Roving ring scotoma & Jack-in-the-box phenomenon are few the most confusing aspects of optics in Ophthalmology.
If you have heard about these but couldn't understand them then this is the post for you,I will explain you these concepts in the most easy manner possible with diagrams.
Surgical treatment of cataract by removal of the crystalline lens of the eye results in an aphakic eye which has very different optical properties from the normal or phakic eye.
In particular, after the removal of the crystalline lens which accounts for about one-thirds of the eye's optical power(about 10 -15 Diopters, the patient requires a spectacle lens about +10.00D -+15.00D .
3 main modalities to correct aphakia :
- Aphakic spectacles
- Contact lenses
- Intraocular lens(IOL). (now most widely used)
We are going to stress about few fascinating problems like Roving ring scotoma & Jack-in-the-box phenomenon due to Aphakic spectacles due to which they were replaced by the IOL.
To correct the refractive error in aphakia about 10 dioptre of convex lenses are required for distance vision and about 13 dioptre for near vision. Such high power lenses are associated with numerous physical and optical problems. The most important of these problems are:
- Magnification: Each dioptre of convex power leads to about 3 % magnification of image and a difference of image size between the two eyes of about 7 % is tolerable. Thus 10 dioptre aphakic spectacles lead to about 30 % magnification of the image which give rise to diplopia i.e., two images of one object are seen one small (from normal eye) and other larger (from aphakic eye). Moreover, when the objects appear larger they appear falsely closer than reality, and this leads to physical in-coordination.
- Roving Ring Scotoma: The edge of a convex lens acts as a prism and the higher the power of the convex lens the greater is the prism angle (alpha). The light falling on the prism bends towards its base by an angle alpha/2 , therefore, greater the angle alpha the more will be the bending. In aphakic spectacles, the angle alpha being large, the light falling at the edge of the lens bends towards the center of the lens (base of prism) and does not reach the pupil and is, therefore, not seen. This results in an area of the visual field which is not visible to the patient, or scotoma. And because the edge of the lens is present all around the lens like a ring, so it gives rise to a ring shaped scotoma. The position of this scotoma is not fixed in the visual field because the eye keeps moving (or roving) in relation to the aphakic spectacle. Hence, the result is a roving ring scotoma.
- Jack-in-the-box Phenomenon: The presence of the above scotoma leads to another interesting phenomenon. If an interesting object appears in the periphery of the patients visual field, it appears blurred (because the light is passing from the side of the spectacle frame). The person tends to move his head towards the object in order to see it clearly. But as he turns the head the object comes to lie in the area of scotoma and thus disappears. As he turns his head further so that the object comes to lie in front of the spectacle in the visible area and so reappears again clear and sharp. This sudden disappearance and sharp reappearance of the objects is called jack-in-the-box phenomenon.
- Pin Cushion Effect: The magnification of image is more at the periphery of the lens due to prism effect. Therefore, all the objects appear stretched out at the corners like a pin-cushion.
- Spherical Aberrations: The light converges more near the edge of the lens than at the center so the rays of light falling near the edge are brought to focus in front of the rays falling at the center. This results poor quality of image despite appropriate correction of refractive error.
- Chromatic Aberrations: The shorter the wave-length the more is the refraction a ray of light undergoes. Therefore, in VIBGYOR the violet end undergoes greater refraction than the red end of the spectrum. This causes diffraction of light and makes the edges of white object appear rain-bow colored.