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a tactile or verbal response. Other tests such as pupil response measurement or ocular pressure measurement are automatically performed with the subject not required to give any verbal or tactile response. An ocular examination will give the ophthalmologist (eye doctor) an indication on the health of the various structures in the eye. Finally, electrodiagnostic techniques are available to obtain objective measurements of function from different stages of the visual pathway.

3.2 Visual Acuity

An assessment of a patient’s visual acuity is commonly performed in high street opticians or in ophthalmology departments. The procedure is easy to perform and gives a subjective measure of central vision. The most common method is the Snellen visual acuity test. The test should be performed in a room at least 6 m long although smaller rooms can be used if mirrors are employed to increase the viewing distance. The Snellen chart consists of graded letter sizes. Let us consider the construction of a modern Snellen visual acuity chart. The letter sizes are based on a construction from a 5×4 grid. For example, Figure 3.1 shows the construction of the letter ‘U’.

The Snellen distance is defined as the distance, d, at which the basic construction unit of a letter subtends 1 of arc. Thus, if the basic construction unit size is denoted h(m) and the distance, d(m), then to subtend 1 of arc the relationship needed is

tan(1 ) = h /d

and this re-arranges to give the Snellan distance, d, as

d = h /tan(1 )

For example, a letter size of 35 mm can be constructed from a basic unit size (h) of 7 mm and this unit would subtend 1 of arc at a distance, d, given by

d = h /tan(1 ) = 0.007 m 2.909 × 10−4 = 24.06 m

Figure 3.1. Construction of a Snellen letter

Figure 3.2. Snellen visual acuity chart (not to scale)

Thus, the Snellen size for this letter is therefore 24 m.

The full Snellen chart is shown in Figure 3.2. The letters of the chart are graded in size, based on the Snellen size as described above. For example the top line has a letter of size 87 mm which equates to a Snellen size of 60 m. The second line has letters of Snellen size 36 m and so on down to a Snellen size of 5 m for the bottom line of the chart.

3.2.1 Using the Chart

The patient will normally stand or sit at a viewing distance of 6 m and read the letters. If for example the patient could read down to line 5 without errors then the visual acuity will be recorded as 6/12. If the patient could read to line 7 but got 2 letters wrong on this line then the visual acuity will be recorded as 6/6-2. A recorded acuity of 6/6 is considered normal. In the USA acuity values are quoted in feet rather than metres with 20/20 vision considered as normal visual acuity.

The nurse or optician will often repeat the procedure with the eye covered by a card with a pinhole through which the patient views the chart. This gives a method of assessing the best corrected vision. Viewing the chart through a pinhole eliminates any refractive errors of the peripheral cornea or the crystalline lens of the eye and the acuity measured in this way simulates that with proper glasses in place.

3.2.2 Variations in Measuring Visual Acuity

Another common method of measuring visual acuity and an alternative to the Snellen acuity test is the use of the Bailey–Lovie chart. This chart has the same

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Table 3.1. Snellen–LogMAR conversions. The table gives the equivalent MAR and LogMAR values for a particular Snellen acuity

number of letters on each line which eliminates any differences due to crowding effects. The letter sizes are also graded in a different way and make use of the minimum angle of resolution (MAR). A Snellen acuity of 6/6 gives a MAR value of 1 and a Snellen acuity of 6/12 gives a MAR value of 2 (12/6). These values are converted to a logarithmic scale as shown in Table 3.1.

LogMAR acuity is often preferred in the scientific literature as it is more meaningful to perform statistical analysis on this type of data. In practice a Bailey–Lovie chart will use letter sizes in increments of 0.1 log units and will therefore not correspond exactly to Snellen acuity sizes. For example a LogMAR acuity of 0.5 will correspond to a Snellen value of 6/19 which is not normally available on a Snellen chart.

For patients who are unfamiliar with the alphabet a single optotype Landold C chart can be used This letter C is constructed in such a way that the gap is equivalent to 1 unit of a Snellen acuity letter. The orientation of the letter is varied and can be rotated through 90◦, 180◦ or 270◦. The patient is asked to identify where the gap is: top, bottom, right or left (Figure 3.3).

Children will be shown picture acuity cards. This will be a card with a single cartoon picture of a familiar object such as a house, dog or a car. The child is simply asked to identify the picture. The individual cards have the same set of pictures of variable sizes equivalent to the size of the Snellen letters.

Figure 3.3. Single optotype chart