Before beginning, let's go through the muscles constricting and dilating the pupil. Constriction of the pupil means the pupil (ie the dark hole) becomes smaller. Dilation of the pupil means the pupil becomes larger.
The pupil is contricted or dilated by the contraction of smooth muscle in the iris. There are two types. One type is arranged in the circle near the pupil. When a circle of muscle contracts, the circle becomes smaller. Thus contraction of the circular muscle causes constriction of the pupil. The second type of smooth muscle is arranged radially. When these muscle cells contract it pulls open the pupil. In other words, contraction of radial smooth muscle causes dilation of the pupil.
This explains the autonomic actions:
Glaucoma, which is the second leading cause of blindness, is a chronic neuropathy of the optic nerve in which there is progressive degeneration at the optic disc. On visual examination, the rim of the optic disc becomes thinner and the optic disc cups inward. Histologically, retinal ganglion cell axons (see handout figure) degenerate, followed by their cell bodies.
The degeneration is linked to increased stress placed on the region of the optic disc by increased intraocular pressure. While increased intraocular pressure is not detected in every case, the reduction of degeneration is closely linked to reducing the pressure, and all the treatments are focused on this.
The increased intraocular pressure is due to excessive accumulation of aqueous humor. Aqueous humor is formed in the ciliary body, flows through the pupil, and exits through the trabecular network, which is found in the angle between the cornea and iris. An unusually good figure is found at this link(http://www.nejm.org.offcampus.lib.washington.edu/action/showImage?doi=10.1056%2FNEJMra0804630&iid=f01). (For off campus use, here is the LINK TO PROXY SERVER. Copy and paste the URL.)
While the trabecular meshwork is the primary, normal means of outflow of the aqueous humor, a second pathway does exist. Refer to the figure at this link showing the two means of outflow(http://physiologyonline.physiology.org/content/18/5/205/F1.large.jpg). While the second pathway, the uveoscleral outflow path through the interstices of the ciliary muscle, is much less important normally, apparently current drugs that increase drainage (prostaglandin analogs and alpha2 agonists) act through this pathway. Hopefully better drugs will be developed that target the trabecular meshwork since it is normally a much more open pathway.
There are two main types of glaucoma.
An 80 year old woman is examined due to a complaint of slowly declining vision. She had a procedure called laser trabeculoplasty five years earlier. Which of the two types of glaucoma would is most likely?
How do you suppose the physician might assess the actual damage that has occurred due to the glaucoma?
Would the woman be slowly developing a blind spot over the part of the visual field detected at the optic disc?
One hypothesis suggests that the transport of a type of substance back to the cell body is blocked as the elevated intraocular pressure pinches the axons at the optic disc. Apoptosis ensues. Based on earlier lecture material, can you speculate what type of substance this might be? (tough question)
Recall there are both alpha adrenergic receptors and beta adrenergic receptors. Some tissues have just one of the two types. Some tissues have both types. Each tissue has to be viewed by itself. There are no general rules. Sometimes both types of receptors do more or less the same thing. But elsewhere the two types of receptors can promote very different things. Thus, below you will see that alpha agonists and beta antagonists are used for the same action. That is just the way the particular tissue is put together.
Drugs used in glaucoma increase the drainage of the aqueous humor and/or reduce its rate of formation at the ciliary body. They are used mainly as eye drops. But enough of the drugs enter the circulation so some of side effects are systemic.
The specific patient described above is being given timolol, which is a beta antagonist, and iatanoprost, which is a prostaglandin analog, as eye drops. Why would these two drugs be complementary?
Risk factors in open-angle glaucoma include older age, black race, family history of glaucoma, myopia and especially elevated intraocular pressure. People with significant risk are usually urged to have a dilated pupil eye examination each year.