Essay
Matthew Trisic
Aline Garcia-Rubio
Senior Project
June 4, 2010
Eye Scream, You Scream, We All Scream for…Trabeculectomies?
When I first started thinking about my senior project I only knew that I wanted to do something related to medicine. I had worked with an ophthalmologist before and went to his clinic and viewed surgeries he had done, so I asked him if he had any recommendations for any connections he might have. He told me about a friend of his, Dr. Steve Mansberger, who works at Good Samaritan in the Devers Eye Institute and specializes in glaucoma. After learning about glaucoma a little bit in school I thought it sounded interesting and I took the opportunity.
Glaucoma is a condition of the eye in which the optic nerve is damaged. This can lead to loss of vision if not properly treated. In most cases, abnormal intraocular pressure causes damage to the nerve. Inside of the eye is the aqueous humor, the fluid between the cornea and the lens, and the vitreous humor, the fluid in the rest of the eye. This fluid is constantly produced by retinal cells at the back of the eye and by the ciliary body at the lens. The fluid drains through the trabecular meshwork and into the Schlemm’s canal. If any of this area becomes inflamed or blocked, the fluid will not be able to drain as quickly. This causes the pressure inside of the eye to increase. Once this occurs the optic nerve becomes damaged and people begin to lose visual acuity, possibly losing light perception.
A trabeculectomy is a surgery done to treat glaucoma. It alleviates the pressure on the eye by increasing the drainage of the aqueous humor. A trabeculectomy involves removing a portion of the trabecular meshwork so fluid can drain more rapidly. The surgeon first punctures the conjunctiva (the protective coating outside of the eye) and inserts small sponges soaked in one of two different antimetabolites. An antimetabolite is a chemical that inhibits normal metabolic processes. Antimetabolites used in a trabeculectomy, mitomycin C (MMC) and 5-fluorouracil (5FU) inhibit fibroblasts to prevent scarring. A cut is then made in the sclera (the white part of the eye) to access the inside of the eye. Portions of the sclera, trabecular meshwork, and Schlemm’s canal are taken out to remove obstruction and provide proper drainage. The fluid will now drain into a small pocket by the cut in the conjunctiva. This is called a bleb and is usually located by the limbus (the meeting of the cornea and the sclera, at the edge of the iris). Once the fluid gathers in the bleb it will be absorbed into the bloodstream and leave the eye. If a bleb leaks or doesn’t fill up then another trabeculectomy can be done at a different site.
A Baerveldt tube surgery bypasses the trabecular network completely. This allows the fluid to drain directly instead of passing through multiple structures. The tube is inserted into the eye and rests inside the conjunctiva. It is not easily visible, but possible to see if scrutinized closely. This surgery is usually an alternative surgery and can be done after a failed trabeculectomy. The tube will help to form a bleb like the trabeculectomy and redirect the fluid to drain out of the eye into the bloodstream.
I did not really know what to expect when I first began my project. When I talked with Dr. Mansberger about what I would be doing during my project he showed me different types of research they were doing and told me which one he would like me to work on. The first project I would be working on dealt with success rates of surgeries. The second dealt with checking the pressure of eyes with an artificial lens implant. Both of these will be discussed later in the essay.
I spent most of my time sitting at my desk and looking through patient files. I looked through a total of 43 files. While I looked through files I had to enter information into a database I created using Microsoft Access. I had to enter demographics such as name, date of birth, type of glaucoma, type of lens, what kind of surgery they had, and race. I also tracked their intraocular pressure, the number of glaucoma medications they used, and their visual acuity over the course of a year. After recording all of these values in my database I analyzed the data to determine whether or not that surgery was a success. There were three possible outcomes for a surgery. Either it was a success, a qualified success, or a failure. Normal pressure usually falls in between 10 mm Hg and 21 mm Hg. Not every patient with glaucoma above the normal range and in this case it is called low-tension glaucoma. A surgery fails if the IOP (intraocular pressure) remains at or above 21 for two consecutive visits three months after surgery, the IOP is not 20% below the preoperative IOP for two consecutive visits three months after surgery, or the patient loses light perception. If none of these situations occur, then the surgery was a success. If the surgery was a success but the patient is still dependent on medication then it is called a qualified success. Before creating my database I read two articles which told me about surgery success rates. The two surgeries I compared were trabeculectomies and Baerveldt tube surgeries. Trabeculectomies tend to have more complete successes and more failures, while tube surgeries tend to have more qualified successes and fewer failures. For trabeculectomies, I found a 66.67% success rate, a 13.89% qualified success rate, and a 19.44% failure rate. For Baerveldt tube surgeries, I found a 0% success rate, a 57.14% success rate, and a 42.86% failure rate.
Keep in mind, however, the small number of patients there were in this sample. I only had the time to look at 43 patients, and out of those 36 of them had a trabeculectomy while only 7 had a tube surgery. 7 patients cannot give an accurate assessment of how many surgeries will fail and succeed. But if we look at the percentages it is easy to see how they will eventually fit the estimations.
The next experiment I helped with was to determine an accurate way of measuring the IOP of an eye with a lens implant. For this project I worked mainly with Dr. Chris Shen. IOP is mainly measured through the cornea, the protective layer at the front of the eye. When an artificial lens is inserted between the aqueous humor and the vitreous humor of the eye to correct vision, however, it will not provide an accurate reading. In this case a different method of measuring IOP must be employed.
Once Dr. Shen obtained a pair of donated eyeballs, he called me over to the main hospital so we could begin the experiment. The first thing we did was measure the pressures of one of the eyes without an implanted lens. We measured the IOP at three places: the cornea (the protective lens in front of the eye), the limbus, and a consistent point on the sclera. A bottle of fluid was hooked up to an IV which ran through a machine and into the eye. The machine could measure the pressure inside the eyeball and we changed the pressure by raising or lowering the IV (raising the IV made the pressure go up and lowering the IV made the pressure go down). We measured the eye at 4 different pressures: 10 mm Hg, 20 mm Hg, 30 mm Hg, and 40 mm Hg. We also used two different instruments to measure the pressure. The first, the Tono-Pen, is used by tapping it against the eye. It gives the average of all of the different readings taken from the taps along with a percent error. If the percent error exceeded 5 or 10 then we did not use the reading. The next instrument we used was a Schiotz tonometer. The Schiotz physically rests on the eye and measures the pressure with a weight system. It was most frequently used in the 50s but may make a comeback if it proves accurate. Each time we used a different instrument on a different location of the eye we did 10 readings, but for the rest we only did 3. Once we finished with the first eye Dr. Michael Straiko joined us and implanted the fake lens while we worked on the second eye. When we were working on the eyes that had received a fake lens we did not measure for IOP on the cornea since we knew it would not work, so they went a lot faster. My job was to raise and lower the IV to adjust the pressure and to record the values as Dr. Shen measured the pressure. I was not allowed to tell Dr. Shen what the pressure was that we were measuring for in case he biased himself while measuring. Unfortunately I did not get to see this experiment to completion since there was not an opportunity for more donor eyeballs.
My project ended up being a very good experience. Although I did not get to do the most exciting work I found it interesting. I am glad I could experience aspects of research. Although I may not want to follow that career path, it definitely allowed me to gain a valuable perspective. Now that I am done I will be helping another student, Christina Ge, continue with the work. I may also be joining Dr. Mansberger again to view some surgeries in the future. I am thankful I was given such a useful opportunity to do this work as my senior project.
