Glaucoma similar to other ischemic posterior segment disorders

Issue: March 2011

To the Editor:

I read with great excitement the article by José M. De Jesús, “Ocular biomechanics, autoregulation potential factors in glaucoma pathogenesis” (December 2010, pages 16 to 20), as this is an area in which I have been doing extensive research since 2007.

An exhaustive review of the literature has enabled me to theorize that glaucoma is similar to other ischemic posterior segment disorders such as age-related macular degeneration, diabetic retinopathy, central serous chorioretinopathy and cystoid macular edema, among others. Let me elaborate.

I would first like to point out that an important distinction must be made between retinal blood flow and choroidal blood flow; they are entirely separate systems. First, the retinal circulation is autoregulated, while the choroidal circulation is not. Furthermore, the blood flow formula given by Dr. De Jesús, that of pressure head divided by resistance, must be explained in slightly further detail.

In the retinal circulation, the blood flow is directly proportional to the pressure head (Pa – Pv), divided by the resistance squared. In the choroid, the formula changes to the pressure head divided by the resistance to the fourth power. Therefore, if you want to increase blood flow to these tissues, it is far wiser and, theoretically, more effective to reduce vascular resistance than to lower intraocular pressure (lowering IOP can only lower the Pv value in the formula and has little effect on the blood flow). In other words, if you can reduce the vascular resistance a little, you could increase the blood flow a lot, especially in the choroid.

As the aforementioned posterior segment disorders are, to an extent, ischemic conditions, they may be theoretically treated by increasing choroidal blood flow. As the choroid is basically erectile tissue (you can check Duke-Elder’s System of Ophthalmology to confirm that), glaucoma is one of the ischemic posterior segment disorders that can be theoretically treated with phosphodiesterase-5 inhibitors (such as Viagra [sildenafil citrate, Pfizer], Cialis [tadalafil, Eli Lilly] and Levitra [vardenafil, Bayer]). Readers, I implore you to consider this.

Stephen R. Keller, OD
Palm Desert, Calif.

Dr. De Jesús responds:

I want to thank Dr. Keller for his interest in my article. Although I can appreciate his theory on the pathogenesis of glaucoma, I differ in some respect. It seems that all the diseases Dr. Keller mentions that he believes are similar to glaucoma are vascular in essence. (I believe that when he mentions age-related macular degeneration he is referring to neovascular AMD).

In my opinion, glaucoma is a multifactorial optic nerve disease where the presence of aberrant vascular flow is only one of those factors. Therefore, I do not feel it is prudent to group glaucoma with ocular diseases that are solely or primarily provoked by microvascular disorders.

I totally agree with Dr. Keller when he mentions that choroidal circulation and retinal circulation are separate systems and choroidal circulation is not autoregulated. However, when discussing glaucoma and autoregulation, explaining such distinction is not necessary. It is a general known fact that when autoregulation is mentioned in relation to glaucoma it refers to retinal circulation supplying the anterior aspect of the optic nerve head and the nerve fiber layer. Moreover, for the same reason, it is not necessary to establish the difference in ocular blood flow formulas between both systems.

Dr. Keller also mentions vasodilating medications that may be beneficial for the treatment of glaucoma. I do not see how this is relevant to the article. The primary idea of the article is to try to determine whether or not autoregulation and altered ocular biomechanics have a definite impact on the incidence of glaucoma, particularly in patients with elevated intraocular pressure.

José M. De Jesús, OD, MA, FAAO
Dean for Academic Affairs,
Inter American University of Puerto Rico
School of Optometry