Astigmatism (from the Greek, "a" meaning absence and "stigma" meaning point) is a refractive error (ametropia) that occurs when parallel rays of light entering the non-accommodating eye are not focused on the retina
Astigmatism is a condition in which uneven curvature of the cornea hinders even refraction. Leads to the placement of the focal point anterior or posterior to the retina, which causes unclear vision at all distances
The cornea of the normal eye has a uniform curvature, with resulting equal refracting power over its entire surface. In some individuals, the cornea is not uniform and the curvature is greater in one meridian (plane) than another, much like a football as a rugby ball. Light rays refracted by this cornea are not brought to a single point focus, and retinal images from objects (both distant and near) are blurred and may appear broadened or elongated. This refractive error is called astigmatism
Types
Astigmatism can be described as regular or irregular
Regular astigmatism
A more common form, the cornea would resemble a football or a rugby ball standing on one end or its side, or less often, tipped to one side
In regular astigmatism, there are two principal meridians separated by 90 degrees
Irregular astigmatism
Less common, the corneal “rugby ball” would appear out of its customary shape and/or bumpy
It is described as a refractive state in which “refraction in different meridians confirms to no geometrical plane and the refracted rays have no planes of symmetry
Types
Symptoms
Asthenopia, transient blurred vision, dull ache in eyes, frontal headache and sometimes nausea and drowsiness -in low astigmatism <1D
Blurred vision and defective vision
Elongation of objects may be noticed in high astigmatism
Keeping the reading material close to the eyes
Signs
Half closure of the lid
Head tilt- In an attempt to bring their axes nearer to the horizontal or vertical meridians
Investigation
Retinoscopy reveals different power in two different axes
Ophthalmoscopy - Oval or tilted optic disc in patients with high degree of astigmatism
Keratometry and Corneal Topography - Reveal different corneal curvature in two different meridians in corneal astigmatism
Astigmatic fan test -Used to confirm the cylindrical correction
Jackson's cross-cylinder test - To verify the strength and axis of the cylinder prescribed
Treatments
Optical treatment - prescription of the appropriate cylindrical lens
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