Monday, March 2, 2015

Study group discussion: Relative afferent pupillary defect (Marcus Gunn pupil) vs Optic nerve lesion

*A picture was posted on the group on which this discussion took place.

Description of the picture for the readers convenience:
In the first picture, we see normal pupils.
In the second picture, light is shone in the left eye. Right and left pupil constrict.
In the third picture, light is shone in the right eye. Right and left pupils do not constrict.
In the fourth picture, light is again shone on the left eye and again, both pupils constrict*

RAPD?
Also know as Marcus Gunn eye!
Aka prostitute's pupil :P

Nope. Optic nerve is affected on the left side. As there are absolutely no afferents from left eye, indirect is absent.

Yes.. The 3rd nerve efferent is intact and optic nerve is affected.

There is crossing of the afferent fibres of each eye. That's why when you stimulate one eye the other eye also dilates via the efferent fibres. That's how the other eye also constricts (indirect). In this picture, there is complete afferent defect. The optic nerve is completely transected.

Why not RAPD?

In RAPD, when light is shone to the diseased eye, direct absent.. Therefore, doesn't constrict.
Mnemonic: Direct Diminishes in a Diseased eye, indirect present.

RAPD is diagnosed by swinging light test. When you alternate the flashlight.. The affected pupil ( less number of afferent fibres ) has a release phenomenon and dilates instead of constricts. The affected pupil initially constricts..But when you swing the flashlight repeatedly it dilates. The pic hasn't shown frequent swinging. And the pupil remains dilated in the first go when the light is shown. So there is no relative afferent defect.

I haven't heard of the release phenomenon, I've thought it's because relative to the normal eye, the disease eye appears to dilate. It always constricts but because you compare it with the normal eye, it seems to dilate.

But why do we call it relative? The direct being present and the indirect being absent?

I dont think we call it relative cause efferent is intact. We call it relative cause, relative to the normal eye..The affected eye has reduced optic nerve fibres. This is done to detect early loss of fibres in optic neuritis.

I'm such type of patient. I mean my left optic nerve is affected.

Really? How were you diagnosed and when?

I met with an accident. It was a severe injury on left  eye. 3rd nerve got damaged at first. Doctors said it's severe. It will heal with time. But doctors were not sure it will heal completely or not.

Oh I'm so sorry!
Can you see?

After 1 yr, I again went for checkup.. Doc said 3rd nerve is alright now but optic nerve is affected due to increased stress. Now, I have only partial vision left eye. It's 6/24 (Normal is 6/6)
And it'll not heal completely for whole life.

Optic field shows that the person can't see through different angles with the defective eye. That's why, it's called Partial Vision.
Because of damage to optic nerve.. Healing depends upon the degree of damage. Meconerv Forte is the medicine for that.. But chances of complete healing are rare..

Difficulties make us more strong! Just keep going no matter what.

Yeah, of course. Thanks!

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