The clear, front part of the eye, called the cornea, is normally shaped like a basketball. This shape allows the incoming light to come to focus at a single point on the retina. If the cornea is shaped like a football or an egg, it causes the light to become distorted and forms a blur on the retina. To understand this disease, let us take the example of the game of darts. A normal eye is like an ace shooter, with all the darts hitting the bull’s eye. An eye with astigmatism is like a novice, with the darts scattered all over the board, some in the center and some out. Further, let’s say you remove the darts and try to reinsert them in the inner circle. The novice keeps pushing out some that were already there and this prevents all the darts from being in the inner circle at the same time.
The brain also tries to achieve accumulation of all the data on the macula. It sends signals to the inner muscles of the eye to change the shape of the lens to achieve this goal. It’s a futile exercise leading to fatigue of the eye muscle and so nutrition is rushed to the eye through the blood vessels, which are red in color and cause the eyes to appear red. The brain becomes exhausted and therefore, people with astigmatism experience headache, redness of the eyes and generally feel their eyes are tired.
Now that you understand what astigmatism is, let’s discuss the less common causes of astigmatism. The natural lens, retina, and rarely, the vitreous jelly can also cause astigmatism. Spoke shaped cataracts, called cortical cataracts, can cause astigmatism. The combination of all these generates the total astigmatism of the eye.
We are fortunate enough to finally have technologies which can detect and eliminate this condition. The most precise way to detect and document changes in astigmatism is by a machine called color corneal topographer. This sends beams of light to be reflected off the front surface of the eye. It detects and color-codes the results for easier interpretation. This yields the astigmatism on the cornea on the front part of the eye. OCT based devices like IOL Master 700, and others like Pentacam, can analyze astigmatism on the posterior corneal surface. Finally, autorefractors and wavefront devices can detect the total astigmatism of the eye by sending beams of light to the retina and back.
Autorefractor calculates the corneal curvature and displays astigmatism.
Astigmatism displayed on colored corneal topography
Non-surgical treatment of Astigmatism
The twentieth century way to counter the effects of astigmatism has been wearing glasses or contact lenses, but they do not cure the condition. They lenses in both are shaped like a football as well and placed perpendicular to the direction of astigmatism of the eye. Now you have two football shaped objects perpendicular to each other. Light that passes through these double football shapes develop various aberrations or defects, preventing perfect vision. This leads to distortions and color changes of objects in the field of vision. If you wear glasses and rotate them in front of your eyes, you will see the world move in an opposite direction.
Contact lenses can also counteract the effect of astigmatism. Hard lenses and soft lenses employ different methods. Hard lenses accumulate a tear film between the lens and the cornea. This tear film acts as a lens of opposite power and negates astigmatism. This method can also treat irregular refraction. Soft lenses, on the other hand, have the power ground on the cornea in a particular axis. It is marked and weighted at the six o’clock position to prevent contact lens rotation.
Why not just eliminate the football shape of the eye? The good news is that in this century astigmatism can be permanently cured!
For younger people, between 18 and 45 years of age, Wavefront astigmatism Lasik is the best option. The Wavefront device uses principles of astrophysics to calculate the imperfections of the eye. Iris registration allows very precise placement of the laser beam. The result is the football shape of the eye being changed to a spherical shape. The vision, especially night vision, improves. The halos and glare at night decrease. Further, the redness and fatigue are eliminated.
Treatment calculation iDesign lasik
iDesign wavefront treatment.
If you are above 45 years of age, we have to determine the quality of tear film, thickness of the cornea, presence of cataracts, and desire to be free from glasses. If you have a cornea with normal thickness, a good tear film and the absence of cataracts, thin flap Lasik can get rid of the astigmatism. If you have dry eye or a thin cornea, Superlasik or Epilasik is a better choice. It is a superficial type of Lasik in which there is no cutting into the cornea to make a flap.
If cataracts are present, the scenario changes. First, we have to study the technical output from our various machines to determine whether the astigmatism is stemming from the cornea, the cataract or the retina. If it is from the cataract, then eliminating it should solve the problem. If it is from the retina, then it is imperative that the health of the retina be confirmed before progressing further.
The treatment options for corneal astigmatism include limbal relaxing incision, Toric IOL, multifocal IOL or Toric multifocal IOL. Limbal relaxing incisions are derived from RK, or radial keratotomy techniques. Incisions in the cornea are made dependent on the amount of astigmatism. The number, length, depth and the distance from the center of the incisions vary depending on the amount of astigmatism which has to be treated. Metal or diamond blades have been used since the last century. Today, femtosecond lasers can perform this function more accurately. These lasers are different from the one used for Lasik. The best part is that these lasers can also perform the cataract surgery. They can make openings in the lens and reduce the lens into small cubes.
The drawback of LRI is that it can treat only a small amount of astigmatism and the effect may also fade away with time. A permanent way of correcting astigmatism is the use of a Toric implantable lens. The lenses are inserted when the cataracts are removed. They are of different powers and must be accurately aligned for best results. If they rotate, the astigmatism correction can be affected. That is why the more stable Tecnis and Acrysof Toric are popular. The Tecnis Toric is white and allows maximum light to pass through. The yellow Toric IOL absorbs the blue light but a person would need more light to read in a dim room.
Despite the monofocal Toric lenses, a person would still need reading glasses. This problem has been solved by merging the benefits of accommodating lenses and multifocal lenses with Toric lenses. The current FDA approved lenses are, Trulign (Crystalens Toric), Symfony Toric, Panoptix Toric and Tecnis Multifocal Toric. Theses lenses will counteract the astigmatism of the eye and will allow you to see sharply at all distances.
Eliminating Astigmatism with PIE
PIE has an arch enemy, it is astigmatism. It is imperative to correct even small amounts of astigmatism to get the best vision after implanting presbyopic implants. When we open the lens bag during PIE or cataract surgery, it resolves the lenticular astigmatism. We then need a strategy for the residual corneal astigmatism. It is logical to treat astigmatism at its source. Therefore, a procedure on the cornea is the best option.
Limbal Relaxing Incisions (LRI) – Manual or Femtosecond laser. Vertical incisions up to 90% depth of the cornea near the junction of the clear cornea and the opaque sclera. The number (one or two) and the length of each determine the final effectiveness of LRI. These incisions have minimal side effects as they are far from the pupil. They have less effect than AK and also effectiveness decreases as the incisions heal by vascularization.
Astigmatic Keratotomy (AK) – Diamond Blade or Femtosecond laser. These are made on 90 % clear cornea. Being closer to the center of the cornea they are more effective. The final effect is dependent on the number, distance and length of each AK incision. The femtosecond laser can make hidden AK. These are ones which do not come to the surface but stop short at the epithelium. These can be opened later to further titrate the final effect. AK can cause progressive effects over years. If the incisions are too close to the pupil, they can cause glare at night.
Operating on Steep Axis – When an incision is made on the steep axis it flattens that axis, decreasing the astigmatism. The length of the incision is directly correlated to the effect. Corneal incisions are more effective than scleral incisions.
Wound suturing on flat axis – When a suture is applied to a corneal wound it causes flattening around the suture but steepens the central cornea.
Lasik/ Lasek – The best and precise way to treat astigmatism is by laser vision correction. Either Lasik or Lasek may be performed. When performing Lasik and creating a flap it is necessary to wait at least three months after lens implantation to avoid destabilizing the lens. Where the surgeon is anticipating Lasik after PIE, he can choose to make a flap before PIE. Then a few weeks after lens implantation, the flap can be lifted, and laser vision correction be performed. With femtosecond lasers a flap can be performed at the time of the laser cataract surgery.
Toric Lens – A lens with built in astigmatism can counter the corneal astigmatism. Special vector-based nomograms are required to predict the exact power and orientation of the Toric lens. A Toric lens will not give the ability to read. It may be an option where laser vision cannot be performed.
Toric Multifocal Lens – A combination of Toric and multifocal lenses can treat astigmatism and yield vision at all distances.