Friday, February 20, 2015

Study group discussion: Why are they called false localizing signs?

This was discussed on our study group. I tried to explain it :D

Can anyone explain why bilateral grasp reflex and bilateral babinski sign are false localizing signs?

There are certain signs which makes you think lesion is at that level so you localize it there but the lesion may be at a different level. So you localized it falsely, just because of that sign. 

Neurological signs maybe called false localizing signs when their appearance reflects pathology distant from the expected anatomical locus.
Many false localizing signs occur in raised Intracranial tension and idiopathic isolated cerebral hypertension.

Say there is raised ICT - the midbrain is compressed all the way to the opposite side. The crus cerebri may be compressed against the contralateral tentorium causing paresis which is contralateral to the damaged crus cerebri but ipsilateral to the mass lesion. (This is called Kernohan's phenomenon.

So if you don't have brain imaging, it's a challenge to find out where is the lesion.
In brainstem lesions, we usually expect paresis to be contralateral, but in this case is will be on the same side of massive lesion (That is, pushing the contralateral crus cerebri, damaging it - that in turn, produces a lesion contralateral to damaged crus but ipsilateral to massive lesion.) That's why, it's called false localization.

Crossed extensor response/bilateral Babinski sign: Unilateral stimulation produces bilateral Babinski in patients with bilateral cerebral disease and spinal cord disease.

So basically, if you're having bilateral Babinski and you're localizing it to the spinal cord when in reality - the lesion may be high up in the brain (Due to raised ICT) it's a false localization!

Raised ICT may also cause uncal falx herniation, which can cause compression of third nerve and abducens nerve. (Abducens against the foramen magnum and clivus). Another example of false localization.

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