What is a normal intraocular pressure (IOP) reading?

Normal Intraocular Pressure (IOP) Reading

The normal intraocular pressure (IOP) range is typically between 10-21 mmHg when measured by Goldmann applanation tonometry, with an average of approximately 15-16 mmHg. 1

Understanding IOP Measurements

Normal Range Considerations

• The standard normal range of 10-21 mmHg is based on population studies
• IOP readings above 21 mmHg may indicate ocular hypertension
• However, many individuals with primary open-angle glaucoma (POAG) have IOP levels below 22 mmHg 1
• Population-based studies suggest that approximately half of all individuals with POAG have IOP levels below the traditional 22 mmHg cutoff 1

Factors Affecting IOP Measurement Accuracy

Central Corneal Thickness (CCT)

CCT significantly impacts IOP readings and varies by ethnicity:

• Caucasian Americans: average 556 μm
• Latinos: average 546 μm
• Asians: average 552 μm
• American Indian/Alaska Natives: average 555 μm
• African Americans: average 534 μm 1

IOP measurement artifacts due to CCT:

• Thinner corneas: IOP tends to be underestimated
• Thicker corneas: IOP tends to be overestimated
• No validated standard nomogram exists for CCT correction 1

Special Corneal Conditions

IOP measurements may be significantly affected in:

• Post-keratorefractive surgery eyes
• Forme fruste keratoconus
• Fuchs endotheliopathy
• Corneal edema (causes underestimation despite thick cornea) 1

Tonometry Methods

Goldmann Applanation Tonometry (GAT)

• Considered the gold standard for IOP measurement 1, 2
• Most reliable in eyes with normal corneal parameters
• Limitations include:
  • Requires local anesthetics
  • Difficult in uncooperative patients, children, and surgical patients
  • Significantly affected by corneal parameters 2

Alternative Tonometry Methods

For corneas with abnormal biomechanical properties, consider:

• Pneumatonometry
• Dynamic contour tonometry
• Rebound tonometry (I-care)
• Noncontact differential tonometry (Ocular Response Analyzer) 1

Recent studies on alternative tonometers:

• Rebound tonometers (I-care ic200) tend to overestimate IOP by approximately 1-2 mmHg compared to GAT, with greater overestimation at higher pressures 3, 4
• Noncontact tonometers may show IOP ranges up to 10 mmHg in the same eye during multiple measurements 5

Clinical Implications

IOP in Glaucoma Diagnosis

• IOP alone is not sufficient for glaucoma diagnosis 1
• Using IOP above 21 mmHg as a screening tool has:
  • Sensitivity of only 47.1%
  • Specificity of 92.4% 1
• Comprehensive glaucoma evaluation should include:
  • Clinical examination of the optic nerve head
  • Imaging of the retinal nerve fiber layer
  • Visual field assessment 1

Diurnal Variation

• Normal eyes can have IOP fluctuations of 2-6 mmHg throughout the day
• Greater fluctuations may be seen in glaucomatous eyes
• Single IOP measurements may not capture the true IOP profile 2

Pitfalls to Avoid

1. Relying solely on IOP for glaucoma diagnosis or screening
2. Failing to consider CCT when interpreting IOP readings
3. Using inappropriate tonometry methods for abnormal corneas
4. Not accounting for diurnal IOP variations
5. Overlooking other risk factors for glaucoma when IOP is within normal range

Remember that while 10-21 mmHg is considered the normal range, individual variations exist, and IOP should be interpreted in the context of other clinical findings, especially when evaluating for glaucoma.