midbrain

pons

The medial lemniscus, which is related with proprioception and vibration of sense

The corticospinal tract can also be seen at all levels of the brainstem.

 In front of the medial lemniscus is the ventral brainstem.

The ventral part of the midbrain and pons contain the corticospinal, corticobulbar, and corticopontine tracts.

corticobulbar tracts

corticopontine tracts.

The caudal medulla contains only the corticospinal tract.

In the midbrain, the corticospinal and corticobulbar tracts are located in the midportion of the cerebral peduncle.

In the pons, the corticospinal tract travels anteromedially.

We are looking from posterior. Behind the medial lemniscus is the dorsal brainstem.

The medial longitudinal fasciculus interconnects the nucleus of the oculomotor, trochlear and abducens nerves to coordinate ocular movements.

 the central tegmental tract 

facial colliculus

The MLF travels next to the midline.

The central tegmental tract is a part of the extrapyramidal system and its damage may result in intention tremor or nystagmus.

The central tegmental and trigeminal mesencephalic tracts are positioned between the superior cerebellar peduncle laterally,and sulcus limitans medially.

After looking at the ventral and dorsal neurocritical fiber tracts of the brainstem,we can go through the safe entry zones.The safe entry zones in the midbrain that we will discuss are the perioculomotor zone used for lesions located in the ventral midbrain

the supra- and infra-collicular areas for lesions in the dorsal midbrain, 

and the lateral mesencephalic sulcus for both ventral and dorsal located lesions. 

In the pons, the peritrigeminal zone is used for ventral,

and the supra- and infra-facial collicular approaches for dorsal pontine lesions.

In the medulla, the anterolateral, postolivary and dorsal medullary sulci have been proposed as safe entry zones.

The surgical target area for reaching the perioculomotor zone is the ventral surface of the midbrain.

The perioculomotor zone is bordered by the posterior cerebral artery superiorly, and the superior cerebellar artery inferiorly.

The transylvian route by the pterional or orbitozygomatic approaches can be used to reach the perioculomotor zone.On the left side, the uncus

 and oculomotor nerve can be seen here

  and the posterior clinoid and basilar artery also seen here.

The perioculomotor approach is directed through the frontopontine fibers,

and is bordered medially by the exit point of the oculomotor nerve

and laterally by the medial edge of the corticospinal tract.

The frontopontine fibers were removed to expose the medial lemniscus, which is the border between the ventral and dorsal mibrain.

  The corticospinal tract is the lateral limit of this exposure.

An entry through the perioculomotor zone can reach the ventral midbrain, which is in front of the medial lemniscus, or dorsal mibrain, which is behind the medial lemniscus.

  Avoiding damage to the intramesencephalic segment of the oculomotor nerve is important.

This segment arises from the oculomotor nucleus and passes medial, and inside the red nucleus,to exit the midbrain through the interpeduncular fossa.

Another entry zone in the midbrain is the lateral mesencephalic sulcus, which is positioned on the dorsolateral surface of the midbrain.The lateral mesencephalic sulcus is accesible through the subtemporal approach.

  This is a left subtemporal view.

  And tentorium is right here.

The lateral mesencephalic vein runs into the lateral mesencephalic sulcus.

Known complications in this approach are the ophthalmoparesis due to the manipulation of CNs III and IV, and the venous infarction caused by damage to the vein of Labbe complex.

 The lateral mesencephalic sulcus can also be reached by the lateral supracerebellar infratentorial approach.

  The sulcus runs on the dorsolateral surface of the midbrain, 

  and extends from the pontomesencephalic sulcus inferiorly, 

and medial geniculate body superiorly.

The sulcus also corresponds to ventral surface of the medial lemniscus.

Critical structures at risk in this area are the corticospinal tract in the ventral midbrain,  and the red, oculomotor and trochlear nuclei in the dorsal midbrain.

oculomotor and trochlear nuclei in the dorsal midbrain.

Entry at a right angle through the tectal surface encounters the red nucleus.

 While, entry at a 45 degree angle reaches the medial lemniscus.

The depth of the red nucleus from the lateral mesencephalic sulcus averaged 5 mm.

The surgical target area for the pericollicular entry zones is the dorsal surface of the midbrain. 

The pericollicular entry zones can be reached by way of the supracerebellar infratentorial approach, which also exposes the pineal gland, superior and inferior colliculi.

Transverse incisions are made just above or below the colliculi depending on the location of lesions.

The nuclei of the superior and inferior colliculi are located just deep to the surface.The superior colliculus is involved in the reflex movement of eyes and head,

  and inferior colliculus involved in the auditory system.

  The deep structure is the red nucleus in the dorsal midbrain, that extends up from the midlevel of the inferior colliculus.

 In further dissection, bilateral red nuclei,and decussation of the superior cerebellar peduncle, which is positioned at the level of the inferior colliculus, have been exposed.

  The central tegmental tract runs just ventral to the trigeminal mesencephalic tract.

This lateral view demonstrates that the oculomotor and trochlear nuclei are positioned ventral to the aqueduct.

  The intramesencephalic segment of the oculomotor nerve 

The intramesencephalic segment of the oculomotor nerve passes through and medial to the red nucleus to exit through the interpeduncular fossa.

  The red nucleus extends up from the midlevel of the inferior colliculus to the lateral wall of the third ventricle.

  The subthalamic nucleus is located ventrolateral,

  and the medial lemniscus is located lateral to the red nucleus.

  Here is the lateral mesencephalic sulcus, which corresponds to the most lateral edge of the medial lemniscus.

This illustration shows the surgical target area for reaching the peritrigeminal zone for removal of the ventral pontine lesions.

Using the retrosigmoid approach in reaching the peritrigeminal zone, one can see the trigeminal

  and facial nerves here.

  The anterior petrosectomy, also called as Kawase approach, can also be used for exposure of the peritrigeminal entry zone.

  The trigeminal and the facial nerves can be seen.

  The peritrigeminal zone incision is made between the trigeminal and facial nerves.

Structures at risk are the intrapontine segment of the abducens and facial nerves inferiorly, intrapontine segment of the trigeminal nerve superiorly, trigeminal spinal tract, trigeminal motor and main sensory nuclei posteromedially, and corticospinal tract anteromedially.

  and facial nerves inferiorly,

  intrapontine segment of the trigeminal nerve superiorly,

   trigeminal spinal tract,

  trigeminal motor and main sensory nuclei posteromedially,

  and corticospinal tract anteromedially.

  The surgical target area for dorsal pontine lesions is the floor of the fourth ventricle.

  The median suboccipital approach can be used to reach the floor of the 4th ventricle.

  The most important structure to avoid in this area is the facial colliculus.37% of facial colliculi did not form a distinctive prominence in the floor.

  The facial colliculus, which is formed by the abducens nucleus and intrapontine segment of the facial nerve

 curves around the abducens nucleus, and then runs forward to exit the brainstem.

   On the floor, the superior fovea has a triangle shape.

  Its superolateral edge is formed by the superior cerebellar peduncle,

  its inferolateral edge formed by the vestibular area,

   and its medial base formed by the sulcus limitans.

  The lateral base of the superior fovea triangle is an important landmark for the upper edge of the facial colliculus in case where the prominence of the facial colliculus is not well defined.

  Also, the trigeminal motor nucleus is located deep to the superolateral edge of this triangle.

  For dorsal pontine lesions, the entry zones above and below the facial colliculus, are used.

 inferiorly by the facial colliculus,

 medially by the medial longitudinal fasciculus,

  and laterally by the sulcus limitans,

  which is situated just medial to the trigeminal nuclei, trigeminal mesencephalic and central tegmental tracts.

  The infrafacial approach is bordered by the facial colliculus superiorly, hypoglossal triangle inferiorly,

  medial longitudinal fasciculus medially, and facial nucleus together with nucleus ambiguus,which are the same coronal plane, laterally.

   The surgical target area for ventral medullary lesions has been shown here.

 The far lateral approach is used to reach the anterolateral and postolivary sulci.

  postolivary sulci

   hypoglossal nerve and rootlets of the C1 

  The longitudinal incisions are made between hypoglossal nerve and rootlets of the C1 in the anterolateral sulcus, or in the postolivary sulcus.

  The critical structure in the approach through the anterolateral sulcus is the corticospinal tract.

  The nucleus ambiguus must be taken into account when entering through the 

postolivary sulcus.

  intermediate sulci

  lateral sulci

We'll start to think about that, if you're gonna expose that area, what approach are you going to use? We're on the lateral side of the peduncle, and you wanna expose this area. What approach are you going to use?Anyone?You'll use...how about subtemporal approach?

  Now, what if you wanna take that subtemporal approach and extend it downward to the area below the trigeminal nerve, what do you call that approach? That will be a... a Kawase's anterior petrosectomy approach.

Now, if you want to expose the interpeduncular fossa, or below basilar bifurcation,coming down the midline and the approach is one that we'll deal with tomorrow that we call transcavernous approach.

But it delivers you from the front through a OZ crainiotomy down to the interpeduncular fossa.

Now, what approach have we looked at today that exposes the area from the lower edge of the trigeminal nerve down to IX and has VII and VIII in the center?What are you going to call that approach? Anyone?We've looked at it already. That's what approach? Translabyrinthine approach, or retrolabyrinthine.

And if you wanna extend it medially, then what do we call that approach up to the midline, along the clivus, adjacent to 6th nerve, that's...transcochlear approach.

And then if you wanna take these approaches...a translabyrinthine or retrolabyrinthine,and extend it along the lateral brainstem, above the tentorium,  that's a combined supra-infratentorial presigmoid approach.

  And you can do a retrolabyrinthine, translabyrinthine. Or if you want to extend it medially,then you can do a transcochlear combined supra-infratentorial presigmoid approach.

Now the workhorse of the posterior fossa we talked about initially and you can see IV, V, it gets you below VII and VIII, to IX, X, XI, and you even have access to XII, in retrosigmoid approach.

So it's really the workhouse approach of posterior fossa.

Now, for this area, this'll be part of the focus tomorrow. But,you can come to collicular plate area,pineal,either supra- or infratentorial approach from the back.

  And if you wanna access 4th ventricle,  we call it... telovelar approach.

  And, so these are the approaches that we often use from the back side.

  And we've talked about this approach which is to lateral medulla, foramen magnum, far lateral approach.

We can also come transoral. And today, some of the endoscopic approaches, 

here's the areas that you can reach at times using the endoscopic approaches.One great thing about the endoscopic approaches is they deliver the operative focus to the area between the cranial nerves.

The subtemporal, retrosigmoid, anterior petrosectomy, translabyrinthine, far lateral, all come in on the lateral side of the cranial nerves.

Anytime you go from lateral to medial, working from lateral medial to the cranial nerves,the risk of the operative approach goes up.

And so, the endoscopic transnasal approaches are gaining wider usage today for...much of this pathology.We still have problems with those, with CSF leak and closure, but still as we understand that anatomy better, we're getting better with lower complication rates with these approaches.

So, let's just take a quick look at each of them in 3D. Here's subtemporal approach.

We're on the left side, occipital back here,

and here we work under the temporal lobe, and we get...it delivers us down to the side of the peduncle

at the midbrain-pons junction.

We see the...Trochlear

And we can get a little bit lower if we divide the tent here

   But it gives us this lateral side of the peduncle here between III and IV,

   posterior cerebral (artery).

Now if we want to extend that approach downward we can use the Kawase's approach. 

So that, we can go to the internal acoustic meatus, 

  or we can drill out the petrous apex under the trigeminal nerve

down to the side of the clivus, along the inferior petrosal sinus and the 6th nerve in the anterior petrosectomy approach.  

for this approach, we want to work medial to the internal acoustic meatus We don't want to enter the meatus that we've looked at.

 We want to preserve the cochlea angle,

and we'll work behind the greater petrosal nerve, and take out the petrous apex,

drill down to the side of the clivus along the inferior petrosal sinus,

under the superior petrous sinus.

We can elevate the temporal lobe then and have access to the peduncle and midbrain.

And then we can work above and below the trigeminal nerve, down to the front of the brainstem, and along the side of the clivus.

Always in these approaches, we want to preserve vein of Labbe, and the...what nerve...trochlear nerve when we divide the tent.

And, it's one of the most commonly used routes to the low basilar bifurcation to cavernomas of the anteolateral brainstem.

Now, transcavernous approach that delivers us to the interpeduncular fossa.

here we're...optic nerve,carotid.

Usually in this area we don't work between the optic nerve and carotid, but we elevate the carotid and work between the carotid and 3rd nerve below the posterior communicating artery.

And we take off the anterior clinoid,

and then we elevate the carotid and work between the carotid and the 3rd nerve.

And the 3rd nerve, for it enters the roof of the cavernous sinus, is in a short cistern. And we can pass a nerve hook into the cistern, and open the oculomotor cistern,

And then we can work between the carotid and 3rd nerve to expose posterior clinoid,dorsum,

and then we can drill off the dorsum and posterior clinoid,

  Then for this part of the brainstem, the collicular plate, the pineal region,

  we can come in ether above or under the tentorium. And...

if we come in under the tent over the apex of the vermis, it's very difficult to retract enough in the midline in a median approach.

to get down from the superior to the inferior colliculus to the 4th nerve.

But this is the median approach, under the tent,over the apex of the vermis.

We can see pineal, internal cerebral (vein),

  But if we move off paramedian, off the apex of the vermis, then we can get in to superior, inferior colliculus. 

  We can see the 4th nerve here, below the inferior colliculus.

We have access to SCA, 

  posterior cerebral artery.

  We can even work all the way to thepeduncle here and the area of the crural cistern.

  But usually to get down to the inferior colliculus, if you're coming in below the tent,you need to come in paramedian off of the apex of the vermis.

  And, you can get in...retract the occipital lobe,

   expose pineal, posterior falx, tent.

  have access to the pineal, and you also get access to superior and inferior colliculus, and tectal plate.

  So these are approaches to the posterior part of midbrain. And you have access also to posterior corpus callosum coming in occipital transtentorial.

   Now we move down in the midline  telovelar approach

  And telovelar approach we've talked about... we retract the tonsil away from the uvula.

  You see the tela below the inferior medullary velum.

  You open the tela, you have access all the way up to the aqueduct.

  You open the velum, you have access to superolateral recess.

  Here's the superior peduncle passing from the dentate that wraps around the superior pole of the tonsil, and is passing up toward the red nucleus.

  But dividing the tela gives you access to most of the floor and accessing the velum, and opening it gives you access to the superolateral recess.

   So that, rather than dividing the vermis which can cause a mutism syndrome, in young people it's usually transient,

  we retract the tonsils away from the side of the uvula.

  We open the tela above Magendie to access the floor of the 4th.

  We open the velum to access the superolateral recess.

   And then if the pathology extends out of the lateral recess.

   Its roof, below the lower half of the roof of the lateral recess, is formed by tela and velum.

  We see the dorsal cochlear nucleus, the site of the auditory brainstem implant,in the lateral recess.

  So to access the lateral recess, we elevate the tonsil,

  we see the tela lining the lower margin between Magendie and Luschka.

  We open the tela, and we have access to the lateral recess.

   So using telovelar approach, we have access to all of the floor of the 4th, the peduncular mass, But it accesses all of these areas of the 4th ventricle.

   the superolateral recess, and over to the lateral recess.

  And, here we see again, the dorsal cochlear nucleus, the site of the auditory brainstem implant,

  here, the velum, a paper-thin membrane 

  the velum,lining the lower margin of the superolateral recess.

   Now, the workhorce of this area, we've talked about is retrosigmoid approach

  gives us access from above V along the lower margin of X down to the foramen magnum. 

  it accesses all of this area of this CP angle formed by cerebellum 

  folding around the lateral pons and the middle cerebellar peduncle.

  So that we can work from IV and SCA down to V along the internal acoustic meatus

   down to IX, X, XI.

  And the approach also gives us access into the internal acoustic meatus,

   the nerves at the fundus. And using the approach,we can drill further lateral and into the labyrinth.

  For a more localized approach to the CP angle, from usually V at the upper edge, IX at the lower edge, maybe you see X, depending on the height of the jugular bulb, we can use a translabyrinthine approach,

  So that, we work through the mastoid. 

  We find the mastoid antrum superficial 

  Or we can drill out the labyrinth, we can do a partial labyrinthectomy, 

  with some chance of preserving hearing in which we drill the superior and posterior canal.

   But if we need to get into the internal acoustic meatus, for pathology that extends into the meatus, we have to sacrifice hearing and drill out the labyrinth, to get into the internal acoustic meatus.

  If the pathology is medial to the porus in the CP angle, we can just open the presigmoid dura,preserve the labyrinth, and do a retrolabyrinthine exposure.

  And if we need to get medial then, we can work around the facial nerve,drill out the cochlea,

  that delivers us through the petrous apex,

  down to the front of the pons,

  along the edge of the clivus and the inferior petrosal sinus,

  to the side of the basilar artery.

  And then if we need to combine exposure of this area, with exposure along the lateral brainstem,then we can do a combined supra-infratentorial presigmoid approach.We can do that retrolabyrinth,translabyrinth,

  or we can extend it medial drilling out the cochlea. 

  So, this is the combined supra-infratentorial presigmoid approach.Here we have the labyrinth.  

   we open the temporal dura, open presigmoid,

   and, the exposure of the posterior fossa.

  We divide the tent, preserving IV,

   an important consideration always is Labbe.

  And then if the pathology is further medial, along the clivus,we can move the facial nerve backwards,

  and work through the petrous apex

  in a transcochlear approach that delivers us down to the side of the clivus along the 6th nerve.

  We divide the tent, preserving IV, in a combined supra-infratentorial presigmoid approach,and in this case, the transcochlear variant.

  The soft palate hides this area, so it really is difficult to get much above the edge of the foramen magnum,

  but it gives you C1, the odontoid.   

  here we have the uvula

   here we see longus colli,

  longus capitis.

   Dividing the uvula gives you access to lower clivus, anterior arch of C1.

  And you can access some of the lower brainstem transoral. But, today many of these approaches are being converted to transnasal endoscopic approaches.

    Here's just a far lateral approach. 

  we work through the suboccipital triangles. 

  We reflect the muscles of the suboccipital triangle.

  And here's just the far lateral exposure. It's better than just retrosigmoid,but still the condyles can give a significant obstruction to the view of the clivus and the front of the brainstem.

  But with the... if you drill out the condyle here,you can pull this dura further lateral for access to the front of the brainstem or to the lower clivus 

  and, you can drill out, then, transcondylar,and get access to the lower clivus,for lesions involving lower clivus,

  or extending across in front of the medulla to the contralateral vertebral artery.

  And then, what can you see with the endoscopic approaches? Well, the endoscopic approaches today are being used to give us access from anterior fossa down along the front of the brainstem, down to the area of foramen magnum,

  and they have the advantage of coming in medial to this line of cranial nerves that you have to work across coming from lateral.

   So, just a quick look at the clivus which we divide into upper, middle and lower parts.

  And, as we look at the clivus, well... you can do very focal approaches to the lower, middle or through the upper part of the clivus and sphenoid sinus to the area of the cavernous sinus.

  But, lower clivus, you can access hypoglossal canal, medial edge of jugular foramen.

  Mid clivus gives you access to VII and VIII,basilar artery.

 And then through upper clivus or sphenoid...through this back wall of the sphenoid sinus, you have access to medial part of cavernous sinus. 

   here's lower clivus. 

   Here's atlanto-occipital condyle,

  hypoglossal canal, here, jugular foramen,

  Here, the hypoglossal canal has been drilled out. We see IX, X, XI.

  Then we can...you can work to the other side, IX, X, XI,

  12th nerve coming through hypoglossal canal. 

 You can...here we just see hypoglossal canal, IX, X, XI,

  and on the other side. 

    here we go up to the upper third of clivus that sits in the posterior wall of sphenoid sinus.

  working through this area, you have access to cavernous carotid,pituitary,

   and here the medial wall of the cavernous sinus.

  Here we see right cavernous sinus, III,

 6th nerve,

  V2 down in this area,

  V1 here.

  And then you can look through this area to the other cavernous sinus, looking over the cavernous carotid,ophthalmic artery.

  And, when you remove that posterior wall of upper clivus, you have access to mamillary bodies,basilar apex,

   posterior communicating,

   P1,P2.

  mid clivus here,

   when we drill it off, you have access to mid basilar artery,

   pons.

   Here you see the vidian nerves preserved in the vidian canal. 

  that pass through the clivus and anterior fossa along the medial side of the cranial nerves.They don't put the nerves at risk from coming from lateral to medial.

  But, here's just the endoscopic view of the area from sella. You can along a  planum all the way down to foramen magnum.You can get to odontoid. 

  But we see anteromedial cavernous sinus,

  6th nerve passing through the dura, entering the cavernous sinus,

  Here we've drilled off to one side, here the petrous apex.And we work below the trigeminal nerve, 

  We see some of the superior petrosal veins. This vein coming off of the pons, 

   VII and VIII from the front side.  

Just think about a focal approach to this area for a decompression of the facial nerve right here, or microvascular decompression of the trigeminal nerve. Is that going to happen by this route? I think someday it probably will,

So that, for coming in along the midline in front, we do have an approach that will deliver us medial to these lateral approaches that cross the cranial nerves.