The pressure inside your eyes is called intraocular pressure (IOP). Normal IOP ranges between 10-21 mmHg. Increased IOP, also known as ocular hypertension, is when the pressure inside the eye is higher than normal. Ocular hypertension is a major risk factor for glaucoma, a group of eye diseases that damage the optic nerve and can cause blindness if left untreated. Understanding what causes IOP to increase is important for identifying people at risk for developing glaucoma.
What is intraocular pressure?
Intraocular pressure is the fluid pressure inside the eye. Aqueous humor, a clear fluid, circulates inside the anterior chamber (front part) of the eye to nourish the cornea and lens. This fluid is constantly produced by the ciliary body behind the iris. It flows from the posterior to anterior chamber, draining out of the eye through a meshwork of tissues called the trabecular meshwork. IOP is maintained by the balance between fluid production and drainage.
Normal IOP ranges between 10-21 mmHg. The average is 15-16 mmHg. IOP can fluctuate throughout the day, usually highest in the morning and lowest at night.each person also has a unique normal IOP range. As long as drainage keeps pace with production, IOP remains stable at healthy levels.
What is ocular hypertension?
Ocular hypertension means the IOP is consistently above the normal range, usually above 21 mmHg. With ocular hypertension, the optic nerve may show some early signs of damage. However, vision remains normal. It is a major risk factor for developing glaucoma but does not always lead to glaucoma.
What causes IOP to increase?
There are several reasons why IOP can become elevated. The main causes include:
Impaired drainage
The most common reason for increased IOP is obstructed drainage of aqueous fluid from the eye. Resistance to aqueous outflow increases pressure inside the eye. Drainage may be impaired due to:
– Blocked trabecular meshwork: The trabecular meshwork can get partially blocked with debris or inflammatory cells. This prevents aqueous humor from exiting the eye through this drainage channel.
– Closed angle glaucoma: The iris can gradually block the drainage angle formed by the cornea and iris. This closes off the trabecular meshwork and prevents drainage through this pathway.
– Scarring of Schlemm’s canal: Schlemm’s canal is the main vessel that drains aqueous out of the trabecular meshwork. Scarring or collapse of this canal can obstruct outflow.
Excess aqueous production
In some cases, the ciliary body produces too much aqueous fluid. This can occur due to:
– Inflammation
– Tumors
– Medications including corticosteroids
The excess fluid builds up because it cannot drain fast enough through the trabecular meshwork.
Pupil block
Pupil block happens when the iris sticks to the lens, blocking the flow of aqueous from the posterior to anterior chambers. As fluid accumulates behind the iris, it pushes the iris forward. This narrows the drainage angle and increases resistance to outflow.
Damage to ciliary body
The ciliary body controls accommodation (focusing) and aqueous production. Damage from trauma, inflammation, or a tumor can cause abnormal fluid production leading to increased IOP.
Other risk factors for elevated IOP
In addition to the above causes, other factors that increase the risk of high IOP include:
– Older age: The risk increases over age 40.
– Thinner corneas
– Family history of glaucoma
– Diabetes
– Hypertension
– Farsightedness
– Long-term corticosteroid use
– Eye surgery or injury
– Sleep apnea
Symptoms of ocular hypertension
Ocular hypertension usually causes no symptoms until it has progressed to glaucoma. Symptoms only occur after significant optic nerve damage and vision loss. That’s why routine eye exams are so important to check for elevated IOP before glaucoma develops.
Some symptoms that may eventually occur with high eye pressure include:
– Blurry vision
– Halos around lights
– Vision loss
– Redness
– Eye pain or headaches
– Nausea and vomiting if pressure is very high
However, many people never notice any symptoms until they have already lost vision from glaucoma. Noticing symptoms does not happen until over 40% of optic nerve fibers are irreversibly damaged. That’s why it is crucial to identify and treat elevated IOP before permanent visual field loss occurs.
Diagnosing high IOP
The only way to detect ocular hypertension is through an eye exam. Tonometry is the measurement of IOP. It is performed as part of a comprehensive eye examination.
There are different methods of tonometry to check eye pressure:
Air puff tonometry
This non-contact method uses a quick puff of air to the surface of the eye. As the air flattens the cornea, an internal sensor calculates the amount of resistance to determine the IOP. This common technique is used for routine glaucoma screenings. However, its accuracy can be affected by corneal thickness and other factors.
Applanation tonometry
Goldmann applanation tonometry is considered the gold standard. The examiner applies a prism instrument directly to the anesthetized cornea. The slight pressure flattens a small area of the cornea. As the pressure equals the IOP, flattening stops and the IOP is measured. It provides the most accurate readings.
Rebound tonometry
This method uses a small plastic-tipped probe that briefly touches the cornea. An internal sensor measures how fast the probe rebounds to calculate the IOP. It does not require anesthetic drops. However, it is not as consistently accurate as Goldmann tonometry.
Transpalpebral tonometry
This device measures IOP through the eyelid without touching the eye. It eliminates the risk of corneal abrasion and avoids the use of anesthetic drops. However, transpalpebral tonometry is not as accurate as Goldmann tonometry.
No single IOP reading can diagnose ocular hypertension or glaucoma. Multiple measurements, at different times of day, are necessary to find the overall highest IOP. Visual field testing and examination of the optic nerve are also needed. If high IOP is detected, regular monitoring is required to assess the risk for vision loss.
Treatment for ocular hypertension
It is important to understand IOP is only a risk factor for glaucoma and does not always cause optic nerve damage or vision loss. Treatment aims to lower IOP to a safer level in order to avoid glaucoma progression and preserve vision.
Treatment options for ocular hypertension include:
Medication
Eye drops are the most common treatment. Different types of prescription eye drops can lower IOP by reducing aqueous production or improving fluid drainage:
– Prostaglandin analogs like Latanoprost or Travoprost increase uveoscleral outflow
– Beta blockers like Timolol reduce aqueous production
– Alpha agonists like Brimonidine also lower fluid production
– Carbonic anhydrase inhibitors like Dorzolamide reduce fluid formation
Typically eye drops must be used daily for life. Multiple medications may be needed to sufficiently reduce IOP. Side effects may include eye irritation or redness.
Laser trabeculoplasty
Selective Laser Trabeculoplasty (SLT) is an outpatient procedure that targets areas of the trabecular meshwork with low energy laser to improve drainage. It can reduce IOP by approximately 20%. Effects may diminish over time. Repeat procedures may be needed.
Surgery
Examples like trabeculectomy or drainage implant surgery create alternative drainage pathways for aqueous fluid to leave the eye. This more aggressively reduces IOP when other methods are inadequate to slow glaucoma progression.
Monitoring and lifestyle management
– Regular eye exams to monitor IOP, optic nerve changes, and visual fields. IOP screening every 1-2 years for those over age 40.
– Medication compliance. Using eye drops daily as prescribed.
– Healthy diet and exercise to maintain healthy eye circulation.
– Managing other medical conditions like diabetes.
– Avoid or limit corticosteroid use if possible.
– Quit smoking which can increase eye pressure.
Risk of developing glaucoma
For people with ocular hypertension, the risk of progressing to glaucoma depends on how high the IOP is and other factors like age and central corneal thickness. Those with moderately elevated IOP around 23-25 mmHg have a 10% risk over 5 years. The risk increases to 40-50% when IOP is consistently greater than 28 mmHg. Lowering IOP lowers the risk of optic nerve damage.
Other factors also impact the risk of converting to glaucoma such as:
| Risk Factor | Level of Risk |
|---|---|
| Age over 50 | Higher risk |
| African ancestry | 2-4 times higher risk than other races |
| Family history of glaucoma | 3-4 times higher risk if sibling has glaucoma |
| Thinner central corneal measurement | Higher risk with corneal thickness below 555 micrometers |
| Myopia | 2-3 times higher risk |
| Diabetes | Up to 2 times higher risk |
| Migraine headaches | 2 times higher risk |
So even with the same eye pressure, some people are at greater risk for developing glaucoma. Regular eye exams, IOP monitoring, and early treatment are keys to preventing vision loss.
Conclusion
Increased eye pressure, called ocular hypertension, is a major risk factor for glaucoma. But what causes high IOP in the first place? The main mechanisms include reduced drainage of aqueous fluid out of the eye, increased fluid production, and pupil block among other causes. There are often no early warning signs, which is why routine eye exams to measure IOP are so important to detect ocular hypertension. Treatment aims to lower pressure through medications, laser, or surgery in order to prevent optic nerve damage from glaucoma and preserve sight. By understanding the causes and treatment of increased IOP, vision loss can often be avoided.