A new step towards early detection of glaucoma

Glaucoma is the second most common cause of blindness in the world, surpassed only by Cataracts

Currently, its global prevalence is estimated at 3.5%, and it is expected that Up to 112 million people worldwide will be affected by this disease by 2040.. The encouraging news is that, with proper treatment and follow-up, 90% of patients can avoid the most serious consequences.

Glaucoma consists of an irreparable injury to the optic nerve that leads to a progressive loss of the visual field. Its main cause is a 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 at the periphery and gradually approaches the center. These visual field losses are measured through an ophthalmological evaluation called campimetry, consisting of the study of injuries and losses of visual field amplitude through ocular fixations. It is, therefore, a vital test to evaluate the progression of glaucoma.

Half of the population affected by glaucoma does not know it

Because this disease does not present symptoms or discomfort in its initial phase, affected people are unaware that they suffer from it. For this reason, it is often described as the “silent enemy”, until permanent and irreversible visual loss occurs.

In addition, several studies They associate the condition with low corneal viscoelastic quality, that is, with greater rigidity in the cornea.

Before addressing in detail the possibility of developing an early detection system based on this last characteristic, let's look at the cornea and what its role is in the development of glaucoma.

The cornea: a true optical window to the brain

The cornea is a vital organ in vision. It is a complex avascular tissue delimited by two epithelia: the anterior one (which interacts with the tear and allows regeneration and healing) and the posterior one or endothelium (which allows the passage of nutrients).

Its particular structure allows the corneal tissue to be transparent and that the light is transmitted correctly in the visible spectrum (between 380 and 780 nanometers). If this transparency is altered (this happens in inflammatory processes such as edema, burns or wounds), the cornea becomes opaque, which generates hazy vision.

On the other hand, the radii of curvature and corneal refractive index allow the light from objects to be focused on the retinal angular field. If, for different reasons, these parameters are altered, the subject will experience blur phenomena (myopia and hyperopia) or astigmatism.

So what role does the cornea play in the development of glaucoma?

Absorbs excess pressure in the eyeball

Another primary function of this part of the eye is to compensate intraocular pressure. On the one hand, it allows the shape of the eyeball to be maintained, in addition to absorbing the energy transferred through external or internal pressure to the visual organ. In this way, the stability of the tissue and, consequently, the rest of the eyeball is preserved.

It could be said that a healthy cornea performs the function of shock absorber in the eye, absorbing fluctuations in intraocular pressure and thus preventing gradual damage to the optic nerve. This fundamental property is due to its viscoelastic character; That is, the corneal tissue recovers its original shape after the cessation of external pressure, but it does so slowly and progressively.

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 recovers its original shape (unloading). With this simple action, the upload and download processes describe the same fireplace in the stress-strain graph. We would speak, then, of an elastic material; Mathematically, the material recovers its original shape following the same slope, according to Hooke's law.

In viscoelastic materials (such as the cornea) it happens in a different way. The loading and unloading processes are not linear and, furthermore, they do not follow the same path. As a consequence, the material absorbs energy in order to recover its original shape, said energy being proportional to the area between both curves (called hysteresis).

Knowing that the cornea is essentially viscoelastic (and that it protects the optic nerve from intraocular pressure variations), how can we characterize this cushioning property of the cornea?

Corneal delay time (Tau)

In our recent article, published in the magazine Biomedical Physics and Engineering Express, we propose a new indicator related to the biomechanical health of the cornea: the corneal delay time (Tau), equivalent to the time it takes for the cornea to recover 63% of its original shape.

High values ​​of this parameter would correspond to highly viscoelastic corneas (and, therefore, well prepared to absorb harmful fluctuations in intraocular pressure). On the other hand, corneas with a low Tau would behave like springs (with hardly any cushioning capacity), with very probable gradual damage to the optic nerve.

For example, a healthy subject without previous pathologies may have a Tau parameter of about 1,15 milliseconds, while another with high intraocular pressure and being treated for glaucoma would have a lower value, about 0,60 milliseconds.

In our work we suggest that patients in the second case should be the most controlled and periodically monitored, in order to evaluate possible vision loss due to glaucoma.

Implications of this new study

The corneal delay time could be used to early detect ocular hypertension diseases such as the one in question, before its serious symptoms manifest.

Furthermore, it is a very easy parameter to obtain clinically, since it only requires one commonly used ophthalmological instrument: the air tonometer.

However, there is still a long way to go. Trials are needed with a larger number of patients with ocular hypertension and adequate follow-up of subjects with low values ​​of the Tau parameter.

In short, one more step towards the early detection of this “silent” disease.

Source: The Conversation

Authors: Óscar del Barco Novillo (Physics Professor, University of Murcia), Conchita Marcellán (Physics Professor, University of Zaragoza), Francisco Javier Ávila Gómez (Applied Physics Professor, University of Zaragoza) and Laura Remón Martín (Applied Physics Professor, University of Zaragoza

Image: University of Zaragoza