2020–2021 BCSC Basic and Clinical Science Course™
2 Fundamentals and Principles of Ophthalmology
Part IV: Biochemistry and Metabolism
Chapter 11: Vitreous
Biochemical Changes With Aging and Disease
Physiologic Changes After Vitrectomy
Most of the changes in ocular physiology that occur after vitrectomy result from altered viscosity in the vitreous cavity; when the vitreous is removed, the viscosity decreases between 300-and 2000-fold. Consequently, growth factors and other compounds, such as antibiotics, transfer between the posterior and anterior segments more easily and are also cleared more quickly from the eye. This effect is proportional to the change in diffusion coefficient, which is of the same magnitude as the change in viscosity.
Fluid currents that move solutes even more rapidly may be present (see Fig 11-1). In particular, oxygen movement is accelerated. The oxygen gradient that exists between the well-oxygenated anterior segment and the posterior segment under normal physiologic conditions is abolished. This leads to increased oxygen tension in the vitreous cavity. Under physiologic conditions, vitreous ascorbate combines with oxygen, forming dehydroascorbate and water. However, after vitrectomy, oxygen levels exceed the capacity of ascorbate, leading to increased oxidative stress at the posterior pole of the lens and the development of cataract (Fig 11-6).
Holekamp NM, Shui YB, Beebe DC. Vitrectomy surgery increases oxygen exposure to the lens: a possible mechanism for nuclear cataract formation. Am J Ophthalmol. 2005;139(2):302–310.
Shui YB, Holekamp NM, Kramer BC, et al. The gel state of the vitreous and ascorbate-dependent oxygen consumption: relationship to the etiology of nuclear cataracts. Arch Ophthalmol. 2009;127(4):475–482.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.