Astigmatism is a refractive condition in which the eye’s optical system is incapable of forming a point image for a point object. This is because the refracting power of the optical system varies from one meridian to another.
In regular astigmatism (which includes the great majority of cases), the meridian of greatest refraction and the meridian of least refraction are 90 degrees apart. The amount of astigmatism is equal to the difference in refracting power of the two principal meridians: in any other meridian, the amount of astigmatism is a function of the square of the sine of the angle between the meridian of least refraction and the meridian in question. (The meridians of greatest and least refraction are defined as the principal meridians.)
The cornea is usually the cause of clinically significant astigmatism, although the crystalline lens tends to cause astigmatism of small amounts. Most corneas are more steeply curved in the vertical meridian in the horizontal meridian, causing the vergence of light to be greater in the vertical than in the horizontal meridian, this condition is called direct or with the rule astigmatism.
When the vergence of light is greater in the horizontal meridian than in the vertical meridian, the condition is called indirect or against the rule astigmatism.
By convention, if the two principal meridians are more than 30 degrees away from 90 or 180 degrees, oblique astigmatism exists. Whereas a refracting surface having the same radius of curvature in all meridians is called a spherical surface, a refracting surface having differing radii or curvature in different meridians is called a toric surface.
Instead of a single point image being formed for each object point, the image consists of two focal lines, one parallel to each of the principal meridians: between the two focal lines, the circle of least confusion is located. At any plane other than that containing one of the focal lines or the circle of least confusion, the image takes the form of a blur ellipse rather than a blur circle.
Pathophysiology of astigmatism
The refractive media of the astigmatic eye are not spherical, but refract differently along one meridian that along the meridian perpendicular to it. Any amount of astigmatism at the axis orientation introduces a Sturm conoid into the image system. This produces two focal points. A punctiform object is therefore represented as a sharply defined lines segment at the focal point of the first meridian, but also appears as a sharply defined line segment rotated 90° at the focal point of the second meridian.
Midway between these two focal points is what is known as the “circle of least confusion.” This refers to the location at which the image is equally distorted in every direction -i.e., the location with the least loss of image definition.
The aggregate system lacks a focal point.
The combined astigmatic components of all the refractive media comprise the total astigmatism of the eye. These media include:
- Anterior surface of the cornea.
- Posterior surface of the cornea.
- Posterior surface of the lens.
- Anterior surface of the lens.
Rarely, nonspherical curvature of the retina may also contribute to astigmatism.
Classification and causes: – Astigmatism can be classified as follows:
- External astigmatism: – Astigmatism of the anterior of the cornea.
- Internal astigmatism: – The sum of the astigmatic components of the other media.
Symptoms of higher astigmatism (>1D)
- i. Blurred vision
- ii. Tilting of the head for oblique astigmatism
- iii. Turning of the head (rare)
- iv. Squinting to achieve “pinhole” vision clarity
- v. Reading material held close to eyes to achieve large (as in myopia) but blurred retinal image.
Symptoms of lower astigmatism (<1D)
- i. Asthenopia (“tired eyes”), especially when doing precise work at a fixed distance. With the rule astigmatism produces more symptoms, but clearer vision than the same amount of against the rule astigmatism.
- ii. Transient blurred vision relived by closing or rubbing the eyes (as in hyperopia) when doing precise work at a fixed distance.
- iii. Frontal headaches with long periods of visual concentration on a task.
Diagnosis and treatment of astigmatism
Astigmatism should be suspected in people who narrow their eyelids almost to a slit in order to see more clearly. Diagnosis usually requires careful assessment of the cornea and specialized testing to determine exactly which part of the image is distorted and how severely. Instruments used to measures astigmatism include the retinoscope, which allows light reflections from the inside of the eye to be studied and the keratometer, which measures the corneal images.
Astigmatism is harder to correct than near or farsightedness. Glasses with cylindrical lenses ground to the proper curvature may be prescribed, although the wearer may still notice some blurring. If astigmatism is accompanied by near or farsightedness, compound spectacle lenses are the answer. Contact lenses may also be used to correct corneal curvature. Severe cases of “irregular” astigmatism caused by damage to the eye may require a corneal graft.
Homeopathic treatment of Astigmatism symptoms – Homeopathy is one of the most popular holistic systems of medicine. The selection of remedy is based upon the theory of individualization and symptoms similarity by using holistic approach. This is the only way through which a state of complete health can be regained by removing all the sign and symptoms from which the patient is suffering. The aim of homeopathy is not only to treat astigmatism symptoms but to address its underlying cause and individual susceptibility. As far as therapeutic medication is concerned, several remedies are available to cure astigmatism symptoms that can be selected on the basis of cause, sensations and modalities of the complaints. For individualized remedy selection and treatment, the patient should consult a qualified homeopathic doctor in person. There are following remedies which are helpful in the treatment of astigmatism symptoms:
Gelsemium, Lilium Tig, Physostigma, Sepia, Tuberculinum, Androctonos, Anhalonium, Ruta, Picric Acid, Heloderma and many other medicines.
Theodore Grosvenor; Primary Care Optometry; 2007; 17
Gerhard K. Lang: Ophthalmology: pocket textbook atlas; 2006 453
Deborah Pavan-Langston; Manual of ocular diagnosis and therapy; 2007; 410
David B. Jacoby. R,. M. Youngson: Encyclopedia of Family Health: 2004; 142