How to localise ? How to localise a lesion in the nervous system can be quite intimidating if one doesn't follow a logical sequence. So i will use this opportunity to help those who may be taking their first steps in neuroanat. First comes the hx and then the physical exam - note down all the positive findings and the significant negatives. the positive findings will help us outline the possible sites - both central and peripheral. The significant negatives will help us narrow down the list of possibilities - for ex. - central vs peripheral, corticospinal tract vs cortex, pons vs lesion in medulla etc. Minimising the list of possibilities by ruling out the significant negatives is called - 'looking for neighborhood signs'...means that if there is a huge fire in a house then the other houses in close proximity will also be affected..so looking at which neighbors are affected / not affected we can rule in or rule out a location ( both in PNS and CNS ). Now let's solve the Spot Diagnosis provided by Egyptian doctor. What we see is an inability to wrinkle the forehead on one side...so that has to do with the innervation of the facial nerve and so all sites below rostral medulla are ruled out assuming that that this is an isolated finding....why ? becos any lesion below the rostral medulla will have nothing to do with the innervation of the facial nerve. so location ruled out - lesions below rostral medulla possible locations - u can go from peripheral to central or viceversa. I will go from central to peripheral and then narrow it down further a) face area of primary motor cortex ( contralateral to the side of facial weakness ) b) genu of innternal capsule ( contralateral to the side of facial weakness ) c) facial nucleus ( ipsilateral to the side of weakness ) d) exiting nerve fibers in the pons or rostral lateral medulla ( ipsilateral to the side of weakness ) e) peripheral facial nerve ( Bell's palsy ) aka as isolated facial weakness ( ipsilateral to the side of weakness ) now let's see which is the most likely site from this list of possibilities a) we know that the upper half of the face receives bilateral innervation so only lesion affecting both primary cortex will cause forehead weakness and in which case it will be an extensive lesion with patient in coma ( and not obeying commands as in our Spot Diagnosis ) and we would not have an isolated forehead weakness. b) genu is ruled out for the same reason ( UMN fibers to upper face comes from both sides ) and ofcourse since the genu is a tightly packed area - u will have subtle arm weakness or dysarthria or leg weakness and the facial weakness has to be UMN type i.e only lower half affected. c) in facial nucleus the forehead and orbicularis oculi are not spared ( LMN type ) + neighborhood signs i.e deficits involving CN 6, 5 or the corticospinal tract. d) in rostral lateral medullary nerve lesions - 'lateral medullary syndrome' will be present. e) so that leaves us with a peripheral facial nerve lesion ( Bell's palsy ) - so in addition to LMN type facial palsy we may have hyperacusis, decreased taste and pain behind the ear on the affected side without any other cranial nerve or long tract abnormalities. To summarise, only cases of isolated facial nerve weakness of a lower motor neuron pattern, possibly with some hyperacusis, loss of taste, or retroauricular pain, can be located with certainit to the peripheral facial nerve. The presence of sensory loss or any other CN or motor abnormalities require evaluation for CNS lesion. Ans: Right peripheral facial nerve palsy (Y)
