Thin Corneas & Corneal Thickness Guide For Patients

BySaya Nagori, MD Board Certified Ophthalmologist & Glaucoma Specialist

A thin cornea is a condition in which the cornea, the clear, dome-shaped front surface of the eye, is thinner than normal. In the vast majority of patients having a thin cornea will not negatively impact visual acuity or their eye health. However, in some cases, a thin cornea can be a sign of more severe ocular disorders such as keratoconus or glaucoma.

Various factors can contribute to the thinning of the cornea, including genetic predisposition, certain medical conditions, and environmental factors. The risks associated with thin cornea vary depending on the severity of the condition and its underlying causes. A thorough examination by an ophthalmologist is crucial for diagnosing and determining the appropriate treatment options.

Treatment for thin cornea depends on the cause and severity of the condition. In the vast majority of cases, thin corneas are not treated unless they are associated with an ocular disorder. Thin corneas can be normal corneas, but they are just thin. For many patients, it doesn’t mean much. 

For more severe cases of corneal thinning that are related to ocular disease, surgical interventions such as corneal transplantation or collagen crosslinking may be necessary to restore the cornea’s structural integrity and prevent further vision loss. Early detection and treatment of corneal issues like keratoconus can help to preserve clear vision.

Causes Of Thin Cornea

Genetics

Most patients who have a thin cornea are born with it. In the vast majority of patients having a thin cornea does not negatively impact their life. However in some cases a thin cornea is related to a corneal disease and genetics can play a role in this. For instance, conditions such as keratoconus and corneal ectasia are often hereditary, leading to gradual corneal thinning and deformation.

Eye Injury

Chronic injury to the eye such as eye rubbing can also lead to a change in the corneal shape and lead to keratoconus. This is more common in patients who have a history of allergies and atopy. These patients may have seasonal allergies, eczema and be constantly rubbing their eye due to itchiness. In keratoconus there is progressive thinning and progressive bulging of the cornea. Patients may find that their prescription is changing frequently and the degree of astigmatism is increasing. There are treatments available such as corneal cross linking. 

Eye Disease

There are also certain diseases that can cause corneal thinning. Dellen, or areas of non-wetting on the cornea, can lead to thinning and breakdown of corneal tissue. Also some eye infections such as herpes keratitis could also damage the cornea and cause it to scar and change thickness.

LASIK Surgery, PRK Surgery, SMILE Surgery

Refractive procedures are also a frequent cause of thin corneas. In the lasik procedure, after a corneal flap is created, a laser is applied to correct the refractive error of the eye. This process of applying laser to the corneal stroma will thin the cornea. In fact, the thickness of the cornea is a factor when deciding if someone is a good candidate for laser vision correction.

Using Pachymetry To Measure A Thin Cornea

An ophthalmologist will use a device called a pachymeter to measure your corneal thickness. Pachymetry is a non-invasive procedure, usually performed as part of routine testing in your doctor’s office. Your doctor is more likely to perform this test if you have other risk factors for glaucoma such as optic nerve cupping, high high pressure, or a history of having vision correcting surgical procedure like LASIK surgery in the past. 

Normal Corneal Thickness VS A Thin Cornea

The average central corneal thickness is 540 microns. Patients corneas can range from thicknesses in the 400s to the 600s. Some patients may also have corneal thickness that is in the 300s or higher than 700 although these extremes are rare and are not occurring naturally. Usually very thick corneas occur from scarring, and usually thin corneas happen after laser vision correction. For example, if someone had laser eye surgery, then it is likely that their corneas are much thinner. This will be important for the eye doctor to keep in mind when they measure a patients intraocular pressure as they will need to adjust the pressure measurement to account for the thin cornea. 

How Does A Thin Cornea Affect IOP Measurement

The thickness of your cornea plays a role in the accuracy of intraocular pressure (IOP) measurements. It is essential to understand its influence on IOP readings to correctly assess the risk of developing glaucoma and other related eye issues.

Thicker corneas tend to yield artificially high IOP readings, whereas thin and soft corneas result in artificially low IOP measurements

[1]. Consequently, patients with thin corneas (less than 555 µm) show artificially low IOP readings, which could be dangerous. If their actual IOP is higher, they might be at risk for glaucoma development without knowing it

[2].Corneal properties affecting IOP measurements include resistance to applanation; for example, scarring could lead to artificially high readings, whereas edema causes lower readings. Prior refractive surgery can also influence IOP readings, as the cornea will be thinner than its pre-surgery state, leading to an underestimation of IOP.

Lastly, it’s essential to consider that thinner corneas can significantly increase the risk of progression from ocular hypertension to glaucoma. This influential factor has been demonstrated through the publication of the Ocular Hypertension Treatment Study (OHTS). This showed that having a thin cornea was an independent risk factor for the development of glaucoma.

Adjusting Intraocular Pressure For A Thin Cornea

Corneal thickness is an important factor in accurately measuring a patients intraocular pressure (IOP). On average, intraocular pressure is adjusted by 1 mm of mercury for every 20 microns of corneal thickness away from the average corneal thickness. For example, if a patient had an IOP measurement of 20 and a corneal thickness of 500, this would result in an adjusted IOP of about 22. This is because 540 – 500 yields 40 and then 40 divided by 20 equals 2. Because the cornea is thinner than the average thickness, it means that the pressure is reading artificially lower than it actually is. This means the IOP measured of 20 in a cornea that is 500 is about 22. It is important to keep in mind that this is not exact and the further away from 540 your corneal thickness is, the adjustment may not be exact. 

Eye Conditions Associated With A Thin Cornea

Glaucoma

Thin corneas are related to glaucoma in two ways:

Independent Risk Factor For Glaucoma

A thin cornea is an independent risk factor for developing glaucoma. Glaucoma is a condition where increased pressure in the eye can damage the optic nerve, leading to vision loss. Corneal thickness in the 400s alerts an eye doctor to be suspicious of development of glaucoma in these patients.

Underestimation Of IOP In Thin Corneas

A thin corneas IOP must be adjusted when a patient has glaucoma. It is important to take corneal thickness into account when a patient is being treated for glaucoma. For example in a patient with a normal corneal thickness, a pressure of 15 may be acceptable. However in a patient with a corneal thickness of 440, the IOP may need to be further lowered to effectively treat the glaucoma. How an eye doctor adjusts your glaucoma medications depends on a variety of factors including your IOP, the results of your visual field testing, and other studies like an OCT (optical coherence tomography). It is unique to each patient. 

Keratoconus

Keratoconus is a disorder caused by thinning of the cornea, leading to a cone-shaped, bulging cornea. This can result in distorted vision, sensitivity to light, and glare. In keratoconus the cornea bulges and becomes thinner over time.

Corneal Transplant

Patients with thin corneas may require a corneal transplant if their cornea’s condition worsens or if they develop complications, such as scarring, tearing, or non-responsive ulcers. A cornea transplant involves replacing the damaged cornea with a healthy donor cornea, which may improve vision and reduce associated symptoms.

Refractive Surgery Complications

Thin corneas can also be associated with an increased risk of complications after refractive surgeries, such as LASIK and PRK, as these procedures involve reshaping the cornea to correct vision. Patients with thin corneas may face a greater risk of damaging the corneal structure during surgery if there is not enough corneal tissue to ablate. This can lead to problems such as irregular astigmatism and corneal ectasia, a progressive thinning and weakening of the cornea. This is why proper patient selection by an eye surgeon for a laser vision correction procedure is so important. Good lasik candidates will have enough corneal tissue so that the laser can be done properly.

Treating A Thin Cornea

There are many patients who have a thin cornea who don’t need any treatment at all. The thin cornea may just be an incidental finding and they do have any other eye problems like glaucoma or keratoconus. This is likely the vast majority of patients who have a central corneal thickness that is thinner than the average of 540. In certain cases, a thin cornea may need to be treated, depending on the eye condition associated with it and individual needs. Here are some popular treatments:

Collagen Cross-Linking

Collagen cross-linking is a procedure used for keratoconsu that aims to strengthen the cornea by increasing the number of collagen cross-links within it. It involves the use of riboflavin (vitamin B2) and ultraviolet light to induce these cross-links. This treatment is particularly effective in delaying or halting the progression of conditions like keratoconus.

Corneal Transplant

When other treatments are unsuitable or fail to provide adequate relief, a corneal transplant, or keratoplasty, might be necessary. A corneal transplant replaces the damaged or thinned cornea with a healthy donor cornea. Though the transplant is generally successful, potential complications include graft rejection, poor vision, infection, and astigmatism. Astigmatism can usually be managed with contact lenses after the transplant.

Can You Prevent A Thin Cornea?

Having a thin cornea is likely something you were born with and it will be unlikely to cause you major issues in life unless it is associated with another eye disease. There is proof however that frequent eye rubbing can lead to keratoconus so there are some things you can do to make sure your corneal health is intact.  to prevent thin corneas, there are some measures that can be taken to reduce the risk of developing or worsening the condition:

  • Regular eye exams: Schedule regular eye exams with a qualified eye care professional. This will help to detect any abnormalities or changes in corneal thickness early, allowing for proper diagnosis and treatment if necessary.  
  • Protect your eyes: Wearing protective eyewear while participating in high-risk activities, such as sports or working with hazardous materials, can help prevent eye injuries that may lead to corneal thinning or other corneal conditions.  
  • Manage underlying conditions: Discuss your eye health with your healthcare provider if you have a history of eye surgery, diabetes, autoimmune disease, or other conditions that may increase your risk of corneal thinning. Proper management of these underlying conditions may help prevent additional harm to the cornea.  
  • Maintain good eye hygiene: Keeping the eye area clean and free from debris is important for overall eye health. Regularly clean your eyelids, particularly if you wear contact lenses, to reduce the risk of infections that can cause corneal thinning. 

By following these prevention tips, individuals can minimize their risk of developing thin corneas and reduce the likelihood of associated complications. Early detection and appropriate treatment remain crucial in managing corneal thinning and safeguarding overall eye health.

Thin Cornea: Summary

The average cornea has a corneal thickness of 540 microns. A thin cornea is an independent risk factor for the development of glaucoma and patients who have a thin cornea are at higher risk for this disase. It is also important that an ophthalmologist takes a thin cornea into account when measuring an intraocular pressure. Thin corneas tend to have artificially low readings which means the pressure in the eye is actually higher than the machine reads. This can be an important measurement especially in patients who have ocular hypertension or glaucoma. Some eye conditions associated with varying levels of corneal thickness are keratoconus, HSV keratitis, corneal scarring, and corneal dellens. The best option to evaluate a patient’s cornea is seeing an eye doctor. Visiting the ophthalmologist regularly is important as there are some treatments available for certain eye conditions related to the shape of the cornea. 

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