• Sam Ireland

Brains and Eyes - 2


Consider a couple possible scenarios that may occur while testing a patient's pupillary reflex:

Scenario 1:

While shining a light into the R eye..

R PUPIL: No response

L PUPIL: Constricts

While shining a light into the L eye now...

R PUPIL: No response

L PUPIL: Constricts

Diagnosis?

Scenario 2:

While shining a light into the R eye..

R PUPIL: Constricts

L PUPIL: Constricts

While shining a light into the L eye now...

R PUPIL: No response

L PUPIL: No response

Diagnosis?

The Anatomy

Before we get into the breakdown of each scenario, let's look at how the eyes and brain normally communicate all these signals.

The first thing that happens is that the retina picks up the message and send is through the Optic nerve (CN II) to both of the Pretectal nucleus. CN II can do this because the Optic Chiasm splits the message before sending it farther back. You can in the illustration (my crudely drawn junk) that each of the Pretectal Nucleus then splits the message two ways. The visual system has a lot of crossover in this initial part of the anatomy.

The next step is that both of the Pretectal Nucleus, having split the message two ways each, connect to each of the Edinger-Westphal Nucleus (green). The Edinger-Westphal Nucleus is what connects to the Oculomotor Nerve (CN III), which is what sends the message to the pupil - causing miosis (pupillary constriction). There is also a Ciliary Ganglia in the pathway between the beginning and end of CN III that I did not include in the illustration (I did not find it relevant to the discussion).

Scenario 1 Breakdown

Just to refresh what scenario 1 was...

R PUPIL: No response

L PUPIL: Constricts

While shining a light into the L eye now...

R PUPIL: No response

L PUPIL: Constricts

Strange, right? The L pupil seems to be working just fine, but did you expect the R pupil (even though not constricting in response to light itself) to still make the L pupil constrict? To diagnose the problem, we have to take a look at the neuroanatomy that connects all of these structures and pinpoint the breakdown in communication.

While shining a light into the L eye, there are no communication problems up until the both of the Pretectal Nucleus sends the message to the R Edinger-Westphal Nucleus. If there is a communication problem in either the Edinger-Westphal Nucleus, or the Oculomotor Nerve, the message to constrict will never make it to that eye, resulting it an unresponsive pupil. What about when we shine the light in the damaged side?

Because the side with the damaged CN III / Edinger-Westphal Nucleus still has a working Optic Nerve, it can still receive messages and send them to the other eye - the message just never makes it back to the eye that is sensing the light in the first place. This will result in consensual miosis, with ipsilateral unresponsiveness.

Congratulations! You've pinpointed the pupillary reflex issue to a problem with either the CN III, or the Edinger-Westphal Nucleus on the R side.

Scenario 2 Breakdown

Just to refresh what scenario 2. was...

While shining a light into the R eye..

R PUPIL: Constricts

L PUPIL: Constricts

While shining a light into the L eye now...

R PUPIL: No response

L PUPIL: No response

This one may seem a little more straight forwards than the first - and you can probably even figure out the problem before the illustrations by now. The L eye is clearly not receiving signals. Let's look at shining our light into the R eye first.

Everything looks normal while shining our light into the R eye. We see expected miosis in the ipsilateral pupil, as well as consensual miosis in the contralateral pupil. Now let's check our L eye.

Just like we thought, the L eye is not able to send out any signals. The Optic Nerve is damaged prior to the split of the Optic Chiasm! If the signal could make it to the Optic Chiasm, the signal could be split and the crossovers in the system could take care of the rest - but this problem is too close to the eye. Since CN II isn't getting the signal far enough, neither pupil will be stimulated to constrict.

Meaning Behind The Diagnosis

In scenario 1 we had a problem with CN III, the Oculomotor Nerve. In my last blog we talked about 'down and out syndrome,' or 'cranial nerve III palsy.' You will not always have all three symptoms associated with down and out syndrome. You may only have Ptosis, or perhaps a lateral gaze, or in this case, a dilated or unresponsive pupil. An acute onset of this presentation may signals that there is a mass or bleed pressing on CN III. Refer to my last blog for where that issue may arise in the brain.

In scenario 2 we had an Optic Nerve problem (CN II). If this does not present as a chronic problem, than compression, ischemia, or detachment of the nerve is most likely (along with retinal issues). Emergent head CT is warranted in both of these cases.

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