Astigmatism is a type of refractive error wherein the refraction varies in different meridian. As a result, the light rays entering the eye cannot converge to a point focus but form focal lines.
As described by Bannon and Walsh (1945), astigmatism was first mentioned in 1727 by Sir Issac Newton. But Thomas Young in 1800 published first description of astigmatism.
Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the lens, and axial length of the eyeball. In emmetropia, these three components of refractive power combine to produce normal refraction to the eye.
Emmetropia is the condition where the eye has no refractive error and requires no correction for distance vision. In an emmetropic eye, rays of light parallel to the optical axis focuses on the retina. The far point in emmetropia (point conjugate to retina in non- accommodating state) is optical infinity, which is 6 meters. Ametropia (refractive error) results when cornea and lens inadequately focus the light rays. The measuring unit for refractive error is dioptre (D), which is defined as the reciprocal of the focal length in meters.
The term ametropia (refractive error) describes any condition where light is poorly focused on light sensitive layer of eye, resulting in blurred vision. This is a common eye problem and includes conditions such as myopia (near- sightedness), hypermetropia (far- sightedness), astigmatism, and presbyopia (age- related diminution of vision). A person who is able to see without the aid of spectacles or contact lenses is emmetropic.
Prevalence and distribution of ametropia vary greatly with age. Majority of children in early infancy are found to be somewhat hypermetropic. During the school years, children begin to become myopic in increasing numbers. Astigmatism change relatively little with age. The majority of children and young adults have a small amount of with-the-rule astigmatism, but in later adult years, there is a tendency for with-the-rule astigmatism to decrease in amount and for against-the-rule astigmatism to increase.
Astigmatism is divided as
- Regular astigmatism: It is correctable with cylindrical or sphero-cylindrical lenses. It may be
– With-the-rule astigmatism: When the steepest corneal meridian is close to 90?.
– Against-the-rule astigmatism: When the steepest meridian is close to 180?.
– Oblique astigmatism: Principal meridians do not lie close to 90? or 180?.
– Bi- oblique astigmatism: The two principal meridians are not at right angle to each other.
- Irregular astigmatism: It is not correctable with cylindrical or sphero-cylindrical lenses. Patients with irregular astigmatism typically suffer from reduced visual acuity and poor quality of vision. Irregular astigmatism may result from corneal diseases, such askeratoconus, trauma or scarring following herpes infection. It may also be produced due to cataract surgery, penetrating keratoplasty or elective kerato-refractive surgery.
Depending on spherical ametropia of a particular eye, astigmatism may be classified as simple or compound based on whether one or both meridians, respectively, are focused outside the retina. If one meridian focuses in front of the retina and the other meridian focuses behind it, the astigmatism is called mixed astigmatism.
In astigmatism, the eye has different refractive powers along different meridians. Light entering in vertical direction gets focused differently than light in the horizontal direction. The meridian of steeper curvature has greater refractive power. The astigmatic eye produces a blurred image because two focal lines of images are being produced. This requires different corrections along each of these meridians to produce a single focused image on the retina.
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Symptoms vary according to the degree and type of regular astigmatism and may include
- Blurring of vision: Patients with low astigmatism show transient blurring of vision, which is relieved by closing or rubbing the eyes. Eyes become fatigued with reading and the letters are described as ‘running together’.
- Symptoms of asthenopia (eye strain): Symptoms of asthenopia (eye strain) include tiredness of eyes, headache, irritability, dizziness and fatigue.
- Narrowing of eyes: Narrowing of eyes may be resorted to in cases having high astigmatism to see clear. Narrowing produces a stenopaeic slit effect which cuts down the light rays in one meridian.
- Head tilt: Some patients with high oblique astigmatism may keep the head tilted to one side to avoid distortion of image.
- Holding books close to eyes: Many patients with high astigmatism may hold the books close to the eyes in a bid to achieve larger image.
Patients with irregular astigmatism present with
- Defective vision.
- Distorted vision.
- Polyopia (seeing multiple images of a single object).
Astigmatism may be natural, traumatic following a wound or surgically induced as is seen following cataract surgery. Tight sutures after surgery further accentuate astigmatism.
- Regular astigmatism may be
- Corneal astigmatism: It is due to abnormalities in corneal curvature. This is the commonest typeand is usually congenital. Acquired corneal astigmatism is not infrequent, but it often gives rise to irregular astigmatism.
- Lenticular astigmatism: This is relatively less common. It may be of following types:
– Curvature astigmatism: Congenital abnormalities of lens curvature may produce small amount of curvature astigmatism. Marked lenticular astigmatism may be seen in condition like lenticonus.
– Positional astigmatism: Congenital tilting or oblique placement of lens may produce small amount of astigmatism. Congenital or traumatic subluxation of lens may also produce astigmatism of varying degrees.
– Index astigmatism: Variable degree of change in refractive index of crystalline lens of patients suffering from diabetes or nuclear sclerosis may occur.
- Retinal astigmatism: Occasionally, obliquity of macula may produce retinal astigmatism.
Optics of regular astigmatism:
In regular astigmatism, the parallel rays of light are not focused at a point but form two focal lines. This configuration of refracted rays through astigmatic surface (or toric surface) is called Sturm’s conoid and the distance between two focal lines is called focal interval of Sturm. Length of focal interval is a measure of the degree of astigmatism.
- Irregular astigmatism may be
- Corneal irregular astigmatism: Extensive corneal scarring or ectatic conditions likekeratoconusmay produce this.
- Lenticular irregular astigmatism: This may be produced due to variable refractive index in different parts of the crystalline lens and rarely may occur during maturation of cataract.
- Retinal irregular astigmatism: It may be produced by distortion of the macular area due to scarring or tumours of retina and choroid pushing the macular area.
Diagnosis of astigmatism depends upon the symptoms and clinical features.
Refractive error may be determined by
- Retinoscopy: Power in two different axes may be determined by retinoscopy.
- Astigmatic fan test: Astigmatic fan test is a sensitive test to find out astigmatism.
- Jackson cross- cylinder test: Jackson cross- cylinder test helps in confirming the power and axis of cylindrical lenses.
- Keratometry: Keratometry reveals different corneal curvature in two separate meridians. Irregular astigmatism shows distorted keratometry.
- Corneal topography: Corneal topography is essential in understanding the shape and curvature of cornea, and helps in establishing diagnosis. Irregular astigmatism patients show irregularities on corneal topography.
- Very high frequency (VHF) digital ultrasound arc scanner: It is capable of obtaining 3- dimensional (3D) layered pachymetry across the central 10 mm of cornea. Resolution of scanner on cornea mode distinguishes individual corneal layers such as epithelium or stroma in all three dimensions, due to multi-meridional scanning.
- Wavefront aberrometry: Wavefront aberrometry may help in diagnosis of irregular astigmatism and the assessment of the optical quality of the eye. Simulations of the retinal images are useful to understand some of the symptoms of irregular astigmatism. With corneal topographic analysis, the origin of irregular astigmatism from the cornea or internally, or both, may be specified by aberrometry. Clinical application of aberrometry is wavefront – guided refractive surgery.
Types of regular astigmatism:
Based upon the axis and the angle between two principal meridia, regular astigmatism may be classified into:
- With-the-rule astigmatism: The two principal meridia are placed at right angle to one another with vertical meridian steeper or more curved than the horizontal one. This is called with-the-rule astigmatism, because similar astigmatic condition exists normally due to eyelid pressure on cornea.
- Against-the-rule astigmatism: The two principal meridia are placed at right angle to one another with horizontal meridian steeper or more curved than the vertical one.
- Oblique astigmatism: It is a type of regular astigmatism where two principal meridia, though placed at right angle, are not in vertical and horizontal meridian. Oblique astigmatism is often found to be symmetrical (e.g. cylindrical lens required at 30? in both eyes) or complementary (e.g. cylindrical lens required at 30? in one eye and at 150? in the other eye).
- Bi-oblique astigmatism: In this type, the two principal meridia are not at right angle to each other.
Refractive types of regular astigmatism: Depending upon the position of two focal lines in relation to retina, regular astigmatism is classified into:
- Simple astigmatism: The rays of light are focused in one meridian only on the retina. The rays of light in other meridian are focused in front or behind retina. Accordingly, it is called
– Simple myopic astigmatism: When the light rays of other meridian are focused in front of the retina.
– Simple hypermetropic astigmatism: When the light rays of other meridian are focused behind the retina.
- Compound astigmatism: The rays of light are focused either in front or behind the retina in both the meridia. Accordingly, it is called
– Compound myopic astigmatism: The rays of light are focused in front of the retina in both the meridia.
– Compound hypermetropic astigmatism: The rays of light are focused behind the retina in both the meridia.
- Mixed astigmatism: The rays of light are focused in front in one meridian and behind the retina in other meridian. Thus, eye is myopic in one meridian and it is hypermetropic in other. Such patients have relatively less symptoms as ‘circle of least diffusion’ forms on retina. It is presumed that a pencil of rays forms a minute circle of clear vision, called circle of least diffusion. Larger is the circle of least diffusion, poorer the vision.
Management should be carried out under medical supervision.
Medical optical therapy:
Optical therapy of regular astigmatism comprises of prescribing appropriate cylindrical lenses. The cylindrical lenses may be prescribed in the form of
- Spectacles: Regular astigmatism may be corrected with spherocylindrical lenses except where the meridians are not perpendicular. Parallel rays falling upon a cylindrical lens are affected in different ways. In the direction of its axis, it is simply a plane lamina with parallel sides, so that it has no effect on the rays. In the direction at right angles to its axis, it is spherical on one side and plane on the other. Therefore, it acts like a planoconvex or a planoconcave.
- Contact lenses.
Guidelines for optical correction:
- Small astigmatism: Small astigmatism (about 0.5 dioptres or less) may be ignored and corrected only if there are symptoms of eye asthenopia or there is deterioration of vision.
- High astigmatism: Cylindrical error should be corrected as full as possible. An under correction may be prescribed to begin with if the patient is not comfortable with full correction. In all cases in which astigmatism produces asthenopic symptoms, full optical correction should be prescribed for constant use i.e. for distance as well as for near.
- Change in astigmatic correction: Change in astigmatic correction should preferably be avoided even if there is some improvement in visual acuity. Change in astigmatic correction may be given only if there is significant improvement in visual acuity and it may take some time to adjust.
Bi- oblique astigmatism, high astigmatism or mixed astigmatism is better treated by prescribing with contact lenses.
Patients with irregular astigmatism typically suffer from reduced visual acuity as well as visual quality. Pinhole occluders may provide some improvement in vision as compared to spectacles.
Optical therapy of irregular astigmatism consists of prescribing contact lenses which replaces the affected anterior corneal surface for refraction. An improvement in vision may be obtained using gas permeable lenses as compared to soft hydrogel lenses.
Indications of surgery: Correction of astigmatism by surgery is done when it significantly affects vision. Typically, visually significant astigmatism is roughly more than 1 dioptres, although many patients may be symptomatic even with lower astigmatism. With improved surgical techniques such as excimer laser, even lower degree of astigmatism is being treated by surgical means. Usually surgery is done for
- Irregular astigmatism.
- Patients who wish to discard glasses or contact lenses.
- Patients who are intolerant of glasses or contact lenses.
- Development of visually significant astigmatism following ocular surgery.
Surgical therapy includes:
- Laser in situ keratomileusis (LASIK): Keratomileusis refers to carving of cornea. Astigmatism is corrected by combining central and peripheral treatments to differentially steepen the flattest corneal meridian and flatten the steepest meridian. Excimer laser ablation is applied directly to the corneal stroma after reflecting a lamellar or partial- thickness corneal flap. The flap is returned to its original position, once ablation is complete. Recovery of vision usually takes few days and some patients even take few weeks.
- Wavefront- guided LASIK: Wavefront- guided LASIK or custom LASIK is a new wavefront technology to program ablation pattern more precisely than the conventional LASIK.
- IntraLASIK (IntraLase): IntraLASIK uses femtosecond laser, which is more accurate than a blade and provides a huge margin in terms of precision and safety. Femtosecond laser creates a corneal flap at a pre-programmed depth and position.
- EpiLASIK: EpiLASIK is designed to create a thin flap in the epithelium. It is an excellent alternative for patients with thin and steep or flat cornea. The layer is preserved and later replaced following reshaping of cornea using excimer laser.
- Photorefractive keratectomy (PRK): PRK also uses excimer laser to reshape cornea, but this procedure does not require lamellar flap. The laser is applied directly to the anterior stromal surface after removal of surface epithelium. A soft contact lens is prescribed after laser treatment. The corneal epithelium typically heals in about 4 to 7 days. Improvement of vision may take few weeks to several months.
- Laser subepithelial keratomileusis (LASEK): It involves cleaving the epithelial sheet at the basement membrane with dilute alcohol, applying laser as in conventional PRK, and repositioning the epithelium afterward.
- Photoastigmatic refractive keratectomy (PARK): Photoastigmatic refractive keratectomy (PARK) has been used to reduce astigmatism.
- Radial keratotomy (RK): Earlier RK as keratorefractive surgery was used, that effectively reduced astigmatism. In RK, spoke- like cuts radiating from central cornea at about 90% corneal depth were given to balloon the peripheral cornea. Although RK surgery was effective, the results were often unpredictable and led to overcorrection, with progressive hypermetropia. RK surgery was soon replaced by laser vision correction with the introduction of PRK.
- Astigmatic keratotomy (AK): Astigmatic keratotomy is a modification of RK surgery and is used to reduce corneal astigmatism. The two forms of corneal incisions, astigmatic keratotomy and limbal relaxing incisions (LRI), or more accurately peripheral corneal relaxing incisions (PCRI), are differentiated by their location on cornea. In both procedures, incisions are made to about 90% depth in cornea to flatten the steep meridian. PCRIs are easier to perform and have a weaker effect in correction because of their more peripheral location. Relaxing incisions, especially arcuate ones (arcuate keratotomy), are one of the commonly performed procedures for reduction of astigmatism. Femtosecond laser- enabled keratotomy has become an alternative to the mechanical techniques because of higher precision of incisions with improved accuracy and safety.
- Refractive lens exchange (RLE): Refractive lens exchange (RLE) is a surgical procedure that may correct refractive error by removing and replacing the clear crystalline lens of the eye with toric intraocular lenses (toric IOLs). RLE is an option for high refractive errors, where laser vision correction is not an option.
- Phakic intraocular lens (phakic IOL) implant: Toric phakic intraocular lens (phakic IOL) may be implanted in front of and attached to the iris or is placed just behind the iris. Toric phakic IOL is an alternative to laser vision correction with keratorefractive surgery. Unlike RLE, all eye structures including crystalline lens, is not touched when a toric phakic IOL is implanted.
- Penetrating keratoplasty: Penetrating keratoplasty may be required in patients with affected cornea producing irregular astigmatism.
Contraindications to surgery:
- Corneal or anterior segment diseases such as keratitis, conjunctivitis or corneal ulcers.