Pathophysiology

Retinal Ischemia

Occlusion of retinal arterioles leads to an acute decrease of the inner retinal Po2 and subsequent injury to the inner retinal cells. Systemic hyperoxia but pure O2 increases inner retina Po2. This is due to the vasoconstrictive effect of 100 percent O2. The ischemic retina may require other metabolites in addition to O2 to resume normal function. In chronic retinal ischemia such as diabetic retinopathy and retinal vein occlusion, modifications of the retinal vascular bed lead to the formation of ischemic areas. The ischemic areas produce vasoproliferative factors such as vascular endothelial growth factor (VEGF), stimulating new vessel formation. The new vessel is leaky because of the scarce intercellular junctions between the endothelial cells. Clinical trials are under way to test the use of intravitreal or subtenon steroids, as well as oral and local anti-VEGF agents, in preventing retinal neovascularization due to ischemia.

Glossary

Autoregulation of retinal blood flow: The ability of the retina to maintain its blood flow relatively constant despite moderate variations of perfusion pressure.

Further Reading

Foundation of the American Academy of Ophthalmology. (2000). Fundamentals and principles of ophthalmology. In Basic and Clinical Science Course, Section 2, pp. 76-88. San Francisco, CA: The Foundation of the American Academy of Ophthalmology. This chapter offers detailed anatomical description of the retinal and choroidal microvasculature.

Harris, A., Bingaman, D. P., Ciulla, T. A., and Martin, B. J. (2001). Retinal and choroidal blood flow in health and diseases. In Retina, 2nd ed. (S. J. Ryan, ed.), pp. 68-88. St. Louis, MO: Harcourt-Mosby. This chapter has good coverage of the pathophysiology of retinal and choroidal microvasculature.

Pournaras, C. J., and Donati, G. (2000). Retinal and choroidal circulations. In Principles and Practice of Ophthalmology, 2nd ed. (D. M. Alberts and F. A. Jakobiec, eds.), pp. 1804-1817. Philadelphia: Saunders. The chapter has in-depth discussion on the level of oxygen at various retinal and choroidal layers, as well as its regulation.

Capsule Biography

Dr. Ming Lu is a Heed Ophthalmic Fellow at the University of Texas Southwestern Medical Center. Winner of the Leslie Nesmith Award from the Schepens International Society in 1999, he is interested in diabetic retinopathy and retinopathy of prematurity.

Dr Adamis is cofounder and Chief Scientific Officer of Eyetech Pharmaceuticals. His research is focused on the mechanisms of diabetic retinopathy and age-related macular degeneration.

CHAPTER 64

Essentials of Human Physiology

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