How to Measure Vision
Shivan

Shivan

How to Measure Vision

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One of the five vital signs of the eye is vision or visual acuity. The other four include pupil response, intraocular pressure, extraocular motility, and confrontational visual fields1. We will be covering the importance of visual acuity and how to measure it.

Why Visual Acuity?

Visual acuity or more strictly, distance visual acuity is measured at almost every optometric and ophthalmic examination. It provides an indication of how well an individual can recognise detail at a distance. For example, being able to recognise street signs at a distance while driving. 

Visual acuity is an important screening tool to determine if an individual requires glasses or if they have any eye diseases such as macular degeneration, and cataract. It aids in providing us an ability to grade its functional severity – an important tool when triaging patients.

However, it will not pick up some other sight-threatening conditions like early-stage glaucoma; this is where a full eye examination is useful.

How to Measure Vision?

Originally, visual acuity was measured using a Snellen chart, developed by Dutch Ophthalmologist, Herman Snellen. Today, the LogMAR chart (also known as Bailey-Lovie chart or Early Treatment Diabetic Retinopathy Study/ETDRS) chart is used due to better accuracy.

The logMAR chart is used for the bulk of the population. Exceptions include those who are illiterate, those who cannot recognise English letters, and very young children (under 5 years).

Steps:

  1. Set the patient at an appropriate distance. The vision chart will normally have a distance rating, usually for 3 m, 4 m, or 6 m. This distance should be measured accurately using a tape measure and marked on the floor for the patient to stand. You may have noticed rooms with a mirror on the wall and a letter chart on the opposing wall. This is a way of doubling a room’s optical distance. For example, a 3 m room with a mirror makes it into a 6 m room optically. 6 m is used as this is considered close to optical infinity or distance. 3 m and 4 m, though not perfect, are close enough, especially for screening. If the chart is not illuminated (chart on the wall as opposed to a computerised system, the room must be well lit. 1300 lux is the standard.
  2. Cover one eye and instruct to read the smallest line. Occlusion of an eye can be performed with the palm of the hand or with a paddle. You can get the patient to hold the paddle or you can hold the paddle in front of the eye yourself. It is important to make sure that the patient has completely occluded their eye. Normally, the left eye is covered and the right eye is tested first as by convention. Additionally, you may have to encourage guessing to make sure the patient is trying. It should be to a point where they struggle. It might also help to get the patient warmed up and read the first letter of each line until error and then testing the line above the one they made an error on and to continue to the smaller line until failure.
  3. Note this down and measure vision for the other eye. Get the patient to read the lines backward as to combat against memorisation. Then test both eyes together.

Other notes: 

  • Generally, a vision check with glasses on is more useful. If the vision is reduced this can be down to a change in their prescription or some eye disease.
  • For children, it might be better to start reading the lines without occlusion. Reading the letters with both eyes together first can improve confidence.

Notation

After, you will get a result for the right eye, left eye and both eyes together. It is important to record this using Snellen notation.

You might have heard of 20/20 vision. This is known as the Snellen notation in feet, which is the most common. There are other forms of notation such as logMAR, but this is mainly used for analysis in research and will not be covered. 

How to Measure Vision
Snellen Fraction

The 20/20 is actually in the imperial form (feet). Since we are in New Zealand, we use the metric version of 6/6 (20 ft ≈ 6 m). The numerator or top number represents the test distance. The denominator or bottom number of the fraction represents the distance at which the detail of the letter would subtend 1 minute of arc.

Angle subtending 1 minute of arc of a letter’s detail

In layman terms, if a person at normal sight was at this distance of the denominator. This is what the person being tested can see at the numerator distance. In other words, 6/60 for the person being tested, they are seeing the same as a person who has normal sight who is at 60m – so this can be considered quite bad! If the denominator is smaller vision is better and if it is larger, the vision is worse.

Each line will have a particular notation (e.g. the 6/24 line or the 6/6 line).

It is also important to note if the patient is wearing their spectacles or contact lens correction. Aided describes with correction and unaided describes without correction.

The smallest line read is recorded. If a few letters were missed or some letter from the smaller lines was read this is recorded as well. This is best described by example. A patient, who does not wear glasses or contacts, identifies 3 out of 5 letters correctly on the 6/6 line with his right eye, and they are able to guess 1 out of the 5 letters smaller than the 6/6 line in addition to the 6/6 line also with their left eye. The notation would be:

Right Eye Unaided VA  6/6 -2 
Left Eye Unaided VA 6/6 +1

The -2 and +1, in this case, represent the 2 letters missed on the 6/6 line with the right eye only seeing and the extra letter seen with the left eye seeing.

What to expect normally:

  • 6/6 – this is considered the gold standard of vision (“20/20”)
  • 6/12 – the requirement for drivers licensing private car and motorbike
  • 6/9 – the requirement for driver licensing heavy trade

If you are measuring vision worse than the chart can possibly give (normally the 6/60 line is the worst line on most charts), formally, you would have to reposition the patient by getting them closer to the chart. However, this would involve recalculating the Snellen fraction which is quite complicated. If we measure the vision to be this bad, a precise measure is not valuable in a triaging situation. So, we can use alternative measurements.

Alternative measurements include ranking from best to worse worse to worst:

  • Counting Fingers (CF) – the patient is able to count the number of fingers in front of them.
  • Hand Motions (HM) – the patient is able to see the hand waving in front of them.
  • Light Projection (LProj) – in a dark room, using a pen torch, shine light from a different direction and the patient is able to correctly identify the location of the light.
  • Light Perception (LP) – the patient is able to tell there is light or not in a dark room but is unable to tell what direction it is coming from.
  • No Light Perception (NLP) – the patient is not able to respond to visual stimuli at all.

If the patient is illiterate or cannot communicate, other optotypes (character used for testing) have to be used, for example, Illiterate E.

Example of Illiterate E Optotype

Pinhole Test

If the vision is reduced, the pinhole test is a good indicator to determine if the cause is due to a change in prescription or there is a pathological issue.

The pinhole test involves putting up a pinhole in front of the patient’s viewing eye. It might be better if the patient positions the pinhole. The pinhole reduces the unfocused, light scattering peripheral rays entering the eye leaving the more-in-focus central rays. This results in a clearer image if the patient has a prescription. 

An improvement in vision is a good indicator that the reduction in vision is caused by a prescription or change in prescription. If there is no improvement, this would indicate eye disease or amblyopia (lazy eye). 

However, pinhole testing is only a guide and relying on it is never a good idea. A full eye examination is always required to determine prescription and health.

Reading Vision

Near visual acuity is an assessment of how well a patient can recognise letters when reading up close. It is generally used in an eye examination for working our a prescription for reading. It is usually left out of preliminary testing as distance visual acuity is sufficient to measure the ability of an eye to recognise detail.

Near visual acuity is important as certain hobbies rely heavily on the state of near vision such as reading, crafting, etc. Near vision affects learning as well as work when reading is required or being able to see detail up close.

Usually, an eye prescription affects distance vision and reading vision differently. However, eye health in most cases affects both distance and reading vision at the same level.

Myopia or short-sightedness can cause poor distance vision but good reading vision.

Presbyopia results in a deterioration in reading vision with age. The lens in the eye changes focus to allow one to focus up close and far away. This is known as accommodation. At the ages of 40 to 50 years, the lens’s ability to change focus diminishes and this results in poor vision for reading – Presbyopia. Reading glasses are usually the remedy for this.

When testing for reading vision or near visual acuity a special reading card is given for the patient to hold and read.

The card will have a recommended distance of use. This is normally at the average working distance of 40cm2.

The patient is normally asked to read the smallest line possible. Normally, both eyes are tested together and testing with one eye occluded is rarely used since most people read with both eyes together, and distance visual acuity is used to pick up any problems of either eye.

There are 3 forms of notation: M, N and J. N notation is what we generally use. This is the notation, which is the point size for Times New Roman font. There is a linear scale, so N8 is twice as big as N4. 

The distance of testing is also measured. For example, N5 @ 40cm. The distance is important and varies with smartphone use tending to be closer than reading and desktop computer use being slightly further away.

The important line is N8 at 40cm – this is the size of the text in most newspapers and books; we like to aim at N6 for comfort with reading. Any reduction would require a reading prescription and further investigation through a full eye examination.

Summary

Visual acuity is the fundamental measurement of how well a person can recognise detail. Distance visual acuity can be used as a screening tool to see if there is a prescription present or monitoring the severity of eye disease. A full eye examination is always required to confirm the presence of both of these. Near visual acuity provides a good idea of what a person can see up close, which can affect their lifestyle.

References
  1. Pearls in Opthalmology for Emergency Nurse – Journal of Emergency Nurse[]
  2. How to prescribe spectacles for presbyopia – Rènée du Toit[]

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