4
Long Sight, Short Sight
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 longsighted person) person has smaller than usual
eyes, with an anteroposterior diameter of less
than 24 mm. To obtain a clear image, this abnormal 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 movement 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
29
30
Common Eye Diseases and their Management
Figure 4.3. Concave lens “with”; convex lens “against”. Try this
for yourself in the clinic.
Figure 4.1. A long-sighted person.
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 contrast, the optic disc of the myope is larger and
paler with well-defined margins and can be mistaken for an atrophic disc.
Hypermetropia is associated with certain eye
conditions, notably narrow-angle glaucoma and
childhood amblyopia of disuse. Myopia is associated with other conditions, particularly retinal
detachment, cataract and myopic retinal degeneration.You must be aware, though, that whereas
Table 4.1. Eye disease and refractive error.
Myopia (“short sight”)
Conditions associated
with myopia
Figure 4.2. A short-sighted person.
Hypermetropia
(“long sight”)
Conditions associated with
hypermetropia
Retinal detachment
Macula haemorrhages
Cataract
Myopic chorioretinal
degeneration
Down’s syndrome
Keratoconus (conical
cornea)
Narrow-angle glaucoma
Concomitant squint
Amblyopia of disuse
Conditions causing
myopia
Conditions causing
hypermetropia
Large eye
Cataract
Diabetes mellitus
Accommodation spasm,
or “pseudomyopia”
Congenital glaucoma
Small eye
Retinal detachment
Orbital tumours
Macula oedema
Long Sight, Short Sight
refractive errors are extremely common, these
particular conditions are relatively rare in the
general population. Table 4.1 shows a more comprehensive 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
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.