132 3 Sight Measurement

3.9 Chapter Summary

A variety of test procedures and investigations are available to examine the human visual system. A simple visual acuity test (Section 3.2) can quickly tell the investigator if a subject has normal central vision. Normal visual acuity provides valuable information on many key components of the visual system. Good acuity implies that the cornea and lens are clear and free from media opacities which may be caused by cataract or internal haemorrhages. The cornea and lens are also able to focus the image on the retina, the retina is able to process the information and transmit signals down the optic nerves to the visual cortex for interpretation. Good acuity therefore implies an intact functioning central visual system. Complementary to visual acuity, visual field testing will also provide subjective information on the integrity of the peripheral visual system. However, if any of these procedures are abnormal, they are unable to provide information on which part of the visual system an abnormality may be present. In order to localise visual dysfunction, the ophthalmologist will conduct a full clinical examination. This will assess the integrity of the cornea and the lens using the techniques of ophthalmoscopic examination, pressure measurements or biometry as described in Sections 3.5 and 3.6. Many diseases of the retina can be detected by the ophthalmic examination. Abnormalities such as exudates, haemorrhages or abnormal pigmentation may be present which could indicate degenerative disease processes or sight threatening conditions such as diabetic retinopathy. However, the ophthalmic examination does not always reveal the site of dysfunction. This may lie under the retinal surface such as an accumulation of fluid as in macular oedema which can be revealed using new imaging techniques such as OCT (Section 3.7). Structure may also appear to be normal but there may still be cellular dysfunction and defects may be further down the visual pathway such as in optic nerve disease. In these areas objective electrophysiology techniques enable a diagnosis to be reached (Section 3.8).

The combination of subjective testing, ophthalmic examination, electrophysiology and imaging techniques enable powerful diagnosis and monitoring of disease processes or treatment strategies.

Glossary

Acuity: Sharpness and clearness of vision

Electrophysiology: Recording of electrical responses from the muscular or nervous system

Interferometry: Measurement of a length in terms of the wavelength of light Macula: The central area of the retina – important for central vision Perimetry: The measurement of visual fields

Retinal pigment epithelium (RPE): An inner retinal cellular layer that absorbs light, provides nutrients and digests ageing visual pigments from the retina

Tonometry: Measurement of intraocular pressure