ICEH PUBLICATION

Refraction Training Manual

4. Refractive Errors

Emmetropia

This is the normal eye, having no refractive error. It is an unaccommodated eye in which the rays of light from a distant object are focussed onto the retina.

The cornea: This is an unchanging refractive surface with a power of about 42.00 DS.

The natural (crystalline) lens: This is a variable, refractive body which can accommodate (focus) to increase its power. In its unaccommodated condition it has a power of around 20.00 DS and this power increases with accommodation.

EMMETROPIA

Ametropia

This is the general term applied to an eye with any refractive error, with the eye unaccommodated. In an ametropic eye, light rays do not focus onto the retina, but focus in front of, or behind the retina.

There are three main classes of ametropia

Hypermetropia
Myopia
Astigmatism

Hypermetropia

(Hyperopia, long sight)

Rays of light are focussed behind the eye, when the eye is unaccommodated. This can be due to the eye being too short or the refractive components of the eye too weak.

Hypermetropia

By accommodation (focussing), objects can be brought into focus and seen clearly.

Accommodation

The constant effort required to maintain accommodation in hypermetropia can result in aesthenopia (eyestrain), giving symptoms of headache, tired and aching eyes, and even watering eyes (lacrimation).

Hypermetropia in more detail

Hypermetropia is divided into several parts, based on the ability of the eye to cope with the refractive error.

Total Hypermetropia is the full amount of hypermetropia
Latent Hypermetropia is the portion of the total error which is easily overcome by accommodation, and any attempt to correct this will result in blurring the vision
Facultative Hypermetropia is the portion that can be corrected by lenses, or by accommodation
Absolute Hypermetropia is the portion that cannot be corrected by accommodation

Example

Let us assume that a patient has vision of 6/24. If we add positive lenses we will reduce the remaining refractive error and improve vision.

We add + 3.00 DS and find that the vision has now improved to 6/6. We then add more positive power to make + 5.00 DS, and the visual acuity remains at 6/6. Adding any more positive power blurs the vision. We then do a refraction under cycloplegia (see later) and find that the patient accepts +6.50.

Cycloplegia gives us the total hypermetropia which is therefore + 6.50 DS

The initial + 3.00 DS which improved the visual acuity corrected the absolute hypermetropia.

The addition of power up to + 5.00 DS reveals more about the hypermetropia. This amount is the sum of the absolute and facultative hypermetropia. Thus the facultative hypermetropia is + 2.00 DS.

Addition of further positive power blurs the patient. The latent hypermetropia is + 1.50 DS. We calculate this by deducting the absolute and facultative from the total hypermetropia.

Therefore,

Total = Latent + Facultative + Absolute

Myopia

(Short sight)

This time rays of light are focussed in front of the retina, whether or not the eye is accommodating. In fact, accommodation will result in even more blurring. Myopia is due to either the eye being too long or the refractive components too strong.

Myopia (Short sight)

Astigmatism

Astigmatism - Regular

Here, the eye has a different refractive power in different meridians of the eye - this means that the eye is not spherical. For example, vertical rays entering the eye may be focussed behind the retina while horizontal rays focus in front of the retina. The two meridians are always at right angles (90°) to each other in regular astigmatism. This type of astigmatism is correctable with cylinders.

Astigmatism - Regular

Between the two focal points there is the circle of least confusion or blur circle. This is the position that gives the least blurring of vision for the eye.

Categories of astigmatism

There are several categories of astigmatism:

'With the rule' where the stronger refracting meridian is vertical and the weaker horizontal. The minus cyl axis is at about 180°
'Against the rule' where the stronger meridian is in the horizontal. The minus cyl axis is at about 90°
Oblique astigmatism, where the axes are around 45° and 135°

Oblique astigmatism has a greater effect on vision than with or against the rule. It is also necessary to prescribe for oblique astigmatism carefully as the distortion caused can be more difficult to adapt to.

Each of these, when combined with a spherical element, can also be sub-divided into 5 groups:

Compound myopic - both meridians are myopic
Simple myopic - one meridian is plano, the other myopic
Mixed - one meridian is hypermetropic, the other is myopic
Simple hypermetropic - one meridian is plano, the other hypermetropic
Compound hypermetropic - both meridians are hypermetropic

Astigmatism may be due to:

Corneal - the corneal surface, which has a refractive power of about 42.00 D, may not be spherical and has radius of curvature which is greater in one meridian than the other
Lenticular - due to the lens tilting within the eye. This is normally a maximum of 0.50 DC and is against the rule

Frequency of astigmatic corrections.

With the rule 80%
Against the rule 10%
Oblique Astigmatism 10%

Astigmatism - Irregular

This is normally due to a medical condition such as keratoconus, pterygium, intra-orbital space occupying lesion, etc. These conditions should be referred.