Ocular Examination

Published on 08/03/2015 by admin

Filed under Opthalmology

Last modified 08/03/2015

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1 Ocular Examination

VISUAL ACUITY

The measurement of visual acuity is the first essential part of any ocular examination and, although the examination technique is simple, the process being assessed is complex and requires the interaction of many factors, both physiological and psychological. Assessment of visual acuity requires the eye to detect the object and resolve it into its component parts. This information is then transmitted to the cerebral cortex where it is matched against existing memory shapes. The patient must then be able to communicate recognition of the object to the examiner. Physiologically, visual acuity measures the capability of the visual system to resolve a target; this is dependent on three main factors: the background illumination, the contrast of the target to the background and the angle that the target subtends at the nodal point of the eye.

In theory the eye has a maximal resolution of 1 minute of arc at the nodal point. In practice, young people normally have a better acuity than this at 20/15 (6/5) which corresponds to the spacing of individual cones in the foveola. Although visual acuity is primarily a function of cones the degree of visual processing in the retina must be considered and, in particular, the receptive fields of the retinal ganglion cells. In the foveola there is a 1:1 relationship of cones to ganglion cells but this increases rapidly more peripherally. There is an increasing loss of visual acuity with age so that in old age 20/30 (6/9) or even 20/40 (6/12) may be considered normal.

Although distance acuity is normally measured clinically near vision is in some ways more important in the daily life of the patient. Near vision is tested by reading test print of standardized sizes with the appropriate spectacle correction and good illumination. Factors of accommodation and magnification are important in the assessment of near vision and the correlation between distance acuity and near acuity is not always good. Patients with 20/60 (6/18) distance vision can often manage to read print of J3(N5) size, provided their macular function is normal. There appears to be a large redundancy of nerve fibres in the visual pathways: probably only approximately 15 per cent of the optic nerve fibres are actually required to be able to read 20/30 (6/9).

Table 1.1 shows the pathological and physiological factors that can limit visual acuity. This process can be influenced by physiological and pathological factors anywhere along this pathway.

Background illumination alters the level of retinal adaptation. Low levels of light stimulate the rod system; the receptor density and level of retinal integration of this system are less than that of the cones and consequently acuity is also low. At high levels of illumination the cone system is stimulated and acuity is maximal. To obtain the best visual acuity illumination should be in the optimal photopic range. Because of the effect of reduced retinal illumination from lens opacities in patients with cataract may be seeing in the mesopic to low photopic range where the acuity is proportional to background illumination. In these patients, an increase in the ambient lighting will give them better vision provided that light scattering by the cataract does not counter this.

MEASUREMENT OF VISUAL ACUITY

Visual acuity is usually measured at a distance of 6 metres (20 feet) to eliminate the contribution from accommodation. It should be performed on each eye in turn without and with full refractive correction. Acuity is usually measured at high contrast. The visual angle refers to the angle subtended by an object at the nodal point of the eye; it depends on the size and distance of the object from the eye. The normal limit of resolution is 1 minute of arc but some individuals see better than this possibly due to a finer cone mosaic, better image processing in the retina or cortex, or fewer optical aberrations.

TESTING ACUITY IN CHILDREN

Visual acuity assessment in children presents particular problems. Good results can be achieved only with time and patience and by selecting the right test for the age of the child. These include qualitative tests such as the child turning to fixate a face or light, suppression of optokinetic nystagmus following rotation or objecting to occlusion of one eye. While semiquantitative measurements are available, for instance picking up ‘hundreds and thousands’ sweets or following small balls quantitative tests are most informative. For infants forced-choice preferential looking or visual evoked potentials (VEPs) can be used; both give different results. Older verbal children can use picture cards (Cardiff cards, Kay’s pictures) and from the age of three may manage matching letter tests (e.g. the Sheridan–Gardiner test; see Ch. 18). Caution is necessary when using Snellen charts with single letters because of the phenomenon of ‘crowding’ – being able to see single letters more easily than rows of letters – which can overestimate true acuity.

PHYSIOLOGICAL LIMITATION OF ACUITY

The physiological limits of visual acuity are essentially set by the sources of error in the system uncorrectable by standard refraction. Light rays passing through the eye are degraded by inbuilt optical aberrations, thereby increasing the blur at the margins of the images. This loss in edge contrast reduces the resolving power of the visual system. Apart from refractive error (sphere and cylinder), the main optical factors are higher-order aberrations, chromatic aberration and diffraction. Glare disability is produced from forward light-scatter from the ocular media and opacities. It casts a veiling luminance over the macula, reducing image contrast. A good clinical example is posterior subcapsular cataract where acuity is relatively well preserved but the patient has a disabling glare in bright light.

WAVEFRONT ANALYSIS

Wavefront analysis plots the total optical aberration of the eye. The low-order aberrations of sphere and cylinder can be corrected by simple optics; higher-order aberrations cannot be corrected by routine refraction. These used to be referred to as ‘irregular astigmatism’ and, in the case of irregular corneal astigmatism, can be corrected only by wearing a contact lens. Wavefront analysis allows detailed analysis of these aberrations; it has become important in understanding patient dissatisfaction following refractive surgery and, by correcting aberrations, offers the possibility of supranormal vision. This has yet to be achieved.

CONTRAST SENSITIVITY

The eye can detect objects by responding to the differing levels of luminance between a target and its background. This is defined in terms of the maximum and minimum luminance at the detected edge.

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Standard visual acuity tests measure acuity under high contrast conditions but do not tell us anything about visual performance under different circumstances such as driving at night or reading in poor light which are often more appropriate to daily life and cause clinical symptoms. It is thus possible for patients to retain good Snellen acuity but have reduced contrast sensitivity at lower levels of illumination. Contrast sensitivity testing is of particular importance in assessing the effect of refractive surgery on visual performance. It can be measured at either a fixed target size with varying contrast or over a range of target sizes (spatial frequencies) and contrast to derive a contrast sensitivity curve, which is an extremely useful way to assess overall visual performance. There are a number of different ways to test contrast sensitivity; they fall into two groups – either differentiating bars, stripes and gratings or, alternatively, letters against a background. Letter tests usually produce a better performance than gratings.

COLOUR VISION

The perception of colour arises from the different cone receptors that are maximally sensitive at three separate wavelengths: red (protan), green (deuteran) and blue (tritan). Light of different wavelengths stimulates each of the cone populations to a different degree so that colours within the visible spectrum arise from their own specific pattern of cone stimulation. Colour brightness (luminosity) and saturation (amount of white light present) are other properties detected by the eye that must be taken into account in colour testing. As colour vision depends on cones it is a property of photopic central vision.

ABNORMAL COLOUR VISION

Abnormal colour vision can either be congenital or acquired; acquired causes include macular and optic nerve damage. Kollner’s rule states that optic nerve disease tends to affect the red–green axis whereas macular damage affects the blue–yellow axis. There are many exceptions to this rule, such as glaucoma and autosomal dominant optic atrophy, which affect the blue–yellow axis; and Stargardt’s disease, which primarily affects the red–green axis. Patients who have abnormal cone populations are not able to match some colours visible to a patient with normal anatomy but have a normal ability within other spectral areas. The most common type of colour deficiency is anomalous trichromacy in which the person has three cone populations but is deficient in one of them. Thus with protanomaly the person is deficient in red cones and needs excess red to match yellow; deuteranomaly requires more green. Dichromats have only two cone systems and thus cannot distinguish certain colours. Protanopes have absence of red cones, deuteranopes green cones. These anomalies are transmitted on the X chromosome and affect about 7 per cent of Caucasian males. Tritanomaly is very rare. These patients have difficulty in distinguishing turquoise blues and greens or yellows and pinks from one another. Achromatopsia is the complete absence of cones. These patients can distinguish colour only in terms of brightness; they have photophobia and poor vision. All patients with congenital colour defects have normal fundi.

Several clinical tests exist for the assessment of colour vision. These include pseudoisochromatic test plates, hue-matching tests and anomaloscopes; the former are the quickest and easiest to use. The most common is the Ishihara test for red–green defects; other types are the Hardy–Ritcher–Rand (HRR) series or the City University System which have the important advantage of testing the blue–yellow axis as well as red–green. If pseudochromatic plates are to be used properly in testing cone function they must be viewed at 75 cm in controlled white light and luminance conditions with appropriate refractive correction. Hue-matching tests, such as the Farnsworth–Munsell test, are more accurate but more time consuming. Testing colour contrast sensitivity with computerized systems is a sensitive, reliable and valid research tool.