#Cornealhysteresis is essentially a measurement of the shock-absorbing capacity of the eye.
Learn more about Corneal Hysteresis here:
In medicine “vital signs” are those measurements considered critical to the health of an individual. Common vital signs include temperature, heart rate, breathing rate, and blood pressure. But there are also vital signs of the eye. Visual acuity (e.g., “20/20”) and intraocular pressure (IOP) are two such measurements. For the last twenty years, those with ocular hypertension or glaucoma have had a third vital sign: central corneal thickness (CCT). Corneal thickness can be used (among other things) to estimate how likely someone may be to develop glaucoma. However, a newly discovered measurement has proven to be even more valuable than CCT. This new measurement is called Corneal Hysteresis (“CH”).
What is Corneal Hysteresis?
The cornea is the clear (transparent) surface at the front of the eye which focuses light onto the lens inside the eye. It can be thought of as a half-millimeter thick soft contact lens. Like a soft contact lens, it is deformable (“bendy”). Many biological tissues that can be flexed or bent without permanent damage can be described as “viscoelastic”. Viscoelastic tissues absorb some of the force used to deform them like the suspension strut in your car or truck works to absorb the impact of a pothole.
Corneal hysteresis essentially measures the “shock absorbing” ability of the cornea. The higher the corneal hysteresis, the greater the shock absorbing capacity of the cornea (and perhaps the eye).
It turns out that a low corneal hysteresis may be predictive of the presence of glaucoma and its progression (worsening over time). In particular, a low corneal hysteresis:
Can help predict who might develop glaucoma
Is more common in eyes with glaucoma than otherwise normal eyes
Is associated with optic nerve and visual field damage in those with glaucoma
Increases the risk of further damage to the optic nerve
Increases the risk of progressive visual field loss
May help in predicting which patients with normal tension glaucoma are more likely to lose vision
May predict who will respond to certain IOP lowering therapies. For example, a low corneal hysteresis is associated with a greater reduction of IOP after treatment with a class of IOP lowering eye drops known as the prostaglandin analogs (latanoprost being the most common). This also appears to be the case with certain surgical treatments such as selective laser trabeculoplasty (SLT).
What might be the reason for these associations?
It’s thought that corneal hysteresis may be similar to whole eye hysteresis. In other words, a low corneal hysteresis may indicate that not only is the cornea unable to adequately deal with changes in force, but the entire eye may be subject to damage from repetitive shocks. The eye suffers from a lot of physical abuse during the day as it is rapidly moved inside the orbit and it is squeezed by the eyelids during blinking.
Both elevated IOP and fluctuations in IOP can result in damage to the optic nerve. We kind of understand why elevated IOP is bad, but why fluctuating IOP is a risk for glaucoma is less well understood. Hysteresis may provide one answer (if not the answer) to this question. After all, if an eye has a limited ability to absorb shock then it makes sense that fluctuating IOP (essentially mini-shocks) would increase the likelihood of damage to the eye.
Low central corneal thickness (CCT) measurements are also known to be associated with glaucoma risk. However, corneal hysteresis is more strongly associated with glaucoma presence, risk of progression, and effectiveness of glaucoma treatments than central corneal thickness. Thus, if you could choose only one corneal measurement, you would be wise to choose Corneal Hysteresis over central corneal thickness.
If you fit into one of the following groups you could benefit from knowing your corneal hysteresis:
You have someone in your family with glaucoma
You have diabetes (which can increase your risk of developing glaucoma)
You are nearsighted (which can increase your risk of developing glaucoma)
You have elevated eye pressure (“ocular hypertension”)
You’ve been told that you are a “glaucoma suspect”
Your doctor is considering placing you on glaucoma medications
You’ve just started a new glaucoma medication
You are considering the option of glaucoma surgery
You have just healed from glaucoma surgery
Should I Worry if my Corneal Hysteresis Number is Low?
A low CH number on its own doesn’t mean that glaucoma is inevitable. However, the combination of a low corneal hysteresis and a high IOP is predictive of loss of vision over time. As such, this combination may suggest to your eye doctor that more aggressive lowering of the IOP is needed.
David Richardson, MD
Medical Director, San Marino Eye
David Richardson, M.D. is recognized as one of the top cataract and glaucoma surgeons in the US and is among an elite group of glaucoma surgeons in the country performing the highly specialized canaloplasty procedure. Morever, Dr. Richardson is one of only a few surgeons in the greater Los Angeles area that performs MicroPulse P3™ “Cyclophotocoagulation” (MP3) glaucoma laser surgery. Dr. Richardson graduated Magna Cum Laude from the University of Southern California and earned his Medical Degree from Harvard Medical School. He completed his ophthalmology residency at the LAC+USC Medical Center/ Doheny Eye Institute. Dr. Richardson is also an Ambassador of Glaucoma Research Foundation.