Topical review: Task shifting and the recruitment and retention of eye health workers in underserved areas.

Many populations experience difficulty accessing eye care, especially in rural areas. Implementing workforce recruitment and retention strategies, as well as task shifting through widening scope of practice, can improve eye care accessibility. This article provides novel evidence on the compatibility of these strategies aimed at enhancing ophthalmic workforce recruitment, retention, and efficacy. PURPOSE: The global burden of blindness is unequally distributed, affects rural areas more, and is frequently associated with limited access to eye care. The World Health Organization has specified both task shifting and increasing human resources for eye health as instruments to improve access to eye care in underserved areas. However, it is uncertain whether these two instruments are sufficiently compatible to provide positive synergic effects. To address this uncertainty, we conducted a structured literature review and synthesized relevant evidence relating to task shifting, workforce recruitment, retention, and eye care. Twenty-three studies from across the globe were analyzed and grouped into three categories: studies exploring recruitment and retention in human resources for eye health in general, studies discussing the relationship between task shifting and recruitment or retention of health workers in general, and studies specifically discussing task shifting and recruitment or retention in eye care workers. FINDINGS: Our findings demonstrate that incentives are effective for initiating task shifting and improving recruitment and retention in rural areas with a stronger effect noted in midlevel eye care professionals and trainees. Incentives can take various forms, e.g., financial and nonfinancial. The consideration of context-specific motivational factors is essential when designing strategies to facilitate task shifting and to improve recruitment and retention.

Exploring eye care pathways, patient priorities and economics in Pakistan: A scoping review and expert consultation study with thematic analysis.

PURPOSE: As the prevalence of eye diseases increases, demand for effective, accessible and equitable eye care grows worldwide. This is especially true in lower and middle-income countries, which have variable levels of infrastructure and economic resources to meet this increased demand. In the present study we aimed to review the literature on eye care in Pakistan comprehensively, with a particular focus on eye care pathways, patient priorities and economics. METHODS: A systematic scoping review was performed to identify literature relating to eye care in Pakistan. Searches of relevant electronic databases and grey literature were carried out. The results were analysed through a mixed methods approach encompassing descriptive numerical summary and thematic analysis. To consolidate results and define priority areas for future study, expert consultation exercises with key stakeholders were conducted using qualitative semi-structured interviews. RESULTS: One hundred and thirty-two papers (published and unpublished) were included in the final review. The majority (n = 93) of studies utilised a quantitative design. Seven interlinked themes were identified: eye care pathways, burden of eye disease, public views on eye-related issues, workforce, barriers to uptake of eye care services, quality of eye care services and economic impact of blindness. Research priorities included investigating the eye care workforce, the quality and efficiency of current eye care services, eye care services available in rural Pakistan and the costs and benefits related to eye care provision and sustaining eye care programmes. CONCLUSIONS: To the best of our knowledge, this is the first review to synthesise evidence from papers across the field relating to eye care in Pakistan. As such, this work provides new insights into the achievements of the national eye health programme, challenges in eye care in Pakistan and priority areas for future research.

Socio-economic differences in accessing NHS spectacles amongst children with differing refractive errors living in Scotland.

BACKGROUND/OBJECTIVES: Adults living in more deprived areas are less likely to attend an eye examination, resulting in greater visual impairment from undiagnosed eye disease and a widening of health inequalities. It is unknown if the introduction of free NHS eye examinations and help with spectacle costs has benefited children in Scotland. This study aimed to explore factors associated with accessing NHS spectacles including level of deprivation, refractive error, urbanity and age. SUBJECTS/METHODS: NHS-financed General Ophthalmic Services (GOS) 3 supplement the cost of spectacles for children under 16 years. Administrative data on the spectacle refraction dispensed were obtained from Information Services Division (ISD) for mainland Scotland, 2018, and categorised by: Emmetropes/low hyperopes (reference group), myopes and moderate/high hyperopes. Data were linked to the Scottish Index of Multiple Deprivation (SIMD) quintile. RESULTS: Data included 108, 043 GOS 3 claims. Greater deprivation was associated with greater GOS 3 claims p = 0.041. This was most evident in emmetropic/low hyperopic children and in moderate/high hyperopic children. GOS 3 claims in the myopes group increased with age across all SIMD and decreased with age in the moderate/high hyperope group (all p < 0.001). GOS 3 claims were not associated with urbanity for all Health Boards (p = 0.13). CONCLUSIONS: Children in areas of greater deprivation and in more rural areas are not disadvantaged in accessing NHS spectacles. This did not vary by refractive error group. This suggests that health policy in Scotland is accessible to those from all deprivation levels and refractive errors.

The role of peripheral ocular length and peripheral corneal radius of curvature in determining refractive error.

PURPOSE: The purpose of this study was to extend the knowledge of peripheral biometric component and its relationship to refractive status in healthy individuals by determining the correlation between peripheral ocular length to peripheral corneal radius ratio and the refractive error. METHODS: This prospective study was conducted on thirty-three healthy adult participants. Refractive error was assessed objectively and subjectively and recorded as the mean spherical equivalent. Central and peripheral ocular lengths at 30° were assessed using partial coherence interferometry under dilation with 1% tropicamide. Central and peripheral corneal radius of curvature was assessed using Scheimpflug topography. Peripheral ocular lengths at 30° were paired with peripheral corneal curvatures at the incident points of the IOLMaster beam (3.8mm away from corneal apex) superiorly, inferiorly, temporally and nasally to calculate the peripheral ocular length-peripheral corneal radius ratio. Descriptive statistics were used to describe the distribution and spread of the data. Pearson's correlation analysis was used to present the association between biometric and refractive variables. RESULTS: Refractive error was negatively correlated with the axial length-central corneal radius ratio (r=-0.91; p<0.001) and with 30° peripheral ocular length-peripheral corneal radius ratio in all four meridians (r≤-0.76; p<0.001). The strength of the correlation was considerably lower when only axial length or peripheral ocular lengths were used. CONCLUSION: Using the ratios of peripheral ocular length-peripheral corneal radius to predict refractive error is more effective than using peripheral corneal radius or peripheral ocular length alone.

COVID-19: ensuring safe clinical teaching at university optometry schools.

The COVID-19 pandemic has been spreading across the globe for several months. The nature of the virus (SARS-CoV-2) with easy person-to-person transmissions and the severe clinical course observed in some people necessitated unprecedented modifications of everyday social interactions. These included the temporary suspension of considerable elements of clinical teaching at optometry schools worldwide. This article describes the challenges optometry schools were facing in early to mid 2020. The paper highlights the experiences of six universities in five countries on four continents. Strategies to minimise the risk of virus transmission, to ensure safe clinical optometric teaching and how to overcome the challenges presented by COVID-19 are described. An outlook on opportunities to further improve optometric education is provided.

Interventions for visual field defects in people with stroke.

BACKGROUND: Visual field defects are estimated to affect 20% to 57% of people who have had a stroke. Visual field defects can affect functional ability in activities of daily living (commonly affecting mobility, reading and driving), quality of life, ability to participate in rehabilitation, and depression and anxiety following stroke. There are many interventions for visual field defects, which are proposed to work by restoring the visual field (restitution); compensating for the visual field defect by changing behaviour or activity (compensation); substituting for the visual field defect by using a device or extraneous modification (substitution); or ensuring appropriate diagnosis, referral and treatment prescription through standardised assessment or screening, or both. OBJECTIVES: To determine the effects of interventions for people with visual field defects after stroke. SEARCH METHODS: We searched the Cochrane Stroke Group Trials Register, the Cochrane Eyes and Vision Group Trials Register, CENTRAL, MEDLINE, Embase, CINAHL, AMED, PsycINFO, and PDQT Databse, and clinical trials databases, including ClinicalTrials.gov and WHO Clinical Trials Registry, to May 2018. We also searched reference lists and trials registers, handsearched journals and conference proceedings, and contacted experts. SELECTION CRITERIA: Randomised trials in adults after stroke, where the intervention was specifically targeted at improving the visual field defect or improving the ability of the participant to cope with the visual field loss. The primary outcome was functional ability in activities of daily living and secondary outcomes included functional ability in extended activities of daily living, reading ability, visual field measures, balance, falls, depression and anxiety, discharge destination or residence after stroke, quality of life and social isolation, visual scanning, adverse events, and death. DATA COLLECTION AND ANALYSIS: Two review authors independently screened abstracts, extracted data and appraised trials. We undertook an assessment of methodological quality for allocation concealment, blinding of outcome assessors, method of dealing with missing data, and other potential sources of bias. We assessed the quality of evidence for each outcome using the GRADE approach. MAIN RESULTS: Twenty studies (732 randomised participants, with data for 547 participants with stroke) met the inclusion criteria for this review. However, only 10 of these studies compared the effect of an intervention with a placebo, control, or no treatment group, and eight had data which could be included in meta-analyses. Only two of these eight studies presented data relating to our primary outcome of functional abilities in activities of daily living. One study reported evidence relating to adverse events.Three studies (88 participants) compared a restitutive intervention with a control, but data were only available for one study (19 participants). There was very low-quality evidence that visual restitution therapy had no effect on visual field outcomes, and a statistically significant effect on quality of life, but limitations with these data mean that there is insufficient evidence to draw any conclusions about the effectiveness of restitutive interventions as compared to control.Four studies (193 participants) compared the effect of scanning (compensatory) training with a control or placebo intervention. There was low-quality evidence that scanning training was more beneficial than control or placebo on quality of life, measured using the Visual Function Questionnaire (VFQ-25) (two studies, 96 participants, mean difference (MD) 9.36, 95% confidence interval (CI) 3.10 to 15.62). However, there was low or very-low quality evidence of no effect on measures of visual field, extended activities of daily living, reading, and scanning ability. There was low-quality evidence of no significant increase in adverse events in people doing scanning training, as compared to no treatment.Three studies (166 participants) compared a substitutive intervention (a type of prism) with a control. There was low or very-low quality evidence that prisms did not have an effect on measures of activities of daily living, extended activities of daily living, reading, falls, or quality of life, and very low-quality evidence that they may have an effect on scanning ability (one study, 39 participants, MD 9.80, 95% CI 1.91 to 17.69). There was low-quality evidence of an increased odds of an adverse event (primarily headache) in people wearing prisms, as compared to no treatment.One study (39 participants) compared the effect of assessment by an orthoptist to standard care (no assessment) and found very low-quality evidence that there was no effect on measures of activities of daily living.Due to the quality and quantity of evidence, we remain uncertain about the benefits of assessment interventions. AUTHORS' CONCLUSIONS: There is a lack of evidence relating to the effect of interventions on our primary outcome of functional ability in activities of daily living. There is limited low-quality evidence that compensatory scanning training may be more beneficial than placebo or control at improving quality of life, but not other outcomes. There is insufficient evidence to reach any generalised conclusions about the effect of restitutive interventions or substitutive interventions (prisms) as compared to placebo, control, or no treatment. There is low-quality evidence that prisms may cause minor adverse events.

Patient-reported severity of dry eye and quality of life in diabetes.

PURPOSE: The purpose of the study was to assess the relationship between patient-reported severity of dry eye disease (DED), quality of life (QoL), presence of diabetic retinopathy (DR) and length of disease duration in people with type 1 diabetes mellitus (DM1) and type 2 diabetes mellitus (DM2). PATIENTS AND METHODS: A survey of 152 people (110 with and 42 without diabetes). All participants completed the Ocular Surface Disease Index (OSDI) and Dry Eye-related Quality of Life Score (DEQS) questionnaires. RESULTS: Forty-four percent of all diabetic subjects reported dry eye symptoms, compared to 29% in the control group. Patients with DM2 reported dry eye symptoms more frequently than those with DM1 (55% and 27% respectively, P=0.001). Dry eye severity was linked to a significant deterioration in QoL in both types of diabetes (DM1, r=0.609 and P=0.036; DM2, r=0.417 and P=0.011). Irrespective of DR, the presence of DED was significantly higher in DM2 compared to DM1 (with DR, P=0.011; without DR, P=0.018). CONCLUSION: Dry eye symptoms are associated with reduced QoL and are more common in people with DM2 than in DM1, irrespective of DR status. Routine clinical screening for severe DED could potentially allow for a timely and more effective treatment and could contribute to mitigating the dry eye-associated reduction in QoL in those with DM2.

Corneal structure, transparency, thickness and optical density (densitometry), especially as relevant to contact lens wear-a review.

Clinical instruments using Scheimpflug image-based methods to obtain optical sectional images of the cornea have been introduced in recent years along with proposals that it should be possible to routinely and reliably measure the optical density (referred to as the densitometry) of the human cornea in situ. Such a concept is reviewed from the perspective of what might be considered as the basic principles underlying the understanding of corneal transparency (from the 1950's) and the progressive changes in these ideas from subjective slitlamp-based clinical observations from the late 1960's, especially in contact lens wearers. Much more has been learned about the overall macrostructure (including corneal thickness) and the ultrastructure of the cornea from contemporary studies in the 1990's, and these aspects of the cornea will be reviewed alongside consideration of the methods of assessing the optical characteristics of the cornea in the living eye. From these perspectives, in this review systematic consideration will be given to what objective (quantitative) output one of these Scheimpflug-based systems provides and how this information might be actually related to corneal transparency characteristics that might be observed clinically, particularly after long-term contact lens wear.

Effect of individual-level and socioeconomic factors on long-term survival after cataract surgery over a 30-year period.

PURPOSE: To evaluate survival and the risk for mortality after cataract surgery in relation to individual-level and socioeconomic factors in Scotland over 3 decades. SETTING: Linked healthcare data, United Kingdom. DESIGN: Representative population-based study. METHODS: A 5% random sample of Scottish decedents linked to hospital records (1981 to 2012) was assessed. Survival time, survival probability, and determinants of mortality were evaluated after the first and second recorded hospital episodes for cataract surgery. Cox proportional-hazards regression models were used to assess the effect of individual-level and socioeconomic factors including age, geographic location, socioeconomic status, and comorbidity on mortality. RESULTS: The study evaluated linked administrative healthcare data from 9228 deceased patients who had cataract surgery. The mean survival time was 2383 days ± 1853 (SD). The survival probability decreased from 98% 90 days after surgery to 22% at 10 years, 2% at 20 years, and 0% after 30 years. The mean age was 77 ± 9 years. Age (hazard ratio [HR] 3.66; 95% confidence interval [CI], 2.97-3.80; P < .001) and severe comorbidity (HR 1.68; 95% CI, 1.47-1.91; P < .001) were associated with an increased risk for mortality; women had a 20% lower risk than men (HR 0.80; 95% CI, 0.76-0.83; P < .001). Socioeconomic status and rural geographic locations were not linked to mortality. CONCLUSIONS: Long-term survival after cataract surgery was determined by individual-level characteristics reflecting the mortality patterns of aging populations. The mortality risk was independent of socioeconomic and geographic factors per se.

Relationship between Corneal Thickness and Radius to Body Height.

PURPOSE: To investigate the possible association between body stature (height) and corneal thickness and radius in younger-adult Caucasians, especially within the context of previously published literature. METHODS: Body height and weight were measured in 109 healthy subjects, with an average age of 24 ± 6 years (mean ± SD). Subjects underwent an ophthalmic assessment including anterior segment imaging by Scheimpflug topography and specular microscopy. Central and peripheral corneal thickness and corneal radius were analyzed. The relationship between body stature and corneal parameters was assessed using simple and multiple regression analysis. Effect size was determined by generating regression and correlation coefficients. RESULTS: Body height ranged from 1.54 to 1.86 m (mean ± SD 1.67 ± 0.08 m), central corneal thickness from 465 to 629 µm (554 ± 33 µm), whereas corneal radius measured between 7.16 and 8.49 mm (7.75 ± 0.24 mm). Body height was weakly associated with central corneal thickness and peripheral corneal thickness (r ≥ -0.180), and moderately with corneal radius (r = 0.351). Based on the regression equations, central corneal thickness decreases by 8 µm, whereas corneal radius increases by 0.11 mm for each 0.1-m difference in body height. No significant correlations were found for similar assessments using body weight or body mass index. CONCLUSIONS: Differences in corneal radius and corneal thickness can be linked to body stature. However, effect sizes were consistently small and no more than 13% of the variability in corneal curvature could be explained by variations in body stature.

Repeatability of Pentacam peripheral corneal thickness measurements.

PURPOSE: To assess the repeatability of corneal thickness (CT) measurements, along the horizontal meridian up to 5mm from centre, in healthy eyes with the Pentacam instrument. METHODS: CT was measured in 82 right eyes (82 healthy subjects) at 11 corneal locations nominally 1mm apart along the horizontal meridian with Scheimpflug topography Pentacam. Two consecutive scans were performed in quick succession. The repeatability of CT was determined by assessing differences between measurements and calculating the coefficient of variation (CV). The relative repeatability (difference) was calculated as the ratio of the absolute peripheral CT difference to the central thickness. A variant of Bland-Altman analysis was carried out to determine the effect of the overall magnitude of CT on the absolute and relative differences. RESULTS: Mean CT (µm) at the temporal 5mm location was 770 ± 51 and 823 ± 56 µm at the corresponding nasal location; central CT was 554 ± 36 µm. Good repeatability (CV < 1.2%) was found at all corneal locations. Differences (arithmetic; absolute and relative) were 3 ± 17 µm; 13 ± 11 µm; 2 ± 2%; respectively temporally and -1 ± 15 µm; 13 ± 8 µm; 2 ± 1% nasally, both at 5mm from the corneal apex. These values decreased for central CT (0 ± 6 µm; 4 ± 4 µm; 1 ± 1%). Differences between both measurements in all corneal locations were not statistically significant (p = 0.308 ANOVA Games-Howell). CONCLUSIONS: Pentacam shows good repeatability for pachymetry measurements up to 5 mm away from the corneal apex across the horizontal meridian, even though repeatability decreases slightly towards the periphery. A single Pentacam scan will be sufficient for most clinical applications.

Peripheral nasal-temporal corneal asymmetry in relation to corneal thickness: a Scheimpflug imaging study.

PURPOSE: To investigate the asymmetry of the peripheral cornea up to 5 mm nasally and temporally from the centre and to assess correlations with regional peripheral corneal thickness. METHODS: Central and peripheral corneal thickness was measured by Scheimpflug imaging (Pentacam) in 113 eyes of 113 healthy, pre-presbyopic Caucasian subjects. Absolute and relative corneal thickness were analysed in 1 mm steps up to 5 mm to the nasal and temporal sides with the corneal apex as the central reference point. Nasal-temporal asymmetry was calculated as the thickness ratio between corresponding off-centre thickness measurements. RESULTS: The mean (±SD) central corneal thickness was 552 ± 36 µm. CT increased by 22% at 4 mm temporally to 672 ± 44 µm, and 32% at 4 mm nasally to 731 ± 45 µm. The nasal-temporal asymmetry became greater with increasing distance from the corneal centre, with a mean difference of 59 ± 22 µm at 4 mm from the apex. The nasal-temporal thickness ratio, based on this difference, was significantly related to the relative temporal (r = -0.41, p < 0.001, simple linear regression) and nasal corneal thickness (r = 0.61, p < 0.001). CONCLUSIONS: A substantial and progressively increasing nasal-temporal asymmetry in corneal thickness has been confirmed by Scheimpflug imaging, which is related to the magnitude of corneal thickness at peripheral locations. Pachymetry output data and models, including volume calculations, that assume symmetry to the corneal thickness profile may not provide optimum metrics for planning and predicting the outcome of corneal refractive surgery procedures.

Data extraction and reporting strategies of studies assessing non-central corneal thickness by Pentacam: a review.

PURPOSE: To review data extraction strategies for the reporting of non-central corneal thickness with the Pentacam system. METHODS: Using predefined search terms, the electronic National Institutes of Health database (PubMed) was searched for studies assessing non-central corneal thickness using the Pentacam instrument. The article titles of the search results were screened for relevance. The abstracts of papers with appropriate titles were retrieved and read. Articles with relevant abstracts were obtained and read in full. The reference list of each article was hand searched to identify further studies. Articles reporting central corneal thickness only were excluded. RESULTS: Seventeen peer-reviewed studies were identified. Considerable differences in data extraction and reporting of non-central corneal thickness were noted, with non-central pachymetry being assessed at locations between 1.5 and 5.0mm away from the centre. The terminology used to describe non-central pachymetry was inconsistent. Ring-averaged and single point pachymetry data have been reported. Ring-averaged pachymetry may obscure considerable regional variability in corneal thickness. CONCLUSIONS: The use of different data extraction and reporting strategies can obscure regional corneal thickness asymmetries. This may influence the monitoring of corneal crosslinking outcomes, the interpretation of corneal swelling in contact lens studies and the clinical decision-making in preoperative assessments of refractive surgery patients. For the reliable identification of regional corneal thickness variations point-pachymetry data appears to be preferable over ring-averaged pachymetry.

The corneal endothelium after keratoplasty for keratoconus.

BACKGROUND: The aim was to assess the corneal endothelium, post-operative visual outcome and complications following keratoplasty for keratoconus. METHODS: Seventy-six corneal grafts (57 per cent penetrating keratoplasties and 43 per cent deep anterior lamellar keratoplasties) were assessed as part of routine follow-up at the Ophthalmology Department at Gartnavel General Hospital in Glasgow. Routine ophthalmic assessment included visual acuity, slitlamp biomicroscopy and scanning slit confocal microscopy of the central cornea. Case records were reviewed and demographic, surgical and post-operative data retrieved. Linear and logistic regression models were used to determine associations between endothelial cell density, visual acuity and post-operative complications. Kaplan-Meier analysis was used to estimate the probability for grafts to retain more than 700 endothelial cells per mm(2) and to have 0.3 logMAR or better vision. Odds ratios were calculated to assess the risk for low cell counts and poor visual outcome. RESULTS: The mean and standard deviation of the endothelial cell count was predictably higher in lamellar (2033 ± 643 cells per mm(2)) than penetrating keratoplasties (898 ± 380 cells per mm(2)) and the mean post-operative cell count for grafts without post-operative complications was higher (1585 ± 775) than those with complications (1312 ± 747 cells per mm(2)). Mean visual acuity was 0.25 ± 0.20 and 0.29 ± 0.41 for lamellar and penetrating keratoplasties, respectively. A higher number of post-operative events was associated with an increased risk for graft rejection (odds ratio 2.40, p = 0.008, multiple logistic regression) and with poorer visual outcome (odds ratio 1.38, p = 0.044). After keratoplasty, the visual outcome and endothelial cell density were not associated, either in penetrating or deep anterior lamellar grafts (p > 0.05). CONCLUSIONS: In keratoplasties performed for keratoconus, visual outcome cannot be predicted reliably by post-operative endothelial cell density but post-operative events can lead to poorer visual outcomes in both types of grafts.

Assessment of the reliability of endothelial cell-density estimates in the presence of pseudoguttata.

PURPOSE: The purpose of this work is to assess the reliability of endothelial cell-density (ECD) estimates in corneas with different severity pseudoguttata. METHODS: Specular microscopy was undertaken on grade 1, 2, or 3 pseudoguttata patients and age-matched controls aged 52-83 years. On high magnification prints of central cornea, areas of complete cells (all sides visible) and partial 'cells' (one or more sides obscured) were measured manually. Sets of 45 complete cells were selected, as well as 75 cells that were a mixture of complete and partial cells on guttate endothelia. ECD was calculated by a progressive averaging technique. RESULTS: Each group comprised 12 patients with similar range of ECD values (1,230-4,587 cells/mm(2)). Based on 40 complete cells, ECD could be estimated to within ±3.1% for grade 3 pseudoguttata versus ±2.0% for controls. If a mixture of complete and partial cells were measured, ECD could be estimated to within ±2.8% for grade 3 pseudoguttata images (n = 70 cells) and ±1.1% for controls. The estimated variability increases to substantial levels of ±20% if only ten cells were measured. No statistical differences in ECD were noted between guttate and normal endothelia if only complete cells were measured, but could be different if partial 'cells' were included. CONCLUSIONS: Providing adequate numbers of complete cells are measured and in the absence of obvious polymegathism, ECD estimates can be made to within around ±3% in the presence of typical but significant pseudoguttata.

Central corneal thickness and intraocular pressure measures in human corneas with endothelial guttata: an observational quality control study.

PURPOSE: The aim of the present study was to assess central corneal thickness (CCT) and intraocular pressure (IOP) in eyes where the corneas were affected by different degrees of severity of endothelial pseudo-guttata or guttata. METHODS: In a prospective, case series observational study, non-contact tonometry and non-contact specular microscopy (NCSM) with pachymetry for central corneal thickness measures were undertaken as routine procedures on predominantly older patients without a history of corneal problems or contact lens wear. For those showing any signs of corneal endothelial abnormalities, images of the central cornea endothelium were further processed to measure the area (as a percentage) occupied by the guttata. RESULTS: Abnormal endothelial images were obtained from 43 patients (seven with bilateral changes) with an average age of 67.5 years. Between 1.5 and 54.9 per cent of the endothelial images were affected by guttata, which were assigned grade 1 (20 eyes), grade 2 (18 eyes) or grade 3 (11 eyes). When assessed by grade, the central corneal thickness increased and the measured IOP decreased as the guttata became more numerous and confluent. Regression analyses revealed only a weak association between central corneal thickness (p = 0.044, r = 0.149) or the measured IOP (p = 0.090, r = -0.244) and the extent of the guttata (percentage). With the apparently contrasting IOP and central corneal thickness effects, no significant IOP-CCT relationship was noted (p ≥ 0.268, r ≤ 0.160). CONCLUSIONS: Where corneas have mild-to-modest non-dystrophic endothelial guttata, there may be a less predictable effect of corneal thickness on the outcome of tonometry.

The orbscan acoustic (correction) factor for central corneal thickness measures of normal human corneas.

BACKGROUND: Orbscan scanning-slit optical pachymetry was introduced over a decade ago and yielded higher central corneal thickness (CCT) values to the "gold standard" of contact ultrasound pachymetry (U/S). An acoustic correction factor (AF) was introduced later to compensate for this difference. The goal of this review was to assess the magnitude and consistency of the difference, as well as to assess how useful the AF had been. METHODS: Using PubMed (Medline)-sourced citations, published articles were identified that included data on CCT from U/S and Orbscan, with the latter data checked to see whether an AF had been applied. Main comparisons were made between (1) Orbscan data without AF and U/S, and (2) Orbscan data with a 0.92 AF applied and the U/S data. RESULTS: From 46 studies involving a total of 6136 eyes (average number per study of 133, range 6-1214), the average CCT values by U/S ranged from 0.520 to 0.580, for a group mean of 0.545 mm. For Orbscan without AF, the average CCT values ranged from 0.557 to 0.624 mm, for a mean of 0.582 mm, a net difference of 0.037 mm from U/S, with all Orbscan data (average values from any particular study) being higher than U/S. With a 0.92 AF applied, the net difference was -0.009 mm. The calculated limits of agreement between the two methods ranged from 0.004 to 0.073 without AF, but from -0.041 to + 0.023 mm with the AF. The overall outcome was essentially the same if weighted for cohort size and sample variability, or if only studies reporting on one eye were considered. CONCLUSIONS: Orbscan pachymetry can be expected to yield CCT data that is approximately 7% higher than U/S. The global application of a 0.92 AF does not robustly align the Orbscan CCT data to that of U/S and, in fact, can easily result in the data being as much as 7% lower. Overall, the level of agreement between Orbscan and U/S is limited, and Orbscan data should simply be reported as measured without any adjustment.