Visual Acuity Test
Visual acuity (VA) indicates the clarity of vision dependent on the sharpness of retinal focus along with interpretation by the brain. Visual Acuity Test involves a test where a person has to read letters on a standardized Snellen chart held about 20 feet away. This test is done individually for both eyes. Visual acuity test is part of the routine eye tests to check for vision. Visual acuity is expressed as a fraction where the numerator is the distance of the chart, usually 20 feet. The denominator is the distance where a person with normal eyesight can read the same line correctly. 20/20 is normal vision. Abnormal results on the visual acuity test indicate the need for corrective glasses.
Coloboma refers to a gap or defect in any of the structures of the eye such as iris, retina, ciliary body, lens, choroid and optic disc. It normally occurs at the lower quadrant of the eye and can involve one or both the eyes. It is a congenital defect and the condition occurs due to the improper development of the eyes during pregnancy. If the the child is born with coloboma, the eye pupil will be shaped like a key hole or a notch. Coloboma can occur in isolation or can occur along with other chromosomal abnormalities. Persons suffering coloboma also might have a condition called microphthalmia. Other eye abnormalities that might accompany coloboma are cataract, glaucoma, myopia, retinal detachment and nystagmus. When coloboma occurs by itself, it is described as nonsyndromic or isolated.
Normally the baby eyes in the womb will develop between 4 and 15 weeks. Before even the eye begins to form, optic fissure is formed to supply the required nourishment to the eye. Optic fissure closes by itself once the eyes are formed completely. Sometimes the optic fissure fails to seal thus leaving a gap, causing coloboma. Coloboma can also result from genetic syndrome, but not all the cases have a family history of coloboma, implying that it can occur even without genetic disorders.
Coloboma can affect different tissues of the eye. It is found in iris and ciliary body if the anterior portion of the optic fissure does not close properly. Iris coloboma is the most common defect that affects the lower part of the colored portion of the eye. Here the pupil of the eye appears like a keyhole and may also looks enlarged. Babies with iris coloboma develop 'cat eyes'. It can also appear as a split in the iris from the pupil to the edge of the iris. However the vision and the functioning of the eye are normally not affected with front eye coloboma. Children with iris coloboma may experience light sensitivity due to the large pupil. The condition may extend to the back of the eye if the posterior portion of the eye does not close properly. In such cases, the retina or the optic nerve that connects the eye to the brain may be affected. The child may have limited or no vision.
Diagnosis of Coloboma
Coloboma is often diagnosed by it's appearance as a notch or a keyhole shape of the pupil. If the coloboma is suspected, a detailed examination by the ophthalmologist is recommended to assess how much of the eye has been affected by coloboma. Diagnostic tests like visual acuity test, refraction test, and detailing of symptoms is done to aid the diagnosis.
Treatment of Coloboma
There is no effective treatment that can correct or fix the missing structure in coloboma. The condition can only be monitored from time to time and be managed effectively to enjoy the optimum possible function of the eye. In case of iris coloboma, children may be prescribed glasses or contact lenses for better vision and appearance. Sunglasses are recommended in case coloboma is causing sensitivity to light.
When the coloboma extends to deeper layers like choroid, retina and optic nerve, patients are at risk for developing retinal detachment. In such cases one has to consult a retinal specialist on a regular basis. Normally patients with retina, choroid and optic nerve colobomas are supported with low vision aids.
The macula is a vital part of the retina that is responsible for our central vision. It is involved in processing the light rays entering the central vision field. It is rich in cells called cones that convert light rays into impulses that travel through the optic nerve to the brain. Macular vision is about fine details, color and reading. Macular vision is intrinsic to our good vision. Problems in macular vision are usually due to drying out of certain areas of the macula or wet patches. Macular degeneration happens largely due to aging. Dry Macular degeneration or non-exudative macular degeneration is what is mostly noticed. It does not cause as much damage as wet macular degeneration. Symptoms of dry macular degeneration appear gradually and there is no treatment for this. Wet macular degeneration happens rather rapidly due to abnormal blood vessels in the retina. Photo dynamic therapy (laser) and anti-VEGF inhibitors are used to treat wet macular degeneration. Retinal photographs, visual acuity test, Amsler Grid and Fluorescein Angiography are tests that help determine macular vision problems.
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Bibliography / Reference
Collection of Pages - Last revised Date: April 18, 2019