Cranial Nerves: The Eye 1 – Pupils, Acuity, Fields

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Cranial Nerves

The Eye 1 – Pupils, Acuity, Fields

BACKGROUND

Examination of the eye can provide very many important diagnostic clues for both general medical and neurological diseases.

Examination can be divided into:

4 Fields

The organisation of the visual pathways means different patterns of visual field abnormality arise from lesions at different sites. The normal visual pathways are given in Figure 7.1.

image

Figure 7.1 Visual pathways

The visual fields are divided vertically through the point of fixation into the temporal and nasal fields. Something on your right as you look ahead is in the temporal field of your right eye and the nasal field of your left eye.

The visual fields are described from the patient’s point of view.

Field defects are said to be homonymous if the same part of the visual field is affected in both eyes. This can be congruous (the field defects in both eyes match exactly) or incongruous (the field defects do not match exactly).

Testing the fields is very useful in localisation of a lesion (Table 7.1).

Table 7.1

Testing the visual fields

Type of defect Site of lesion
Monocular field defect Anterior to optic chiasm
Bitemporal field defect At the optic chiasm
Homonymous field defect Behind the optic chiasm
Congruous homonymous field defect Behind the lateral geniculate bodies

The normal visual fields for different types of stimuli are very different. The normal field for moving objects or large objects is wider than for objects held still or small objects. The normal field for recognition of coloured objects is more limited than for monochrome. It is useful to test this on yourself. Look straight into the distance in front of you and put your hands out straight to your side. Wiggle your fingers and, keeping your arms straight, gradually bring your arms forward until you can see your moving fingers. Repeat this holding a small white object, and then with a red object until you can see that it is red. You will appreciate the different normal fields for these different stimuli.

2 PUPILS

WHAT TO DO IN A CONSCIOUS PATIENT

(For pupillary changes in an unconscious patient, see Chapter 27.)

Look at the pupils.

Shine a bright light in one eye.

Place your finger 10 cm in front of the patient’s nose. Ask the patient to look into the distance and then at your finger.

Look at the pupils for their reaction to accommodation.

WHAT YOU FIND

See Figure 7.2.

FURTHER TESTING

3 ACUITY

WHAT TO DO AND WHAT YOU FIND

Can the patient see out of both eyes?

Acuity can be tested in several ways.

(i) Using Snellen’s chart

For example: 6/6 when the letter is read at the correct distance or 6/60 when the largest letter (normally seen at 60 m) is read at 6 m, or 20/20 and 20/200 when these acuities are measured in feet.

(iii) Using bedside material such as newspapers

Test as in (ii) and record the type size read (e.g. headlines only, all print).

If unable to read largest letters:

See if the patient can:

Ask the patient to look through a pinhole made in a card.

If acuity improves, the visual impairment is refractive in origin and not from other optical or neurological causes.

A new development

Ophthalmologists are increasingly using LogMAR (logarithm of the minimum angle of resolution) charts to measure acuity. There are a number of different LogMAR chart designs. They are read in the same way as a Snellen chart. However, the result is expressed as the logarithm of the minimum angle or resolution, which in turn is the inverse of the Snellen ratio. For example, for Snellen acuities:

4 FIELDS

WHAT TO DO

Assess major field defects

If one side is ignored when both fingers are moved together but is seen when moved by itself, then there is visual inattention.

Test each eye individually

What to test with?

Large objects are more easily seen than small objects; white objects are more easily seen than red. Thus, fields will vary according to the size and colour of the target used.

Central vision is colour (cones) and peripheral vision is monochrome (rods).

A combination of wiggling fingers (described above) and red pin provide the most sensitive and specific bedside test for field defects.

Once you find a field defect

Define the edges.

Bring the pin from where it cannot be seen to where it can be seen.

imageTIP

The edges are often vertical or horizontal (Fig. 7.6).

When there is a homonymous hemianopia

The macula needs to be tested.

Bring the pin horizontally from the side with the defect towards the point of fixation.

Describe the field loss from the patient’s point of view.

Central field defects—scotomas—and the blind spot (the field defect produced by the optic disc) are usually found using a red pin.

imageTIP

If a patient complains of a hole in his visual field, it is often easier to give him the pin and ask him to place it in the hole in his vision.

WHAT YOU FIND

See Figure 7.7.