Understanding Visual Acuity

• Visual acuity (VA) indicates the eye's capacity to discern and resolve fine object details, it is directly related to retinal image sharpness, which technically measures the ability to perceive fine details.
• The purpose of a visual acuity test is to quantitatively assess the capacity to identify black symbols on a white backdrop at a consistent distance, the size of the symbols varies.
• Visual acuity measures central vision and overall visual system function from the cornea to the occipital cortex, achieved by having patients identify images or read letters on a chart.

The importance of visual acuity

• Indicates refractive status, macula, and neural integrity.
• Compares the sharpness of vision between both eyes.
• Assesses an individual’s visual ability to other optometrists.
• Compares VA in the same patient at different times.
• Compares VA between different patients.

Factors Affecting Visual Acuity

• Refractive errors include diffraction and aberrations in the shape of the eyeball or cornea, and are also due to illumination and contrast.
• Refraction error within the eyeball is a primary consideration for patients with vision-related issues (ametropia).
• Visual disorders are of two types optical and neural

Types of visual disorders

• Optical factors: Form image on eye retina (reduced ability of the lens to focus light).
• Neural factors: Involve health and function of the retina, photoreceptor density, neural pathways to the brain, and interpretive faculty of the brain.

Requirements for Normal Vision

• Both eyes must be in alignment with functioning extraocular muscles.
• Clear cornea, lens of the eye and ocular media (aqueous and vitreous).
• Intact retina, optic nerve, and visual pathway.

Common references to visual acuity

• Distance acuity (e.g., "20/20 vision") signifies ability to recognize small details at a distance, which is often compromised in individuals with myopia.
• Near acuity indicates the capability to recognize small details up close, which is often compromised in cases of hyperopia.

Total eye care Examination

• Measures refractive error.
• Assesses individual's visual needs
• Prescribes suitable correction to facilitate visual tasks, using trial lenses and visual acuity charts.

Visual Acuity and Refractive Error Testing

• Tests for recording visual acuity usually include the Snellen chart and E-test.

Classification of visual acuity

• Recognition acuity-depends on minimum legible resolution and can be separated by form sense or true minimum legible.

Recognition acuity: Form sense and true minimum legible

• Form sense uses a simple form with orientation to determine acuity.
• True minimum legible uses complex patterns like letters or numerals.

Visual Acuity Principle

• Depended on the Snellen and logMAR Principles

Snellen Principle

• Objects must be separated by a minimum angle of resolution of one minute of arc to be distinguished.
• Threshold to resolve detail is represented by the foveal cone diameter (0.002mm).
• For detail to be discernible, information must be separated by at least 1 minute of arc.
• Height of a (6/6) or (20/20) letter subtends 5 min at 6m or 20 feet.
• The height of a (6/6) or (20/20) letter should be 8.73mm.

Snellen Fraction

• Snellen's fraction describes visual acuity as the ratio of the testing distance to the distance at which a normal eye can see the letter.

Common Letter Sizes on Visual Acuity Charts

• Range from 8.75 mm (20/20 or 6/6) to 175.20 mm (20/400 or 6/120).
• If a patient discerns a 35.04mm letter at 6m, their VA is the same as a normally sighted person at 24m, denoted as VA = 6/24.

LogMAR Principle

• LogMAR Principle utilizes a uniform letter size progression.
• Letters increase in a constant ratio of X1.25.
• Separation between letters is standardized and related to letter size.
• Letter size changes in units of 0.1log units per row.
• The final LogMAR score considers every read letter, with each row having five letters.
• A patient can read 5 letters if the chart is brought close enough.
• Crowding is consistent
• Results remain legitimate regardless of test distance.
• Large letters make VA assessment meaningful in low vision patients (LVPs).