Continuing with our series on corneal transplants and why someone might need one, we will be discussing a condition today called Keratoconus.
You may recognize keratoconus from one of our previous articles regarding a treatment called corneal crosslinking.
Corneal crosslinking is a great treatment for early stages of keratoconus, however when keratoconus progresses to the severe stages, the last logical step in the treatment process is a corneal transplant—hence why it is a part of our corneal transplant series.
What is Keratoconus?
Keratoconus is a condition that affects the front most structure of the eye, the cornea.
The cornea is composed of 5 layers—epithelium, Bowman’s layer, stroma, Descemet’s membrane, and the endothelium.
Keratoconus is thought to affect the front half of the cornea—epithelium, Bowman’s layer, and the anterior half of the stroma.
It is thought that the underlying mechanism of keratoconus is a breakdown of Bowman’s layer, as a result the epithelium and stroma are affected as well.
Bowman’s layer is a relatively thin layer—only about 10 micrometers thick.
It is composed of type 1 collagen (the same type that makes up our bones) and its primary function is to provide structural support to the cornea and is hence responsible for maintaining its spherical shape.
Bowman’s layer cannot regenerate. Once it becomes damaged, it is permanently affected and stuck in a weakened state.
In keratoconus, the cornea loses a significant amount of collagen, resulting to an overall weakening of the cornea.
As this affects Bowman’s layer, the layer begins to fragment, losing its ability to keep the cornea in a stiffened, strong, and stable state.
The pressure from the stroma and posterior parts of the cornea add pressure to the weakened Bowman’s layer leading to the cornea protruding forward, creating a cone-like shape rather than the typical spherical shape.
As the disease continues to progress, thinning of both the epithelium and stroma are noted.
Keratoconus therefore gets its name from the “cone-shaped” corneal appearance seen as the disease progresses.
Keratoconus causes decreased vision as the cornea is the first source of light refraction in the eye.
Light refraction is a crucial part of vision creation. Incoming light rays must be bent (refracted) in a very particular manner so that the light rays come to a focused point source on the back most structure of the eye called the retina.
The retina contains specialized light-detecting cells called photoreceptors. If the light does not come to a focused point on the retina, it will create a larger light circle which stimulates more photoreceptors than necessary.
When this occurs, it creates a blurry image instead of a clear image.
In keratoconus, the cone shape of the cornea causes light rays to be refracted in an asymmetric manner, creating what is called irregular astigmatism.
Essentially, a normal spherical cornea refracts light to a point source—this point source can land in front of or behind the retina creating blur, however this blur is easily correctable by glasses or contact lenses.
In a conical cornea, the light is refracted in different amounts depending upon where it hits the cone—this causes some of the light to come to a point at one length and the rest to come to a point at another length.
The creation of multiple blur points is registered in the brain as streaky, ghost-image like, massive blur circles which often cannot be corrected by glasses and contact lenses in the more advanced stages.
Findings Associated with Keratoconus
At this time, the underlying cause of keratoconus is unknown. However, there are known risk factors that increase the likelihood of developing keratoconus.
The process of keratoconus typically begins around puberty and continues to progress into the mid-30s. Around reaching mid-adulthood, the condition begins to stabilize and will slow its progression.
Advancement of the disease at younger ages is the primary predictor for disease severity.
Currently, risk factors for developing keratoconus include genetics, those suffering from connective tissue disorders such as Marfan’s, and individuals who rub their eyes a lot (often seen in those with chronic allergies, inflammatory disorders, or other conditions resulting in chronic eye inflammation).
The current thought behind why regular eye rubbing induces keratoconus is that the physical rubbing of the eyes weakens the structure of the cornea—hence affecting Bowman’s layer and leading to keratoconus development later in life.
Some individuals with keratoconus may be only mildly affected, however others can have drastically reduced vision as the cornea continues to thin.
Besides decreased vision secondary to irregular astigmatism, other symptoms associated with keratoconus include light sensitivity and glare or haloes around lights.
Fortunately, keratoconus is typically a painless disease. Many individuals may not even realize they have keratoconus until the disease has advanced to the more moderate stages—hence why yearly eye exams are so important, as eye doctors are trained to pick up on some of the more subtle early signs of the condition.
Keratoconus can become painful, however in the advanced stages, if something called a corneal hydrop forms.
Corneal hydrops are essentially blisters that form in the cornea when the cornea thins too much and the fluid from inside the eye—aqueous humor—leaks into the cornea, creating a fluid filled pocket.
Corneal hydrops, like regular skin blisters, can pop, causing extreme pain and further hazy vision and scarring.
Corneal hydrops are not a typical finding in keratoconus, however they can, and do happen, in severe forms.
Typically a doctor will recommend a corneal transplant before keratoconus reaches this stage to prevent these types of situations—hence why regular eye exams, especially if one knows he/she has keratoconus, are crucial.
Treatment of Keratoconus
As mentioned before, one of the newest treatment options for keratoconus is corneal crosslinking.
For more information about corneal crosslinking, check out our article “Treating Keratoconus with Corneal Crosslinking” published in August of 2021.
Essentially, corneal crosslinking is a treatment that can be performed in early stages of the condition to help strength the cornea, preventing keratoconus from advancing to more severe stages.
Another treatment option is called corneal INTACs. INTACs are small plastic rings that can be inserted into the cornea to help stabilize it and provide extra support.
For many, keratoconus can be controlled with observation and the above treatment methods. For others, the cornea continues to thin to dangerously thin levels, requiring a corneal transplant as treatment.
Recent studies have shown that only 15-20% of those who have keratoconus will require a corneal transplant as treatment. However, while it may be uncommon, it is good to know a treatment option is available for those with advanced stages of the disease.
Following a corneal transplant, an individuals’ vision is expected to fluctuate for up to a year, depending on the type of transplant received.
While transplants are ever improving, some kind of prescription correction is expected post–surgery, although often times vision is significantly improved in comparison to pre-operation numbers without correction.
For keratoconus, the most common types of corneal transplant surgeries performed are penetrating keratoplasty (PK) or deep anterior lamellar keratoplasty (DALK).
We will be sure to discuss both of these options in more detail in our article regarding various types of corneal transplants, so stay tuned to our corneal transplant diseases series for more information!