Introduction:
Corneal hysteresis is the measurement of the ability of the cornea to absorb and disperse energy. This measurement is carried out in individuals who tend developing glaucoma. A low corneal hysteresis suggests progressive optic nerve and visual field damage, which can be a risk factor for glaucoma. It is a part of corneal biomechanics that can be measured. The ocular response analyzer can measure corneal hysteria and predicts the progression rate and how the person reacts to the therapy in primary open-angle glaucoma.
What Is Cornea?
The cornea is the clear outer layer at the front of the eye. The cornea helps your eye to focus light so you can see clearly. It permits the light to enter the eyes. It comprises five layers: the epithelium, Bowman's layer, the stroma, Descemet's membrane, and the endothelium. Of these layers, the stroma comprises 90 percent of the total cornea.
What Is Glaucoma?
Glaucoma is a group of eye diseases that can lead to blindness by damaging the optic nerve at the back of the eye. It is the second leading cause of irreversible blindness in the world. Many people continue to lose sight despite disease control. Glaucoma is an irreversible disease; early detection is important to avoid the loss of sight. It is a common disease in the elderly. The symptoms start very slowly and often go unnoticed. A comprehensive dilated eye examination is the only way to discover the disease. Glaucoma is incurable; however, early detection can halt eye damage, thus protecting the vision.
What Is Corneal Hysteresis?
Cornea is a viscoelastic structure that is it exhibits both the properties of viscosity and elasticity. The viscoelastic system is characterized by hysteresis, a lapse between the input and output in a system after a direction change. Corneal hysteresis measures the viscoelastic damping of the corneal tissues. The various eye structures like the cornea, sclera, peripapillary ring, and lamina cribrosa comprise an extracellular matrix coded by the genes creating collagen. Thus, an eye with a deformed cornea has a lamina cribrosa which cannot dampen the pressure changes to a great extent and leads to damage from increased intraocular pressure.
The word hysteresis means lagging and is the difference between how the material deforms and reforms after a force is applied. The cornea consists of glycosaminoglycans and proteoglycans and is viscoelastic. This helps the cornea absorb and dispose of energy after applying force. Corneal hysteresis is not a physical property of the cornea but shows how it reacts to an external force. Corneal hysteria of the normal eyes ranges from 9.6 to 10.7 mm of Hg, while the value in primary open-angle glaucoma ranges from 8 to 10 mm of Hg.
How Is Corneal Hysteresis Measured?
Corneal hysteresis is measured with the help of two devices: an Ocular Response Analyzer (ORA) and a Corneal Visualization Scheimpflug Technology tonometer (Corvis ST).
-
Ocular Response Analyzer (ORA) - This device uses a quick airflow to the cornea. An electro-optical system calculates the abnormal flattening of the convex corneal surface of the eyes or applanation pressure. This pressure is measured twice, once when the cornea is displaced inwards and once when it is displaced outwards. Since the cornea has viscoelastic properties, thus, it resists inward movement and returns to its original position. There is a lag between these events. The inward pressure is P1, and the outward pressure is P2. The intraocular pressure is the average of the two values. The difference between P1 and P2 calculates corneal hysteresis. The ORA can also help determine the corneal-compensated intraocular pressure and a corneal resistance factor. Corneal-compensated intraocular pressure uses corneal hysteresis to find out the value of pressure that is not influenced much by the intrinsic properties of the cornea.
-
Corneal Visualization Scheimpflug Technology Tonometer - This device works the same as the ORA but utilizes a high-speed Scheimpflug camera to calculate the movement of the cornea. The camera records the reaction of the cornea to the air pressure. The camera can take around 4300 images per second. It also gives a detailed evaluation of the biomechanical properties of the cornea. The intraocular pressure is calculated the same way as ORA. This device does not give the corneal hysteresis result as such, but the effects of the corneal hysteresis influence the intra-ocular pressure. The corneal hysteresis is measured in mm of Hg (millimeters of mercury). The values change from person to person and their ethnicity.
How Are Corneal Thickness and Intraocular Pressure Related to Corneal Hysteresis?
Central corneal thickness and intraocular pressure are the two important factors considered when evaluating corneal hysteresis. The relation between corneal hysteresis and central corneal thickness in healthy individuals is stronger than in glaucoma patients. In healthy individuals, the cornea is thick with a healthy stroma increasing viscous damping. Intraocular pressure is inversely proportional to corneal hysteresis. With increased intraocular pressure, the cornea stiffens, thus reducing its damping ability.
How Is Corneal Hysteresis Applied in the Management of Glaucoma?
Corneal hysteresis is reduced in glaucoma and is found to be associated with structural and functional changes in glaucoma. Corneal hysteresis also predicts the progression of glaucoma and response to glaucoma treatment.
Conclusion:
As people age, the ocular tissues become less compliant, and corneal hysteresis is a marker for ocular compliance. Low corneal hysteresis value is associated with damage to the optic nerve and loss of visual field. Corneal hysteresis plays an important role in glaucoma management.
Corneal hysteresis is the difference between the pressure at which the cornea bends inwards and outwards during a jet of air is passed. It is measured in mm of Hg by the ocular response analyzer and Oculus, known as the Corvis ST. ORA uses an air jet tonometer to measure the pressure, whereas, Corvis-ST uses a high-speed Scheimpflug camera to check the cornea. Other investigations are required in this field to diagnose and monitor the detrimental condition called glaucoma.
