Patrick Egan

Intraocular Pressure, Blood Pressure, and Retinal Blood Flow Autoregulation: A Mathematical Model to Clarify Their Relationship and Clinical Relevance

PURPOSE This study investigates the relationship between intraocular pressure (IOP) and retinal hemodynamics and predicts how arterial blood pressure (BP) and blood flow autoregulation (AR) influence this relationship. METHODS A mathematical model is developed to simulate blood flow in the central retinal vessels and retinal microvasculature as current flowing through a network of resistances and capacitances. Variable resistances describe active and passive diameter changes due to AR and IOP. The model is validated by using clinically measured values of retinal blood flow and velocity. The model simulations for six theoretical patients with high, normal, and low BP (HBP-, NBP-, LBP-) and functional or absent AR (-wAR, -woAR) are compared with clinical data. RESULTS The model predicts that NBPwAR and HBPwAR patients can regulate retinal blood flow (RBF) as IOP varies between 15 and 23 mm Hg and between 23 and 29 mm Hg, respectively, whereas LBPwAR patients do not adequately regulate blood flow if IOP is 15 mm Hg or higher. Hemodynamic alterations would be noticeable only if IOP changes occur outside of the regulating range, which, most importantly, depend on BP. The model predictions are consistent with clinical data for IOP reduction via surgery and medications and for cases of induced IOP elevation. CONCLUSIONS The theoretical model results suggest that the ability of IOP to induce noticeable changes in retinal hemodynamics depends on the levels of BP and AR of the individual. These predictions might help to explain the inconsistencies found in the clinical literature concerning the relationship between IOP and retinal hemodynamics.

The role of retrobulbar and retinal circulation on optic nerve head and retinal nerve fibre layer structure in patients with open-angle glaucoma over an 18-month period

Background/aims Evidence suggests that vascular abnormalities play a role in the pathogenesis of open-angle glaucoma (OAG) in some patients. This study aims to assess changes in retrobulbar and retinal blood flow over time in patients with glaucoma and examine their relationship to glaucomatous progression, as determined by retinal and optic nerve structure. Methods In this observational study, 103 patients with OAG were examined at baseline and 18 months follow-up. Retrobulbar blood flow was measured by colour Doppler imaging in the ophthalmic, central retinal and temporal posterior ciliary artery (TPCA) and nasal short posterior ciliary artery. Retinal capillary blood flow was measured by confocal scanning laser Doppler. Peripapillary retinal nerve fibre layer thickness was assessed by optical coherence tomography. Non-parametric Wilcoxon signed ranks tests were used to assess for any statistically significant changes between the baseline and 18-month visits for the retrobulbar and retinal flow, as well as the structural parameters. Results In general, retinal and retrobulbar blood flow parameters decreased over 18 months. Thinning of the optic disc rim and increase in cup area were associated with a higher resistance index (p=0.0334) and lower peak systolic velocity of TPCA (p=0.0282), respectively. A higher amount of retinal zero pixel blood flow correlated with a greater increase in cup/disc ratio (p=0.0170). Conclusions Reductions in retrobulbar and retinal blood flow over time were associated with structural glaucomatous progression, as indicated by retinal and optic nerve changes.

Generic medications in ophthalmology

The purpose of this review is to discuss the process of genericisation of medications in the US and Europe with a focus on ophthalmic drugs. Regulatory guidelines of the US Food and Drug Administration and the European Medicines Agency will be discussed, and the advantages and concerns of genericisation will be explored. We will look at various studies concerning the safety and efficacy of generic drugs compared to their branded counterparts. In particular, the challenges of assuring bioequivalence and therapeutic equivalence in topical ophthalmic drugs will be examined.

Flipping the Switch Power, Social Dominance, and Expectancies of Mental Energy Change

Research suggests that high levels of interpersonal power can promote enhanced executive functioning capabilities. The present work explored whether this effect is contingent upon expectancies concerning power’s downstream cognitive consequences. Study 1 showed that social dominance orientation (SDO) predicted idiosyncratic expectancies of mental energy change toward interpersonal power. Study 2 showed that SDO moderated the executive functioning changes associated with interpersonal power and that this moderation effect was contingent upon changes in perceived mental depletion. Study 3 showed that directly manipulating expectancies of mental energy change concerning interpersonal power moderated the executive functioning consequences of power and that this moderation effect was contingent upon SDO and changes in perceived mental depletion. Together, the present findings underscore the importance of expectancies and individual differences in understanding the effects of interpersonal power.

Goldmann applanation tonometry and dynamic contour tonometry are not correlated with central corneal thickness in primary open angle glaucoma

PurposeTo compare intraocular pressure (IOP) measured by Goldmann applanation tonometry (GAT) and dynamic contour tonometry (DCT) and assess their relationship to central corneal thickness (CCT) in patients with primary open angle glaucoma (OAG). Patients and MethodsGAT, DCT, and CCT were assessed in 116 patients with OAG [mean age 65.9 (10.5); 59% female] participating in the Indianapolis Glaucoma Progression Study. GAT and DCT were measured in a randomized order followed by CCT (ultrasonic corneal pachymetry) during a single study visit. Bland-Altman plots were used to evaluate the limits of agreement between tonometery methodologies whereas multivariate regression analysis was used to evaluate the influence of CCT on GAT and DCT IOP measurements. ResultsIOP values obtained by DCT and GAT showed a strong positive correlation in patients with OAG (r=0.93; P<0.001). Mean IOP measured with DCT [18.4 (5.1) mm Hg] was significantly higher (P<0.001) than GAT IOP measurements [16.5 (4.5) mmHg]. CCT did not seem to influence either GAT or DCT measurements (r=0.1025, P=0.16; r=0.05, P=0.46), respectively. The Bland-Altman data showed that the amount of disagreement between IOP assessment techniques varied, suggesting a proportional bias. ConclusionsIn this group of patients with OAG, there was a strong correlation between GAT and DCT measurements of IOP. IOP measured with DCT was consistently higher than IOP measured with GAT. Neither GAT nor DCT measurements were correlated with CCT. This data suggests that factors other than CCT may be involved in the tendency of DCT to produce higher measures of IOP than GAT.

The Malleable Efficacy of Willpower Theories

Emerging research documents the self-control consequences of individuals’ theories regarding the limited nature of willpower, such that unlimited theorists consistently demonstrate greater self-control than limited theorists. The purpose of the present research was to build upon prior work on self-validation and perceptions of mental fatigue to demonstrate when self-control is actually impaired by endorsing an unlimited theory and—conversely—enhanced by endorsing a limited theory. Four experiments show that fluency reinforces the documented effects of individuals’ willpower theories on self-control, while disfluency reverses the documented effects of individuals’ willpower theories on self-control. Moreover, these effects are driven by differential perceptions of mental fatigue—perceptions altered by individuals’ level of confidence in their willpower theory—and are bound by conditions that promote effortful thought. Collectively, these findings point to the malleable efficacy of willpower theories and the importance of belief confidence in dictating this malleability and in modulating subsequent self-control behavior.