3 Ophthalmoscopy for Technical Failures and Sporadic Screening
Ophthalmoscopy is an alternative method to photography for examining the retina. The Health Technology Board for Scotland recognises that indirect ophthalmoscopy using a slit-lamp is sensitive and specific enough for retinal screening but notes that it carries the disadvantage that there is no permanent record of the image for quality assurance or for monitoring progressive changes. However, this technique will be essential for screening failures of digital photography. Direct ophthalmoscopy has high specificity but its sensitivity is too low to form the basis of a national screening programme. Direct ophthalmoscopy still has a role though in opportunistic screening for those who persistently default from the systematic national programme.
DIRECT OPHTHALMOSCOPY
The direct ophthalmoscope was invented in 1828 by Charles Babbage, a mathematician, but was not used clinically until its independent reinvention by Hermann von Helmholtz in 1853.
The direct ophthalmoscope relies on the fact that light rays emanating from the retina of an emmetropic (no refractive error and therefore no glasses required) patient will be focused on the retina of an emmetropic observer.
The alignment of the illuminating light source is very important as the retina can only be observed when both the illuminated area and observed area of the fundus coincide. This is accomplished by the lower half of the patient’s pupil being illuminated with reflected light from a mirror or prism and the upper half of the pupil being used for observation. When both patient and the observer are emmetropic the image magnification is 15x. Where this is not the case, the interposition of lenses creates a Galilean telescope. A Galilean telescope consists of a weak plus lens in front of a strong minus lens resulting in a magnified upright image. In direct ophthalmoscopy, the retina of the myopic patient is magnified more than that of the hypermetropic patient.
Due to the relatively high magnification obtained with the direct ophthalmoscope, a detailed examination of the optic disc is possible. This instrument will also give a useful assessment of the extent of any lens opacities present when used at a slightly greater distance from the patient’s eye using a plus 10D lens within the ophthalmoscope. The opacities appear silhouetted against the red reflex seen through the pupil.
SLIT-LAMP BIOMICROSCOPY AND INDIRECT OPHTHALMOSCOPY
The slit-lamp biomicroscope is the primary instrument used by ophthalmologists and optometrists to examine the eye.
Used in the normal way, it is possible to view the anterior structures of the eye, the lens and the anterior third of the vitreous. The illumination system is essentially a short focus projector coupled to the observation system. It is possible to view a cross section of the transparent structures (cornea and lens) using a narrow beam and adjusting the angle between the illumination system and the observation system. Not only can a narrow slit be projected on the eye, but also any shaped patch from a slit to a circle and of varying sizes. Changing the objective lenses and/or the eyepieces can alter the magnification and anything from 10x to 40x is usually possible. Some instruments have a zoom to vary magnification. The level of illumination can be varied and there are usually cobalt blue and red free (green) filters available also.
In order to obtain a view of the posterior vitreous and retina, it is necessary to incorporate a supplementary lens known as a Volk lens. There are several of these lenses available ranging from plus 60 diopters to 90 diopters in strength and depending on which lens is used differing amounts of magnification and field of view are achieved. Pupil dilation is usually required and the peripheral retina can be seen by asking the patient to look in different directions.
The lens is held in front of the patient’s eye and unlike the direct ophthalmoscope, an inverted (upside down) and laterally reversed (back to front) image is seen through the microscope.
An important.htmect of this method is that a binocular view is seen and therefore depth perception can be appreciated. Retinal thickening or oedema can be seen as “bumps” and it is possible whether new vessels flat and growing along the surface of the retina or forward and growing into the vitreous cavity.
The head mounted indirect ophthalmoscope utilises the same principles as the slit lamp and Volk lens combination to again provide an inverted aerial image of the retina.
The headset contains not only the illuminating light source required but also a separate observer for each eye. Each observer piece contains a converging lens that allows the image to be viewed binocularly.
The image is independent of the patient’s refractive error and the magnification is determined only by the power of the converging lens.
This is the most effective method to view the far periphery of the retina.