Evaluation of Visual Acuity

Visual acuity is a crucial aspect of the ophthalmic examination. A complete 8-point eye examination includes testing visual acuity, pupillary examination, evaluation of ocular motility and alignment, intraocular pressure measurement, confrontation perimetry, external examination, slit-lamp examination, and examination of the ocular fundus.  Visual acuity is a quantification of the sharpness or clarity of vision. Three terms frequently used when documenting the visual acuity portion of the ophthalmic examination are the Latin abbreviations OU, OS, and OD. The full form of these abbreviations is as follows: Oculus uterque (OU) - both eyes. Oculus sinister (OS) - left eye. Oculus dexter (OD) - right eye. Other abbreviations include DVA (distance visual acuity), IVA (intermediate visual acuity), and NVA (near visual acuity). The distance for near and intermediate should be mentioned. The DVA, IVA, and NVA may be uncorrected (UDVA, UIVA, UNVA), corrected (CDVA, CIVA, CNVA), or binocular (BDVA, BIVA, BNVA). The numbers obtained from a visual acuity examination are all based on the visual clarity of a normal person at a distance of 20 feet or 6 meters. Suppose visual acuity is documented as 20/20 or 6/6. In that case, this number describes that the patient being examined possesses vision equivalent to that of a normal-sighted person at a distance of 20 feet or 6 meters. If vision is documented as 20/40 or 6/12, a person with this visual acuity can seean image or object at 20 feet that a normal-sighted person could have recognized at a distance as far as 40 feet or at 6 meters compared to 12 meters, respectively. Visual acuity specifies a threshold. The smallest appreciable retinal image determines the acuity of vision and is the measure of the tiniest object visible at a certain distance. To appreciate the form of an object, its several parts must be recognized. The angle created at the eye's nodal point by the smallest resolvable object is known as the minimum angle of resolution (MAR). Minimum (detectable) visible acuity: This signifies the detection of an object's presence or absence, quantified by the visual angle made at the nodal center of the eye by the tiniest detectable object. Minimum (separable) resolvable acuity: This indicates the smallest separation between 2 parts of an object or 2 different objects resolvable as 2 different parts by the visual system. A person of normal sight, measured as 20/20 Snellen equivalent, can differentiate 2 objects that cast a visual angle of 1 minute (60 seconds or 0.017 degrees). Resolvable acuity is determined by the spacing of cones in the retina. However, some people can resolve a stimulus as small as 30 seconds of arc. Minimum recognizable acuity: This is denoted by the smallest feature that can be identified or recognized, such as identifying a letter. Although 20/20 is considered the gold standard minimum recognizable acuity in humans, the mean visual acuity in age groups from 18 to 80 may be better than 20/20. The sharpest mean visual acuity was noted in individuals aged 25 to 29. Minimum discriminable acuity: This refers to the smallest change in appreciable orientation, position, or size, also known as hyperacuity. The smallest misalignment we can realize is Vernier acuity, named after Pierre Vernier, who invented a scale used to navigate ships. Humans can proficiently detect whether adjacent lines are aligned (as in the Vernier scale), leading to the widespread use of this scale. The Vernier threshold for humans can be as low as 2 to 5 seconds of arc.