Ocular Potpourri

An Ocular Dynamic Duo

Each rectus extraocular muscle (EOM) consists of dual layers: the global layer inserts via a tendon on the eye, while the orbital layer inserts on the EOM’s connective tissue pulley instead of the eye. Oh et al. (p. 10) counted rectus EOM fibers in human and monkey cadavers, finding that the orbital layer contains about half of total fibers in the deep orbit. This suggests that much of each rectus EOM’s total effort is devoted, not to directly rotating the eye, but to precisely positioning the EOM pulley that controls the EOM’s rotational axis and serves as its functional origin. 

The Egg and Eye

The photodynamic therapy of choroidal neovascularization associated with age-related macular degeneration (AMD) with“ visudyne” as photosensitizer has recently been shown to be an improved treatment modality for a large fraction of the patients with“ wet” AMD. The latter is the leading cause of blindness in the population over 50 years of age in the Western world. In the search for better drugs for this application, finding an optimal screening procedure is essential. Lange et al. (p. 38) demonstrate how the chick’s chorioallantoic membrane model can serve as high throughput drug screening for the preliminary in vivo testing of photosensitizers. 

SEE-ing More than Visual Acuity

Most large-population studies of vision and aging have relied solely on visual acuity tests. Acuity measures the eye’s ability to resolve fine detail but does not capture the full scope of vision changes in later life. The Salisbury Eye Evaluation (SEE) study administered a broad range of vision tests to a group of 2520 older adults in Salisbury, MD. The SEE study shows that disability is related to all of the vision measures, not just acuity. In fact, some tests, such as contrast sensitivity, are at least as important as acuity for predicting difficulty with everyday activities. Rubin et al. (p. 64) suggest that national and international guidelines for establishing visual impairment should consider a wider range of visual function instead of relying so heavily on visual acuity. 

The Disappearing Vitreous

In a survey of adult Chinese living in Singapore, Wong et al. (p. 73) found that older people and women tend to have shorter axial lengths and vitreous chamber depths. Further, vitreous chamber depth was shown to be the most “important” determinant of refraction in adults, and shorter vitreous chamber depths in older compared to younger persons may explain the “hyperopic shift” seen in previous refractive error surveys among adult populations. After 60 years, lens nuclear opacity becomes an additional determinant of refraction, possibly explaining why the “hyperopic shift” becomes less prominent in the elderly. 

A New Rock Star

Rho GTPase is involved in diverse physiological functions associated with cytoskeletal rearrangements, and several putative target molecules of the Rho including ROCK have been identified recently. In the study by Honjo et al. (p. 137), a specific ROCK inhibitor, Y-27632 resulted in a reduction in intraocular pressure and an increase in the outflow facility in the rabbit eyes. Y-27632 also caused retraction and rounding of cell bodies as well as disruption of actin bundles and impairment of focal adhesion formation in cultured human trabecular meshwork cells. These studies suggest that ROCK inhibitors may have great potential to be developed for treatments of glaucoma and other ocular diseases. 

Disappearing Filters

The human lens contains UV filter compounds that protect against UV-induced photodamage. These compounds, however, are intrinsically unstable and bind to lens proteins, contributing to age-dependent coloration and fluorescence of lenses. The human lens also contains the antioxidant glutathione. Bova et al. (p. 200) present a comprehensive study on the quantification of UV filters and glutathione in human lenses with age. Levels of most of the UV filters and glutathione were found to decrease markedly with age. This may have important consequences, in terms of our susceptibility to lenticular and retinal photodamage, and oxidative modification of lens proteins, particularly in later years. 

An Angiogenic Balancing Act

Retinal neovascularization (NV) is an important cause of visual loss as seen in diabetic retinopathy and retinopathy of prematurity. Majka et al. (p. 210) evaluated the role of proteinases and inhibitors in the regulation of retinal NV in a mouse model of proliferative retinopathy. The matrix metalloproteinases (MMP-2, MMP-9 and MT1-MMP) are increased in the retinas of animals with NV, whereas the inhibitor, tissue inhibitor of metalloproteinase (TIMP-2) level remained low. Such a balance between proteinases and their inhibitors appears to be crucial in the process of retinal NV. 

The Little Engine that Could

Fluid transport out of the subretinal space is mediated by a variety of paracrine, autocrine and hormonal signals. The retinal pigment epithelium “engine” for transporting fluid out of the subretinal space consists of apical membrane cotransport proteins and adrenergic receptors that are coupled via second messengers to basolateral membrane Cl channel activity. Quinn et al. (p. 255) suggest that appropriate retinal agonists could activate this engine via different receptors. Paracrine activation provides a potential means of removing the abnormal accumulation of fluid produced in a wide variety of ocular diseases that cause retinal detachment or macular edema. 

Phosphene Prognostications

Electrical currents delivered to the eye through the closed eyelids can easily and safely generate phosphenes. Pulses of different duration yield orderly-arranged thresholds in healthy individuals and in some retinitis pigmentosa patients. This normal behavior of the visual pathways to electrical stimulation has been used as a major inclusion test for identifying blind persons who might benefit from an artificial electrical prosthesis placed on the optic nerve or the retina. In Delbeke et al. (p. 291), profoundly modified responses have been reported in some patients suggesting that the method might provide interesting information about the pathophysiology of retinitis pigmentosa.