Need Help With Question

[drmajumdar]

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Hi,

desperately need help with a question

If a person undergoes lens extraction, without implantation of artificial lens, and instead uses spectacles (aphakic glasses)...........he faces some problems. eg. due to Spherical abberation - he suffers from something called Roving Ring Scotoma or Jack in the Box Phenomenon.

and due to Prismatic aberration....he suffers from "Pin-cushion effect"?

what WHAT ARE THESE GOD-FORSAKEN THINGS??

 

[Richard_Hom]

Hi,

desperately need help with a question

If a person undergoes lens extraction, without implantation of artificial lens, and instead uses spectacles (aphakic glasses)...........he faces some problems. eg. due to Spherical abberation - he suffers from something called Roving Ring Scotoma or Jack in the Box Phenomenon.

and due to Prismatic aberration....he suffers from "Pin-cushion effect"?

what WHAT ARE THESE GOD-FORSAKEN THINGS??

Despite your emphasis, the question probably needs clarification. You will not get this phenomena of optical distortion or effects if the patient is given or fit with aphakic contact lenses. This is inherent in spectacles. However, some of the hyperaspheric designs not only look thinner but also minimize these optical effects.

Where are you a student?

 

[drmajumdar]

i am a medical student in India. since in our course we have a year of ophthalmology.....we are sometimes asked the above questions.

also, the question deals solely with "simple spectacles" given for Aphakic patients.

 

[orangezero]

a simple answer to at least one, off the top of my head, is that pincushioning is a result of plus lenses. a high plus lens would be needed when no intraocular lens or implant is in the eye.

 

[drmajumdar]

but could someone please explain to me the actual optics/physics of these two phenomenons.

I kinda came up with explanation of Roving ring scotoma, from my high-school physics. Could someone tell me if it is correct?

Here goes.......... aphakic glasses are high-powered conves glasses, therefore due to Spherical aberation, the rays incident on it at the periphery are refracted to a slightly greater degree, and converge at a point a little in front of the focus (retina). therefore objects in the periphery are always slightly out of focus. hence there is a peripheral area of myopia (came up with that). so when moving objects come from the periphery to the centre, they suddenly pop up into focus when near the centre, and again become hazy when in the periphery (Jack-inthe-box). and since this peripheral are of haziness/myopia moves along with the movements of the eyeball....hence it is called Roving Ring Scotoma.

All criticisms and explanations are welcome.

Pin-cusioning......what is it?......anyone??? please!!!

 

[PBEA]

but could someone please explain to me the actual optics/physics of these two phenomenons.

I kinda came up with explanation of Roving ring scotoma, from my high-school physics. Could someone tell me if it is correct?

Here goes.......... aphakic glasses are high-powered conves glasses, therefore due to Spherical aberation, the rays incident on it at the periphery are refracted to a slightly greater degree, and converge at a point a little in front of the focus (retina). therefore objects in the periphery are always slightly out of focus. hence there is a peripheral area of myopia (came up with that). so when moving objects come from the periphery to the centre, they suddenly pop up into focus when near the centre, and again become hazy when in the periphery (Jack-inthe-box). and since this peripheral are of haziness/myopia moves along with the movements of the eyeball....hence it is called Roving Ring Scotoma.

All criticisms and explanations are welcome.

Pin-cusioning......what is it?......anyone??? please!!!

These issues are of little value in the clinic, as Dr. Hom mentioned aphakics are best corrected with contact lenses, eliminating all of the above optic phenomenon. You only need to recognize that they occur. Your explanation of jackinthebox and ring scotoma is fairly accurate, although I think the scotoma is the result of extreme prismatic effect, in other words, the image from this area of visual field never makes into the eye, hence scotoma. Once the image is "moved" from this extreme primatic effect area then it pops into view (jackinthebox). Pin cushion occurs for the basically the same reasons as above. Higher peripheral mag (or prismatic effect) stretches the edges of the image, stretching a square's edges into a "pincushion" shape. Although I always said it looked like a ninja's throwing star. Hiiiii Ya! All of these can collectively be called spherical abberation IMHO. You do use IOL's in India, right???

 

[UABopt]

Hi,

desperately need help with a question

If a person undergoes lens extraction, without implantation of artificial lens, and instead uses spectacles (aphakic glasses)...........he faces some problems. eg. due to Spherical abberation - he suffers from something called Roving Ring Scotoma or Jack in the Box Phenomenon.

and due to Prismatic aberration....he suffers from "Pin-cushion effect"?

what WHAT ARE THESE GOD-FORSAKEN THINGS??

A very nice description of these problems is discussed in great detail in Clinical Optics by Fannin and Gosveneor on page 329.

In short, the gullstrand's schematic emetropic eye has a refracing power of 58.64D removing the lens decreases the power to +43.05. This deficit must be corrected with a very high plus lens. The size of the retinal image is directly proportional to the anterior focal length and inversely proportional to the refracting power of the eye. Which means (retinal image size in aphakia) / (retinal imgage size in emmetropia) = 58.64/43.05, this leads to a 36 percent increase in image size in the aphakic. This is assuming that you place the lens at the anterior focal length 23.33 mm in front of the eye. Then you have to take into account spectacle magnification as well.

Decreased field of view results from the fact that a plus lens also acts as a prism which increases in power toward the periphery and the base in the direction of the center of the lens. That is to say above the optical center there is a base down effect, and below the optical center there is a base up effect. Pretice's law statest that the amount of prismatic power is given by the product of the decentration in cm and the power of the lens in that given meridian in diopters. The jack in the box or ring scotoma effect is a result of both this prismatic effect and having spectacle magnification in the central field of view, but not having this magnification in the periphery where the spectacles do not cover your vision. What results is an image in the center is larger than in the periphery, and you are essentially missing the peripheral vision in that area at the junction between the periphery of the lens and the space around it.

The book listed above does a great job explaining all of these issues. In optometry school we have 4 or 5 optics courses, it is hard to condense so much information into a single post. Not to mention that lots of the information that is covered in these courses is promptly forgotten upon completion of part 1 of boards. Knowing all of the formulas and the reasons behind these effects for an optometry student is an ok academic exercise, but for a medical student it seems knowing that these effects exist should be enough, knowing the minutia of exactly why seems slightly esoteric and unnecessary to me. Again to limit these effects it is best to decrease the vertex distance, the best solution is contact lenses.

 

[drmajumdar]

THANK YOU SO MUCH FOR THE RESPONSE AND EXPLANATIONS.

Yes, use of IOLs is very much prevalent in India and rather the usual norm. Its just that inspite of that, our professors often ask us questions which have frankly VERY little application in the real world....hence we have to dig up obsolete stuff.

 

[drmajumdar]

thanx again to the senior members for answering the question.

But alas! i have been grounded again with another question!!

In the visual field defects characterstically seen in Glaucoma........is there any explanation as to why a Roenne's Nasal Step occurs??

Is it something to do with the anatomical dispostion of the Median Raphe?

 

[PBEA]

thanx again to the senior members for answering the question.

But alas! i have been grounded again with another question!!

In the visual field defects characterstically seen in Glaucoma........is there any explanation as to why a Roenne's Nasal Step occurs??

Is it something to do with the anatomical dispostion of the Median Raphe?

Now your just being silly. Lets merge a couple of nasal steps and create a ring scotoma, shall we!! Sorry, but ring scotoma is still not that relevant (clinically). I dont care if you have one thats repeatable. Either your already treating the glaucoma or you cant remove the artifact. OK fine roving ring scotoma is valid to learn as a student. I was wrong before when I said it was not. Do I have to "answer" your above question?

 

[drmajumdar]

Sorry, for the long absence.

Hehe, 😉 🙄 the less we go into what our professors expect of us and how much truly clinically relevant and applicable stuff is taught to us the better.

the rebuke is appreciated, as is the sarcasm.

Moving on, I fail to come us with a better answer for the glaucomatous field defect artefact Roenne's Nasal step, except that as with damage to Retina nerve fibres occurring in Superior and temporal poles is not symmetrical, hence the field defects, which progress in an circumrential fashion, do not progress in the same arc, hence when they meet, they form the right-angled roenne's nasal step.

thanx again.....😀 😀 😎

 

[PBEA]

Sorry, for the long absence.

Hehe, 😉 🙄 the less we go into what our professors expect of us and how much truly clinically relevant and applicable stuff is taught to us the better.

the rebuke is appreciated, as is the sarcasm.

Moving on, I fail to come us with a better answer for the glaucomatous field defect artefact Roenne's Nasal step, except that as with damage to Retina nerve fibres occurring in Superior and temporal poles is not symmetrical, hence the field defects, which progress in an circumrential fashion, do not progress in the same arc, hence when they meet, they form the right-angled roenne's nasal step.

thanx again.....😀 😀 😎

I was being sarcastic because I thought you were😳 I thought you answered your own question the first time. Anyway, yes, nasal steps occur because of the midline. I agree with you regarding the asymmetry, different arcs form because of asymmetric damage (as well as asymmetric nerve fiber layout) above and below the midline. Hope this helps, you may want to look into it some more though. Seems pretty detailed for someone who isnt going to practice ophthalmology! I'd be happy to help with any of your questions, Good luck

 

[drmajumdar]

!! thanks a lot anyway.

and thanks again for the cooperation. had my semester exam viva today...........was quizzed about...🙄.... role of accomodation in Aphakic and pseudophakic eyes!!!

 

[PBEA]

!! thanks a lot anyway.

and thanks again for the cooperation. had my semester exam viva today...........was quizzed about...🙄.... role of accomodation in Aphakic and pseudophakic eyes!!!

Ha, they are trying to trick you. Of course there is no accomodation in these pts. Although there are implants which attempt to provide accom., it cant occur otherwise.

 

[acurarte]

Put on a pair of +12 glasses and you will understand.