To investigate refractive error, vision is examined by means of a stenopeic opening [Figure 12].
Procedure
- The patient stands 5m away from the visual acuity chart. One eye is covered and the stenopeic opening is held in front of the other eye.
- The examination continues until the patient is unable to read a line correctly. From here, move to the next line using the above mentioned method. Repeat this until the patient again is incapable of correctly naming two-thirds or more of the indicated symbols.
- Make a note on the card: VODcc or sc = … + SO = … where SO means stenopeic opening.
- Repeated for the other eye.
Figure 12
If the patient’s symptoms are caused by a refractive error, you will notice a marked improvement in vision when the stenopeic opening is applied.
Near Vision
Near vision should be determined when the patient has symptoms related to reading, and when examining people over the age of 40. Provide the patient with a reading card and ask them to read the text that reduces in font size. The patient is allowed to keep the card at a distance comfortable for them. The last text that can be read without difficulty indicates the near vision.
A point-shaped light source emits light that diverges in all directions. If the distance between the observer and light source is far enough, the angle between the light rays is so small that the diverging light rays can be considered parallel.
The refractive media of the eye form a positive, i.e. converging lens.
This property refracts parallel light rays to the focal point of the lens. A second property is that light rays which enter the middle of the lens will continue to travel without being refracted [Figure 13].
For the following illustrations, these abbreviations are used:
B = Light Source
B’ = Picture
F = Focal Point Of The Lens
h = Horizontal
v = Vertical
Figure 13
Emmetropia
The retina can be seen as a projection screen. When this projection screen is located exactly at the focal point of the lens, each point-shaped light source will form a point-shaped image on the screen. This is the case in an emmetropic eye [Figure 14]. The length of the eye axis and the strength of the refractive media are in accordance with each other.
Figure 14
Myopia
If the retina is located behind the focal point of the lens, a point-shaped light source is projected as a blotch on the screen. There is also occurrence of scattered light. This is a myopic eye. The refractive media refract too much, relative to the eye in which they are located [Figure 15]. The length of the eye axis is too long for the refractive media (axis myopia), or the refractive media are too strong for the length of the eye axis (systemic myopia).
Figure 15
Hypermetropia
If the retina is located in front of the focal point of the lens, a point-shaped light source is also projected as a blotch on the screen. There is also occurrence of scattered light. This is a hypermetropic eye. The refractive media refract too little, relative to the eye in which they are located. The length of the eye axis is too short for the refractive media (axis hypermetropia), or the refractive media are too weak for the length of the eye axis (systemic hypermetropia). This applies to the non-accommodated eye.
Because we are capable of increasing the refraction of our refractive system through accommodation (we make use of this when looking at closer objects), a not too severe hypermetropia can be compensated by adults by means of accommodation. Because the accommodating capacity of the eye is already engaged when looking in the distance, it will already be used to maximum capacity when looking near.
When the accommodation capacity decreases from the age of forty, patients will also develop problems looking into distance [Figure 16].
Figure 16
Presbyopia
When looking at closer objects, the light source (object that is looked at) is so close that the eye no longer considers the entering light source as being parallel; it is now seen as being divergent. This means that the eye needs to accommodate to project the focal point onto the retina. When growing older, the accommodation capacity of the eye decreases. The eye can no longer be made refractive enough to sufficiently converge the divergent rays that need to be processed, to obtain a sharp image on the screen of the retina. This is called presbyopia [Figure 17].
Figure 17
Astigmatism
The refractive media should form a globe-shaped structure with equal curvature in all directions. It is characterised by one, single focal point. In some patients, this is not the case and one meridian has a deviating curvature. This means it will have a different optical effect than that of the polar opposite meridian. This results in two focal points in two directions, perpendicular to each other.
The retina can, however, be located in just one plane. In this plane, the image is projected sharply. In contrast, in the polar opposite plane it is not. The outcome of this is that the patient will see, for example, vertical lines in focus but horizontal lines blurred. This is known as astigmatism [Figure 18].
Figure 18
Effect Of The Stenopeic Opening
When we look through a stenopeic opening, a large part of the incoming rays responsible for the scattered light is stopped. Because of this, the projection image on the retina becomes sharper. That is why there is an improvement in vision in the case of a refractive error [Figure 19].
Vision will also improve in the case of a peripheral blurring of the refractive media. Here, it is because the scattering of light by the blurring is no longer occurring.
Figure 19