History and exam
Key diagnostic factors
common
presence of risk factors
Key risk factors include East Asian, Hispanic, Brazilian, or Native American ethnicity; certain genetic syndromes; and history of eye trauma or surgery.
blurred vision
Visual acuity may remain acceptable with low degrees of astigmatism, due to an almost constant action of accommodation. However, with high degrees of astigmatism, vision is usually significantly impaired and sometimes cannot be brought to normal, even with the appropriate correction.
Other diagnostic factors
common
asthenopia (eye strain)
Frontal and temporal headaches, tearing, and eye strain are common in people with astigmatism. It is probably due to the need for constant accommodation in an attempt to achieve clearer vision. Even a low-degree astigmatism can produce asthenopia.
uncommon
distorted retinal reflex
A scissors motion reflex or otherwise distorted reflex on retinoscopy is an early sign of high-powered astigmatism or keratoconus. It is possible to view this distorted reflex even through an undilated pupil, though it is more obvious with a dilated pupil.[36]
Munson's sign
In advanced keratoconus, due to the conic configuration of the cornea, the lower lid protrudes on down gaze.[36]
Risk factors
strong
East Asian, Hispanic, Native American, African-American, or indigenous Brazilian ethnicity
Several studies have reported increased risk for astigmatism among Hispanic, African-American, and Asian children.[10][11] Asian people have a relatively high prevalence of astigmatism, perhaps due to the greater tightness of the Asian eyelids and narrower palpebral apertures.[2]
One prospective cohort study indicated that 19.3% of Singaporean children had astigmatism with a cylinder power of ≥1 dioptre.[12] In preschool Chinese children, a prevalence of 21.1% was found, and follow-up evaluation in a subset of these children showed that a significant percentage of children had stable and even increased astigmatism.[13] Native American people have an increased prevalence of high levels of astigmatism (>1 dioptre), possibly due to hereditary and nutritional factors.
Astigmatism has been reported to be a prevalent refractive error in indigenous Brazilians, and other Brazilian populations.[14][15][16]
eye trauma or surgery
genetic syndromes
Several genetic syndromes are known to be associated with an increased prevalence of astigmatism, including Down's syndrome and Treacher-Collins syndrome.
The high prevalence of astigmatism in these patients is believed to result from the characteristic shape of the eyelids in these conditions, which causes pressure to be exerted on the cornea.[19][20]
eyelid pathology
Corneal distortion and astigmatism may be related to certain eyelid pathologies, among which are chalazia, eyelid haemangiomas, ptosis, or eyelid surgeries. The mechanism is through eyelid pressure on the cornea.[21]
weak
positive family history
Studies into the genetics of astigmatism present conflicting results. One study comparing monozygotic and dizygotic twins found that the genetic contribution to astigmatism is low.[17] Subsequent studies conducted in northern Europe found that the correlation in monozygotic twins for astigmatism is greater than in dizygotic twins, suggesting a significant genetic effect.[18][25]
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