Intraocular pressure and axial length changes during altitude acclimatization from Beijing to Lhasa

Purpose

To investigate changes in intraocular pressure (IOP) and axial length (AL) on the ascent to high altitude from Beijing to Lhasa.

Patients and methods

Twenty volunteers (17 men, 3 women) who had been sent to work in Lhasa, Tibet for more than 6 months were enrolled. One of their journeys from Beijing to Lhasa was chosen as the time for the examination. IOP, AL, corneal curvature (K), and blood pressure (BP) were measured in Beijing (altitude 43 m) and Lhasa (altitude 3658 m). Their first examination was conducted at least 1 day before arriving in Lhasa and the second examination after they had stayed in Lhasa for 7 days. The data from the highland and lowland examinations were analyzed with a paired-sample T test and Pearson’s correlation coefficient was calculated for the association between IOP and average BP.

Results

The mean IOP was 12.65±2.34 mmHg in Beijing and 14.85±3.1 mmHg in Lhasa. The mean AL was 24.61±1.50mm in Beijing, and 24.98±1.45 mm in Lhasa. The IOP and AL showed a significant elevation in highland compared with lowland (P<0.05). The mean K was 43.58±2.25 D in Beijing and 43.56±2.21 D in Lhasa and no significant difference was found in this study (P>0.05). A positive correlation between variance of IOP and ACD was found (r = 0.475, P<0.05) and no correlation between IOP and average BP was noted.

Conclusions

High altitude may lead to a small but significant change in IOP and axial length. However, the shape of the corneal surface was not influenced by the hypobaric and hypoxic conditions.

Associations between anthropometric indicators and both refraction and ocular biometrics in a cross-sectional study of Chinese schoolchildren

OBJECTIVE

To identify associations between anthropometric indicators (height, weight and body mass index (BMI)) and both refraction and ocular biometrics in Chinese schoolchildren in Tianjin, China.

DESIGN

Cross-sectional study.

PARTICIPANTS

A total of 482 (86.07%) students (6-15 years old) with no history of ocular or systemic pathologies were enrolled in this study.

METHODOLOGY

Height and weight were measured using standardised protocols. Ocular biometrics (axial length (AL), vitreous chamber depth (VCD) and corneal curvature (CC)) were measured by a low-coherence optical reflectometry device. Cycloplegic refraction was measured using autorefraction. The AL/CC ratio and spherical equivalent refraction (SER) were calculated. Myopia was defined as SER ≤-0.50 dioptres (D). Multiple linear regression analysis was performed to explore the associations between anthropometric indicators (height, weight and BMI) and both refraction and ocular biometrics.

RESULTS

The overall prevalence of myopia was 71.16%. Overall, only height was associated with ALs, VCDs, AL/CC ratios and refractions after controlling for age, gender, parental myopia, family income, reading and writing distance and time spent outdoors. Furthermore, age-specific results demonstrated that height and weight were independently associated with refraction in participants aged 6-8 years and 9-11 years participants. Higher heights in schoolchildren were associated with longer ALs (regression coefficient b=+0.25 for each 10 cm difference in height, p<0.01), deeper VCDs (b=+0.23, p<0.01), higher AL/CC ratios (b=+0.04, p<0.01) and more negative refractions (b=-0.48, p<0.01). Heavier weights were also associated with longer ALs (+0.29 mm, p<0.01), deeper VCDs (+0.29 mm, p<0.01), higher AL/CC ratios (+0.04, p<0.01) and more negative refractions (-0.48 D, p<0.01).

CONCLUSIONS

Height and weight remained independently related to refraction and various ocular biometrics during the early adolescent growth period after adequately controlling for covariates, which could support the idea that a shared mechanism may regulate the coordinated growth of body and eye size in children.

Ex vivo biophysical characterization of a hydrogel-based artificial vitreous substitute

Purpose

To characterize the biophysical properties of an artificial vitreous body substitute (VBS), which consists of a biocompatible, cross-linked, hyaluronic acid (HA)-based hydrogel, by analysing the VBS’s influence on intraocular pressure (IOP) and retinal integrity in distinct ex vivo eye models in order to evaluate the its potential for in vivo biocompatibility testing.

Methods

Pig eyes were obtained immediately postmortem, and VBS was injected after core-vitrectomy. IOP was followed for 24 h (n = 5). VBS influence on retinal integrity was investigated using isolated bovine retinas superfused with an oxygen saturated nutrient solution. An electroretinogram (ERG) was recorded on explanted bovine retinae using silver/silver chloride electrodes; after application of VBS for 2 min, a washout period of 70 min was employed. The percentage of a-and b-wave reduction at the end of the washout phase was compared to baseline values (n = 5). Data were calculated throughout as the mean and the standard deviation. qRT-PCR (Bax/Bcl–2-ratio, GFAP- and PGP9.5-levels) or western blot analysis was used to test for toxicity of Princess Volume after 24 h (and β-3 tubulin with GAPDH as a control gene). Significance was estimated by Student´s t-test; p ≤0.05 was considered to be statistically significant.

Results

The IOP increased non-significantly by 10% after 24 h. Short-term biocompatibility testing using isolated superfused bovine retinas showed neither significant reductions of the b-wave nor the a-wave amplitudes (b-wave reduction 14.2%, p>0.05; a-wave reduction 23.9%, p>0.05). qRT-PCR and western blot analysis did not reveal significant toxicity after 24 h.

Conclusions

The manufactured HA-based hydrogel showed highly favourable biophysical characteristics in the explored ex vivo models, justifying in vivo studies enabling the assessment of biocompatibility.

A swarm of slippery micropropellers penetrates the vitreous body of the eye

The intravitreal delivery of therapeutic agents promises major benefits in the field of ocular medicine. Traditional delivery methods rely on the random, passive diffusion of molecules, which do not allow for the rapid delivery of a concentrated cargo to a defined region at the posterior pole of the eye. The use of particles promises targeted delivery but faces the challenge that most tissues including the vitreous have a tight macromolecular matrix that acts as a barrier and prevents its penetration. Here, we demonstrate novel intravitreal delivery microvehicles-slippery micropropellers-that can be actively propelled through the vitreous humor to reach the retina. The propulsion is achieved by helical magnetic micropropellers that have a liquid layer coating to minimize adhesion to the surrounding biopolymeric network. The submicrometer diameter of the propellers enables the penetration of the biopolymeric network and the propulsion through the porcine vitreous body of the eye over centimeter distances. Clinical optical coherence tomography is used to monitor the movement of the propellers and confirm their arrival on the retina near the optic disc. Overcoming the adhesion forces and actively navigating a swarm of micropropellers in the dense vitreous humor promise practical applications in ophthalmology.

Retinal Biochemistry, Physiology and Cell Biology

The vitreous, the vasculature of the retina, macular pigments, phototransduction, retinal pigment epithelium, Bruch's membrane and the extracellular matrix, all play an important role in the normal function of the retina as well as in diseases. Understanding the pathophysiology allows us to target treatment. As ocular angiogenesis, immunity and inflammation are covered elsewhere, those subjects will not be discussed in this chapter.

Vitreous humor rheology after Nd:YAG laser photo disruption

This work aimed to consider the hazardous side effect of eye floaters treatment with Q-switched Nd:YAG laser on the protein and viscoelastic properties of the vitreous humor, and evaluate the protective role of vitamin C against laser photo disruption. Five groups of New Zealand rabbits were divided as follows: control group for (n = 3) without any treatment, the second group (n = 9) treated with Q-switched Nd:YAG laser energy of 5 mJ × 100 pulse delivered to the anterior, middle, and posterior vitreous, respectively (n = 3 for each). The third group (n = 9) received a daily dose of 25 mg/kg body weight vitamin C for 2 weeks, and then treated with laser as the previous group. The fourth group (n = 9) treated with 10 mJ 9 50 pulse delivered to the anterior, middle, and posterior vitreous, respectively (n = 3 rabbits each). The fifth group (n = 9) received a daily dose of 25 mg/kg body weight vitamin C for 2 weeks, and then treated with laser as the previous group. After 2 weeks of laser treatment, the protein content, refractive index (RI), and the rheological properties of vitreous humor, such as consistency, shear stress, and viscosity, were determined. The results showed that, the anterior vitreous group exposed to of 5 mJ × 100 pulse and/or supplemented with vitamin C, showed no obvious change. Furthermore, all other treated groups especially for mid-vitreous and posterior vitreous humor showed increase in the protein content, RI and the viscosity of vitreous humor. The flow index remained below unity indicating the non-Newtonian behavior of the vitreous humor. Application of Q-switched Nd:YAG laser should be restricted to the anterior vitreous humor to prevent the deleterious effect of laser on the gel state of the vitreous humor.

[Proliferative vitreoretinopathy: pathophysiology and clinical diagnosis]

Proliferative vitreoretinopathy (PVR) remains one of the most common causes of failed retinal detachment (RD) surgery. Many histological and clinical studies have highlighted the chain of events leading to PVR: cellular migration into the vitreous cavity, cellular differentiation, myofibroblast proliferation and activation, synthesis of extracellular matrix proteins, then contraction of preretinal tissues. The development of PVR can be explained schematically by cellular exposure to growth factors and cytokines (particularly retinal pigment epithelial cells and glial cells), in the context of break-down of the blood-retinal barrier (inflammation, choroidal detachment, iatrogenic effect of cryotherapy and surgery) and of cellular contact with the vitreous. Although the pathophysiology of PVR is now better understood, its severity remains an issue. A systematic search for preoperative PVR risk factors allows the most suitable therapeutic option to be chosen.

Is neutralizing vitreal growth factors a viable strategy to prevent proliferative vitreoretinopathy?

Proliferative vitreoretinopathy (PVR) is a blinding disorder that occurs in eyes with rhegmatogenous retinal detachment and in eyes that have recently undergone retinal detachment surgery. There are presently no treatment strategies to reduce the risk of developing PVR in eyes with retinal detachment, and surgical intervention is the only option for eyes with retinal detachment and established PVR. Given the poor visual outcome associated with the surgical treatment of PVR, considerable work has been done to identify pharmacologic agents that could antagonize the PVR process. Intensive efforts to identify molecular determinants of PVR implicate vitreal growth factors. A surprise that emerged in the course of testing the 'growth factor hypothesis' of PVR was the existence of a functional relationship amongst growth factors that engage platelet-derived growth factor (PDGF) receptor α (PDGFRα), a receptor tyrosine kinase that is key to pathogenesis of experimental PVR. Vascular endothelial cell growth factor A (VEGF), which is best known for its ability to activate VEGF receptors (VEGFRs) and induce permeability and/or angiogenesis, enables activation of PDGFRα by a wide spectrum of vitreal growth factors outside of the PDGF family (non-PDGFs) in a way that triggers signaling events that potently enhance the viability of cells displaced into vitreous. Targeting these growth factors or signaling events effectively neutralizes the bioactivity of PVR vitreous and prevents PVR in a number of preclinical models. In this review, we discuss recent conceptual advances in understanding the role of growth factors in PVR, and consider the tangible treatment strategies for clinical application.

A fully-integrated high-compliance voltage SoC for epi-retinal and neural prostheses

This paper presents a fully functionally integrated 1024-channel mixed-mode and mixed-voltage system-on-a-chip (SoC) for epi-retinal and neural prostheses. Taking an AC input, an integrated power telemetry circuits is capable of generating multiple DC voltages with a voltage conversion efficiency of 83% at a load of 100 mW without external diodes or separate power integrated circuits, reducing the form factor of the prosthetic device. A wireless DPSK receiver with a novel noise reduction scheme supports a data rate of 2 Mb/s at a bit-error-rate of 2 ×10⁻⁷. The 1024-channel stimulator array meets an output compliance voltage of ±10 V and provides flexible stimulation waveforms. Through chip-clustering, the stimulator array can be further expanded to 4096 channels. This SoC is designed and fabricated in TSMC 0.18 μm high-voltage 32 V CMOS process and occupies a chip area of 5.7 mm × 6.6 mm. Using this SoC, a retinal implant bench-top test system is set up with real-time visual verification. In-vitro experiment conducted in artificial vitreous humor is designed and set-up to investigate stimulation waveforms for better visual resolution. In our in-vivo experiment, a hind-limb paralyzed rat with spinal cord transection and implanted chronic epidural electrodes has been shown to regain stepping and standing abilities using stimulus provided by the SoC.

Correspondence: Spatial variations of viscoelastic properties of porcine vitreous humors

Using a microbubble-based acoustic radiation force approach, spatial variations of Young's modulus and shear viscosity of the porcine vitreous humors in two groups--young pigs (6 months old) and mature pigs (2 to 3 years old)--were measured in situ. The measurements in these groups (4 specimens in each group) were performed in several positions along an anterior-to-posterior direction. At each position, microbubbles were generated by focusing a nanosecond pulsed laser beam and the displacement of each microbubble in response to an impulsive acoustic radiation force was measured every 10 µs using a custom-made high-pulse-repetition-frequency ultrasound system. Based on measured dynamics of the microbubble, Young's modulus and shear viscosity at various locations of the vitreous were reconstructed. Young's moduli of the young and mature porcine vitreous at anterior region were the highest, whereas the central region had the lowest values, indicating the clear spatial variations in the vitreous humor elasticity in both groups.

Effects of the chromatic defocus caused by interchange of two monochromatic lights on refraction and ocular dimension in guinea pigs

To investigate refractive and axial responses to the shift of focal plane resulting from the interchange of two monochromatic lights separately corresponding to the peak wavelengths of the cones absorption spectrum in retina, fifty 2-week-old pigmented guinea pigs were randomly assigned to five groups based on the mode of illumination: short-wavelength light (SL), middle-wavelength light (ML) and broad-band white light (BL) for 20 weeks, SL for 10 weeks followed by ML for 10 weeks (STM), as well as ML for 10 weeks followed by SL for 10 weeks (MTS). Biometric and refractive measurements were then performed every 2 weeks. After 10 weeks, SL and STM groups became more hyperopic and had less vitreous elongation than BL group. However, ML and MTS groups became more myopic and had more vitreous elongation. After interchange of the monochromatic light, the refractive error decreased rapidly by about 1.93D and the vitreous length increased by 0.14 mm in STM group from 10 to 12 weeks. After that, there were no significant intergroup differences between STM and BL groups. The interchange from ML to SL quickly increased the refractive error by about 1.53D and decreased the vitreous length by about 0.13 mm in MTS group after two weeks. At this time, there were also no significant intergroup differences between MTS and BL groups. The guinea pig eye can accurately detect the shift in focal plane caused by interchange of two monochromatic lights and rapidly generate refractive and axial responses. However, an excessive compensation was induced. Some properties of photoreceptors or retina may be changed by the monochromatic light to influence the following refractive development.

Effect of choroidal blood perfusion and natural convection in vitreous humor during transpupillary thermotherapy (TTT)

During retinal surgical treatment often the gel-like vitreous humor is replaced by aqueous substitutes. A two-dimensional computational model is developed for simulating transpupillary thermotherapy (TTT) process in a human eye under post-retinal treatment. The model accounts for natural convection in vitreous humor and the choroidal blood perfusion. Time dependent and steady state forms of Pennes bio heat transfer and the natural convection governing energy equations are solved using finite volume formulation. The results for steady state and at the end of 60 s of the laser irradiated TTT process show that flow in vitreous humor is significant. The velocity contours indicate strong natural convection on the upper half of the vitreous chamber. Compared with the stationary vitreous case, the peak temperature in retina during TTT, drops by 15 K and 12.5 K due to natural convection flow in the vitreous humor under steady and transient states, respectively. The choroidal blood perfusion also reduces the peak retinal temperature by 6 K and 1.5 K in steady state and transient cases, respectively. The vitreous humor convection enhances heat transfer in the regions adjacent to the laser spot. The temperature rise and the associated thermal damage in the neighboring regions resulting from the flow of vitreous humor is presented.

Quality of image of grating target placed in vitreous of isolated pig eyes photographed through different implanted multifocal intraocular lenses

PURPOSE

To determine the quality of the image of a grating target placed in the vitreous of isolated pig eyes and photographed through implanted refractive and diffractive multifocal intraocular lenses (IOL).

METHODS

Refractive multifocal (NXG1, PY60MV), diffractive multifocal (ZM900, SA60D3) and monofocal (SA60AT, ZA9003) IOL were implanted in the capsular bag of isolated pig eyes. A grating target was placed in the vitreous and photographed through a flat or a wide-field viewing contact lens. The contrast of the grating targets of different spatial frequencies was measured.

RESULTS

With the flat corneal contact lens, the gratings appeared clear and not distorted when viewed through the optics of the NXG1 and PY60MV for far vision but were distorted with reduced contrast when viewed through the optical zone for near vision. The images through the diffractive zone of the ZM900 and SA60D3 were more defocused than with the monofocal IOL (p < 0.005). Ghost images oriented centrifugally of the original image were seen with the ZM900 resulting in lower contrast at higher spatial frequencies than with the SA60D3 with less defocused images only in the central area. With the wide-field viewing contact lens, the images were less defocused and the contrast was comparable to both refractive and diffractive multifocal IOL.

CONCLUSION

Both refractive and diffractive multifocal IOL reduced the contrast of the retinal image when viewed through a flat corneal contact lens but less defocused when viewed through a wide-field viewing contact lens.

Intraocular expression of thymosin β4 in proliferative diabetic retinopathy

PURPOSE

To examine an association between thymosin β4 as potentially angioproliferative factor and proliferative diabetic retinopathy.

METHODS

The clinical study part included 62 patients with proliferative diabetic retinopathy (PDR) (study group) and 24 patients with non-diabetic pre-retinal membranes (control group). All patients underwent pars plana vitrectomy. We examined the thymosin β4 concentration in vitreous and plasma; and the expression of thymosin β4, glial fibrillary acidic protein (GFAP) and CD31 (PECAM-1 or Platelet Endothelial Cell Adhesion Molecule) and the levels of thymosin β4 mRNA and vascular endothelial growth factor (VEGF) mRNA in the excised membranes. The experimental study part consisted of 24 Sprague--Dawley rats with streptozotocin-induced diabetes mellitus and 24 age-matched control animals without diabetes. We determined the mRNA concentrations of thymosin β4, VEGF and GFAP in the rat retinas.

RESULTS

In the clinical study part, the vitreal and plasma thymosin β4 concentrations were significantly higher in the study group than control group (p =0.04 and p=0.01, respectively), and were significantly (p=0.028) correlated with each other. Co-expression of thymosin β4 and CD31 was observed in the diabetic fibrovascular membranes. Thymosin β4 mRNA and VEGF mRNA levels were significantly (p<0.01) higher in diabetic membranes than in non-diabetic membranes. In the experimental study part, the diabetic retinas showed co-localization of thymosin β4 and GFAP. The mRNA levels of thymosin β4, VEGF and GFAP were significantly (p<0.01) higher in diabetic rats than in control animals.

CONCLUSIONS

Thymosin β4 was produced in intraocular fibrovascular membranes of patients with PDR and in rats with experimental diabetes mellitus. Thymosin β4 may play a role in diabetic retinal neovascularization.

[Potential of liposomes for the intravitreal injection of therapeutic molecules]

Intravitreal administration has been widely used since 20 years and has been shown to improve the treatment of diseases of the posterior segment of the eye with infectious origin or in edematous maculopathies. This route of administration allows to achieve high concentration of drug in the vitreous and avoids the problems resulting from systemic administration. However, two basic problems limit the use of intravitreal therapy. Many drugs are rapidly cleared from the vitreous humor; therefore, to reach and to maintain effective therapy repeated injections are necessary. Repeated intravitreal injections increase the risk of endophthalmitis, damage to lens, retinal detachment. Moreover, some drugs provoke a local toxicity at their effective dose inducing side-effects and possible retinal lesions. In this context, the development and the use of new drug delivery systems for intravitreal administration are necessary to treat chronic ocular diseases. Among them, particulate systems such as liposomes have been widely studied. Liposomes are easily injectable and permit to reduce the toxicity and to increase the residence time of several drugs in the eye. They are also able to protect in vivo poorly-stable molecules from degradation such as peptides and nucleic acids. Some promising results have been obtained for the treatment of retinitis induced by cytomegalovirus in human and more recently for the treatment of uveitis in animal. Finally, the fate of liposomes in ocular tissues and fluids after their injection into the vitreous and their elimination routes begin to be more known.

Simultaneous measurement of intraocular pressure in the anterior chamber and the vitreous cavity

PURPOSE

To simultaneously measure, using manometry, the changes of intraocular pressure (IOP) in the anterior chamber and in the vitreous cavity of a single porcine eye, induced by an external column of water.

METHODS

We prospectively measured IOP simultaneously in seven freshly enucleated porcine eyes in both the anterior chamber and the vitreous cavity. Measurements were obtained through blood-pressure transducers connected to 21-gauge catheters. A third cannula was inserted into the anterior chamber to increase the IOP; when the cannula was connected to the water column, the pressure increased rapidly from 0 to 180 mmHg. Changes were registered every 5 seconds during the first minute. Statistical analysis was performed using the Wilcoxon signed-rank test. P ≤ 0.05 was considered significant.

RESULTS

Before the IOP was increased, the median basal IOP value was 18 mmHg in the anterior chamber and 23 mmHg in the vitreous cavity (P > 0.05). Sixty seconds after the end of the experiment, the median IOP value was 135 mmHg (range 122-145) in the anterior chamber and 55 mmHg (range 16-68) in the vitreous cavity (P < 0.01).

CONCLUSION

Cannulation of the anterior chamber measures the actual IOP; however, the IOP measured by cannulation in the vitreous cavity using the same catheter diameter was different, possibly because of the viscosity of the vitreous. The viscosity of the vitreous probably makes the cannulation method of IOP evaluation in the vitreous cavity inaccurate.

The in vitro response of human retinal endothelial cells to cytokines and other chemically active agents is altered by coculture with vitreous-derived hyalocytes

BACKGROUND

Ocular angiogenesis is regulated by polypeptides including cytokines, which are known to affect vascular endothelial cells. We have reported that hyalocytes interact with vascular endothelial cells, and some cytokines affect these interactions.

AIMS

To determine the effect of various chemically active agents on the viability of endothelial cells alone and cocultured with hyalocytes.

METHODS

The viability of human retinal endothelial cells (HRECs) was determined after exposure to IL-1alpha, IL-1beta, IL-6, TNFalpha and VEGF using the MTT assay. These results were compared to the viability when the HRECs were cocultured with porcine hyalocytes that had been exposed to different types of cytokines. The effects of bevacizumab, fenofibrate and dexamethasone on the viability of HRECs in coculture with hyalocytes were also assessed.

RESULTS

Ten micrograms/millilitre of bevacizumab decreased the percentage of living HRECs stimulated by VEGF without hyalocytes, but with the hyalocytes, 100 microg/ml of bevacizumab was required to decrease the percentage of viable HRECs stimulated by VEGF. Fenofibrate, at 5 microg/ml, decreased the viability of HRECs stimulated by IL-1beta and VEGF without hyalocytes but could not decrease the viability of HRECs cocultured with hyalocytes. Dexamethasone, at 50 microg/ml, decreased the viability of HRECs stimulated by IL-1alpha, IL-1beta, IL-6 and VEGF without hyalocytes but could not decrease the viability of HRECs cocultured.

CONCLUSIONS

Coculturing HRECs with vitreous-derived hyalocytes depressed the effects of cytokines, bevacizumab, fenofibrate and dexamethasone. This suggests that the vitreal hyalocytes may play a role in pathogenic endothelial cell proliferation in vivo. Future studies to better understand this pathobiology should utilize coculture systems of HRECs and vitreal hyalocytes.

Mathematical model of flow in the vitreous humor induced by saccadic eye rotations: effect of geometry

Saccadic eye rotations induce a flow in the vitreous humor of the eye. Any such flow is likely to have a significant influence on the dispersion of drugs injected into the vitreous chamber. The shape of this chamber deviates from a perfect sphere by up to 10-20% of the radius, which is predominantly due to an indentation caused by the lens. In this paper we investigate theoretically the effect of the domain shape upon the flow field generated by saccades by considering an idealized model. The posterior chamber geometry is assumed to be a sphere with a small indentation, undergoing prescribed small-amplitude sinusoidal torsional oscillations, and, as an initial step towards understanding the problem, we treat the vitreous humor as a Newtonian fluid filling the chamber. The latter assumption applies best in the case of a liquefied vitreous or a tamponade fluid introduced in the vitreous chamber after vitrectomy. We find the flow field in terms of vector spherical harmonics, focusing on the deviation from the flow that would be obtained in a perfect sphere. The flow induced by the departure of the domain geometry from the spherical shape has an oscillating component at leading order and a smaller-amplitude steady streaming flow. The oscillating component includes a circulation cell formed every half-period, which migrates from the indentation towards the center of the domain where it disappears. The steady component has two counter-rotating circulations in the anterior part of the domain. These findings are in good qualitative agreement with the experimental results of Stocchino et al. (Phys Med Biol 52:2021-2034, 2007). Our results predict a significant reduction in the expected time for drug dispersal across the eye compared with the situation in which there is no fluid flow present.

Imposed anisometropia, accommodation, and regulation of refractive state

PURPOSE

To determine the effects of imposed anisometropic retinal defocus on accommodation, ocular growth, and refractive state changes in marmosets.

METHODS

Marmosets were raised with extended-wear soft contact lenses for an average duration of 10 weeks beginning at an average age of 76 d. Experimental animals wore either a positive or negative power contact lens over one eye and a plano lens or no lens over the other. Another group wore binocular lenses of equal magnitude but opposite sign. Untreated marmosets served as controls and three wore plano lenses monocularly. Cycloplegic refractive state, corneal curvature, and vitreous chamber depth were measured before, during, and after the period of lens wear. To investigate the accommodative response, the effective refractive state was measured through each anisometropic condition at varying accommodative stimuli positions using an infrared refractometer.

RESULTS

Eye growth and refractive state are significantly correlated with the sign and power of the contact lens worn. The eyes of marmosets reared with monocular negative power lenses had longer vitreous chambers and were myopic relative to contralateral control eyes (p < 0.01). Monocular positive power lenses produced a significant reduction in vitreous chamber depth and hyperopia relative to the contralateral control eyes (p < 0.05). In marmosets reared binocularly with lenses of opposite sign, we found larger interocular differences in vitreous chamber depths and refractive state (p < 0.001). Accommodation influences the defocus experienced through the lenses, however, the mean effective refractive state was still hyperopia in the negative-lens-treated eyes and myopia in the positive-lens-treated eyes.

CONCLUSIONS

Imposed anisometropia effectively alters marmoset eye growth and refractive state to compensate for the imposed defocus. The response to imposed hyperopia is larger and faster than the response to imposed myopia. The pattern of accommodation under imposed anisometropia produces effective refractive states that are consistent with the changes in eye growth and refractive state observed.

Pharmacokinetic and toxicity study of intravitreal erythropoietin in rabbits

AIM

To study the pharmacokinetics and toxicity of intravitreal erythropoietin (EPO) for potential clinical use.

METHODS

For toxicity study, 4 groups (60 rabbits) with intravitreal injection (IVit) of EPO were studied (10 U, 100 U, or 1,000 U) per eye for single injection and 0.6 U/eye (the designed therapeutic level in rabbits) for monthly injections (6X). Eye examination, flash electroretinogram (ERG), and fluorescein angiography (FA) were carried out before and after injection. The rabbits were killed for histological study at different intervals. For the pharmacokinetic study, after IVit of 5 U EPO into left eyes, 44 rabbits were killed at different intervals, and the EPO levels in vitreous, aqueous, retina and serum were analyzed by enzyme-linked immunosorbent assay.

RESULTS

At all of the time points examined, the eyes were within normal limits. No significant ERG or FA change was observed. The histology of retina remained unchanged. The pharmacokinetic profile of EPO in ocular compartments was summarized as follows. The half-life times of EPO in vitreous, aqueous and serum were 2.84, 3.24 and 2.12 d, respectively; and Cmax were 4615.75, 294.31 and 1.60 U/L, respectively. EPO concentrations in the retina of the injected eye peaked at 1.36 U/g protein at 6 h following injection, with the half-life observed to be 3.42 d.

CONCLUSIONS

IVit of EPO in a wide range is well tolerated and safe for rabbit eyes. At doses up to 10-fold higher than therapeutic levels, EPO has a pharmacokinetic profile with faster clearance, which is favorable for episodic IVit.

To see the invisible: the quest of imaging vitreous

PURPOSE

Imaging vitreous has long been a quest to view what is, by design, invisible. This chapter will review important historical aspects, past and present imaging methodologies, and new technologies that are currently in development for future research and clinical applications.

METHODS

Classic and modern histologic techniques, dark-field slit microscopy, clinical slit lamp biomicroscopy, standard and scanning laser ophthalmoscopy (SLO), ultrasonography, optical coherence tomography (OCT), combined OCT-SLO, magnetic resonance and Raman spectroscopies, and dynamic light scattering methodologies are presented.

RESULTS

The best available histologic techniques for imaging vitreous are those that avoid rapid dehydration of vitreous specimens. Dark-field slit microscopy enables in vitro imaging without dehydration or tissue fixatives. OCT enables better in vivo visualization of the vitreoretinal interface than SLO and ultrasonography, but does not adequately image the vitreous body. The combination of OCT with SLO has provided useful new imaging capabilities, but only at the vitreoretinal interface. Dynamic light scattering can evaluate the vitreous body by determining the average sizes of vitreous macromolecules in aging, disease, and as a means to assess the effects of pharmacologic vitreolysis. Raman spectroscopy can detect altered vitreous molecules, such as glycated collagen and other proteins in diabetic vitreopathy and possibly other diseases.

CONCLUSIONS

A better understanding of normal vitreous physiology and structure and how these change in aging and disease is needed to develop more effective therapies and prevention. The quest to adequately image vitreous will likely only succeed through the combined use of more than one technique to provide better vitreous imaging for future research and clinical applications.

Intravitreal triamcinolone for cystoid macular edema related to branch retinal vein occlusion

We retrospectively evaluated the effectiveness of intravitreal triamcinolone in treating 19 eyes with macular edema related to branch retinal vein occlusion (BRVO). Eyes with nonischemic BRVO respond more favorably than those with ischemic BRVO. A single injection of intravitreal triamcinolone led to elevated intraocular pressure in 3 of 19 eyes (16%). Half of phakic eyes had progression of cataract. Retreatment may be necessary.

Decompression-induced ocular tear film bubbles reflect the process of denitrogenation

PURPOSE

To confirm that the tear film bubbles observed after decompression from hyperbaric exposure are due to denitrogenation and to assess the time course of denitrogenation based on the number of ocular tear film bubbles.

METHODS

The study comprised two parts. In the first, subjects (n = 8) were compressed to a pressure of 2.0 ATA (atmospheres absolute; depth of 10 meters of sea water [msw]) for 60 minutes in a hyperbaric chamber on two separate occasions. On one occasion they breathed air, whereas on the second occasion they inspired pure oxygen. Before and within 30 minutes after each dive, the subjects' tear film was examined with a slit lamp microscope and the average number of bubbles recorded. Ultrasound reflectivity of the lens-vitreous humor compartments was also examined. In the second part of the study, subjects (n = 8) participated in two simulated dives in the hyperbaric chamber: 4.0 ATA (30 msw) for 15 minutes and 2.5 ATA (15 msw) for 180 minutes. The former was a no-stop decompression dive, whereas the latter required a 43-minute decompression stop at 3 msw. Ocular tear film examinations were conducted before the dive, as well as 30 minutes and 1 day, 2 days, and 3 days after the dives.

RESULTS

The number of tear film bubbles increased significantly (P < 0.05) after the air dives to 2.0 ATA for 60 minutes, whereas there was no significant postdecompression increase in tear film when oxygen was inspired by the subjects during the dive. Posterior lens-vitreous humor reflectivity increased significantly after decompression from 2 ATA, when air was the breathing mixture, whereas no change in reflectivity was observed when oxygen was inspired during the dive. In the second part of the study, there was a significant elevation in tear film bubbles for 2 days after the two dives (P < 0.05). There was no significant difference in the number of ocular tear film bubbles between the two dives.

CONCLUSIONS

After a hyperbaric air exposure, tear film bubbles reflect the process of denitrogenation, which may persist for up to 2 days after the decompression.

Different biocompatibility of crystalline triamcinolone deposits on retinal cells in vitro and in vivo

Epiretinal deposits of triamcinolone acetonide (TA) can be detrimental to retinal cells in vitro as several laboratory studies have shown. This contrasts with the good clinical experience of intravitreal TA use. We investigated the effect of TA crystals on retinal cells concerning the critical dose range, a potential cell recovery, the drug-tissue interaction and what protective biological factors could explain the discrepancy between in vivo and in vitro results. A human retinal pigment epithelium cell line (ARPE19) and transformed rat retinal ganglion cells (RGC5) were used. Purified TA crystals were either added directly on top of the cell cultures or on top of membrane filter inserts, basement membrane sheets or porcine vitreous with the cells growing underneath. To determine the number of live versus dead cells fluorescent stains were used. Proliferation and viability were measured using the MTT assay and the mean inhibitory dose (ID(50)) calculated with or without a filter. Cell recovery was measured after transient TA exposure (0.01-1 mg/ml) compared to continuous exposure after 7 days. To exclude a mere mechanical effect of epicellular deposits the TA crystals were replaced by glass pearls in a serum-free medium and the MTT toxicity assay was performed after 24 h. Without direct contact of TA crystals with the cells only a moderate decrease of mitochondrial activity was observed that fully recovered after transient exposure and showed a clinically safe ID(50) of 7.7 mg/ml. In contrast, direct exposure to even minute crystalline deposits for 7 days caused a rapid progressive and irreversible cell death being significant far below clinically used concentrations (ID(50) 0.058 mg/ml). Direct exposure to glass pearls did not show any loss of viability. Both basement membrane sheets and vitreous reliably prevented direct cytotoxicity to underlying retinal ganglion cells. Our findings suggest that irreversible TA cytotoxicity in a cell culture setting occurs earlier than previously assumed in the presence of even minute epicellular deposits. But in most clinical situations epiretinal TA deposits seem not to be harmful to ocular cells as protective biological factors may prevent close apposition of TA crystals to susceptible retinal cells. However, in eyes that have undergone vitrectomy with ILM peeling epimacular deposits could be critical.

Proteomic analysis of human vitreous fluid by fluorescence-based difference gel electrophoresis (DIGE): a new strategy for identifying potential candidates in the pathogenesis of proliferative diabetic retinopathy

AIMS/HYPOTHESIS

The aim of this study was to compare the protein profile of vitreous fluid from diabetic patients with proliferative diabetic retinopathy (PDR) with that from non-diabetic patients with idiopathic macular holes (MH). The mRNA of proteins differentially produced was also assessed in the retinas from diabetic and non-diabetic donors.

MATERIALS AND METHODS

Vitreous humour from type 1 diabetic patients with PDR (n = 8) and from non-diabetic patients with MH (n = 10) closely matched in terms of age were studied. The comparative proteomic analysis was performed using fluorescence-based difference gel electrophoresis (DIGE). Differentially produced proteins (abundance ratio >1.4, p < 0.05) were identified by mass spectrometry. Expressions of mRNA were measured by real-time RT-PCR in retinas from ten human eyes obtained at post-mortem (five eyes from diabetic subjects and five eyes from non-diabetic subjects).

RESULTS

Eight proteins were highly produced in PDR patients in comparison with non-diabetic subjects: zinc-alpha(2)-glycoprotein (ZAG), apolipoprotein (apo) A1, apoH, fibrinogen A, and the complement factors C3, C4b, C9 and factor B). We found three proteins that were underproduced in PDR subjects: pigment epithelial derived factor (PEDF), interstitial retinol-binding protein (IRBP) and inter-alpha-trypsin inhibitor heavy chain (ITIH2). There was no overlap in the vitreous levels of the above-mentioned proteins between PDR patients and non-diabetic control subjects. The differential production of ZAG, C3, factor B, PEDF and IRBP was further confirmed by western blot, and was in agreement with mRNA levels detected in the retina.

CONCLUSIONS/INTERPRETATION

Proteomic analysis by DIGE, which permits an accurate quantitative comparison, was useful in identifying new potential candidates involved in the pathogenesis of PDR.

Functional characteristics of connective tissue growth factor on vitreoretinal cells

Connective tissue growth factor (CTGF) level is elevated in eyes with proliferative vitreoretinal diseases, such as proliferative diabetic retinopathy and proliferative vitreoretinopathy (PVR), as we previously reported, but its functional characteristics on vitreoretinal cells are yet to be clarified. In this study, we demonstrated a growth-promoting effect of CTGF on cultured hyalocytes and bovine retinal pigment epithelial cells (BRPEs) with the induction of p44/p42 mitogen-activated protein kinase phosphorylation and [(3)H]thymidine incorporation. CTGF also stimulated the synthesis of fibronectin by hyalocytes and BRPEs without significant effect on collagen gel contraction by these cells. On the other hand, CTGF had no direct effects on the proliferation, migration, or in vitro tube formation by vascular endothelial cells. Nevertheless, CTGF promoted vascular endothelial growth factor (VEGF) gene expression by hyalocytes and BRPEs. Although the concentrations of both CTGF and VEGF in the human vitreous samples with proliferative vitreoretinal diseases were elevated, there was no significant correlation between these concentrations. These findings indicate that CTGF appears to be involved in the formation of proliferative membranes without direct regulation of their cicatricial contraction in the pathogenesis of proliferative vitreoretinal diseases. Whereas CTGF might have no direct effects or minimal effects, if any, on retinal neovascularization, it is possible that CTGF has indirect effects by modulating the expression of VEGF.

Aspects of eye accommodation evaluated by finite elements

Axisymmetric nonlinear finite models of accommodation incorporating the posteriorly sloped force and vitreous effects have been studied by means of their effectiveness in mechanical and optical performances. All materials were assumed to be linearly elastic, vitreous and lens matrices were incompressible. The present model is subjected to certain indicated shortcomings, however, the behavior of the model is predictable, reasonable and favourably consistent with different published data, supporting the Helmholtz theory of accommodation.

In vitro generation of functional lens-like structures with relevance to age-related nuclear cataract

PURPOSE

To investigate the capacity of lens epithelial cells, maintained in a modified explant culture system, to mimic normal patterns of lens cell differentiation and to regenerate lens structure and function.

METHODS

Lens epithelial explants were set up in pairs with their apical surfaces facing each other. These explant pairs (EPs) were then cultured in vitreous for up to 43 days to promote their growth and differentiation. Immunohistochemistry and conventional light and electron microscopy were used to assess structural and functional properties of the lens-like structures that developed from EPs.

RESULTS

EPs that were asymmetrically exposed to vitreous routinely produced biconvex, lens-like structures composed of ordered epithelial and fiber cells that were transparent and had some focusing and magnifying ability. In addition, characteristic of the lens in vivo, fiberlike cells that were peripherally situated in EPs contained markers of the relatively early stages of fiber differentiation, whereas centrally situated cells contained markers of terminally differentiated fibers. During long-term culture of the EPs, a central opacity appeared that had structural features similar to those reported for the early stages of human, age-related nuclear cataract.

CONCLUSIONS

This study shows that, given appropriate culture conditions, lens epithelial cells can regenerate ordered lens-like structures with functional properties. This system represents a valuable new tool for the investigation of factors involved in the generation of normal lens structure and function and lens opacification.

Vitreous Modulation of Gene Expression in Low-Passage Human Retinal Pigment Epithelial Cells

PURPOSE

In proliferative vitreoretinopathy (PVR), retinal pigment epithelial (RPE) cells enter the vitreous and proliferate. They become fibroblast-like and participate in the formation of contractile membranes, which can lead to retinal detachment. Vitreous treatment of RPE cells in vitro results in similar morphologic changes. This study was conducted to examine vitreous-induced modulation of gene expression in RPE cells.

METHODS

Low-passage human RPE cell lines derived from three donors were each treated for 6, 12, 24, or 48 hours with complete medium or complete medium containing 25% vitreous. Changes in mRNA levels were examined by using microarrays. Real-time quantitative PCR (qPCR) was used to measure mRNA expression of a subset of genes in cells from three additional donors. Immunohistochemistry and immunoblot analysis were used to examine protein expression.

RESULTS

Vitreous treatment caused a progressive reprogramming of gene expression. qPCR confirmed vitreous modulation of mRNA levels of 10 of 10 genes. Changes consistent with a transition from an epithelial to a mesenchymal phenotype were observed. Downregulated genes included genes associated with differentiated RPE cells. Upregulated genes included genes associated with stress and inflammation. Pathway analysis indicated that the transforming growth factor-beta/bone morphogenetic protein (BMP) pathway and the focal adhesion pathway may play a role in this process. BMP-2 protein and mRNA were increased.

CONCLUSIONS

Despite the biological variation in vitreous and RPE donors, vitreous reproducibly modulated a limited number of mRNAs. Many of these changes were consistent with the more fibroblast-like appearance of vitreous-treated cells and with the pathobiology of PVR. TGF-beta and BMP-2 may be important modulators of vitreous-induced changes in gene expression.

Duration of ERK1/2 phosphorylation induced by FGF or ocular media determines lens cell fate

The ocular environment is important for the establishment and maintenance of lens growth patterns and polarity. In the anterior chamber of the eye, the aqueous humour regulates lens epithelial cell proliferation whereas in the posterior, the vitreous humour regulates the differentiation of the lens cells into fiber cells. Members of the fibroblast growth factor (FGF) growth factor family have been shown to induce lens epithelial cells to undergo cell division and differentiate into fibers, with a low dose of FGF able to induce cell proliferation (but not fiber differentiation), and higher doses required to induce fiber differentiation. Both these cellular events have been shown to be regulated by the MAPK/ERK1/2 signalling pathway. In the present study, to better understand the contribution of ERK1/2 signalling in regulating lens cell proliferation and differentiation, we characterized the ERK1/2 signalling profiles induced by different doses of FGF, and compared these to those induced by the different ocular media. Here, we show that FGF induced a dose-dependent sustained activation of ERK1/2, with both a high (fiber differentiating) dose of FGF and vitreous, stimulating and maintaining a prolonged (up to 18 hr) ERK1/2 phosphorylation profile. In contrast, a lower (proliferating) dose of FGF, and aqueous, stimulated ERK1/2 phosphorylation for only up to 6 hr. If we selectively reduce the 18 hr ERK1/2 phosphorylation profile induced by vitreous to 6 hr, by specifically blocking FGF receptor signalling, the vitreous now fails to induce lens fiber differentiation but retains the ability to induce lens cell proliferation. These findings not only provide insights into the important role that FGF plays in the different ocular media that bathe the lens, but enlighten us on some of the putative molecular mechanisms by which one specific growth factor, in this case FGF, can elicit a different cellular response in the same cell type.

[Genetic and ophthalmological assessment of patients with type II Stickler syndrome]

PURPOSE

To diagnose, evaluate and describe the clinical, genetic and ophthalmic characteristics of a family with type II Stickler syndrome.

METHODS

X-rays for bone age, clinical and genetic evaluation were performed in all patients with ocular alterations. The Stickler syndrome diagnosis was established after correlating these examinations.

RESULTS

Type II Stickler syndrome was found in 11 patients. The most important ocular findings were: high myopia (80%), lens subluxation (70%), exotropia (50%) and vitreoretinal abnormalities (80%) including vitreous cavity (50%). The clinical genetic examination disclosed that 30% of the patients had micrognathia, 50% hearing loss, 40% nasal depression and 60% high palate. Seven cases had articular hypermotility and long fingers and arthropathy was present in 3 cases.

CONCLUSION

Diagnosis of the Stickler syndrome is difficult due to its phenotypic variability and the existence of other genetic syndromes with similar characteristics. Hand and wrist radiographs are of particular importance in the diagnosis of this syndrome.

Transforming growth factor-beta2 and connective tissue growth factor in proliferative vitreoretinal diseases: possible involvement of hyalocytes and therapeutic potential of Rho kinase inhibitor

The critical association of connective tissue growth factor (CTGF), which is thought to be one of the downstream mediators of transforming growth factor-beta (TGF-beta), with vitreoretinal diseases remains to be clarified. In the current study, we first demonstrated the correlation between the concentrations of TGF-beta2 as well as CTGF in the vitreous and CTGF gene regulation in cultured hyalocytes. Concentrations of TGF-beta2 and CTGF in the vitreous from patients with proliferative vitreoretinal diseases were significantly higher than in those with nonproliferative diseases, and there was a positive correlation between their concentrations (r = 0.320, P < 0.01). Cultured hyalocytes expressed CTGF mRNA, which was enhanced in the presence of TGF-beta2, associated with nuclear accumulation of Smad4. TGF-beta2-dependent Smad4 translocation and CTGF gene expression were mediated through Rho kinase and at least partially via p38 mitogen-activated protein kinase. Finally, fasudil, a Rho kinase inhibitor already in clinical use, inhibited both Smad4 translocation and CTGF gene expression. In conclusion, combined effects of TGF-beta2 and CTGF appear to be involved in the pathogenesis of proliferative vitreoretinal diseases. Hyalocytes may be a possible source of CTGF and thus might play a role in vitreoretinal interface diseases. Furthermore, Rho kinase inhibitors might have therapeutic potential to control fibrotic disorders in the eye.

Optical imaging of electrically evoked visual signals in cats: I. Responses to corneal and intravitreal electrical stimulation

Microelectronic retinal prostheses have been shown to restore the perception of light to the blind through electrical stimulation. Conventional recording techniques such as recording electrode arrays on the visual cortex can give a basic understanding of the events that occur during such stimulation events, but their finite size and number limits the spatial resolution achievable with them. Optical imaging of intrinsic signals (OIS imaging) allows for greater resolution (approximately 50 microm) of the activity in the cortex. This can be used to facilitate a greater understanding of the complex neurophysiological events that allow prosthetic vision. This paper shows responses to visual and electrical stimulation of the retina, and demonstrates that OIS imaging may be an effective technique in further refining stimulation techniques and implant designs for retinal prostheses.

Experimental analysis of lens-forming capacity in Xenopus borealis larvae

Previously, the only anuran amphibians known to have the capacity to regenerate a lens after lentectomy were Xenopus laevis and Xenopus tropicalis. This regeneration process occurs during the larval life through transdifferentiation of the outer cornea promoted by inductive factors produced by the retina and accumulated inside the vitreous chamber. However, the capacity of X. tropicalis to regenerate a lens is much lower than that of X. laevis. This study demonstrates that Xenopus borealis, a species more closely related to X. laevis than to X. tropicalis, is not able to regenerate a lens after lentectomy. Nevertheless, some morphological modifications corresponding to the first stages of lens regeneration in X. laevis were observed in the outer cornea of X. borealis. This suggested that in X borealis the regeneration process was blocked at early stages. Results from histological analysis of X. borealis and X. laevis lentectomized eyes and from implantation of outer cornea fragments into the vitreous and anterior chambers demonstrated that: (i) in X. borealis eye, the lens-forming competence in the outer cornea and inductive factors in the vitreous chamber are both present, (ii) no inhibiting factors are present in the anterior chamber, the environment where lens regeneration begins, (iii) the inability of X. borealis to regenerate a lens after lentectomy is due to an inhibiting action exerted by the inner cornea on the spreading of the retinal factor from the vitreous chamber towards the outer cornea. This mechanical inhibition is assured by two distinctive features of X. borealis eye in comparison with X. laevis eye: (i) a weaker and slower response to the retinal inducer by the outer cornea; (ii) a stronger and faster healing of the inner cornea. Unlike X. tropicalis and similar to X. laevis, in X. borealis the competence to respond to the retinal factor is not restricted to the corneal epithelium but also extends to the pericorneal epidermis.

Surgical options in the face of positive pressure

Positive pressure during cataract surgery can adversely affect the clinical outcome if the surgeon is unprepared. A variety of surgical maneuvers are described, including dry insertion of the phacoemulsification needle, capsule protection with the second instrument, dry cortical aspiration in an ophthalmic viscosurgical device (OVD) environment, intraocular lens restraint with an injection cannula at the end of OVD removal, aspiration or vitrectomy through the pars plana, a technique of OVD clearance to manage extreme cases of positive pressure, and intraoperative ophthalmoscopy using a lens designed to function with the operative microscope. Mastery of these important surgical maneuvers will help the surgeon manage challenging cases of positive pressure.

Regulation of eye size by the retinal basement membrane and vitreous body

PURPOSE

Congenital high myopia is an early-onset enlargement of the eye globes that carries a high risk for retinal detachment. The genetic basis for congenital high myopia has frequently been connected to mutations in genes encoding extracellular matrix proteins of the vitreous body (VB) and the inner limiting membrane (ILM). Why defective or missing VB and ILM proteins lead to an increase in eye size is unknown. The present study introduces the chick embryo as a model to study the role of ILM and VB in regulating eye size.

METHODS

The ILM and VB were disrupted by injecting collagenase into the eyes of E5 chick embryos. The digestion of VB and ILM proteins was monitored by Western blot and immunocytochemistry. Eye size was assessed up to 9 days after the enzyme injections.

RESULTS

Intraocular injection of collagenase led to the disruption of the ILM and the VB by digesting their collagen constituents. Once disrupted, the ILM and the collagen II fibrillar network failed to regenerate despite continued synthesis of VB and ILM proteins. ILM and VB disruption resulted in eye enlargement of 50% within 4 days. The increase in eye size was greatly reduced by reconstituting the ILM.

CONCLUSIONS

The present data show that the ILM and the VB play major roles in the early regulation of eye size. The authors speculate that the integrity of the vitreoretinal border is an important factor in preventing congenital high myopia.

Evaluation of acoustic wave propagation velocities in the ocular lens and vitreous tissues of pigs, dogs, and rabbits

OBJECTIVE

To evaluate propagation velocity of acoustic waves through the lens and vitreous body of pigs, dogs, and rabbits and determine whether there were associations between acoustic wave speed and age, temperature, and time after enucleation.

SAMPLE POPULATION

9 pig, 40 dog, and 20 rabbit lenses and 16 pig, 17 dog, and 23 rabbit vitreous bodies.

PROCEDURE

Acoustic wave velocities through the ocular structures were measured by use of the substitution technique.

RESULTS

Mean sound wave velocities in lenses of pigs, dogs, and rabbits were 1,681, 1,707, and 1,731 m/s, respectively, at 36 degrees C. Mean sound wave velocities in the vitreous body of pigs, dogs, and rabbits were 1,535, 1,535, and 1,534 m/s, respectively, at 38 degrees C. The sound wave speed through the vitreous humor, but not the lens, increased linearly with temperature. An association between wave speed and age was observed in the rabbit tissues. Time after enucleation did not affect the velocity of sound in the lens or vitreous body. The sound wave speed conversion factors for lenses, calculated with respect to human ocular tissue at 36 degrees C, were 1.024, 1.040, and 1.055 for pig, dog, and rabbit lenses, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Conversion factors for the speed of sound through lens tissues are needed to avoid underestimation of the thickness of the lens and axial length of the eye in dogs during comparative A-mode ultrasound examinations. These findings are important for accurate calculation of intraocular lens power required to achieve emmetropia in veterinary patients after surgical lens extraction.

Tissue deformation induced by radiation force from Gaussian transducers

Imaging techniques based upon the tissue mechanical response to an acoustic radiation force are being actively researched. In this paper a model for predicting steady-state tissue displacement induced by a radiation force arising from the absorption of Gaussian ultrasound beams is presented. A simple analytic expression is derived that agrees closely with the numerical quadrature of the displacement convolution integrals. The analytic result reveals the dependence of the steady-state axial displacement upon the operational parameters, e.g., an inverse proportional relationship to the tissue shear modulus. The derivation requires that the transducer radius be small compared to the focal length, but accurate results were obtained for transducer radii comparable to the focal length. Favorable comparisons with displacement predictions for non-Gaussian transducers indicate that the theory is also useful for a broader range of transducer intensity profiles.

Angiogenic and antiangiogenic factors in proliferative diabetic retinopathy

Diabetic retinopathy continues to be the leading cause of legal blindness among working-age individuals. The earliest histological features of diabetic retinopathy include neuroretinal damage, capillary basement membrane thickening, loss of pericytes and loss of endothelial cells. At advanced stages, neovascularization, the hallmark of proliferative diabetic retinopathy (PDR) occurs, and blindness can result from relentless abnormal fibrovascular proliferation with subsequent bleeding and retinal detachment. Macular oedema is another retinal complication of diabetes that is responsible for a major part of vision loss, particularly in type 2 diabetes. The breakdown of the blood retinal barrier and the consequent vascular leakage and thickening of retina are the main events involved in its pathogenesis. Although a tight control of both blood glucose levels and hypertension are essential to prevent or arrest progression of the disease, the recommended goals are difficult to achieve in many patients. Laser photocoagulation treatment soon after the onset of PDR significantly reduces the incidence of severe vision loss. However, the optimal timing for laser treatment is frequently passed and, in addition, it is not uniformly successful in halting visual decline. For all these reasons, new pharmacological treatments based on the understanding of the pathophysiological mechanisms of diabetic retinopathy have been developed in recent years. There is mounting evidence to suggest that angiogenic factors play a crucial role in PDR development, vascular endothelial growth factor (VEGF) being the most relevant. Other growth factors or cytokines such as insulin-like growth factor I (IGF-1), hepatocyte growth factor (HGF), basic fibroblast growth factor (b-FGF), platelet derived growth factor (PDGF), pro-inflammatory cytokines and angiopoetins, are also involved in the pathogenesis of PDR. However, the intraocular synthesis of angiogenic factors is counterbalanced by the synthesis of antiangiogenic factors. Therefore, the balance between the angiogenic and antiangiogenic factors rather than angiogenic factors themselves will be crucial in determining the progression of PDR. The main antiangiogenic factor is the pigment epithelium derived factor (PEDF) but the transforming growth factor beta (TGF-beta), thrombospondin (TSP) and somatostatin are also among the intraocullary synthesized antiangiogenic factors.

Protective ocular mechanisms in woodpeckers

AIMS

Woodpeckers possess mechanisms protecting the eye from shaking/impact. Mechanisms available to woodpeckers but not humans may help explain some eye injuries in Shaken Baby syndrome (SBS).

METHODS

Gross dissection and histologic examination of eyes and orbits of seven woodpeckers.

RESULTS

All birds showed restricted axial globe movement due to the tight fit within the orbit and fascial connections between the orbital rim and sclera. The sclera was reinforced with cartilage and bone, the optic nerve lacked redundancy, and the vitreous lacked attachments to the posterior pole retina.

CONCLUSIONS

Woodpecker eyes differ from human infants by an inability of the globe to move axially in the orbit, the sclera to deform, and the vitreous to shear the retina. These findings support current hypotheses that abusive acceleration-deceleration-induced ocular injury in human infants may be related to translation of vitreous within the globe and the globe within the orbit. The woodpecker presents a natural model resistant to mechanical forces that have some similarity to SBS.

Vitreous: a barrier to nonviral ocular gene therapy

PURPOSE

Intravitreal injection of therapeutic DNA, complexed to nonviral carriers such as cationic liposomes, may be promising in the treatment of many severe retinal eye diseases. However, after intravitreal injection, such DNA/cationic liposome complexes-called lipoplexes (LPXs)-which are typically hundreds of nanometers in size, must first diffuse through the vitreous before they can reach the retina. The aim of this study was to elucidate whether vitreous is a barrier for the LPXs and to find strategies to overcome this barrier.

METHODS

Fluorescent polystyrene nanospheres and LPXs were mixed with vitreous, and their mobility was monitored by fluorescence recovery after photobleaching (FRAP), a microscopy-based technique. The stability of LPXs and naked plasmid DNA in vitreous was studied by gel electrophoresis.

RESULTS

We showed that polystyrene nanospheres, in our first experiments used as a model for the LPXs, do not diffuse freely into the vitreous but adhere to fibrillar structures in the vitreous, most likely to collagen fibers. Making the surfaces of the polystyrene nanospheres hydrophilic by attaching hydrophilic polyethylene glycol (PEG) chains at their surfaces circumvented the binding to fibrillar structures in the vitreous. FRAP revealed that "pegylated" polystyrene nanospheres, as long as they are smaller than 500 nm, are indeed mobile in the vitreous. It was further demonstrated that LPXs severely aggregate in vitreous and strongly bind to biopolymers in the vitreous, which immobilizes them completely. However, as observed for the polystyrene nanospheres, coating of the LPXs with PEG averted their aggregation in the vitreous and their binding to fibrillar structures.

CONCLUSIONS

Modifying the surfaces of LPXs with hydrophilic PEG chains prevents them from aggregating in vitreous. In this way, LPXs are obtained that can freely move in vitreous, an absolute criterion for reaching the retina after intravitreal injection.

Experimental investigation of vitreous humour motion within a human eye model

We present an experimental study of the vitreous motion induced by saccadic eye movements. A magnified model of the vitreous chamber has been employed, consisting of a spherical cavity carved in a perspex cylindrical container, which is able to rotate with a prescribed time law. Care has been taken to correctly reproduce real saccadic eye movements. The spherical cavity is filled with glycerol and the flow field is measured on the equatorial plane orthogonal to the axis of rotation, through the PIV technique. Visualizations of the fully three-dimensional flow suggest that it essentially occurs on planes perpendicular to the axis of rotation, the motion orthogonal to such planes being smaller by three to four orders of magnitude. Theoretical results, based on a simplified solution, are in very good agreement with the experimental findings. The maximum value of the shear stress at the wall, which is thought to play a possibly important role in the pathogenesis of retinal detachment, does not significantly depend on the amplitude of saccadic movements. This suggests that relatively small eye rotations, being much more frequent than large movements, are mainly responsible for vitreous stresses on the retina. Results also illustrate the dependence of the maximum shear stress at the wall from the vitreous viscosity.

Age dependence of dielectric properties of bovine brain and ocular tissues in the frequency range of 400 MHz to 18 GHz

In order to identify possible age-dependent dielectric properties of brain and eye tissues in the frequency range of 400 MHz to 18 GHz, measurements on bovine grey and white matter as well as on cornea, lens (cortical) and the vitreous body were performed using a commercially available open-ended coaxial probe and a computer-controlled vector network analyser. Freshly excised tissues of 52 animals of two age groups (42 adult animals, i.e. 16-24 month old and 10 young animals, i.e. 4-6 month old calves) were examined within 8 min (brain tissue) and 15 min (eye tissue), respectively, of the animals' death. Tissue temperatures for the measurements were 32+/-1 degrees C and 25+/-1 degrees C for brain and eye tissues, respectively. Statistical analysis of the measured data revealed significant differences in the dielectric properties of white matter and cortical lens tissue between the adult and the young group. In the case of white matter the mean values of conductivity and permittivity of young tissue were 15%-22% and 12%-15%, respectively, higher compared to the adult tissue in the considered frequency range. Similarly, young cortical lens tissue was 25%-76% higher in conductivity and 27%-39% higher in permittivity than adult cortical lens tissue.

Intraocular humidity immediately after fluid–air exchange in pars plana vitrectomy

PURPOSE

To study vitreous cavity humidity during fluid-air exchange in pars plana vitrectomy.

METHODS

Intraocular humidity in the vitreous cavity was recorded for 2 min in six artificial eyes, six enucleated pig eyes, and ten patient eyes, after the eyes had been filled with either humidified air (75% humidity) or dry air (8% humidity).

RESULTS

In artificial eyes the humidity levelled off at a value that was approximately equal to the humidity of the infused air, i.e., a mean of 71.9% when humidified air was used and a mean of 14.4% when dry air was used. In enucleated pig eyes humidity increased slightly with humidified air and remained stable with dry air. In patients intraocular humidity increased to over 90%, regardless of whether humidified or dry air was used.

CONCLUSION

In the living eye, dry air deprives the retinal tissue of humidity, which is lost into the vitreous cavity. This effect can be reduced by using humidified air.