CORNEAL HYSTERESIS: THE VITAL GLAUCOMA SIGN THAT IS OFTEN MISSED
Technically, hysteresis is defined as the difference between the pressure at which the cornea bends inward during an airjet applanation and the pressure at which it bends out again. Hysteresis is a measure of corneal viscoelasticity. 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. Ocular Response Analyzer (ORA) is the instrument used to measure corneal hysteresis.
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).
Corneal hysteresis is one of the important assessments in the diagnosis of Glaucoma.
HYSTERESIS, GLAUCOMA AND IOP MEDICATION
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).
It is also important to note that:
Hysteresis can give some idea of how much a person’s pressure will come down when you start him on an eye drop.
- Hysteresis is helpful, because sometimes when the IOP of a patient doesn’t come down, we’re tempted to start the patient on a second drop.
•High hysteresis means that the patient has a lower risk of progression: the knowledge means that you may not see a big pressure drop when you start a new medicine. In this situation, the doctor can take a step back and monitor the patient; instead of adding more drops.
•Low hysteresis has a nice pressure response to a new drop; yet you can’t let down your guard.
The low hysteresis means the patient is at greater risk of glaucoma progression, so you still have to watch him carefully.
Corneal hysteresis is measured in mmHg.
Interestingly, corneal hysteresis is not a static number. It can change with time. Corneal hysteresis may increase in eyes after IOP – lowering interventions are implemented.