In a lesion affecting the descending sensory fibres of trigeminal nerve (i.e. fibres carrying pain and temp. that synapse in the spinal nucleus) it also affect the anterolateral system, however the trigeminothalamic tract runs closely to the anterolateral system. Why then in such a lesion will you not lose sensation in both sides of the face? The descending fibres from the ipsilateral side is affected and the trigeminothalamic fibres from the contralateral side running close to the anterolateral system is affected
If you recall the picture at 1:00:00, you can see the spinal nucleus carries general somatic afferent input fibers from the ipsilateral part of the face. They haven't crossed over, to the trigeminothalamic tract that runs on the opposite side of the brainstem, yet. Therefore you will knock out the fibers on the ipsilateral side of the face, but also the contralateral spinothalamic fibers on the opposite side of the body (remember the spinothalamic tracts already decussated at the level of the spinal cord), these fibers run just next to the spinal nucleus in the medulla coming in from the opposite side of the body, hence a checkerboard fallout of sensation will occur.
Hi Dr, with regards to the INO (54:22) why is the lesion then called an ipsilateral adduction deficit? Is the 'ipsilateral' part named as such because of the MLF being on the same side as the deficit? The fact that the opposite abducent nerve is supplying the initial stimulus, is confusing me with regards to the contra-/ipsilateral naming. Henry Jordaan
Yes you are correct. The ipsilateral part is due to the lesion sitting in the MLF ipsilateral to the lesion in the brainstem. 52:21 shows the picture nicely (lightning strike is the lesion). therefore the 3rd CN nerve won't know that the 6th CN nerve is working therefore it won't adduct, but the 6th CN nerve will be the one that is still doing all of the work - abduction, but the 6th CN will now be on the opposite side of the lesion.
Slide 45 - how would you decide whether the most likely localization is left or right as the patient has left facial muscle weakness, but opposite (right-sided) arm and leg weakness?
Cranial nerves always have ipsilateral fallout (except for the pesky trochlear nerve), therefore if you have a patient with cranial nerve dysfunction you have to think the dysfunction is ipsilateral to the nucleus, fascicle or nerve itself. So if the patient has ipsilateral facial weakness but opposite arm and leg weakness then the lesion has to be in the pons because you have that checker-board pattern (ipsilateral face, contralateral body)
Are you referring to the case study of the 3rd CN or 4th. The 3rd CN had a lesion in the ventral medial pons. The pictures that I showed at 33:52 for the trochlear nerve should have had a lesion on the left dorsal midbrain at the level of the inferior colliculus if you wanted to place the lesion in the midbrain (remember, Trochlear nerve is the only one that exists dorsally and decussates to the opposite side)
In a lesion affecting the descending sensory fibres of trigeminal nerve (i.e. fibres carrying pain and temp. that synapse in the spinal nucleus) it also affect the anterolateral system, however the trigeminothalamic tract runs closely to the anterolateral system. Why then in such a lesion will you not lose sensation in both sides of the face? The descending fibres from the ipsilateral side is affected and the trigeminothalamic fibres from the contralateral side running close to the anterolateral system is affected
If you recall the picture at 1:00:00, you can see the spinal nucleus carries general somatic afferent input fibers from the ipsilateral part of the face. They haven't crossed over, to the trigeminothalamic tract that runs on the opposite side of the brainstem, yet. Therefore you will knock out the fibers on the ipsilateral side of the face, but also the contralateral spinothalamic fibers on the opposite side of the body (remember the spinothalamic tracts already decussated at the level of the spinal cord), these fibers run just next to the spinal nucleus in the medulla coming in from the opposite side of the body, hence a checkerboard fallout of sensation will occur.
Hi Dr, with regards to the INO (54:22) why is the lesion then called an ipsilateral adduction deficit? Is the 'ipsilateral' part named as such because of the MLF being on the same side as the deficit? The fact that the opposite abducent nerve is supplying the initial stimulus, is confusing me with regards to the contra-/ipsilateral naming.
Henry Jordaan
Yes you are correct. The ipsilateral part is due to the lesion sitting in the MLF ipsilateral to the lesion in the brainstem. 52:21 shows the picture nicely (lightning strike is the lesion). therefore the 3rd CN nerve won't know that the 6th CN nerve is working therefore it won't adduct, but the 6th CN nerve will be the one that is still doing all of the work - abduction, but the 6th CN will now be on the opposite side of the lesion.
Slide 45 - how would you decide whether the most likely localization is left or right as the patient has left facial muscle weakness, but opposite (right-sided) arm and leg weakness?
Cranial nerves always have ipsilateral fallout (except for the pesky trochlear nerve), therefore if you have a patient with cranial nerve dysfunction you have to think the dysfunction is ipsilateral to the nucleus, fascicle or nerve itself. So if the patient has ipsilateral facial weakness but opposite arm and leg weakness then the lesion has to be in the pons because you have that checker-board pattern (ipsilateral face, contralateral body)
Hi Dr, I would just like to confirm the answer for the case study of the trochlear nerve. Was it a left ventral pons lesion?
Are you referring to the case study of the 3rd CN or 4th. The 3rd CN had a lesion in the ventral medial pons. The pictures that I showed at 33:52 for the trochlear nerve should have had a lesion on the left dorsal midbrain at the level of the inferior colliculus if you wanted to place the lesion in the midbrain (remember, Trochlear nerve is the only one that exists dorsally and decussates to the opposite side)