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Keratoconus

More about Cornea

Introduction

Keratoconus is an eye condition in which the normally round dome-shaped clear cornea progressively thins causing a cone-shaped bulge to develop.  This leads to gradually worsening distorted vision and vision loss   Keratoconus (from Greek: kerato- horn, cornea; and konos cone) means a cone-shaped cornea.  Exactly why this happens is unknown, but genetic factors play a role and it is more common in people with allergic diseases such as asthma.  It is also seen in Down’s syndrome and in some disorders of connective tissue such as Marfan’s disease. It affects up to one in 1,000 people and is more common in people of Asian heritage. It is usually diagnosed in teenagers and young people.

What happens in Keratoconus?

The cornea is the clear window on the front of the eye. It is usually a regular spherical dome in shape. The substance of the cornea consists of hundreds of layers that are linked to each other by a substance called collagen. If these collagen cross-links between layers are lost due to keratoconus, there is progressive corneal thinning and stretching which gradually progresses, often in both eyes. Normal pressure within the eye causes the cornea to bulge forward into an irregular cone shape.

When light enters the eye, it first passes through the cornea. A conical cornea distorts the image. The eye develops irregular astigmatism (cylindrical errors) and myopia (shortsightedness). This changes the eyesight power.  In other words, the number of the eye changes or the  glass power changes.  This eyesight power change happens as and when the shape of the cornea changes.  So in the initial stages, the person complains of slight blurring.  This is because of a mild astigmatism.  This blurry vision is corrected by a change of glasses and then the process repeats itself. As the condition advances blurring of vision worsens and a change in glasses no longer works.  The astigmastism increases and one may get what is known as oblique astigmatism.  The patient may then need soft contact lenses or special contact lenses.  These special lenses are scleral lenses or Rose K lenses.    Patients may also complain of irritation and watering. In advanced stages, the cornea may be significantly thinned out.  This thinning is mainly corneal stromal thinning and can be appreciated when the doctor examines the eye on a slit lamp.

As progression of Keratoconus continues soft lenses no longer work.  Sometimes in advanced keratoconus the innermost layer of cornea tears.  This layer, the descemet's Membrane, is responsible for sucking out any fluid from the cornea.  In other words it keeps the cornea transparent.  Once it tears the fluid from within the eye rushes into the cornea and the cornea becomes edematous and results in clouding of vision.  This causes sudden worsening of vision.  The cornea also looks white, more in the center than in the periphery.  This is known as Corneal hydrops and can lead to loss of vision.  It is also known as Acute Hydrops because of how suddenly this occurs.  Hydrops can lead to a corneal scar and then the only thing that can get rid of the scar is a corneal transplantation.

It is not safe for someone to walk around with a very thin cornea.  This thin cornea may rupture which may lead to loss of eyesight.

What are the risk factors of keratoconus

Why some patients develop keratoconus and some dont is not very clearly known.  However, we do know that keratoconus occurs in some individuals more than others.  Those with asthma or downs syndrome have a higher risk of keratoconus.  Eye Allergies is also a risk factor which may inturn be dependant on environmental factors.  Which is why its important to treat chronic allgergies and prevent the individual with the allergy to rub the eyes.  Other rarer conditions associated with Keratoconus include Leber congenital Amaurosis and mitral valve prolapse.

Usually, Keratoconus is seen in late teens but may occur earlier or even later than this age group.  

Sometimes there is associated family history of keratoconus or a genetic predisposition.

Symptoms of Keratoconus

Early stages just having the need for spectacle correction. As the disease progresses, the vision deteriorates. Visual acuity becomes impaired at all distances, and night vision is sometimes quite poor. Some individuals have a vision in one eye that is markedly worse than that in the other eye. Some develop photophobia (sensitivity to bright light), eye strain from squinting in order to read, or itching in the eye. It is usually little or no sensation of pain. Keratoconus can cause substantial distortion of vision, with multiple images or double vision, streaking, and sensitivity to light. Despite its uncertainties, keratoconus can be successfully managed with a variety of clinical and surgical techniques, and often with little or no impairment to the patient’s quality of life.

Signs of Keratoconus

External Signs
  1. Munson’s sign: V-shaped conformation of the lower lid produced by the ectatic cornea in down gaze
  2. Rizzuti’s sign: Sharply focused conical beam of light near the nasal limbus, produced by lateral illumination of the cornea from temporal side, in patients with advanced keratoconus
Slit Lamp Signs
  1. Vogt’s striae: Fine vertical stress lines in the deep stroma and descemet’s membrane that are parallel to the steep axis of cone. These lines disappear transiently on gentle digital pressure
  2. Fleischer ring: Deposition of iron in the basal epithelial cells in a ring shape at the base of the conical protrusion. This ring is faint and broad in early keratoconus and becomes thinner and more discrete as the condition advances. It is best appreciated in cobalt blue or green filter.
  3. Corneal thinning: Slit lamp examination in keratoconic eyes show central or inferior corneal thinning. Maximum corneal thinning corresponds to the site of maximum steepening.
  4. Corneal apical scarring: Approximately 20 percent of eyes with moderate or severe keratoconus develop corneal scarring. It occurs as a part of natural progression of the disease but is worsened by the wearing of rigid contact lenses. 
  5. Hydrops: An acute rupture in Descemet’s membrane causing imbibition of aqueous into corneal stroma causing it to swell. It is a complication usually seen in patients with advanced keratoconus with associated allergy or eye rubbing.
  6. Prominent corneal nerves: It is not likely that the nerve fibers are more numerous in keratoconic patients, but only that they are more easily seen due to changes in density. An increased visibility of the corneal nerve fibers cannot be considered a singular distinction of keratoconus. 
  7. Rupture in Bowman’s layer: These are irregular superficial opacities that develop due to breaks in Bowman’s layer. They can cause significant visual loss due to scarring in advanced cases.

Diagnosis of Keratoconus ?

This is usually done by an ophthalmologist with a detailed eye examination. Diagnosing early keratoconus can be tricky since the mild disease often does not show any identifiable signs on slit-lamp examination. However, recent and a more definitive diagnosis can be obtained using corneal topography, in which an automated instrument projects an illuminated pattern onto the cornea and determines its shape from analysis of a digital image. The topographical map reveals corneal surface irregularity, distortions or scarring in the cornea, with keratoconus revealed by a characteristic steepness of curvature which is usually below or around the center of the cornea. The topography record of the degree and extent of the deformation is used for assessing its rate of progression.

Tear fluid can also be used as a biomarker for Keratoconus.

Unilateral cases tend to be uncommon. Sometimes it’s a mild condition in the better eye, below the limit of clinical detection. It is common for keratoconus to be diagnosed first in one eye and not until later in the other. 

Progress of the disease

Keratoconus is always, almost a progressive condition. In about 10% to 20% of patients, the keratoconus becomes very advanced. The cornea may become extremely steep, thin, and irregular or the vision cannot be improved and also posing a risk of transplant rejection.

Treatment of Keratoconus

Keratoconus is a known progressive disorder.  In the intial stages its important to try and prevent progression.  As mentioned earlier excessive eye rubbing can lead to progression.  Its important to treat this vigorous eye rubbing with antiallergic eye drops.  These drops could be lubricants or even steroids.  You should know that steroid eye drops should be used only after a doctors prescription.  If not used correctly these drops can cause cataract or glaucoma.

As mentioned earlier a few types of corrective types of contact lenses are used to improve vision.  Gas permeable contact lenses or RGP corrective lenses are used for mild to moderate keratoconus.  In the later stages of keratoconus scleral lenses or Rose K lenses are used.  These speciality lenses help with improving vision because tear fluid fills the gaps between the lenses and the cornea affording adequate vision correction to the patients.

There are a few surgical options too.  Intacts and phakic IOLs.  Intacs are corneal implants or corneal ring implants which may stabalize the cornea while phakic IOLs are lenses that are placed in the eye over or in front of our natural lens. 

Reason tells us that anything which is progressive first needs to be stopped. There is only one treatment that is available for progressive keratoconus which is C3R or CXL or collagen crosslinking. Corneal collagen cross-linking (C3R/ CXL) is a well-accepted and welcome addition to treatment options for keratoconus. There are some limitations of C3R Before C3R was approved, treatment options to rehabilitate vision were quite limited, mostly to contact lenses, and when they failed one needed to undergo a surgical procedure like the corneal transplant procedures.  You can read more about cornea transplant

Frequently Asked Questions ?

Will I loose my eyesight because of Keratoconus?
If left untreated, it can lead to sever vision loss.

Are contact lenses helpful in Keratoconus?
Contact lenses will not help in reducing the progression of Keratoconus however patient with moderate to advanced case of keratoconus may not achieve great vision with spectacle due to irregular astigmatism and can achieve superior vision with specialized contact lenses.

Will my vision improve after C3R surgery?
As mentioned in above question, Vision can only be improved with contact lenses however to reduce the progression of keratoconus and to strengthen the cornea C3R surgery is required.

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