It is useful to distinguish between long-sighted and short-sighted patients as you will see later in this chapter, but straight away we come across a problem with terminology. Think of the
“short-sighted” old man who cannot see to read without glasses and, at the same time, the
“short-sighted” young lady who cannot see clearly in the distance. The term “short sight” is used in these instances unwittingly by the layman to mean two different situations; either it can mean presbyopia (caused by diminished focusing power with ageing, as in the case of the old man) or it can mean myopia (caused by a larger eyeball, as in the case of the young lady).
Leaving aside presbyopia for the time being, we need to realise that the myopic person has physically larger than normal eyes, with an anteroposterior diameter of more than 24 mm, and, by contrast, the hypermetropic (or long- sighted person) person has smaller than usual eyes, with an anteroposterior diameter of less than 24 mm. To obtain a clear image, this abnor- mal length of the eye needs optical correction with a lens to bring light rays to a focus on the retina. The hypermetropic requires a convex lens to converge the rays, whereas the myopic person requires a concave lens to make light rays diverge before reaching the eye.
Glasses with convex lenses in them make the eyes look bigger and glasses with concave lenses in them make the eyes look smaller. Figure 4.1 shows a long-sighted (hypermetropic) patient whose glasses seem to enlarge the eyes and Figure 4.2 shows a short-sighted (myopic)
patient. The clinical importance of this is that with a little practice the physician can tell the difference at a glance as the patient enters the room. This often helps with the diagnosis because certain eye diseases are associated with myopia and others with hypermetropia.
The nature of the spectacle correction can be verified by moving the lens from side to side in front of one’s hand. If the hand appears to move in the opposite direction to that of the move- ment of the spectacle lens, it is convex (Figure 4.3). The spectacles of the myopic patient contain concave, or diverging, lenses and, if these are moved to and fro in front of one’s hand, the hand appears to move in the same direction as the movement. As a further clue, when we look at the hypermetrope from a slight angle, the line of the cheek goes out behind the magnifying lenses and vice versa for the myope (see Figures 4.1 and 4.2).
Here, again, let us remind ourselves that hypermetropia and myopia have nothing to do with presbyopia, which is the failure of the eyes to focus on near objects, appearing in middle age. This is nothing to do with the length of the eyeball but is related to a diminished ability to change the shape of the lens. It is corrected in otherwise normal eyes by using a convex lens.
Obviously myopes, hypermetropes and those with no refractive error are all susceptible to presbyopia.
When we examine hypermetropic and myopic eyes with the ophthalmoscope, we find that there are physical differences between the
4
Long Sight, Short Sight
29
30 Common Eye Diseases and their Management
Figure 4.1. A long-sighted person.
Figure 4.2. A short-sighted person.
Figure 4.3. Concave lens “with”; convex lens “against”. Try this for yourself in the clinic.
two. The optic disc of the hypermetrope tends to be smaller and pinker, and in extreme cases, especially in children, the disc can appear to be swollen when in fact it is quite normal. By con- trast, the optic disc of the myope is larger and paler with well-defined margins and can be mis- taken for an atrophic disc.
Hypermetropia is associated with certain eye conditions, notably narrow-angle glaucoma and childhood amblyopia of disuse. Myopia is asso- ciated with other conditions, particularly retinal detachment, cataract and myopic retinal degen- eration.You must be aware, though, that whereas
Table 4.1. Eye disease and refractive error.
Myopia (“short sight”) Hypermetropia (“long sight”)
Conditions associated Conditions associated with
with myopia hypermetropia
Retinal detachment Narrow-angle glaucoma Macula haemorrhages Concomitant squint
Cataract Amblyopia of disuse
Myopic chorioretinal degeneration Down’s syndrome Keratoconus (conical
cornea)
Conditions causing Conditions causing
myopia hypermetropia
Large eye Small eye
Cataract Retinal detachment
Diabetes mellitus Orbital tumours Accommodation spasm, Macula oedema
or “pseudomyopia”
Congenital glaucoma
Long Sight, Short Sight 31
with seeing flashes of light, he may be about to have a retinal detachment.
If we take note of whether a patient is long sighted or short sighted at an early stage, this information can influence the type of questions that are best asked when taking a history.
Finally, it is worth remembering that the myopic patient can see objects close at hand and read without glasses at any age, whereas the hypermetropic patient has to focus to see at all distances. If the hypermetrope has good focusing power (i.e., the younger patient), the distance vision may be clear without glasses but when hypermetropia is more severe, the unaided vision is poor at all ranges.
refractive errors are extremely common, these particular conditions are relatively rare in the general population. Table 4.1 shows a more com- prehensive list of these associations.
Having observed the nature of the spectacle lenses, we have now made a small step towards diagnosing the eye condition. If the patient is middle aged and complaining of evening headaches, seeing haloes around street lights and, at the same time, blurring of vision, narrow-angle glaucoma is the wrong diagnosis if the patient is myopic. It could well be the right diagnosis if the patient is hypermetropic. If the patient in Figure 4.2 were to complain of the sudden appearance of black spots combined
