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NBME 20 Answers

nbme20/Block 1/Question#16
A 30-year-old woman with multiple sclerosis ...

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submitted by thecatguy(11),

This is a very nitpicky question. As I see it, the 3 main concepts tested are:

  • This patient has trigeminal neuralgia (sharp, brief, episodic pain in the face), which is caused by a lesion to primary sensory fibers that carry pain sensation from the face.
  • Multiple sclerosis, which the patient has, is a demyelinating disease (i.e., it affects white matter).
  • The myelinated axons carrying pain sensation from the ipsilateral face enter the brainstem at the level of the pons and then descend (become the spinal tract of the trigeminal). These white matter fibers pass through the pons to synapse on the spinal nucleus of the trigeminal, which is in the medulla (nucleus = gray matter). (picture here). Therefore, a lesion in the white matter (i.e., plaque) in the pons could cause trigeminal neuralgia, and this phenomenon has been observed.

Above the level of the brainstem (thalamus & cerebral cortex), you have second order sensory neurons. Lesions in this part of the circuit are not generally in trigeminal neuralgia. I suppose they also want us to assume that once the spinal tract of the trigeminal enters the medulla, it's not myelinated anymore. I don't think this is completely true, but given the logic described above, pons would still be the better answer.

As people have pointed out, the primary sensory fibers carrying light touch sensation from the face synapse on the chief sensory nucleus in the pons immediately after they enter the pons. This question is not asking about those fibers though.

I got the question wrong too..

mightymito  Wow this is the best explanation yet! Thanks so much for very clearly walking us through a tricky question. +2  

 +6  upvote downvote
submitted by whossayin(13),

The mnemonic I like for remembering the locations of the cranial nerves is the "2,2,4,4 rule"

Above brainstem= CN I + II Midbrain= CN III, IV Pons= CN V, VI, VII, VIII Medulla= CN IX, X, XI, XII

Principle Sensory Nucleus of the Trigeminal is located in the Pons, as is the Motor Trigeminal Nucleus of the pons. This presentation is probably dealing more with the Principle Sensory Nucleus.

 +2  upvote downvote
submitted by strugglebus(129),

The pons has nerves 5-8, so the trigeminal would be affected here

masonkingcobra  Thalamic pain syndrome would involve dysesthesias on the entire contralateral body so more than just the face. Also it occurs often after post-stroke. Additoinally, these dysesthesias appear weeks/months later +3  

I do understand that the principal nucleus of the trigeminal nerve is located at the pons. But since this patient is having trigeminal neuralgia wouldn't you be disrupting the pathway involved in pain and temperature, which in this case would be the spinal nucleus (located in the medulla)?

Besides, I found this at an article

A recent hypothesis attributes the pain of trigeminal neuralgia to a central mechanism involving the pars oralis of the spinal trigeminal nucleus.[5]


pg32  I literally had medulla selected the whole time and then changed it to pons simply because i felt the test writers were just seeing if we knew where the trigeminal nerve was located. bummer because I think your logic is way better. it's what i first thought when i read the case. +  

I also choose medulla, b/c this patient presented w/ pain, but not any abnormalities in sensation. Pain is mediated by spinal trigeminal nucleus located in Medulla, not Pons

+/- ssc505684708(-3),

MS is a demyelinating disease. That's why I didn't pick cortex or thalamus; they are grey matter. I picked medulla because I the spinotrigeminal nerves traverse the medulla. I guess we are just supposed to assume the demyelination is happening in the pons? I don't know.

woodenspooninmymouth  Sorry, I meant to say that the pons would have the cell bodies for the pain/temp neurons, no? +  

 +0  upvote downvote
submitted by mamed(0),

Per boards and beyond, in the lateral pons is there is the spinal V nucleus which carries contralateral pain and temperature. Knowing that + CN V arises it in the pons, I went with pons. The only other option I had left not crossed out was cerebral hemisphere but that seemed to broad.

theres an association made in 2017 with trigeminal neuralgia and MS - which is what i think this pt has. key thing to note is that while all motor and sensory fibers of CN 5 enter at the level fo the pons (however some also do enter at the level of medulla and even the sc via the spinal tract of 5 to synapse with the long sensory nucleus of 5) heres the link of trigeminal neuralgia and MS: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5649347/

 +0  upvote downvote
submitted by lilyo(32),

I was thinking along the lines of facial sensation which is mediated by the trigeminal nerve and the fact that the trigeminal nerve is located in the pons.

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