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Sunday, February 22, 2009

Roving ring scotoma &Jack-in-the-box Phenomenon in Aphakic spectacles.

jack in the box phenomenon

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.

Aphakic Spectacles:

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:

  1. 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.
  2. 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. roving ring scotoma
  3. 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. jack in box phenomenon
  4. 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. Pin_Cushion
  5. 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.
  6. 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.

90 comments:

Anonymous said...

I ve searched for the explatnation of this jack in the box phenomenon in many books and websites but nowhere did i find such an easy and clear explanation. Great job. Thanks a lot. Dr. Munneeb from Pakistan.

Anonymous said...

Now the phenomenon of jack in the box n roving ring scotoma is very much clear to me thru this explanation. Dr. NOreen from Pakistan.

pradeep n rao said...

super dude u are really a jack u rowed me around

Anonymous said...

great explaination. Dr. Sarita from India

zulfz said...

thank you so much.. finally i could understand dis phenomenon! FINALLy. great gob thnk u. dr.zulfeen frm india

Anonymous said...

thank u so much. i could finally understand dis phenomenon. finally! great job,thank u:) dr. zulfeen frm india

kiru said...

super o super

Dr.rajesh said...

good explination.outstanding

Anonymous said...

Very Good Explanation, Thank you

Muhammed Kunzi, india

Anonymous said...

kindly give reference of your explanation

Anonymous said...

amazing job......thank you very much:)

Erum Zafar Malik,
Pakistan

Roshni said...

Superb.... easy and cool illustrations...
thanks a lot!!!

Anonymous said...

great work man.......

Anonymous said...

thanks for such an easy way of describing things....:)
i would like to add something to the modalities to treat aphakia....:
it can also be done by keratophakia and epikeratophakia

Anonymous said...

Thanks a lot ....... I wish I could have explanation like this to every topic I read......I'm short of words to describe how grateful I m ......
Swati Chitkara

sundaram said...

thanks a lot...

Jino Joy said...

Thanks a lot !
Understood the concept

St. johns Medical college
Bangalore

Dr. Gunjesh said...

great job & keep it up.

Anonymous said...

thank u sir...very very simple concept only bcoz of u...

Anonymous said...

simply explained and i got it all. Thanks. Final year DMC student.

Anonymous said...

Excellent, so simple explanation, thanx!
Dr. Irshad khan J&K India.

Anonymous said...

thanks a lot.explanation is very simple

Anonymous said...

Simply One word... SUPERB.

Anonymous said...

thanx a lot sir.
SUSHIL, Mangalore

Anonymous said...

THANX A LOT!!!!!!!
DR A SARKAR,KOLKATA

Anonymous said...

heartful thanks for u hav made me clear of such greek nd latin terms.THANK U VERY MUCH...

Anonymous said...

really superb explanation...:):)really superb explanation...:):)

Anonymous said...

wow wat a way 2 explain.....really i must say if dis s d way to explain den i must nw m ur fan..wow wat a way 2 explain.....really i must say if dis s d way to explain den i must say nw m ur fan
..
dr.om frm india..

Anonymous said...

thank u....

Anonymous said...

i had to make a presentation on aphakia and was going crazy trying to find an easy explanation for these phenomena, thank you so much!!! :D
3rd year AKU student

Anonymous said...

Outstanding..thank u very much..dr. Rajesh scbmc

Anonymous said...

awesome! hunted in parson's, couldnt get it. has helped me a lot a day before my practs! thanks a ton........


Christian medical college,Vellore, India

Anonymous said...

Good Work Dude...Keep it up.

Nagaraj Umachagi said...

SUPERB SIMPLE!!! Thanks a lot :)

Anonymous said...

there is visual field that is not visible 2 patient in periphery.......then u said there is more magnification at periphery...????please explain.....????

Anonymous said...

Thank You so much...understood it in just the first reading!!!

Anonymous said...

great work...........do you have some other confusing concepts to explain?

Anonymous said...

Beautiful explanation.i didnt understand when i read this in PArson.

Anonymous said...

Thanks. I love you

Anonymous said...

Excellent web site pg aspirent
Dr. Ajeet chamania

Anonymous said...

A wonderful explanation please continue the good job

Dr. Aashish said...

Great explanation. Understood everything easily. Thanks a lot

Dr.jassim said...

bindaaas explanation....no where did i get such an interesting nd easy xplanation..sambhavam colour aayintundu ketaa !!!thnx a lot
Dr.Jassim Abdulla, calicut medical college,kerala.,india

rise n shine said...

Thank u so much
Dr Bhawna

Anonymous said...

thanx a lot! u made it seem so simple

Anonymous said...

JAZAKALLAHU KHAIRAN:) AYESHA NAZIR-4TH YEAR STUDENT FROM PAKISTAN

Anonymous said...

thank u very useful information

Anonymous said...

Thank you so much ! That was a concise :D

Anonymous said...

Brilliant is the word . Good explanation.

Anonymous said...

Seriously...its beautiful to understand it this way....thanks a tonnn

Anonymous said...

Such a wonderful explanation.. Thanks a lot..

Vineela Cherukuri said...

Thank u.... thank u soooo much sir... love this explination... Even the professors who teach us never gave us such clear explination.. That was really awesome :) <3 Cherukuri.Vineela
I.M.Sc Optometry.
H.C.U

Anonymous said...

So crystal clear
Hats off professor

Vishnu
Mbbs
Jnmc

Anonymous said...

Thnq so much fr dis post...helped me a lot
Pearl
MBBS
India

Anonymous said...

explained in very good manner.Dr Azhar Mahmood, Pakistan

Anonymous said...

Thanks a lot for such a lucid explanation...

Kaushik Vora said...

Thans a lot

Anonymous said...

G8 explanation thank u sooooo much for making it so easy

Anonymous said...

Really very explicitly said... Easy to understand

Anonymous said...

very good explanation..great job

Anonymous said...

Great explanation

Anonymous said...

Excellent description.....wid perfect diagrams

kalai selvi said...

Amazed to get an clear overview ... Thnk u

Anonymous said...

Thanks for such an easy explanation!

suresh rasaly said...

...

jaya said...

Thank you very much doctor. You made it so easy to understand. Depicted pictures were of great help .

Sakar Subedi said...

Can you clarify more on pin cushion effect.. In which lens does this effect take place and whats its difference with barrel shaped effect.

Anonymous said...

You really spelled out the concept to the reader!! Amazing! Thanks!

Anonymous said...

really u made me to understand this concept easily....thnq so much....from Mahesh R.M

Juhi Jasmine said...

You're freaking awesome

Unknown said...

Very niceee

Unknown said...

Very niceee

Ved said...

Woah! U the man. Thanks!

veeru's blog said...

Thank you ,explained it in a very easy manner

Jahnawi said...

Thank you!

Jahnawi said...

Thank you!

rajrakesh ricky said...

Thankyou .. Smiplified ...

rajrakesh ricky said...

Thankyou .. Smiplified ...

Unknown said...

Thanks a lot.. very easily understandable~~

Sinchana S Kumbar said...

Very gud and easy explanation.. . thnq

Dr Verma. Sandeep said...

thank you for so easy explaination of complex concept.

The Riparian said...

I've worn very strong aphakic glasses (+20 about) my whole life and I finally understand some things about my vision, like why its hard to walk in crowds with people moving all about.

The Riparian said...

I've worn very strong aphakic glasses (+20 about) my whole life and I finally understand some things about my vision, like why its hard to walk in crowds with people moving all about.

Unknown said...

Amazing! Made my concept crystal clear! Thank you so much.

Anonymous said...

This explanation is really easy to understand...... Thanku so much..... really awesome

Unknown said...

Dude you nailed it

aamir said...

very nice explanation bro.............. great job...............

Koroman Koro said...

Thank you for this wonderful explanation

Unknown said...

thank you...

Anonymous said...

Good explanation

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