Glaucoma is the second most common cause of blindness in the world, second only to cataracts. Its global prevalence is currently estimated at 3.5%, and up to 112 million people worldwide are expected to suffer from the disease by 2040. The encouraging news is that with proper treatment and follow-up, 90% of patients can avoid the most serious consequences.
Glaucoma is irreparable damage to the optic nerve that results in progressive loss of the visual field. Its main cause is high intraocular pressure, although there are other risk factors such as diabetes, family history or high blood pressure.
Patients suffer from so-called “tunnel vision,” that is, a gradual loss of vision that begins in the periphery and gradually approaches the center. These visual field losses are measured using an ophthalmic assessment called campimetry, which consists of examining damage and loss of visual field amplitude due to eye fixation. Thus, it is a vital test for assessing the progression of glaucoma.
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Half of the population suffering from glaucoma does not know about it
Because the disease does not cause symptoms or discomfort in the early stages, affected people are unaware that they are suffering from it. For this reason, it is often called the “silent enemy” until permanent and irreversible vision loss occurs.
In addition, numerous studies have linked this condition to poor corneal viscoelastic quality, i.e., greater corneal stiffness.
Before we look in detail at the possibility of developing an early detection system based on this last characteristic, let’s take a look at the cornea and its role in the development of glaucoma.
The cornea: a true optical window to the brain
The cornea is a vital organ of vision. It is a complex avascular tissue delimited by two epithelia: the anterior epithelium (which interacts with the tear and ensures regeneration and healing) and the posterior, or endothelium (which ensures the passage of nutrients).
Its special structure allows the corneal tissue to be transparent and correctly transmit light in the visible spectrum (from 380 to 780 nanometers). If this transparency is altered (this occurs due to inflammatory processes such as swelling, burns or wounds), the cornea becomes opaque, resulting in blurred vision.
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University of Zaragoza
On the other hand, the radii of curvature and refractive index of the cornea allow light from objects to be focused into the angular field of the retina. If these parameters change for various reasons, the subject will experience blurred vision (nearsightedness and farsightedness) or astigmatism.
So what role does the cornea play in the development of glaucoma?
Absorbs excess pressure in the eyeball.
Another main function of this part of the eye is to compensate for intraocular pressure. On the one hand, it allows you to maintain the shape of the eyeball, and also absorbs energy transmitted through external or internal pressure on the visual organ. In this way, the stability of the tissue and therefore the rest of the eyeball is maintained.
We can say that a healthy cornea performs the function shock absorber in the eye, absorbing fluctuations in intraocular pressure and thereby preventing gradual damage to the optic nerve. This fundamental property is due to its viscoelastic nature; That is, the corneal tissue restores its original shape after the cessation of external pressure, but does so slowly and gradually.
Let us now analyze this particular biomechanical characteristic of the cornea.
Elastic and viscoelastic materials
Suppose we stretch a spring to a certain length (loading process) and then relax it until it regains its original shape (unloading). Thanks to this simple action, the upload and download processes are described in the same way. path on a stress-strain graph. Then we would talk about an elastic material; Mathematically, the material restores its original shape along the same slope in accordance with Hooke’s law.
University of Murcia
In viscoelastic materials (such as corneas) this happens differently. The loading and unloading processes are not linear and, moreover, do not follow the same path. As a consequence, the material absorbs energy to restore its original shape, this energy being proportional to the area between both curves (called hysteresis).
Knowing that the cornea is inherently viscoelastic (and that it protects the optic nerve from changes in intraocular pressure), how can we characterize this shock-absorbing property of the cornea?
Corneal delay time (Tau)
In our recent article published in the magazine Biomedical Physics and Engineering Expresswe propose a new metric related to corneal biomechanical health: corneal retention time (Tau), equivalent to the time it takes the cornea to recover 63% of its original shape.
High values of this parameter would correspond to a cornea with high viscoelasticity (and therefore well prepared to absorb harmful fluctuations in intraocular pressure). On the other hand, corneas with low levels of Tau will behave like springs (with almost no shock-absorbing capacity), leading to a very likely gradual damage to the optic nerve.
For example, a healthy person with no preexisting conditions may have a Tau value of about 1.15 milliseconds, while another person with high intraocular pressure and treatment for glaucoma will have a lower value, about 0.60 milliseconds.
In our work, we believe that patients in the second case should be under maximum controlled and periodic observation in order to assess possible vision loss due to glaucoma.
Implications of this new study
Corneal dwell time can be used for early detection of ocular hypertension diseases like the one in question, before its severe symptoms manifest.
Moreover, this parameter is very easy to obtain clinically because it requires only one commonly used ophthalmic instrument: the air tonometer.
However, there is still a long way to go. Studies with larger numbers of patients with ocular hypertension and adequate follow-up of subjects with low Tau values are needed.
In short, another step towards the early detection of this “silent” disease.