Neuro-ophthalmology questions

[flv83]

Couple questions relating to neuro-ophthalmology as I am a veterinary ophthalmology resident and wanting to know more about differences in human anatomy vs. canine.
1. One of the reflexes we use to test depth of anesthesia is the palpebral reflex. The lateral palpebral reflex (maxillary branch of trigeminal n.) is typically lost before the medial palpebral reflex (ophthalmic branch of trigeminal n.). Does anyone know why this is?
2. I was told by a mentor that there are different Marcus Gunn signs and was asked what they are and what the different ones mean?

Thanks for any help you can give me, I wasn't able to find these answers searching on the web.

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[neurodoc]

Re question #1, I do not know about the medial and lateral palpebral reflexes...we routinely check the blink reflex as part of the human neuro exam. The blink reflex has CNV as its afferent limb and CNVII as its efferent limb. It is evoked by touching the cornea (CNV-1) and watching for a blink on the same as well as the opposite side. It can be quantitated by using a corneal aesthesiometer. I never heard of using a differential response to V2 versus V1 stimulation to test the "depth of anesthesia," but I suppose that it may be possible to do so.

Re question #2, so far as I know the Marcus Gunn (MG) sign refers to an afferent pupillary defect (APD). The APD is due to a lesion in the afferent limb of the pupillary reflex to light. It is classically tested by the "swinging flashlight test. There is both a direct and indirect (consensual) constriction of the pupil when light is shined on one eye. If there is an APD you will note an "escape" (dilation) when the flashlight is swung from the eye without an APD to the eye with an APD. Just review the functional neuroanatomy of the pupillary reflex to understand why this is so.

It's important to understand that the afferent defect is relative. The MG sign is present only when one side has an asymmetric lesion. It's possible that there may be symmetric abnormalities in the afferent limb of the reflex, in which case no APD would be clinically apparent at routine bedside testing. There are special neurophysiological tests that could identify defects in the afferent limb of the reflex.

The only other bedside test for APD that I know of involves looking for "asymmetric hippus."