The cornea, the transparent front of the eye, has physical properties that allow it to respond to force. One such property is corneal hysteresis (CH), which measures the cornea’s ability to absorb and dissipate energy, acting as the eye’s shock absorber. A higher CH value indicates a cornea that is more effective at absorbing shock, while a lower value suggests it is less capable of doing so.
The Biomechanical Properties of the Cornea
The cornea is a viscoelastic tissue, meaning it has properties of both a liquid and a solid. Its elastic qualities allow it to return to its original shape after being deformed, while its viscous properties cause it to absorb energy when a force is applied. This dual nature is fundamental to how the eye handles fluctuations in internal pressure.
For example, a bouncy rubber ball has low hysteresis; it springs back quickly, returning most of the impact’s energy. In contrast, a piece of memory foam has high hysteresis; it absorbs the impact and slowly returns to its original shape, dissipating the energy. The cornea’s hysteresis level provides information about its structural integrity.
This biomechanical property is different from central corneal thickness (CCT), a simple anatomical measurement. While a thicker cornea was once thought to be stronger, this is not always true. Corneal hysteresis directly assesses the cornea’s functional resilience, a metric distinct from its physical size. A cornea can be thick but have poor viscoelastic qualities, making it less effective at handling pressure changes.
Measuring Corneal Hysteresis
Corneal hysteresis is measured non-invasively with an instrument called an Ocular Response Analyzer (ORA). The ORA delivers a gentle puff of air to the cornea’s center.
As the air pulse deforms the cornea inward, an electro-optical system records the pressure required to flatten it (inward applanation). As the cornea recovers its shape, the system records the pressure at which it passes through a second flattened state (outward applanation).
The difference between the inward and outward applanation pressures is the key to the measurement. This difference results from the cornea’s energy absorption and is calculated as the CH value.
Clinical Significance in Glaucoma Management
Corneal hysteresis is significant in managing glaucoma, a disease that causes progressive optic nerve damage. A low CH value is an independent risk factor for glaucoma’s development and progression. Even with normal eye pressure, a low CH can indicate higher susceptibility to optic nerve damage.
A cornea with low hysteresis is less effective at dampening daily fluctuations in intraocular pressure (IOP). This may reflect a similar weakness in the structures supporting the optic nerve, making it more vulnerable to damage. For every 1 mmHg decrease in CH, the rate of visual field loss can accelerate, highlighting its predictive value.
This data also allows for a more refined assessment of a patient’s true eye pressure. Standard IOP measurements can be influenced by corneal properties. The ORA technology uses CH to calculate a cornea-compensated IOP (IOPcc), a more accurate reflection of the true pressure inside the eye, independent of corneal thickness. This helps clinicians make more informed treatment decisions for glaucoma patients.
Implications for Other Eye Conditions and Surgeries
The applications of corneal hysteresis extend beyond glaucoma. The measurement is valuable for corneal ectatic disorders like keratoconus, which involve a progressive thinning and weakening of the cornea. Eyes with keratoconus have very low CH, reflecting their compromised stability. Monitoring CH can help track disease progression and evaluate the effectiveness of treatments like corneal cross-linking.
Assessing corneal biomechanics is also part of the evaluation for refractive surgery candidates. Procedures like LASIK and PRK involve reshaping the cornea with a laser to correct vision. A surgeon must confirm the cornea is strong enough to withstand the surgical alteration for a safe outcome. A low CH value indicates a weaker cornea that may be at higher risk for post-surgical complications.
By incorporating CH into pre-surgical screenings, doctors can better identify suitable candidates and avoid adverse outcomes. This adds a layer of safety by providing a more functional assessment of corneal strength.