Intraocular pressure (IOP) is the fluid pressure inside your eye. It is maintained by aqueous humor, a clear liquid filling the front part of the eye. The continuous production and drainage of this fluid help the eye maintain its spherical shape and function properly. Maintaining this pressure within a healthy range is important for eye health.
The Purpose of Measuring Intraocular Pressure
Measuring intraocular pressure is a routine part of eye examinations, primarily to assess glaucoma risk. Glaucoma is a group of eye conditions that damage the optic nerve, which transmits visual information to the brain. Elevated intraocular pressure is the most significant modifiable factor linked to its development. High pressure can gradually compress and harm delicate optic nerve fibers over time.
Since glaucoma often progresses without noticeable symptoms, regular IOP measurement helps detect issues before significant vision loss. Early identification allows eye care professionals to monitor the condition and preserve vision.
Common Measurement Techniques
Eye care professionals use various tonometry methods to measure intraocular pressure. Each technique assesses corneal resistance. These quick tests are a standard part of a comprehensive eye examination.
Non-Contact Tonometry (NCT)
Non-Contact Tonometry (NCT), or the “air puff” test, involves a gentle puff of air directed at your eye while you rest your chin on a support. The machine measures the eye’s response to estimate pressure. This method requires no numbing eye drops and is suitable for quick screening, especially for children or those sensitive to eye contact.
Applanation Tonometry
Applanation Tonometry, or Goldmann tonometry, is a highly accurate method. Anesthetic eye drops and fluorescein dye are applied to numb the eye. A small, blue-light-illuminated probe then gently touches the cornea. The eye care professional observes through a microscope how much force is needed to flatten a specific corneal area, which helps calculate intraocular pressure.
Other Tonometry Devices
Other tonometry devices offer portable alternatives for measuring IOP. Rebound tonometers, like the iCare device, use a lightweight, disposable probe that briefly taps the cornea. The device measures the probe’s rebound speed, converting it into an IOP reading without numbing drops. The Tono-Pen is another handheld device that gently touches the cornea to provide a pressure reading.
Interpreting the Results
Intraocular pressure readings are typically measured in millimeters of mercury (mmHg). A generally accepted range for normal IOP is between 10 and 21 mmHg. However, this is not a strict boundary, as individual normal ranges can vary.
A reading above this typical range, usually over 21 mmHg, may indicate ocular hypertension. This means the eye pressure is higher than average, but without observable damage to the optic nerve or vision loss. Individuals with ocular hypertension have an increased likelihood of developing glaucoma, so close monitoring by an eye care professional is advised.
It is important to remember that a single high IOP reading does not automatically confirm a glaucoma diagnosis. Eye doctors consider a comprehensive set of findings, including the appearance of the optic nerve, visual field tests, and repeated pressure measurements over time. Conversely, unusually low IOP, or hypotony, typically defined as 5 mmHg or lower, can also be a concern and may signal other underlying eye issues that require investigation.
Factors Influencing Pressure Readings
Intraocular pressure is not constant; it can fluctuate throughout the day and be influenced by various factors. These variations mean a single measurement provides only a snapshot of the eye’s pressure at that moment. Understanding these influences helps in a more complete assessment of eye health.
Diurnal Variation
Diurnal variation refers to natural changes in IOP levels over a 24-hour period. For many, eye pressure tends to be highest in the early morning and may decrease later in the day. While these daily fluctuations are normal, significant variations can be relevant in certain eye conditions.
Corneal Thickness
The thickness of the cornea can also affect the accuracy of IOP measurements. A thicker-than-average cornea may cause tonometers to report a falsely high reading, while a thinner cornea might result in an artificially low reading. Eye care professionals often measure corneal thickness using a procedure called pachymetry to interpret IOP results more accurately.
Other Variables
Other variables can temporarily influence intraocular pressure readings. These can include physical actions like holding one’s breath or straining, certain body postures such as lying flat, and even external factors like wearing a tight shirt collar. Lifestyle elements like caffeine intake or stress levels have also been noted to cause minor, temporary shifts in eye pressure.