How Soft Lenses Affect Eye Pressure Measurement
Imagine if your blood pressure monitor gave different readings depending on whether you were wearing a cotton shirt or a wool sweater. This is precisely the challenge ophthalmologists face when measuring intraocular pressure (IOP) through contact lenses. As the only modifiable risk factor for glaucoma—a leading cause of irreversible blindness worldwide—accurate IOP measurement is crucial for diagnosis and treatment 1 .
The Goldman applanation tonometer has been the gold standard for IOP measurement since the 1950s.
Millions worldwide rely on contact lenses for vision correction, making this issue increasingly relevant.
The device operates on the Imbert-Fick principle, which states that the pressure inside a thin-walled sphere equals the force necessary to flatten its surface divided by the area of flattening.
The tonometer uses a double-prism head that applanates exactly 3.06mm of the corneal diameter, designed so that the resistance of the cornea itself is balanced by the capillary attraction of the tear meniscus.
Goldman himself recognized that corneal properties would influence measurements. The relationship can be expressed as:
Measured IOP = Actual IOP + (Corneal resistance factor)
This corneal resistance factor is influenced by central corneal thickness, curvature, biomechanics, and surface hydration.
When we place a contact lens on the eye, we effectively create a new anterior surface with its own thickness, curvature, and biomechanical properties. Different lens materials—with varying water content, modulus of elasticity, and thickness profiles—interact differently with the tonometer head.
The contact lens creates a physical separation between the tonometer and the cornea.
Lens material properties change how force is transmitted to the cornea.
The lens-tear film interface affects the applanation process.
A comprehensive study published in Eye (2012) investigated this issue 2 3 . Researchers recruited 40 subjects with no ocular diseases and measured their IOP using three different tonometers:
Air puff applanation
Dynamic contour tonometry
Gold standard applanation
The experimental design was meticulous, with measurements taken before and after lens insertion, with adequate adaptation time and multiple measurements to minimize bias.
The findings revealed fascinating device-specific effects:
The study revealed three crucial findings 2 3 :
| Tonometer Type | Principle | Effect of Soft CL | Best For | Limitations |
|---|---|---|---|---|
| Goldman Applanation | Force to flatten cornea | Not possible | Gold standard, most clinical settings | Requires removal of CL |
| Non-Contact (NCT) | Air puff applanation | Minimal effect | Screening, patients who cannot tolerate contact | Slight overestimation compared to GAT |
| Rebound | Probe deceleration | Slight overestimation | Pediatrics, through some CLs | Limited clinical experience |
| Dynamic Contour | Contour matching | Significant effect | Research, corneas after surgery | Affected by CL presence |
| Condition | CCT Change | IOP Measurement Error | Clinical Significance |
|---|---|---|---|
| Normal variation | ±10μm | ±0.2-0.7 mmHg | Minimal |
| 2 hours closed-eye CL wear | +54.1μm | +2.7 mmHg | Potentially significant |
| Corneal edema (10%) | +54μm | +1.0 mmHg/10μm | Likely significant |
These lenses with high oxygen permeability minimize hypoxia-induced corneal edema during experiments.
Measures IOP by detecting the deceleration of a lightweight probe as it makes contact with the cornea.
Measures central corneal thickness, a critical variable since thicker corneas can cause GAT to overestimate IOP.
Uses contour matching rather than applanation, theoretically making it less dependent on corneal properties.
Used to study the pure effect of lens thickness with standardized parameters to eliminate variables.
Another significant study examined the use of rebound tonometry through contact lenses 1 . Researchers measured 50 eyes with silicon hydrogel contact lenses using rebound tonometry both over the lens (RTCL) and directly on the cornea (RT), comparing these to GAT measurements.
RTCL measurements were significantly higher than both RT and GAT, with a difference of 1.52 ± 2.32 mmHg between RTCL and GAT.
Despite statistical significance, this difference may be clinically negligible in normal populations.
Separate from the mechanical effect of contact lenses themselves, extended lens wear can induce corneal edema (swelling), which independently affects IOP measurement 4 .
This finding is particularly important for clinical practice, as it suggests that IOP measurements taken shortly after contact lens removal may be artificially elevated due to residual corneal edema rather than actual pressure changes.
Beyond refractive correction, soft contact lenses are frequently used for therapeutic purposes—protecting corneas with persistent epithelial defects, surface irregularities, or after surgeries 5 .
With NCT, contact lenses may not need removal, as the effect appears minimal.
Allow appropriate waiting period after lens removal to resolve corneal edema.
With GAT, lens removal remains necessary for accurate results.
Rebound tonometry or pneumatonometry can provide reliable alternatives when lens removal is undesirable.
Looking forward, technological innovations may eventually resolve these challenges. Smart soft contact lenses with embedded sensors capable of continuous IOP monitoring represent perhaps the most promising development 5 . These devices aim to provide 24-hour IOP monitoring even during sleep without interfering with corneal physiology or measurement accuracy.