Keratoconus

Prevalence of the disease:

Many studies estimate that the prevalence (number of cases in a given population at a given time) is between one in 2,000 and one in 500 people.

Variations are due to different diagnostic criteria and detection methods, with some mild forms of keratoconus being able to go unnoticed. In addition, although it is often isolated, this eye deformity can be associated with other more complex diseases.

The exact causes of keratoconus are not yet known, but it is probably a genetic disease: even if no specific gene has been identified to date, the fact that several members of the same family can be affected and that the disease almost systematically develops in both eyes supports the theory of a genetic involvement. Furthermore, keratoconus often occurs in people with another disease, genetic or ophthalmological, such as Marfan syndrome, Down syndrome, retinitis pigmentosa or Leber congenital amaurosis (a serious familial eye disease). Thus, 5 to 6% of people with Down syndrome have keratoconus.

 

Symptoms :

The effects of keratoconus vary from person to person and can range from mild to severe. Most often, both eyes are affected (in 90% of cases), but it is common for keratoconus to be diagnosed on only one side, with the time it takes for the second eye to appear being highly variable and can take up to several years.

In the early stages, keratoconus causes blurred (foggy) and distorted vision (astigmatism). This decrease in visual acuity is mainly felt in distance vision (myopia) and is often associated with excessive sensitivity to light (photophobia), glare and eye irritation (watery eyes). As keratoconus progresses, astigmatism and myopia become more pronounced.

Vision is blurred, similar to what you see through a window when it rains heavily. Images may even appear double or multiple. People with keratoconus must change their glasses frequently, as they quickly become powerless to correct astigmatism.

In advanced stages, the cornea becomes deformed and thinner so much that scars appear and cloud it, making vision even more blurred.

But because progression can stop at any time, many keratoconus patients do not reach such a stage.

The Ophthalmologist’s diagnosis is based on examinations:

Measurement of visual acuity, keratorefractometer, pachymetry, corneal elevation topography such as Orbscan, the interpretation of which is obvious at advanced stages and can sometimes pose some difficulties at an early or even subclinical stage.

 

Evolution :

Keratoconus, often discovered at puberty, generally progresses until the age of 30-40 years before stabilizing. The progression is irregular, with some keratoconus remaining stable for a few years, others worsening and then stabilizing again. Keratoconus can stop progressing at any stage. The stages of progression can be classified according to different criteria, but we often distinguish:

• Stage I: visual discomfort, decreased visual acuity with astigmatism
• Stage II: myopia is associated with astigmatism
• Stage III: worsening of visual disturbances and appearance of a clearly visible deformation (protrusion) of the cornea
• Stage IV: thinning associated with loss of transparency (scars or opacities at the apex of the cone) and risk of acute keratoconus.

In rare cases (less than 3%), a particular complication, acute keratoconus, can occur. The thinning of the cornea is such that the transparent and elastic membrane that covers the internal surface of the cornea (endothelium) becomes perforated. This rupture causes a sudden loss of visual acuity by opacifying the cornea (visible white spot corresponding to significant corneal scars).

Treatment to lower the fluid pressure inside the eye and reduce inflammation may be considered, but only a corneal transplant (see ‘Treatments’ page) can restore vision after acute keratoconus.

The evolution from one stage to the next is not obligatory and is not linear in time.

It is often difficult to predict and asymmetrical (only 13% of cases are unilateral). Some patients therefore have severe and rapidly progressive forms, others keep a mild and isolated form throughout their lives.

 

It is important to note that the rate of worsening of the disease is accelerated by the patient himself when he rubs his eyes frequently. Similarly, all refractive surgeries of the cornea (such as lasik to correct myopia) risk aggravating or decompensating the disease. Therefore, they are strictly contraindicated for this condition.

 

Other forms of Keratoconus: (Source SNOF)

The differential diagnosis of keratoconus is made with other conditions involving non-inflammatory thinning of the cornea.

 

Marginal Pellucid Degeneration (MPD):

This is a bilateral condition that results in thinning of the peripheral cornea in the lower sector, usually between 4 and 8 o'clock. This results in a flattening of the vertical axis of the cornea with significant and often irregular astigmatism. The thinning involves a narrow band 1 to 2 mm wide, itself separated from the limbus by a zone of normal cornea 1 to 2 mm wide. The central cornea is of normal thickness and protrudes above the zone of thinning. This zone of corneal thinning is clear, always epithelialized, avascular, without lipid deposits, which distinguishes it from conditions such as Mooren's ulcer or Terrien's marginal degeneration.

It can be complicated by acute attacks, resulting in a corneal scar that is sometimes vascularized.

This condition is usually encountered between the second and fifth decades of life. It is rare in Western regions but much more common in Japan, and an association with keratoconus has been clearly established. Of 20 cases of pellucid marginal degeneration, coexistence of keratoconus was noted in 17 eyes.

Studies have established that keratoconus and pellucid marginal degeneration are only two clinical forms of the same condition.

 

Keratoglobus:

It is a rare, bilateral corneal ectasia, characterized by a globular protrusion of a diffusely thinned cornea, of normal or moderately increased diameter.

Corneal thinning can reach 20% of normal corneal thickness

Although acquired keratoglobus has been described, particularly in Graves' disease, it most often appears as a condition present from birth.

Although it is accompanied by high myopia and high astigmatism, visual acuity can sometimes be relatively preserved. The evolution of keratoglobus is marked above all by the risks of corneal perforations and ruptures, very frequent for minimal trauma.

 

Posterior keratoconus:

This is a rare, non-inflammatory corneal condition that can pose diagnostic problems with keratoconus.

Posterior keratoconus can take two forms: total or localized.

It presents as an increase in curvature of the posterior surface of the cornea with thinning while the anterior surface is normal. While anterior astigmatism may be present, it never has the great irregularity of that encountered in anterior keratoconus. The loss of corneal thickness may approach one-third of a normal cornea. The parenchyma is clear, but stromal scars may exist. Cornea gutatta is frequently encountered in affected areas of the cornea.

It is usually a unilateral congenital condition although bilateral or post-traumatic cases have been described.

Posterior keratoconus is very often associated with other ocular anomalies, particularly affecting the anterior segment: cleavage anomaly, aniridia, uveal ectropion, iris atrophy, glaucoma, anterior lenticonus, ectopia lentis, anterior crystalline opacity.

 

Treatments:

The usual timeline for therapeutic care initially involves fitting glasses and then, if this fails, fitting with contact lenses. Finally, in the event of intolerance or if vision remains insufficient, corneal surgery is required.

The usual timeline for therapeutic care initially involves fitting glasses and then, if this fails, fitting with contact lenses. Finally, in the event of intolerance or if vision remains insufficient, corneal surgery is required.

To give an idea of ​​the scale, approximately 10% of patients with recognized keratoconus will never require treatment, 15% will only wear glasses, 50% will only wear contact lenses and 25% will benefit from a corneal transplant.

 

Contact lenses:

This is the weapon of choice for the majority of patients with keratoconus. However, adaptation is difficult or even impossible in some cases.

The advantage of contact lenses over glasses is related to the interface they create with the tear film. The latter makes it possible to erase a large part of the irregularities of the cornea.

Rigid lenses are much more effective but more complex to adapt. Sometimes in order to stabilize the lens on the cone, it is useful to superimpose a rigid lens on top of a soft lens (Piggyback System).

When the lens is not or no longer tolerated, it is permissible to offer a surgical alternative.

 

Intracorneal rings:

Intracorneal rings are small, rigid semicircular segments made of PMMA (plastic). Their place in therapeutic management is between failure of lens adaptation and recourse to corneal grafting. They have been used for keratoconus since 1997.

Introduced into the thickness of the cornea itself, intracorneal rings are designed to strengthen it. When properly positioned, they can tighten the central cornea. The extent and irregularity of corneal astigmatism are thus reduced. Two models are marketed: INTACS® (Addition Technology, KeraVision, USA) and Ferrara® rings (Mediphacos, Brazil).

A pair of rings is usually placed on the mid-periphery of the cornea. The center of the cornea is therefore completely preserved.

They are introduced during a short surgical procedure. A small 2 mm incision is made at the entry point of the rings and then two channels are dissected with either a calibrated mechanical instrument or using the femtosecond laser. This dissection is performed three-quarters of the way down the cornea, it carries a low risk of perforation of the posterior surface of the cornea which disappears completely with the use of the femtosecond laser. Once the channels have been dissected, the rings are put in place very simply, they are then perfectly well tolerated. The procedure is also considered reversible. The position of the rings is important: if placed too deeply they are dangerous to introduce, if placed too superficially they are not very effective.

The result is generally good if the indication is well established. However, the predictability is average because each patient has a different case. This result is only truly assessed 6 months after the operation because the keratoconic cornea put under tension by the rings takes a long time to stabilize. Then, either the result is compatible with a satisfactory adaptation to glasses or lenses, or the result is insufficient and the rings will simply be removed at the same time as a corneal transplant is performed.

In almost 10 years of development, only a few rare complications have been described: epithelial invasion, infection, extrusion, corneal perforation.

The judicious use of intracorneal rings is useful in advanced stages of keratoconus, often delaying the time of corneal transplantation.

 

Toric implants:

When the astigmatism is too severe to be corrected with glasses or lenses, if the central cornea is clear and the astigmatism is not too irregular, it is possible to offer a corrective intraocular implant.

This implant is toric to correct astigmatism, it is clipped by the surgeon to the iris.

It is not incompatible with the presence of insufficiently effective intracorneal rings. In case of intolerance or ineffectiveness it can be removed. This procedure is possible alone or at the same time as a corneal transplant. Although potentially useful, these implants are still rarely placed but could in the future take a more codified place in the management of the disease.

 

Corneal Cross Linking (CXL):

Cross Linking is a new treatment option for keratoconus. It is a major innovation in the treatment of the cornea in order to stiffen the cornea which is weakened or deformed in the context of certain pathologies such as keratoconus. After wide epithelial abrasion of the cornea, a photosensitive product (Riboflavin) is deposited on the keratoconic cornea and then a UVA (long ultraviolet) session is applied to the cornea for several minutes. The aim of the treatment is to stiffen the collagen by creating chemical bridges between the collagen fibrils thanks to the polymerizing action of phototherapy (Cross Linking principle). The idea is to stabilize or regress the disease.
Cross-Linking (CXL) appears unanimously by all specialists to be very safe and painless.
It can be proposed either in the treatment of keratoconus or in refractive surgery, essentially in addition to PKR (or TransPKR), or even for some in addition to Lasik in order to reinforce corneal rigidity and prevent deformation after refractive surgery.
Cross-linking has received since April 2016 the long-awaited approval of the FDA (Food and Drug Administration), the vigilant health watchdog in the USA.

 

Currently, CXL is most often offered to a young adult who has progressive keratoconus and a corneal thickness greater than 400 microns.

Corneal transplants:

The principle of corneal transplantation is simple. It involves exchanging the central part of the diseased cornea with the same part of the healthy cornea of ​​a deceased donor. The collection, storage and delivery of grafts are very strictly codified. Corneal transplantation is a surgical procedure that is now well mastered. Keratoconus represents approximately 1/3 of corneal transplant indications. Only patients whose visual acuity is insufficient with contact lenses or who cannot benefit from improvement with intracorneal rings are placed on the waiting list for a corneal graft.

Several variations of corneal transplant:

The graft can replace the entire thickness of the cornea or keep the deepest part.

We speak of transfixing graft when the entire thickness of the cornea is replaced, of anterior lamellar graft when only the external part of the cornea is grafted and we speak of pre-Descemet graft when the entire cornea is grafted except its deepest layer (Descemet's membrane + endothelial layer). The endothelium is a key element for corneal transplantation because it ensures the transparency of the graft thanks to a contingent of cells that cannot renew themselves. If it is damaged, the cornea will never be transparent.

The advantage of lamellar and pre-Descemetic grafts is that they are performed with a closed globe and therefore with a much lower risk of infection. In addition, they present a lower risk of rejection because the patient's endothelial part is not grafted. A final advantage is that the endothelial layer, which is not diseased in keratoconus, is preserved. This point ensures a much greater longevity of the graft.

As with any surgery, there are operating risks that the surgeon must make known to his patient. They are quite low but they do exist.

Post-operative risks are numerous but well controlled.

 

Post-operative monitoring is then planned almost every month for 6 to 12 months and then it is spaced regularly. The lifespan of the graft depends on the risk of graft rejection, the initial quality of the graft (cell density), possible inflammatory and blood pressure complications. The risk of graft rejection is all the greater when the endothelium is grafted, the subject is young and the graft is of large diameter. It is maximum in the first year but exists throughout life. Caught in time, graft rejection can be inhibited by high-dose cortisone infusions for 3 days. Caught too late, it requires a new graft. On average, a transfixing graft lasts 20 to 30 years. A patient who is grafted very young may therefore need two or three grafts per eye in their life.

There are some complications to be aware of following a corneal transplant: cataract can be induced by prolonged use of cortisone eye drops, it causes a loss of vision that only surgery can resolve.

A secondary glaucoma related to the same parameters can occur. Appropriate treatment with eye drops, reduction of cortisone and possibly glaucoma surgery can stabilize the situation. The pre-Descemetic graft method reduces the risks of cataracts and glaucoma because it is a closed eye surgery and because the corticosteroid therapy is very short.

If both eyes are to be transplanted, the operation on the second eye is only possible one year after that of the first. The same applies in the event of graft rejection.

The visual results are generally good (on average 5/10) but the speed of visual rehabilitation is slow. It takes 3 months before a transparent cornea is restored and 4 to 6 months before selective removal of the stitches can begin. This step allows the astigmatism to be adjusted and the subject to be fitted with either spectacle lenses or contact lenses again.

Overall, the results of corneal transplantation are very satisfactory for keratoconus and allow a good quality of life for patients, although at the cost of rigorous monitoring.

Source / for more information: keratoconus association

Site : http://www.keratocone.net