How Eye Pressure Is Measured: What Patients Should Know About Accuracy and Technology

How Eye Pressure Is Measured: What Patients Should Know About Accuracy and Technology

Eye pressure, or intraocular pressure (IOP), is one of the most important measurements in eye care, especially when screening for and managing glaucoma. IOP is one of the eye’s vital signs.

Yet many patients are surprised to learn that there is more than one way to measure eye pressure, and that not all methods are equally accurate.

Understanding how eye pressure is measured — and the strengths and limitations of each method — can help you better interpret your results and advocate for your eye health.

Palpation: The Oldest Method

Palpation involves gently pressing on the eye through the eyelids with the fingers to estimate pressure. This method dates back centuries and is still occasionally used in emergency or low-resource settings.

While palpation can identify very high or very low pressures, it is not precise and cannot detect subtle or moderate elevations. It is never sufficient for diagnosing or monitoring glaucoma, but it may provide a rough estimate when no instruments are available.

Schiøtz Tonometry: Historical but Largely Obsolete

The Schiøtz tonometer is an indentation-based device that measures how much the cornea indents under a known weight. It played an important historical role in ophthalmology but is now rarely used in modern clinical practice.

Its accuracy is limited because results are influenced by:

• Corneal thickness and rigidity

• Eye size and scleral elasticity

  
  

Today, Schiøtz tonometry is primarily of historical interest or used in limited settings.

Goldmann Applanation Tonometry (GAT): The Gold Standard

Goldmann applanation tonometry is widely considered the gold standard for measuring eye pressure. It measures the force required to flatten a small, standardized area of the cornea.

Why Goldmann is trusted:

• Extensive validation over decades

• Strong correlation with glaucoma outcomes

• High reproducibility when performed correctly

  
  

That said, Goldmann readings can still be influenced by corneal thickness, corneal curvature, and prior eye surgery, which is why interpreting eye pressure always requires clinical context.

Tono-Pen: Portable and Practical

The Tono-Pen is a handheld, portable device that uses applanation principles and is especially useful when patients cannot sit at a slit lamp.

Advantages:

• Works well in children and bedridden patients

• Useful after corneal surgery

• Quick and convenient

  
  

Limitations:

• Slightly more variable than Goldmann

• Accuracy can decrease at very high or very low pressures

  
  

In clinical practice, it is a reliable alternative when Goldmann is not feasible.

iCare Rebound Tonometry: No Drops Needed

iCare tonometry uses a small probe that briefly contacts the cornea and rebounds back. It does not require numbing drops, making it popular for children and home monitoring.

Advantages:

• Comfortable and fast

• No anesthesia required

• Useful for screening and trend monitoring

  
  

Limitations:

• May underestimate pressure at higher IOP levels

• Still influenced by corneal properties

  
  

iCare is excellent for screening and follow-up trends, but abnormal readings often warrant confirmation with Goldmann tonometry.

Newer and Emerging Technologies

Several newer technologies aim to capture eye pressure more accurately or more continuously:

• Dynamic contour tonometry: Less affected by corneal thicknessOcular Response Analyzer: Provides corneal biomechanics along with IOP

• Contact lens sensors: Measure IOP-related changes over 24 hours

  
  

These tools offer valuable insights into pressure fluctuations, especially overnight, but they are typically adjuncts—not replacements—for standard tonometry in routine care.

So…Which Method Is the Most Accurate?

For most patients and clinical decisions:

Goldmann applanation tonometry remains the most accurate and trusted method when performed properly and interpreted in context.

However, accuracy is not just about the device. It also depends on:

• Corneal thickness and biomechanics

• Time of day

• Body position

• Holding one’s breath or straining

• Repeated measurements over time

• Eyelid squeezing

• Examiner error

  
  

This is why glaucoma care focuses on trends, not single numbers.

Bottom Line

Eye pressure measurement is more nuanced than it appears. Different tools serve different purposes, and no single number tells the whole story.

The most important question is not 

“What was my eye pressure today?” but rather:

 “How does my eye pressure fluctuate over time, and how is my optic nerve responding?”

That is where expert interpretation and comprehensive glaucoma care truly matter.