Keratoconus is a type of corneal ectasia, which refers to a group of diseases in which the cornea is thinned and weakened. The name is derived from the Greek word for “horn” (keratos) and “cone” (konos) – “I have a horn-cone on my eye” (said no one, ever). As the corneal tissue thins, the pressure inside the eyeball may push outward and distort the cornea into a cone shape, leading to distorted vision. Think of a balloon with a weakness in one particular spot – this area will bulge outwards more than the rest of the balloon. While keratoconus does not cause total blindness, vision impairment may progress to the point of causing legal blindness. In the US, legal blindness is defined as a visual acuity of less than 20/200, or a visual field of 20 degrees or less.
Around the world, keratoconus has a prevalence ranging from 1 in 750 to 1 in 2000 depending on your source. It affects males and females equally, and tends to onset around the teenage years in addition to various other puberty woes, or sometimes in the early twenties, when other woes come into play, such as trying to find a job post graduation. It can sometimes be present only in one eye (unilateral keratoconus) but studies have found that approximately 50% of “normal eyes” in these cases will progress to demonstrating keratoconus in the following 16 years from diagnosis. Associations have been made with allergy and eye rubbing as well as other systemic immune diseases such as rheumatoid arthritis, ulcerative colitis, asthma, and irritable bowel syndrome. Sleep apnea and Down syndrome have also been linked to increased odds of keratoconus. A study conducted across the US found African Americans had a 57% higher chance of being diagnosed with keratoconus compared to Caucasian Americans, while Latinos had 43% higher odds than Caucasians. There are genetic links to keratoconus; however, as with many other diseases with genetic associations, having a family member with keratoconus doesn’t spell a definite diagnosis of corneal ectasia for subsequent generations. A US study found a family history of keratoconus in only 13.5% of patients.
Keratoconus may start sub clinically, meaning it is so early that there are no easily detectable signs or symptoms of the disease. The patient still achieves excellent vision with regular spectacles and/or contact lenses and may very well believe themselves to be a generally excellent and regular person. However, an astute clinician can detect signs of subclinical keratoconus by conducting a scan of the cornea using the attractively-named Scheimpflug imaging. This produces a 3D image of the cornea, including information on its elevation and curvature (corneal topography) as well as thickness (corneal pachymetry), amongst other things. It is particularly important to rule out subclinical keratoconus in undiagnosed patients prior to undergoing refractive surgery, such as LASIK or PRK, as these procedures can result symptomatic corneal ectasia when the corneal tissue is surgically thinned during these laser techniques.
So what are the symptoms of keratoconus, you may ask?
- Blurred vision that is no longer perfectly corrected with glasses
- Frequent changes in prescription
- Soft contact lenses no longer fit well on the eye or provide adequate vision
- Increasing astigmatism – a result of the cornea no longer being perfectly spherical like a soccer ball and instead becoming more cone-shaped
- Increasing glare sensitivity – this can manifest as difficulty driving at night, or halos and ghosting around lights
- Eyestrain and headaches
It is important to consult with an ophthalmologist for a diagnosis of keratoconus rather than conducting a self-diagnosis based on this list of symptoms or by asking someone to look for a horn-cone on your eye.
There are several options available for the management and treatment of keratoconus. One is the use of rigid gas permeable contact lenses (RGPs). These hard lenses come in various sizes and can even cover the whites of the eye in a type of RGP known as a scleral or mini-scleral lens. RGPs tend to be more useful than soft contact lenses in correcting vision in keratoconic patients as fluid can fill the spaces between the irregular cornea and the hard lens to create a more even medium for light to enter the eye. As a soft contact lens wraps and conforms to the surface of the eye, they are of limited value in a symptomatic keratoconic patient.
Intacs are a medical device originally designed to surgically correct short-sightedness but in the early 2000’s were also approved for use to treat keratoconus. They appear as two arcs of inert plastic, which are surgically implanted into the stroma (the middle section) of the cornea in an effort to flatten the tissue into a more regular shape to improve the vision. In mild cases of keratoconus this treatment can, in fact, eliminate the need for spectacles or contact lenses (at least until the disease progresses); in more moderate to severe keratoconus, glasses or contacts may still be necessary to provide acceptable vision. Inserting the Intacs is just a day procedure and while vision is usually already improved by the following day, it can take a few months for it to heal fully. Because the implants are located below the cornea’s nerves, you will not be able to feel the Intacs there, and they are not visible to other people unless they’re gazing very deeply into your eyes (probably still looking for your horn-cone).
Unlike the Intacs cornea inserts, another procedure known as corneal cross-linking has been shown to actually strengthen the cornea and slow progression of keratoconus, and in some cases can also improve vision. During the cross-linking procedure, the top epithelial layer of the cornea is removed under anesthesia. Riboflavin (vitamin B2) is soaked into the underlying tissue and then ultraviolet light is shined onto the cornea for about half an hour. The combination of riboflavin and UV light helps to strengthen the bonds between the collagen fibres of the cornea, which makes the cornea less susceptible to thinning and bulging and more able to hold a regular shape. Only in early 2016 was corneal cross-linking approved by the US FDA (Food and Drug Administration) for treatment of progressive keratoconus. Cross-linking may be performed subsequent to Intac implants to further stabilize the cornea and improve vision. However, the procedure is not suitable for patients with corneal thicknesses of under 400 microns and in these cases alternative therapies must be considered.
Both Intacs and corneal cross-linking aim to delay the need for the last resort treatment – corneal transplantation. In corneal transplantation, also known as corneal grafting, the keratoconic cornea is replaced by a healthy donor cornea. The transplantation can involve the entire cornea and all its five layers (known as penetrating keratoplasty), or in more recent times, involve replacement of only the top three layers of the cornea (deep anterior lamellar keratoplasty, or DALK), while leaving intact the patient’s own corneal endothelium and Decemet’s membrane. DALK has the benefit of reducing the likelihood of future rejection of the graft, but also carries its own complications. Studies have found penetrating keratoplasty graft survival rates after 25 years to be as high as 93% but transplantation patients should be mindful that rejection of the graft can happen at any time and a repeated transplant may be required.
Advancements in medicine are being made all the time, so a diagnosis of keratoconus doesn’t need to spell all doom and gloom. As we’ve seen, there are many effective options for the treatment and management of keratoconus; it is definitely possible to live a fulfilling life even with a horn-cone.
Is Keratoconus Genetic? http://journals.lww.com/internat-ophthalmology/Citation/1993/03320/Is_Keratoconus_Genetic_.23.aspx
Study finds associations between keratoconus and systemic immune disorders. https://www.healio.com/ophthalmology/cornea-external-disease/news/online/%7Beb4f2460-a822-414c-844f-1457ef437795%7D/study-finds-associations-between-keratoconus-and-systemic-immune-disorders
The Association Between Sociodemographic Factors, Common Systemic Diseases, and Keratoconus: An Analysis of a Nationwide Health Care Claims Database. http://www.sciencedirect.com/science/article/pii/S0161642015012579
Epidemiology of Keratoconus. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775068/
Longitudinal Study of the Normal Eyes in Unilateral Keratoconus Patients. http://www.sciencedirect.com/science/article/pii/S0161642003014970
Keratoconus Symptoms and Diagnosis. http://www.nkcf.org/about-keratoconus/keratoconus-symptoms-and-diagnosis/
Intacs Corneal Inserts or Implants: A Proven Treatment for Keratoconus. http://www.allaboutvision.com/conditions/inserts.htm
Intacs Technical Sheet. http://intacsforkeratoconus.com/professionals/product-technical-sheet/intacs/
Treating Keratoconus with Collagen Cross-linking. http://www.visioneyeinstitute.com.au/article/treating-keratoconus-collagen-cross-linking/
FDA Approves First Corneal Cross-linking System for Treatment. http://www.visioneyeinstitute.com.au/article/treating-keratoconus-collagen-cross-linking/
Refractive and Topographic Results of Transepithelial Cross-linking Treatment in Eyes with Intacs. http://journals.lww.com/corneajrnl/Abstract/2009/08000/Refractive_and_Topographic_Results_of.1.aspx
Corneal Transplantation for Keratoconus: A Registry Study. http://jamanetwork.com/journals/jamaophthalmology/fullarticle/427371
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