evaluate which parameters may affect endothelial cell loss after Descemet membrane endothelial keratoplasty (DMEK) by comparing eyes in the low vs high quartile of endothelial cell loss over a follow-up period of 4 years. DESIGN: Retrospective cohort study. METHODS: Donor endothelial cell density (ECD) decline was evaluated for 351 eyes of 275 patients up to 4 years after DMEK for Fuchs endothelial corneal dystrophy (FECD). Eyes with a postoperative endothelial cell loss in the lower quartile at all available follow-up moments were assigned to Group 1 (n = 51) and those in the upper quartile to Group 2 (n = 42). Multinomial regression was used to assess which covariates were related to greater ECD decline. RESULTS: Mean endothelial cell loss as compared to preoperative donor ECD for the entire study group was 33 (±16)%, 36 (±17)%, and 52 (±18)% at 1, 6, and 48 months postoperatively. Endothelial cell loss of Group 1 was 12 (±7)%, 13 (±6)%, and 26 (±8)% at, respectively, 1, 6, and 48 months postoperatively, and 59 (±10)%, 64 (±9)%, and 75 (±5)% in Group 2. Partial graft detachment, donor death cause cardiovascular/stroke (vs cancer), postoperative complications other than graft detachment, and severity of preoperative FECD (all P < .01) showed the strongest relation with greater ECD decline. CONCLUSIONS: DMEK eyes with a completely attached graft and operated in an early stage of FECD may show the lowest endothelial cell loss postoperatively.

Clinical Outcome of Rebubbling for Graft Detachment After Descemet Membrane Endothelial Keratoplasty.

PURPOSE: To assess the clinical outcome after successful rebubbling procedures for visually significant graft detachment after Descemet membrane endothelial keratoplasty (DMEK). METHODS: From a total of 760 consecutive DMEK surgeries, 41 eyes required rebubbling. Of those, 33 eyes of 31 patients were successful and were included in our retrospective outcome analysis study. Main outcome measures were compared with those of matched controls with uneventful primary DMEK (attached DMEK grafts without rebubbling). Rebubbling was performed on average 25 (±20) days (range 7-91 days) after DMEK. All eyes were evaluated for best-corrected visual acuity, endothelial cell density (ECD), pachymetry, and complications up to 6 months after rebubbling. RESULTS: At 6 months after DMEK, best-corrected visual acuity in rebubbled eyes did not differ from that in control eyes (P = 0.514). The mean ECD decrease was higher in rebubbled than in control eyes (54% vs. 35%, respectively, P = 0.001). Pachymetry did not differ between both groups (P = 0.153). After rebubbling, one buphthalmic eye showed temporary intraocular pressure elevation and 5 eyes had minor graft edge detachment that did not require further treatment. CONCLUSIONS: Rebubbling for DMEK graft detachment may result in similar visual outcomes as in uncomplicated DMEK, when performed within the first 6 to 8 postoperative weeks. However, rebubbled eyes may have lower ECD, which may be attributed to additional air bubble trauma and/or selection bias through more extensive manipulation during initial DMEK or higher risk of graft detachment in more complicated eyes.

Two-Year Clinical Outcome of 500 Consecutive Cases Undergoing Descemet Membrane Endothelial Keratoplasty.

PURPOSE: To evaluate the clinical outcome of 500 consecutive cases up to 2 years after Descemet membrane endothelial keratoplasty (DMEK) and to assess which parameters may have influenced the clinical outcome. METHODS: From a group of 500 eyes (393 patients), which underwent DMEK for Fuchs endothelial corneal dystrophy (FECD), bullous keratopathy, failed corneal transplants and other indications, clinical outcomes [best-corrected visual acuity (BCVA), central endothelial cell density (ECD), and central corneal thickness] were evaluated before, and at 6, 12, and 24 months after DMEK and postoperative complications were documented. RESULTS: At 12 months postoperatively, 81% of eyes reached a BCVA of ≥20/25 (≥0.8), 49% ≥20/20 (≥1.0), and 15% ≥20/18 (≥1.2) (n = 396) and remained stable up to 24 months (P = 0.828). Compared with preoperative ECD, mean postoperative ECD decreased by 37 (±18)%, 40 (±18)%, and 45 (±18)% at 6, 12, and 24 months, respectively (P < 0.05 for all time points). Surgery indication and graft attachment status were related to postoperative BCVA and ECD results. Eyes with FECD and attached grafts showed better BCVA outcomes and higher ECD (P < 0.05). Central corneal thickness decreased by 20 (±11)% to 525 (±46) µm from preoperative to 6 months postoperatively and remained stable thereafter (P > 0.05). Within the study period, retransplantation was required in 32 eyes (6.4%). Principal longer-term complications were secondary graft failure (1.4%) and allograft rejection (1.4%). CONCLUSIONS: Clinical outcomes remain excellent up to 2 years after DMEK, in particular for eyes operated on for FECD and with completely attached grafts.

Association Between Graft Storage Time and Donor Age With Endothelial Cell Density and Graft Adherence After Descemet Membrane Endothelial Keratoplasty.

IMPORTANCE: After retrospectively evaluating the clinical outcome of 500 consecutive cases after Descemet membrane endothelial keratoplasty (DMEK), we extended the analysis in this study by assessing the effect of donor-related parameters on endothelial cell density (ECD) decline and detachment rate in this group. OBSERVATIONS: This retrospective case series included 500 cases who had undergone DMEK from October 2007 to September 2012 at the Netherlands Institute for Innovative Ocular Surgery (NIIOS), Rotterdam, the Netherlands. Logistic regression analysis (n = 332 eyes) showed that donor age might be associated with a 3% increase in the risk for a detachment (odds ratio, 0.97; 95% CI, 0.94-1.00; P = .049) (ie, higher donor age seems to be associated with lower chances of a detachment). In addition, linear regression analysis indicated that graft storage time in medium was associated with ECD decrease (ie, the longer the storage time, the larger the decrease at 6 months after DMEK) (P = .01). CONCLUSIONS AND RELEVANCE: We showed an association between graft storage time and ECD decline after DMEK and possibly between donor age and graft detachment. Therefore, donor storage times should be kept as short as possible to improve short-term ECDs. More research is needed to draw definite conclusions on the possible effect of donor age on the chance of a detachment after DMEK.

Endothelial Survival After Descemet Membrane Endothelial Keratoplasty: Effect of Surgical Indication and Graft Adherence Status.

IMPORTANCE: This study evaluates the longevity of Descemet membrane endothelial keratoplasty (DMEK) grafts in terms of endothelial survival and endothelial failure. OBJECTIVE: To determine endothelial survival and its association with the indication for surgery and/or partial graft detachment in DMEK. DESIGN, SETTING, AND PARTICIPANTS: Retrospective cross-sectional study of data collected from August 8, 2006, until June 17, 2015, at a tertiary referral center. A total of 352 eyes were evaluated up to 8 years after DMEK for Fuchs endothelial corneal dystrophy (FECD; n = 314), bullous keratopathy (BK; n = 31), and failed previous endothelial graft (n = 7), of which 314 eyes had complete graft attachment and 38 eyes had partial graft detachment (one-third of the graft surface area or less). Endothelial cell density was measured with specular microscopy, and Kaplan-Meier survival estimates were based on eyes with endothelial failure. Endothelial survival was followed up to 8 years after DMEK. MAIN OUTCOMES AND MEASURES: Endothelial cell density, endothelial failure, and endothelial survival. RESULTS: Endothelial cell density decreased to a mean (SD) of 952 (366) and 771 (321) cells/mm2 at 7 and 8 years postoperatively, respectively. Higher endothelial cell densities were found in eyes with FECD compared with those with BK (estimated mean difference, 261 cells/mm2; 95% CI, 118-404; P = .003) and in eyes with attached grafts compared with those with partially detached grafts (estimated mean difference, 330 cells/mm2; 95% CI, 208-452; P < .001), until 8 years. In 11 eyes (3.1%) that had concomitant ocular pathology, endothelial failure occurred within 4 years after DMEK. The overall graft survival probability was 0.96 at 5 and 8 years (95% CI, 0.94-0.99). At 8 years, better survival rates were found in eyes with FECD than in those with BK (survival probability, 0.97 [95% CI, 0.95-0.99] vs 0.84 [95% CI, 0.70-0.99], respectively); until the same follow-up, survival probabilities in eyes with attached and partially detached grafts were 0.97 (95% CI, 0.95-0.99) and 0.91 (95% CI, 0.82-0.99), respectively. CONCLUSIONS AND RELEVANCE: Endothelial decay was higher in eyes with a partial graft detachment than in those with attached grafts and lower in eyes with FECD than in those with BK. Endothelial failure only occurred in eyes with concomitant ocular pathology. These results suggest that eyes with DMEK that have undergone surgery for FECD with a completely attached graft may have an excellent prognosis.

Association of Childhood Complex Trauma and Dissociation With Complex Posttraumatic Stress Disorder Symptoms in Adulthood.

This study replicates and extends prior research on the relationship of childhood complex trauma (CCT) and complex posttraumatic stress disorder (cPTSD) in adulthood, examining the role of psychoform and somatoform dissociation as a potential mediator. CCT, dissociation, and cPTSD were assessed in a large sample of adult psychiatric inpatients. Almost two thirds of participants reported having experienced CCT. Path analyses with bootstrap confidence intervals demonstrated a relationship between CCT, psychoform (but not somatoform) dissociation, and cPTSD. In addition, psychoform dissociation partially mediated the relationship between CCT and adult cPTSD symptoms. Dissociation (pathological or nonpathological psychoform and somatoform symptoms) warrants further clinical and scientific study as a potential link between CCT and the presence of adult cPTSD symptoms and/or the dissociative subtype of PTSD.

Prevalence of use of performance enhancing drugs by fitness centre members.

Studies on the use of performance enhancing drugs (PED) in fitness centres rely predominately on conventional survey methods using direct questioning. However, research indicates that direct questioning of sensitive information is characterized by under-reporting. The aim of the present study was to contrast direct questioning of different types of PED use by Dutch fitness centre members with results obtained with the Randomized Response Technique (RRT). Questionnaires were conducted among members of fitness centres. PED were classified into the following categories: anabolic steroids, prohormones, substances to counteract side-effects, growth hormone and/or insulin, stimulants (to reduce weight), and miscellaneous substances. A total of 718 athletes from 92 fitness centres completed the questionnaire. The conventional method resulted in prevalences varying between 0% and 0.4% for the different types of PED with an overall prevalence of 0.4%. RRT resulted in prevalences varying between 0.8% and 4.8% for the different types of PED with an overall prevalence of 8.2%. The overall prevalence of the two survey methods differed significantly. The current study showed that the conventional survey method using direct questioning led to an underestimation of the prevalence. Based on the RRT results, the percentage of users of PED among members of fitness centres is approximately 8.2%. Stimulants to lose weight had the highest prevalence, even higher than anabolic steroids. The key task for future preventive health work is to not only focus on anabolic steroid use, but also include interventions focusing on the use of stimulants to lose weight.

Dual imputation model for incomplete longitudinal data.

Missing values are a practical issue in the analysis of longitudinal data. Multiple imputation (MI) is a well-known likelihood-based method that has optimal properties in terms of efficiency and consistency if the imputation model is correctly specified. Doubly robust (DR) weighing-based methods protect against misspecification bias if one of the models, but not necessarily both, for the data or the mechanism leading to missing data is correct. We propose a new imputation method that captures the simplicity of MI and protection from the DR method. This method integrates MI and DR to protect against misspecification of the imputation model under a missing at random assumption. Our method avoids analytical complications of missing data particularly in multivariate settings, and is easy to implement in standard statistical packages. Moreover, the proposed method works very well with an intermittent pattern of missingness when other DR methods can not be used. Simulation experiments show that the proposed approach achieves improved performance when one of the models is correct. The method is applied to data from the fireworks disaster study, a randomized clinical trial comparing therapies in disaster-exposed children. We conclude that the new method increases the robustness of imputations.

Incidence of irregular astigmatism eligible for contact lens fitting after Descemet membrane endothelial keratoplasty.

PURPOSE: To evaluate the incidence and causes of anterior corneal surface irregularities after successful Descemet membrane endothelial keratoplasty (DMEK) and the efficacy of contact lens fitting in these cases. SETTING: Tertiary referral center. DESIGN: Retrospective study of prospectively collected data. METHODS: Eyes with a subnormal visual outcome or monocular diplopia after successful DMEK were fitted with a contact lens. These cases were evaluated with Pentacam rotating Scheimpflug camera imaging preoperatively and 6 months postoperatively, and outcomes were compared with those in a randomly selected DMEK control group. RESULTS: In a series of 262 surgeries, 23 eyes (21 patients) were fitted with contact lenses; the control group comprised 23 eyes. Indications for contact lens fitting included (1) obvious corneal surface irregularities due to preexisting corneal scarring, (2) surface irregularities associated with longstanding preoperative stromal edema, and (3) undetectable optical imperfections. The postoperative corrected distance visual acuity (CDVA) with spectacles improved after contact lens correction (P<.001). Preoperative and postoperative irregularity indices were significantly higher in the contact lens group than in the DMEK control group (P<.05). Positive correlations were found between the duration of preoperative stromal edema and postoperative Scheimpflug camera indices (P<.02). CONCLUSIONS: After successful DMEK, 23 of 262 eyes (9%) showed subnormal spectacle CDVA and/or monocular diplopia due to corneal scarring, surface irregularities, or undetectable optical imperfections that could be managed by contact lens fitting. Prolonged preoperative corneal edema for more than 12 months may be a risk factor for diffuse irregular astigmatism after DMEK. FINANCIAL DISCLOSURE: Dr. Melles is a consultant to DORC International BV/Dutch Ophthalmic USA. No author has a financial or proprietary interest in any material or method mentioned.

Outcomes of Descemet membrane endothelial keratoplasty in phakic eyes.

PURPOSE: To determine the clinical outcomes of isolated Descemet membrane transplantation (ie, Descemet membrane endothelial keratoplasty [DMEK]) in phakic eyes. SETTING: Tertiary referral center. DESIGN: Cohort study. METHODS: Phakic eyes from a larger group of consecutive eyes that had DMEK for Fuchs endothelial dystrophy were examined. The examination included corrected distance visual acuity (CDVA), subjective and objective refractions, endothelial cell density (ECD), and intraoperative and postoperative complications at 1, 3, and 6 months. RESULTS: The study enrolled 52 phakic eyes from a group of 260 DMEK eyes. Of the phakic eyes, 69% reached a CDVA equal to or better than 20/40 (≥0.5) within 1 week and 85% reached equal to or better than 20/25 (≥0.8) at 6 months. Compared with an age-matched control group of pseudophakic eyes, phakic eyes had a similar visual rehabilitation rate, final visual outcome, mean ECD at 6 months (1660 cells/mm(2) ± 470 [SD]), minor hyperopic shift (+0.74 diopter), and graft detachment rate (4%). Visual acuity equal to or better than 20/13 (≥1.5) was limited to phakic eyes, suggesting better optical quality with the crystalline lens in situ. Temporary mechanical angle-closure glaucoma due to air-bubble dislocation behind the iris was the main complication (11.5%). Two eyes (4%) required phacoemulsification after DMEK. CONCLUSIONS: In phakic eyes, DMEK may give excellent visual outcomes without an increased risk for complications. Visual acuities equal to or better than 20/13 (≥1.5) may indicate that the almost anatomic repair after DMEK is associated with near perfect optical quality of the transplanted cornea. FINANCIAL DISCLOSURE: Dr. Melles is a consultant to D.O.R.C. International/Dutch Ophthalmic USA. No author has a financial or proprietary interest in any material or method mentioned.

Towards an evidence-based treatment model for cognitive behavioral interventions focusing on chronic fatigue syndrome.

OBJECTIVE: The purpose of the present study was to develop a treatment model for cognitive behavioral interventions focusing on chronic fatigue syndrome (CFS) based on the model of perpetuating factors introduced by Vercoulen et al. [Journal of Psychosomatic Research 1998;45:507-17]. METHODS: For this purpose, we reanalyzed the data of a previously conducted randomized controlled trial in which a low intensity cognitive behavioral intervention was compared to a waiting list control group. Structural equation modeling was used to test a treatment model in which changes in focusing on symptoms, perceived problems with activity, and sense of control over fatigue were hypothesized to mediate the effect of our intervention on fatigue severity and disability. RESULTS: In the final model, which had a good fit to the data, the effect of treatment was mediated by a decrease in perceived problems with activity and an increase in sense of control over fatigue. CONCLUSION: Our findings suggest that cognitive behavioral interventions for CFS need to change the illness perception and beliefs of their patients in order to be effective.

Prevention and management of graft detachment in descemet membrane endothelial keratoplasty.

OBJECTIVE: To describe the prevention and management of various types of graft detachment after Descemet membrane endothelial keratoplasty. METHODS: In 150 consecutive eyes that underwent Descemet membrane endothelial keratoplasty, the incidence and type of graft detachment were studied at 1, 3, 6, 9, 12, and 24 months after surgery in a nonrandomized, prospective clinical study at a tertiary referral center. Four groups of detachments were identified: a partial detachment of one-third or less of the graft surface area (n = 16; group 1); a partial detachment of more than one-third of the graft surface area (n = 8; group 2); a graft positioned upside down (n = 4; group 3); and a free-floating Descemet roll in the host anterior chamber (n = 8; group 4). RESULTS: Partial or complete graft detachment was found in 36 cases (24%), of which 18 (12%) were clinically significant. All 24 eyes with a partial detachment (groups 1 and 2) showed spontaneous corneal clearance, and all but 6 of these eyes (75%) reached visual acuity of 20/40 or better (≥0.5). A reversed clearance pattern and interface spikes were observed in eyes with the graft positioned upside down (group 3). Eyes with a free-floating graft (group 4) showed persistent corneal edema. Detachments were associated with inward folds (12 eyes [33%]), insufficient air-bubble support (7 eyes [19%]), upside-down graft positioning (4 eyes [11%]), use of plastic materials (2 eyes [6%]), irido-graft synechiae (1 eye [3%]), poor endothelial morphology (1 eye [3%]), and stromal irregularity under the main incision (1 eye [3%]); 14 (58%) of the partial detachments were localized inferiorly. CONCLUSIONS: Awaiting spontaneous clearance may be advocated in eyes with a partial detachment. Minor adjustments in surgical protocol as well as careful patient selection may further reduce the incidence of graft detachment after Descemet membrane endothelial keratoplasty to 4% or less. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00521898.

Patterns of corneal endothelialization and corneal clearance after descemet membrane endothelial keratoplasty for fuchs endothelial dystrophy.

PURPOSE: To describe various endothelial migration healing patterns after Descemet membrane endothelial keratoplasty (DMEK), and to determine the contribution of the donor and host endothelium in the clearance of a transplanted cornea. DESIGN: Nonrandomized, prospective clinical study. METHODS: In a total of 150 consecutive eyes that underwent DMEK (ie, transplantation of an isolated Descemet graft, for Fuchs endothelial dystrophy), re-endothelialization patterns were studied. Of these eyes, 36 showed a "stromal gap" between the "descemetorhexis edge" and the graft, or (partial) graft detachment. Endothelialization patterns of the host posterior stroma were documented at 1, 3, 6, 9, 12, and 24 months after surgery with Pentacam imaging, specular microscopy, optical coherence tomography, confocal microscopy, and slit-lamp biomicroscopy. RESULTS: Complete corneal clearance was seen in 28 of 36 eyes (78%) with a stromal gap, or (partial) detachment, progressing from the periphery toward the center; and 27 of 34 eyes (79%) with normal visual potential reached a visual acuity of ≥20/40 (≥0.5) or better. In 3 eyes that had the Descemet graft implanted upside-down, a "reversed corneal clearance pattern" was observed (ie, persistent edema where the graft was attached), while the area overlying the detachment cleared. One case that had a "descemetorhexis" performed without endothelial graft implantation showed persistent stromal edema. CONCLUSION: The presence of donor endothelium in the recipient anterior chamber may be required for endothelial migration and/or recovery of corneal clarity. Re-endothelialization may be associated with massive endothelial migration and some form of cell signaling to draw donor endothelial cells toward the recipient posterior stroma ("homing").

Refractive change and stability after Descemet membrane endothelial keratoplasty. Effect of corneal dehydration-induced hyperopic shift on intraocular lens power calculation.

PURPOSE: To determine the refractive change and stability of the transplanted cornea after Descemet membrane endothelial keratoplasty (DMEK) through a 3.0 mm clear corneal incision. SETTING: Tertiary referral center. DESIGN: Cohort study. METHODS: Subjective and objective refractive data from pseudophakic eyes were obtained before and 3 and 6 months after DMEK. RESULTS: The study comprised 50 eyes, 7 were phakic and 43 pseudophakic. Six months postoperatively, the corrected distance visual acuity was 20/25 (0.8) or better in 38 eyes (74%). The mean increase in spherical equivalent at 6 months (N = 50) was +0.32 diopter (D) ± 1.01 D (SD) (P=.0304) and in refractive cylinder, -0.48 ± 1.02 D (P=.001). Although Scheimpflug imaging showed a stable anterior corneal curvature, the posterior curvature increased from 5.50 ± 0.5 D preoperatively to 6.40 ± 0.4 D at 6 months and pachymetry decreased from 672 ± 82 µm to 540 ± 59 µm, respectively (both N = 32) (both P=.000). CONCLUSIONS: After DMEK, a slight preoperative to postoperative refractive change and stabilization at 3 months occurred that may induce a hyperopic shift that was not the result of the negative lenticule effect of DSEK/DSAEK. Thus, in DMEK, the hyperopic shift may result from a reversal of a preceding myopic shift induced by stromal swelling in endothelial disease. If so, normal intraocular power nomograms apply for cataract surgery before or during DMEK.

Efficacy of descemet membrane endothelial keratoplasty: clinical outcome of 200 consecutive cases after a learning curve of 25 cases.

OBJECTIVE: To evaluate Descemet membrane endothelial keratoplasty for management of corneal endothelial disorders. METHODS: Descemet membrane endothelial keratoplasty was performed in 200 patients with Fuchs endothelial dystrophy or bullous keratopathy. Best-corrected visual acuity, subjective and objective refractive outcome and stability, and endothelial cell density were evaluated at 1, 3, and 6 months postoperatively, and intraoperative and postoperative complications were documented. RESULTS: At 6 months, 94% reached a best-corrected visual acuity of 20/40 or better (≥0.5); 77%, 20/25 or better (≥0.8); 47%, 20/20 or better (≥1.0), and 16%, 20/17 or better (≥1.2) (n = 159). The preoperative to 6 months' postoperative spherical equivalent showed a mean (SD) +0.38 (1.2) diopter hyperopic shift (P = .001) that correlated with a decrease in central corneal thickness (n = 143) (P = .047). Two-thirds of eyes showed refractive stability at 3 months. Donor endothelial cell density showed a decrease from mean (SD) 2560 (186) cells/mm(2) preoperatively to 1690 (520) cells/mm(2) at 6 months after surgery (n = 173) (P < .001). Graft detachment was the main complication and occurred in 18 eyes (9%). Recipient Descemet membrane remnants were present in 12 eyes (6%). Secondary glaucoma was seen in 8 eyes (4%), of which 4 showed air-bubble dislocation behind the iris. In 2 of 33 phakic eyes (6%), a secondary cataract developed requiring phacoemulsification. CONCLUSIONS: Descemet membrane endothelial keratoplasty may offer complete visual rehabilitation within 1 to 6 months after surgery in a majority of eyes. Similar to earlier keratoplasty techniques, Descemet membrane endothelial keratoplasty may be associated with a one-third decrease in donor endothelial cell density in the early postoperative phase. Incidence of (partial) graft detachment stabilized at about 5% but could be further reduced by patient selection and/or technique modification. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00521898.

Feature selection in feature network models: finding predictive subsets of features with the Positive Lasso.

A set of features is the basis for the network representation of proximity data achieved by feature network models (FNMs). Features are binary variables that characterize the objects in an experiment, with some measure of proximity as response variable. Sometimes features are provided by theory and play an important role in the construction of the experimental conditions. In some research settings, the features are not known a priori. This paper shows how to generate features in this situation and how to select an adequate subset of features that takes into account a good compromise between model fit and model complexity, using a new version of least angle regression that restricts coefficients to be non-negative, called the Positive Lasso. It will be shown that features can be generated efficiently with Gray codes that are naturally linked to the FNMs. The model selection strategy makes use of the fact that FNM can be considered as univariate multiple regression model. A simulation study shows that the proposed strategy leads to satisfactory results if the number of objects is less than or equal to 22. If the number of objects is larger than 22, the number of features selected by our method exceeds the true number of features in some conditions.

Estimating standard errors in feature network models.

Feature network models are graphical structures that represent proximity data in a discrete space while using the same formalism that is the basis of least squares methods employed in multidimensional scaling. Existing methods to derive a network model from empirical data only give the best-fitting network and yield no standard errors for the parameter estimates. The additivity properties of networks make it possible to consider the model as a univariate (multiple) linear regression problem with positivity restrictions on the parameters. In the present study, both theoretical and empirical standard errors are obtained for the constrained regression parameters of a network model with known features. The performance of both types of standard error is evaluated using Monte Carlo techniques.

Effects of Group Size and Lack of Sphericity on the Recovery of Clusters in K-means Cluster Analysis.

K-means cluster analysis is known for its tendency to produce spherical and equally sized clusters. To assess the magnitude of these effects, a simulation study was conducted, in which populations were created with varying departures from sphericity and group sizes. An analysis of the recovery of clusters in the samples taken from these populations showed a significant effect of lack of sphericity and group size. This effect was, however, not as large as expected, with still a recovery index of more than 0.5 in the "worst case scenario." An interaction effect between the two data aspects was also found. The decreasing trend in the recovery of clusters for increasing departures from sphericity is different for equal and unequal group sizes.