WHAT IS INTRAOCULAR PRESSURE?High intraocular pressure causes damage to the optic nerve, which can lead to glaucoma. But what is it? What makes it happen? What can you do about it? In order for you to gain a better sense of what glaucoma is and how it can effect the optic nerve of your eye, I need to explain intraocular pressure to you. First, let's dissect the term.
A BASKETBALL ANALOGYI like to watch basketball. It is an exciting game that requires both individual and team skill. But to advance our discussion here, let's talk about the basketball itself. The National Basketball Association (NBA) sanctions just one official ball to be used in their competitions. Why are the regulations so strict? The NBA has about 430 players distributed over 30 teams. At this elite level of competition, even a slight difference in the size, weight, or internal pressure of the ball could have a huge impact on player performance and the outcome of the game. Think about it. A basketball is what we call a closed system in science. This means that air inside the ball is contained by its walls and is not permitted to escape. Materials used to make the ball have limited flexibility and expansion capabilities. Therefore, as more air is introduced into the ball, the pressure inside (force exerted on the walls) increases.Small reductions or increases in ball pressure would mean that the bounce potential would change and influence three key dimensions of the game: height of return after the ball strikes the floor, bounce off the backboard, and bounce off the rim of the basket. This could have a significant impact on the performance of the NBA athletes. For this reason, ball specifications are strictly monitored.
HOW DOES THIS APPLY TO THE EYE?Like the basketball, the eye is a closed system. However, it is not filled with air, but rather with liquid substances called aqueous humor and vitreous humor. Aqueous humor is confined to a small space in the front part of the eye. The remainder of the eye is filled with vitreous humor.
Similar to the basketball, the materials of the eye that contain the aqueous and vitreous humors in a closed space have limited flexibility and expansion capabilities. This means that additional aqueous humor introduced into the eye increases the pressure inside the eye.
NORMAL INTRAOCULAR PRESSUREI define a normal intraocular pressure as one that falls between 10-20 mmHg. At this point, you are probably wondering, "What does 10-20 mmHg mean?" The mmHg refers to millimeters of mercury. But what this actually means is too technical for our discussion here and really is not necessary. Instead, let's make a simple comparison. Normal atmospheric pressure is 760 mmHg. We do not even notice this pressure as we preform our daily activities. Hmm . . . this means that the pressure inside the eye (10-20 mmHg) is only about 2% above normal atmospheric pressure. Do you think that you could feel that? No. We are not able to detect fluctuations in atmospheric pressure this small. But the optic nerve is hypersensitive to even small changes in pressure inside the eye. This example illustrates just how delicate the eye is. We do not even feel normal atmospheric pressure at 760 mmHg, yet even slight differences in intraocular pressure above 20 mmHg can have devastating consequences on the optic nerve and our eyesight.
WHERE DOES THE AQUEOUS HUMOR COME FROM?Aqueous humor is produced continuously by a structure in the front part of the eye (anterior chamber) called the ciliary body. Remember, as we increase the quantity of aqueous humor, we also increase the force exerted on the walls and on other structures in the eye. In order for the eye to remain healthy, the rate of aqueous humor production must be balanced by an equal rate of drainage from the eye. Similar to our basketball analogy, small variations in the production or outflow of aqueous humor can have large influences on intraocular pressure and therefore impact the optic nerve and functioning of the eye.Elevated eye pressure normally is prevented by another eye structure called the trabecular meshwork, which filters aqueous humor before it exits the eye and returns to the bloodstream. Aqueous humor flow is depicted in the diagram above as a long red arrow that goes from the ciliary body, traverses around the iris and through the pupil, and ends up at the trabecular meshwork, where it is filtered before entering Schlemm's canal. The trabecular meshwork is assisted to a small degree by a backup mechanism called the uveoscleral pathway. But only about 5-10% of the aqueous humor is returned to the bloodstream through this route. A number of glaucoma medications work to increase the use of the uveoscleral pathway. The trabecular meshwork is kind of like a drain strainer at the bottom of your kitchen The analogous structures to the drain pipe in the eye are the episcleral veins. So far, we have learned two important things. Intraocular pressure depends on the quantity of aqueous humor produced by the ciliary body and the ease with which it leaves the eye through the trabecular meshwork. Decreased outflow of aqueous humor can be due to an abnormal trabecular meshwork or to damage caused to it by injury or infection.
OCULAR HYPERTENSIONBecause of the extreme sensitivity of the optic nerve, it is absolutely essential that intraocular pressure be maintained within the range of 10-20 mmHg. Like with our basketball example, pressures outside this range, most notably those above 20 mmHg, potentially cause damage to the optic nerve and negatively impact the eye's performance. Ocular hypertension refers to the specific case in which intraocular pressure is at or above 21 mmHg, but there is no detectable loss of visual field or damage to the optic nerve. Individuals with ocular hypertension often are referred to as glaucoma suspects, a significant percentage of which develop glaucoma. However, some people with ocular hypertension never develop glaucoma. Do not confuse ocular hypertension with the more commonly known use of the term hypertension, which is associated with blood pressure, or the pressure that is exerted by the flow of blood within the blood vessel walls. The two are not the same. Those diagnosed with high intraocular pressure do not necessarily also have high blood pressure, and vice versa. Ocular hypertension likely is a new term for most of you. In fact, you probably are becoming overwhelmed by the number of terms to remember. But wait a minute! You do not have to remember all of these terms. You can just revisit glaucoma-eye-info.com whenever you need information and they all will be here. There is no getting around it. The eye is a complex structure, which can make things pretty confusing. But I will try to simplify technical language as much as I can.
NORMAL FLUCTUATIONS IN INTRAOCULAR PRESSURENormal intraocular pressure is a range (10-20 mmHg). Everyone experiences variations within this range throughout the day and night. For example, a person's intraocular pressure might be 16 mmHg in the morning, drop to 14 mmHg in the afternoon, and then rise to 15 mmHg at night. This represents a 3 mmHg variation. Ideally, fluctuations should not exceed 6 mmHg. Each of us also experiences variations in intraocular pressure that can extend outside the normal range; for example, when we ride in an elevator, an airplane (takeoff and landing), and even when we sneeze. However, these fluctuations are transient and are not sufficient to cause significant damage to the optic nerve. An exception to this is suppressing a sneeze. I am not talking about covering your mouth, which helps to contain the spread of germs from a sneeze. This is a good thing to do. Instead, I am referring to attempts to suppress the sneeze entirely by holding your breath, pinching your nose shut with your fingers, or both. This can result in a condition called orbital emphysema. The extreme pressures that result from suppressing a sneeze can cause small hairline fractures in the orbit of the eye, allowing pockets of air to be introduced where they do not belong. In some cases, the air pockets can interfere with normal trabecular meshwork functioning and significantly raise intraocular pressure, resulting in potential damage to the optic nerve.
Research studies have found that variations throughout the day of 6 mmHg or more are common in patients with glaucoma. For example a person with glaucoma may start with a pressure of 22 mmHg during the morning, have it rise to 29 mmHg in the afternoon, and then drop to 25 mmHg at night. Intraocular pressure is the main risk factor in glaucoma that can be modified. For this reason, it is of critical importance that you have it checked regularly, especially if you have been diagnosed with glaucoma. Over recent years, different classes of drugs have been developed to help glaucoma patients. These drugs lower intraocular pressure by altering the two factors explained above: production of aqueous humor by the ciliary body and/or its drainage from the eye into the bloodstream.
REFERENCE:Tsai JC, Denniston AKO, Murray PI, Huang JJ and Aldad TS. Oxford American Handbook of Ophthalmology. New York: Oxford University Press, 2011: 262-268.
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It is manufactured by the Spalding Company. Specifications require that it be 29.5 inches in circumference, weigh 22 ounces, and be inflated to a pressure that is between 7.5-8.5 pounds per square inch (PSI).
sink that helps to trap debris before it can enter the drainpipe. If the debris is not cleaned from the strainer, it will clog and water will back up into the sink. If the strainer either is faulty or is not used at all, then debris will accumulate in the drain pipe and it also will clog. 
