Midbrain Syndromes : Weber’s Syndrome, Benedict’s Syndrome and Parinaud Syndrome.

            

In this article we're gonna talk about and learn about mid-brain syndromes. In order to talk about mid-brain syndromes we first need to revise the rules of four for the brain stem which is a simplified method that is used to understand brain stem anatomy and brain stem vascular syndromes.

Rule of Four

So the rules of four as the name suggests there are four rules and within each of these rules it is all related to four . Basically  the first rule of the rules of four is that :

First  Rule:

There are four structures in the mid line of the brain stem beginning with m and these are the motor pathways or the cortical spinal tract which passes the midline .The medial Le meniscus the medial longitudinal fasciculus and the pathway as well as the motor nuclei and cranial nerves which are also located the nuclei in the midline.

Second Rule:

The second rule of the rule of fours is that there are four structures to the side or lateral beginning with s. So s for sides and these structures are the spinocerebellar pathway the spinal thalamic pathway the sensory nuclei of the fifth cranial nerve are located laterally as well as the sympathetic pathway.

Third Rule:

The third rule of the rule of fours is that there are four cranial nerves in the medulla which is the bottom part of the brain stem there are four cranial nerves in the pons and four cranial nerves above the pods. Two in the midbrain and two above. So again above the pons there are cranial nerves one, two, three and four cranial nerves three and cranial nerves four are within the midbrain in the ponds .

There are cranial nerves five ,six , seven and eight and in the medulla there are cranial nerves 9 10 11 12. 

Fourth Rule:

The last rule the fourth rule of the rule of force is that the four cranial nerve nuclei that are in the midline can be easily remembered by being divisible or that can divide equally into 12 except one and two so the four cranial nerves. 

Division of Cranial Nerve:

I'm talking about that are in the midline and divide equally into 12 include cranial nerves three cranial nerves four,  six and twelve cranial nerves 5, 7, 9 and 11 are in a lateral aspects of the brain stem what makes this even more interesting is that the cranial nerves at the midline so 3, 4 , 6 and 12 are all motor nerves.

So now that we have revised the rule of force of the brain stem we can now use what we have learned to look at the midbrain syndromes and we're going to look at four midbrain syndromes in particular.

Cases

So in the first syndrome,,  let's talk about a case .The case is of a woman that presents with sudden right-sided diplopia and ptosis with a Down and  Out eye on that right side .She also complains of left-sided weakness of her Upper Limb and lower limbs .

Weber's Syndrome:

These signs and symptoms are caused  by occlusion of a branch of the posterior cerebral artery causing what is called Weber's syndrome or ventromedial midbrain syndrome foreign is on the right side and what this results in is ipsilateral oculomotor nerve palsy .

So right-sided in this scenario causing diplopia ptosis and the eye going down and out.  It also causes right pyramidal motor track lesion involvement in the midbrain and so this will result in contralateral hemiparesis.  Because remember the motor fibers actually decassate in the medulla.

Benedict's Syndrome:

The second Syndrome again a case of a man that presents with right-sided diplopia and ptosis and a Down and Out eye but this time the man actually presents also with left-sided sensory changes specifically vibration and proprioception loss he also has left-sided hemiparesis and Associated ataxia this presentation is caused by a condition called Benedict syndrome .

Cause:

Benedict's syndrome occurs due to a lesion in the tegmentum of the midbrain as well as the cerebellum The lesion can be due to infarction Hemorrhage tumor or infiltrative conditions such as tuberculosis.  It can result from occlusion of the posterior cerebral artery or its penetrating branches as well as the branches of the basilar artery in this case the lesion is on the right side again .

Right-sided Benedict syndrome:

Current textbooks vary considerably in the definition of the structures involved with this lesion what you get again with this right-sided Benedict syndrome is ipsilateral so right-sided ocular motor nerve palsy causing diplopia ptosis and the dad and out eye the right limit is  contraction  the midbrain will cause contralateral sensory changes to vibration and proprioception the red nucleus involvement will cause contralateral Ataxia.

Claude Syndrome:

Specifically the motor pathway the right pyramidal motor tract involvement in the midbrain will cause contralateral hemiparesis. What I have not drawn here is that the superior cerebellar peduncle can also be affected and this will cause ipsilateral sensory ataxia.

The third midbrain syndrome without looking into a case is called Claude syndrome and it's used to describe ocular nerve palsy on the same side of the lesion with contralateral ataxia this is caused by lesions involving the third nerve fascicle itself .

Reason of  Contralateral Ataxia:

Contralateral Ataxia is due to involvement of the red nucleus which is motor but again in the same area one can get ipsilateral Ataxia so on the same side due to involvement of the superior cerebellar peduncle this might get a bit confusing but I hope that made sense.

Parinaud Syndrome:

The fourth and final midbrain Syndrome again we will look at a case here you have a 75 year old man who presents with impaired upward gaze nystagmus and bilateral lid retraction.  When you examine the old man you notice that his pupils do not react to light , so there is impairment of light reaction . 

However accommodation response is intact this last part specifically where one does not respond to the light reflex but can accommodate and people's constrict is known as Argo Robertson pupil.

Paranoid Syndrome:

And all these signs and symptoms are due to a condition that can collectively be called paranoid syndrome also known as the dorsal midbrain injury to the amazing Catholic tectum. The back of the midbrain causes of paranoid syndrome are brain tumors in the pineal or midbrain demyelination within the area or stroke of the upper brain stem .

Convergence Retraction Nystagmus:

Another manifestation of paranoid syndrome is Convergence retraction nystagmus and in order to understand this you have to appreciate the diagram which shows you that you have these Supra nuclear centers which have nerve fibers that inhibit the cranial nerve 3 nucleus so damage of the midbrain supranuclear fibers which normally inhibit the third nerve nucleus .

Extraocular Muscles

Essentially preventing activation of the extraocular muscles innervated by cranial nerve 3 will be disrupted and therefore these extraocular muscles ,  the medial,  Superior and inferior rectus muscles will have constant stimulation the eyes will retract. 

Thanks to the superior and inferior rectus contraction and the eye will look medially due to medial rectus contraction and again this is all because your cranial nerve 3 is active and there's no inhibition to that cranial nerve 3 nucleus.

Finally what you see in paranoid syndrome is Argyl Robertson pupil also known as light near dissociation where one can accommodate but one does not respond to light the pupils do not constrict to light and this again is due to injury or lesion to that posterior aspect or the back of the midbrain.

Summary

So in summary in this article in particular we talked about the rules of four and mid-brain-syndromes ,specifically Weber's syndrome Benedict syndrome Claude syndrome and lastly paranoid syndrome.