Pupilary reflex question

[MudPhud20XX]

Here is a question from Kaplan neuroanatomy:

A neurologic exam of a patient reveals a direct and a consensual light reflex in the Lt. eye, but neither a direct nor a consensual light reflex in her Rt. eye. Which of the following is the most likely location of her lesion?

1. Rt. CN II
2. Rt. CN III

The answer is 2: Rt. CN III.

I chose 1: Rt. CN II

Since the patient responded to both direct and a consensual light reflex in the Lt. eye, doesn't that mean that both Lt. and Rt. CN III are normal since the sensory information that travels via Lt. CN II will synapse at pretectal area and project bilaterally to both CN III?

I still think the lesion is in the Rt. CNII, which does explain the fact that the pt. has neither a direct nor a consensual light reflex in the Rt. eye.

What am I missing here?

Many thanks in advance.

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[ChessMaster3000]

Here is a question from Kaplan neuroanatomy:

A neurologic exam of a patient reveals a direct and a consensual light reflex in the Lt. eye, but neither a direct nor a consensual light reflex in her Rt. eye. Which of the following is the most likely location of her lesion?

1. Rt. CN II
2. Rt. CN III

The answer is 2: Rt. CN III.

I chose 1: Rt. CN II

Since the patient responded to both direct and a consensual light reflex in the Lt. eye, doesn't that mean that both Lt. and Rt. CN III are normal since the sensory information that travels via Lt. CN II will synapse at pretectal area and project bilaterally to both CN III?

I still think the lesion is in the Rt. CNII, which does explain the fact that the pt. has neither a direct nor a consensual light reflex in the Rt. eye.

What am I missing here?

Many thanks in advance.

Yeah I would agree with you. I would be curious to see what their explanation is but there is NO way that is correct.

 

[ChessMaster3000]

Yeah I would agree with you. I would be curious to see what their explanation is but there is NO way that is correct.

Also, as a neuroanatomy tie in, what is the structure the signal crosses going from the pretectal nucleus to the contralateral edinger-westphal nucleus?

 

[MudPhud20XX]

That would be the preganglionic parasympathetic CNIII fiber, right?

Anyway, here is what the answer says from Kaplan:

"The pupilary light reflex is assessed by shining a light in one eye and observing pupillary constriction in the same eye (direct response) and in the other eye (consensual response).

Clearly, the left CN II and left CN III are working because a direct response is seen in the left eye. Also, the right CN II must be working to produce a consensual response in the left eye. However, there must be a lesion of the right CN III because pupillary constriction is not observed under any circumstance in the right eye."

 

[syma]

Here is a question from Kaplan neuroanatomy:

A neurologic exam of a patient reveals a direct and a consensual light reflex in the Lt. eye, but neither a direct nor a consensual light reflex in her Rt. eye. Which of the following is the most likely location of her lesion?

1. Rt. CN II
2. Rt. CN III

The answer is 2: Rt. CN III.

I chose 1: Rt. CN II

Since the patient responded to both direct and a consensual light reflex in the Lt. eye, doesn't that mean that both Lt. and Rt. CN III are normal since the sensory information that travels via Lt. CN II will synapse at pretectal area and project bilaterally to both CN III?

I still think the lesion is in the Rt. CNII, which does explain the fact that the pt. has neither a direct nor a consensual light reflex in the Rt. eye.

What am I missing here?

Many thanks in advance.

Think about each response. The left eye has both direct and consensual. The direct response means the left optic nerve is in tact. The consensual response (shining the light in the right eye and watching the response in the left eye) means the RIGHT optic nerve is also in tact AND the left CIII

Now the right eye has neither direct or consensual. We already know the right optic nerve is working because we are getting a consensual response in the left eye. However we are not getting a direct response, so it must be your fibres from the right CIII.

 

[ChessMaster3000]

Think about each response. The left eye has both direct and consensual. The direct response means the left optic nerve is in tact. The consensual response (shining the light in the right eye and watching the response in the left eye) means the RIGHT optic nerve is also in tact AND the left CIII

Now the right eye has neither direct or consensual. We already know the right optic nerve is working because we are getting a consensual response in the left eye. However we are not getting a direct response, so it must be your fibres from the right CIII.

Oh I see, they are saying a consensual response in the left eye is not the right eye constricting in response to left afferent stimulation (which is what both of us though), but rather a consensual response in the left eye is due to an afferent response in the right eye. You understand the concept, its just a matter of whether you understand what is meant by "consensual respsponse"

 

[ChessMaster3000]

That would be the preganglionic parasympathetic CNIII fiber, right?

Anyway, here is what the answer says from Kaplan:

"The pupilary light reflex is assessed by shining a light in one eye and observing pupillary constriction in the same eye (direct response) and in the other eye (consensual response).

Clearly, the left CN II and left CN III are working because a direct response is seen in the left eye. Also, the right CN II must be working to produce a consensual response in the left eye. However, there must be a lesion of the right CN III because pupillary constriction is not observed under any circumstance in the right eye."

Oh, and I dont think they are considered preganglionic sympathetic fibers yet because they still have to synapse at the E-W nucleus. Anyway, I was thinking of the posterior commissure. Not the best worded question though.

 

[syma]

Consensual response relies on the afferent fibres of the contra lateral eye (CII) and then efferent fibres (CIII) of the ipsilateral eye.

So if you say someone has a direct, consensual (and near) response of the left eye, you are shining the light in the left eye for direct response, and then the right eye for consensual response.

So yes, it depends on your understanding of consensual, but that underpins your understanding of the swinging light test when checking for a (relative) afferent pupillary defect.

HTH