Ординатура / Офтальмология / Английские материалы / Oxford American Handbook of Ophthalmology_Tsai, Denniston, Murray_2011

ASSESSMENT OF VISION: ACUITY (2) 7

Assessment of vision: acuity (2)

Distance acuity (cont.)

LogMAR charts

This records the logarithm of the minimum angle of resolution (LogMAR). Based on the Bailey–Lovie logMAR chart, the actual chart in common usage is the Ferris modification known as the ETDRS chart (Fig. 1.2). LogMAR testing has marked advantages over Snellen, notably that 1) all letters are equally legible; 2) it controls the crowding phenomenon with 5 letters on each line and appropriate separation; and 3) there is a logical geometric progression of resolution. Starting with the LogMAR 1.0 line (Snellen 20/200), each letter is read. It is usually read at 20 feet. Each correct line (worth 0.1 units) or each correct letter (worth 0.02 units) is subtracted from 1.0 to give the final score.

Table 1.2 Distance acuity scoring systems

Crowding is a phenomenon by which neighboring targets interfere as proximity increases. Amblyopic patients are particularly susceptible and may score better with single optotype tests (e.g., Sheridan-Gardiner), than on a multiple test (e.g., Snellen). This has led to the use of multiple optotype forms of letter matching or picture tests.

Although other tests may approximate to a Snellen acuity reading, they are not exactly equivalent. It is therefore important to document which test has been used.

Pinhole acuity

A pinhole (stenopeic aperture, 1.2 mm diameter) can neutralize up to 3D of refractive error.

Near (reading) acuity

Various charts are available. Most have paragraphs of text that are read by the patient at his/her usual reading distance (usually around 30 cm). The notation used is N, this corresponding to the point size of the text being read. The range of the booklets is from N5 to N48.

10 CHAPTER 1 Clinical skills

images (e.g., bird + cage l bird in a cage), to the fusion of two images (e.g., rabbit with tail + rabbit with flowers lrabbit with tail and flowers), and finally to the perception of depth in a fused image (e.g., two disparate images of a bucket l 3D bucket). Normal disparity perceived is 60 sec of arc but may be up to 15 sec.

Figure 1.3 Schematic example of Keeler acuity cards.

Table 1.3 Tests of binocular status

ASSESSMENT OF VISION: CONTRAST AND COLOR 11

Assessment of vision: contrast and color

Contrast sensitivity

While visual-acuity charts (e.g., Snellen) test high contrast (black letters on white background), most daily visual tasks require resolution of low to medium contrast. Contrast sensitivity may be reduced in the presence of normal Snellen acuity. It may be measured by a number of charts, all of which score the minimum contrast detectable for a specified target size.

The Vistech chart has rows of broken circles that decrease in contrast across the row and diminish in size from row to row. Identification of target orientation is plotted on a template to give a graph of contrast vs. spatial frequency. Charts are available for use at 1.5 feet and 10 feet.

Alternative charts maintaining a constant target size include the Pelli– Robson chart (triplets of capital letters, read until 2 or 3 mistakes in 1 triplet; Fig. 1.4) or Cambridge chart (square wave gratings, usually read at 20 feet, forced choice as to which of two luminance matched pages the grating is on).

Color vision

Red desaturation

The perception of redness (e.g., of a red pin) in both eyes is compared, occluding one at a time. This can be done for central vision (reduced in an optic neuropathy) or peripheral field (bitemporally reduced in a chiasmal lesion). An approximate score can be assigned by the patient to the washed-out image in relation to the normal image, e.g., 50%.

Ishihara pseudo-isochromatic plates

These are used at 2 feet under good illumination in patients with VA20/60. The first test plate (seen even by achromats with sufficient acuity) is followed by a series of plates testing red–green confusion. Some of the plates differentiate whether the defect is of the protan (red) or deutan (green) system. It does not test the tritan (blue) system. Patients with congenital red–green color blindness (protanopia, deuteranopia) tend to make predictable mistakes, whereas in acquired disease (optic neuropathy), the mistakes do not follow a specific pattern.

Hardy–Rand–Rittler plates

These are less commonly used, but they have the advantage of testing tritan as well as protan and deutan discrimination.

Farnsworth-D15 test

This is a test of confusion, giving limited information on the protan, deutan, and tritan systems. It may be used as a screening test of color vision (e.g., for military personnel).

12 CHAPTER 1 Clinical skills

Farnsworth–Munsell 100-Hue test

This is a time-consuming test of color discrimination in which the patient attempts to order 85 colored caps by hue. When this is plotted onto a dedicated chart, it provides detailed information on protan, deutan, and tritan systems. This test is often used as the final arbitrator for color vision–requiring professions.

V R S K D R N H C S O K S C N O Z V C N H Z O K N O D V H R C D N Z S V

Figure 1.4 Schematic example of Pelli–Robson chart.

14 CHAPTER 1 Clinical skills

The beam height and width are adjusted by apertures; the beam height is recorded (in mm) and may be useful in measurement (e.g., disc size, corneal ulcer, etc.).

require different angulations of the illu-

the beam from the center of the field of view. Tilting the beam vertically may reduce troublesome reflections when using handheld lenses.

Illumination: mirrors

In certain situations, such as when using small angulations (3–10°), the standard long mirror may partially obscure the view. If this is troublesome, it can be replaced by the short mirror.

Figure 1.8 Mirror.

Fixation lamp

Many slit lamps have a fixation target, either a standard fixation lamp or an annular target with a focusing range of –15 to +10. This is adjusted to the patients’ refractive error, enabling them to see the target clearly.

Stereovariator

Some slit lamps have a stereovariator that changes the angle of convergence from 13° to 4.5°. The conventional 13° provides better stereopsis, but the 4.5° provides a larger binocular field of view and thus improved acuity (binocular acuity > monocular acuity). This means that the 4.5° setting may be advantageous for detailed examination of certain ocular surfaces (e.g., corneal endothelium).

BIOMICROSCOPY: USE OF THE SLIT LAMP 15

Biomicroscopy: use of the slit lamp

Box 1.3 Outline of slit-lamp examination

Set-up

•Adjust patient chair, slit lamp (Fig. 1.9), and your chair so that you and the patient can be comfortable during the examination.

•Adjust the chin rest until the patient’s eyes are at the level of the marker (on the side of the head rest).

•Adjust the eyepieces:

1)Dial in your refraction: use the nearer scale for the 10x eyepieces and the further scale for the 16xeyepieces;

2)Fine-tune eyepieces: focus each eye in turn on a focusing rod placed in the central column (requires removal of the tonometer plate); this may be more “minus” than expected because of induced accommodation.

•Adjust the interpupillary distance.

Examination

•Start examination with lowest magnification (1xsetting and 10x eyepieces) and low illumination. Rather than inadvertently dazzling your patient, first test brightness (e.g., on your hand).

•Start examination with direct illumination (usually fairly thin beam, angled 30–60°).

•Examine in a methodical manner from outside in, i.e., orbit/ocular adnexa, lids, anterior segment, posterior segment.

•Throughout the examination:

1)Adjust illumination: adjust filter, orientation, and angulation

and illumination technique (direct illumination, retroillumination, scleral scatter, specular reflection) to optimize visualization.

2)Adjust magnification: to optimize visualization (e.g., of cells in the anterior chamber).

•At the end of the examination, do not leave your patient stranded on the slit lamp. Switch the slit lamp off (for the sake of the patient and the bulb) and encourage the patient to sit back.

Additional techniques

•Tonometry: Goldmann tonometer with fluorescein and blue light.

•Gonioscopy and indirect funduscopy: performed with appropriate handheld lenses.