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Question about cranial nerve lesions

Discussion in 'Medical Students - MD' started by Fah-Q, May 31, 2002.

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  1. Fah-Q

    Fah-Q Senior Member
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    I am studying for step 1 and First-Aid has a page about cranial nerve lesions and the associated physical findings. I am having trouble understanding/believing their description of CN V and XI lesions. It says in CN V lesion the jaw deviates toward the side of the lesion. Is this during opening? closing? at rest? all of the above? And why? Also, it says that a CN XI lesion causes the head to deviate away from the side of the lesion. This one makes no sense to me. Please help.
     
    #1 Fah-Q, May 31, 2002
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  2. Chris_P

    Chris_P Senior Member
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    As far as the accessory nerve is concerned, if the lesion is above the point where it enters sternocleidomastoid, that muscle as well as trapezius will be paralysed and the unopposed action of the other scm will cause the head to be turned away from the side of the lesion. This won't happen if the lesion is below sternocleidomastoid (in the posterior triangle - most common area of injury). In this case the patient will be weak on the side of injury when asked to shrug shoulders.

    I'll look up about the CNV injuries - more often they are sensory than motor (ie trigeminal neuralgia).

    Chris
     
    #2 Chris_P, May 31, 2002
  3. Zeffer

    Zeffer "My dog ate em. I swear thats the truth!"
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    For CN V the V3 branch (mandibular portion)innervates the medial, and lateral pterygoids, masseter, and temporalis muscles (muscles of mastication). The muscles of mastication not only chew the food but maintain the position of the mandible by supporting the TMJ (Temporal Mandibular Joint). When lesioned muscles on the same side of the lesion no longer have tone on the side of the lesion, thus there is no opposing force to the musculature on the normal side.

    The reason for the injury to XI is similar. With no innervation to the musculature (SCM and trapezius) tone is lost to the side of injury while the other side is just fine. With no opposing action to the unlesioned side the head will deviate to the unlesioned side.

    Hope this helps. Good Luck
     
    #3 Zeffer, May 31, 2002
  4. Fah-Q

    Fah-Q Senior Member
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    </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Chris_P:
    <strong>As far as the accessory nerve is concerned, if the lesion is above the point where it enters sternocleidomastoid, that muscle as well as trapezius will be paralysed and the unopposed action of the other scm will cause the head to be turned away from the side of the lesion. This won't happen if the lesion is below sternocleidomastoid (in the posterior triangle - most common area of injury). In this case the patient will be weak on the side of injury when asked to shrug shoulders.

    I'll look up about the CNV injuries - more often they are sensory than motor (ie trigeminal neuralgia).

    Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Here is what I am having a problem with. First of all, when I say "turn" I mean to rotate the head like you were looking to your left. Also, when I say "tilt" (ie, tilt head to left) I mean to basically put your left ear on your left shoulder. As far as I understand, the action of the SCM contracting unilaterally is to turn the head to the opposite direction (the LEFT SCM turns the head to the RIGHT). Contracting SCM bilaterally cuases flexion of the neck. So, losing your LEFT SCM due to LEFT CNXI lesion would cause the unopposed action of the RIGHT SCM to turn the head to the LEFT (towards the side of the lesion). I have also read in Moore that the SCM also tilts the head when contracted unilaterally. I am assuming that if the deviation is really TOWARDS the side of the lesion that it is because the tilting action of the unopposed SCM is more powerful than the turning action. Is this correct? Are we thoroughly confued yet?

    As for the jaw deviation, I understand now. I had forgotten the origin is the sphenoid bone and the insertion (movable end) is the mandible. So, contracting the right pterygoids would cuase the jaw to move to the left. So a lesion of the right would cuase the unopposed action of the left to cause a deviation to the right (same side as lesion). Right?
     
    #4 Fah-Q, May 31, 2002
  5. Dodge This

    Dodge This Senior Member
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    Here's my understanding of CN XI.

    Corticobulbar innervation of the accessory nucleus is primarily ipsilateral. If you have an upper motor neuron lesion on the LEFT, the LEFT sterncleidomastoid will be paralyzed, making you unable to turn your head to the RIGHT. Similarly, a LMN lesion of the LEFT accessory nerve, will paralyze the LEFT SCM, also causing inability to turn the head to the RIGHT.

    I thought that when the head is midline, neither SCM is contracting, so it's not like the unopposed SCM would act to pull the head in one direction. The deficit would be seen when the patient actively moves the head.
     
    #5 Dodge This, Jun 1, 2002
  6. Chris_P

    Chris_P Senior Member
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    Hi

    You are right - the action it is more of a tilt than a turn towards its own side. The rotation towards the opposite side. You can see the action of scm demonstrated in someone with torticollis. <img src="http://www.clenchingsyndrome.com/18593730.png" alt=" - " /> It looks like the head is pulled to the side of the contraction but there is a slight rotation to away from the side of contraction.

    The classic test for scm injury is to ask the patient to turn their head against the force of your hand. If they have a weakness it indicates the scm OPPOSITE to the direction of movement may be weak.

    There are plenty of other muscles involved in rotation and postitioning of the head so Dodge This is right when he says that it would be difficult to spot when the head is held in the midline.

    Chris
     
    #6 Chris_P, Jun 2, 2002
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