ASTIGMATISM Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular. REGULAR ASTIGMATISM The astigmatism is regular when the refractive power changes uniformly from one meridian to another (i.e., there are two principal meridia). Etiology 1. Corneal astigmatism is the result of abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism. 2. Lenticular astigmatism is rare. It may be: i. Curvatural due to abnormalities of curvature of lens as seen in lenticonus. ii. Positional due to tilting or oblique placement of lens as seen in subluxation. iii. Index astigmatism may occur rarely due to variable refractve index of lens in different meridia. 3. Retinal astigmatism due to oblique placement of macula may also be seen occasionally. Types of regular astigmatism Depending upon the axis and the angle between the two principal meridia, regular astigmatism can be classified into the following types : 1. With-the-rule astigmatism. In this type the two principal meridia are placed at right angles to one another but the vertical meridian is more curved than the horizontal. Thus, correction of this astigmatism will require the concave cylinders at 180° ± 20° or convex cylindrical lens at 90° ± 20°. This is called 'with-the-rule' astigmatism, because similar astigmatic condition exists normally (the vertical meridian is normally rendered 0.25 D more convex than the horizontal meridian by the pressure of eyelids). 2. Against-the-rule astigmatism refers to an astigmatic condition in which the horizontal meridian is more curved than the vertical meridian. Therefore, correction of this astigmatism will require the presciption of convex cylindrical lens at 180° ± 20° or concave cylindrical lens at 90° ± 20° axis. 3. Oblique astigmatism is a type of regular astigmatism where the two principal meridia are not the horizontal and vertical though these are at right angles to one another (e.g., 45° and 135°). 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). 4. Bioblique astigmatism. In this type of regular astigmatism the two principal meridia are not at right angle to each other e.g., one may be at 30o and other at 100°. Optics of regular astigmatism As already mentioned, in regular astigmatism the parallel rays of light are not focused on a point but form two focal lines. The configuration of rays refracted through the astigmatic surface (toric surface) is called Sturm’s conoid and the distance between the two focal lines is known as focal interval of Sturm. The shape of bundle of rays at different levels (after refraction through astigmatic surface) is described on page 25. Refractive types of regular astigmatism Depending upon the position of the two focal lines in relation to retina, the regular astigmatism is further classified into three types: 1. Simple astigmatism, wherein the rays are focused on the retina in one meridian and either in front (simple myopic astigmatism – Fig. 3.30a) or behind (simple hypermetropic astigmatism – Fig. 3.30b) the retina in the other meridian. OPTICS AND REFRACTION 37 2. Compound astigmatism. In this type the rays of light in both the meridia are focused either in front or behind the retina and the condition is labelled as compound myopic or compound hypermetropic astigmatism, respectively (Figs. 3.30c and d). 3. Mixed astigmatism refers to a condition wherein the light rays in one meridian are focused in front and in other meridian behind the retina (Fig. 3.30e). Thus in one meridian eye is myopic and in another hypermetropic. Such patients have comparatively less symptoms as 'circle of least diffusion' is formed on the retina (see Fig. 3.15). Symptoms Symptoms of regular astigmatism include: (i) defective vision; (ii) blurring of objects; (iii) depending upon the type and degree of astigmatism, objects may appear proportionately elongated; and (iv) asthenopic symptoms, which are marked especially in small amount of astigmatism, consist of a dull ache in the eyes, headache, early tiredness of eyes and sometimes nausea and even drowsiness. Signs 1. Different power in two meridia is revealed on retinoscopy or autorefractometry. 2. Oval or tilted optic disc may be seen on ophthalmoscopy in patients with high degree of astigmatism. 3. Head tilt. The astigmatic patients may (very exceptionally) develop a torticollis in an attempt to bring their axes nearer to the horizontal or vertical meridians. 4. Half closure of the lid. Like myopes, the astigmatic patients may half shut the eyes to achieve the greater clarity of stenopaeic vision. Investigations 1. Retinoscopy reveals different power in two different axis (see page 548) 2. Keratometry. Keratometry and computerized corneal topotograpy reveal different corneal curvature in two different meridia in corneal astigmatism (see page 554) 3. Astigmatic fan test and (4) Jackson's cross cylinder test. These tests are useful in confirming the power and axis of cylindrical lenses (see pages 555, 556). Fig. 3.30. Types of astigmatism : simple myopic (A); simple hypermetropic (B); compound myopic (C); compound hypermetropic (D); and mixed (E). 38 Comprehensive OPHTHALMOLOGY Treatment 1. Optical treatment of regular astigmatism comprises the prescribing appropriate cylindrical lens, discovered after accurate refraction. i. Spectacles with full correction of cylindrical power and appropriate axis should be used for distance and near vision. ii. Contact lenses. Rigid contact lenses may correct upto 2-3 of regular astigmatism, while soft contact lenses can correct only little astigmatism. For higher degrees of astigmatism toric contact lenses are needed. In order to maintain the correct axis of toric lenses, ballasting or truncation is required. 2. Surgical correction of astigmatism is quite effective. For details see page 48. IRREGULAR ASTIGMATISM It is characterized by an irregular change of refractive power in different meridia. There are multiple meridia which admit no geometrical analysis. Etiological types 1. Curvatural irregular astigmatism is found in patients with extensive corneal scars or keratoconus. 2. Index irregular astigmatism due to variable refractive index in different parts of the crystalline lens may occur rarely during maturation of cataract. Symptoms of irregular astigmatism include: Defective vision, Distortion of objects and Polyopia. Investigations 1. Placido's disc test reveales distorted circles (see page. 471) 2. Photokerotoscopy and computerized corneal topography give photographic record of irregular corneal curvature. Treatment 1. Optical treatment of irregular astigmatism consists of contact lens which replaces the anterior surface of the cornea for refraction. 2. Phototherapeutic keratectomy (PTK) performed with excimer laser may be helpful in patients with superficial corneal scar responsible for irregular astigmatism. 3. Surgical treatment is indicated in extensive corneal scarring (when vision does not improve with contact lenses) and consists of penetrating keratoplasty.
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