Dorsal midbrain syndrome, Parinaud syndrome

Summary

The video script discusses the dorsal midbrain syndrome, focusing on the midbrain's dorsal aspect and its impact on ocular functions. Key features include lid retraction, light-near dissociation in pupils, convergence retraction movements, and potential gaze palsies. It explains how lesions in this region can disrupt various ocular motor functions due to the involvement of the oculomotor nucleus and other related structures.

Takeaways

• 🧠 The dorsal midbrain syndrome involves the dorsal part of the midbrain, specifically the superior colliculus and the tectum and tegmentum areas.
• 👁️‍🗨️ The ophthalmology-related actions in the dorsal midbrain include the ocular motor nucleus of the third cranial nerve and the Edinger-Westphal nucleus.
• 👁️‍🗨️ A dorsal midbrain lesion can lead to lid retraction, known as the Collier's lid retraction sign, due to the inhibition of the inhibitory fibers of the third nerve nucleus.
• 🌟 Light-near dissociation can occur where the light pathway is disrupted but the near pathway is preserved, leading to different reactions to light and near stimuli.
• 👀 Convergence retraction nystagmus is a unique eye movement associated with dorsal midbrain lesions, characterized by the eyes converging and globes retracting.
• 🔄 The convergence retraction is due to the simultaneous firing of all muscles innervated by the third nerve.
• 👆 Upgaze palsy is a common finding in dorsal midbrain syndrome, affecting the vertical gaze center located at the rostral midbrain-thalamic junction.
• 👇 Downgaze palsy is less common and usually requires a larger lesion to disrupt the bilaterally represented downgaze pathway.
• 🌅 The 'setting sun' sign, where the eyes are forced into a downward position, can be seen with hydrocephalus due to unopposed downgaze innervation.
• 🔎 The constellation of findings in dorsal midbrain syndrome helps diagnose the condition and includes lid retraction, light-near dissociation, convergence retraction nystagmus, and gaze palsies.

Q & A

What is the dorsal midbrain syndrome?

The dorsal midbrain syndrome is a neurological condition that results from damage to the dorsal part of the midbrain, affecting the ocular motor functions and related structures.

What are the key structures in the dorsal midbrain?

The key structures in the dorsal midbrain include the superior colliculus, the tectum (roof), the tegmentum (floor), and the cerebral peduncle.

What is the role of the Edinger-Westfall nucleus in the dorsal midbrain syndrome?

The Edinger-Westfall nucleus is involved in the parasympathetic control of the pupil. In the dorsal midbrain syndrome, damage to this nucleus can lead to light-near dissociation.

What is lid retraction and how is it related to the dorsal midbrain syndrome?

Lid retraction, also known as the Collier's lid retraction sign, is a feature of the dorsal midbrain syndrome. It occurs due to the disruption of the supranuclear input to the third nerve nucleus, leading to bilateral and symmetric lid retraction.

What is light-near dissociation and how does it manifest in the dorsal midbrain syndrome?

Light-near dissociation refers to the separation of the light and near reflex pathways. In the dorsal midbrain syndrome, the light pathway may be disrupted while the near pathway is preserved, leading to an abnormal pupillary response to light but not to near stimuli.

What is convergence retraction nystagmus and how is it associated with the dorsal midbrain syndrome?

Convergence retraction nystagmus is an abnormal eye movement characterized by convergence and globe retraction. It is associated with the dorsal midbrain syndrome due to the firing of muscles innervated by the third nerve, which is affected by lesions in this region.

Why does convergence retraction nystagmus occur in the dorsal midbrain syndrome?

It occurs because the medial rectus muscle fires due to the innervation to the third nerve nucleus, causing the eyes to converge. Simultaneously, the superior and inferior rectus muscles also fire, leading to globe retraction.

What is the significance of the vertical gaze center in the dorsal midbrain syndrome?

The vertical gaze center, located at the pontomesencephalic junction, is responsible for controlling upward and downward eye movements. Lesions in the dorsal midbrain can affect this center, leading to upgaze or downgaze paresis.

Why is upgaze palsy more common than downgaze palsy in the dorsal midbrain syndrome?

Upgaze palsy is more common than downgaze palsy in the dorsal midbrain syndrome because the fibers that innervate both third nerve nuclei cross in the posterior commissure, making them more susceptible to damage from a unilateral lesion.

What is the role of the posterior commissure in the dorsal midbrain syndrome?

The posterior commissure is a fiber tract that connects the third nerve nuclei. Damage to this structure can result in an upgaze palsy, as it is involved in the control of upward eye movements.

How can hydrocephalus affect eye movements in the context of the dorsal midbrain syndrome?

Hydrocephalus can cause the eyes to be forced into a downward position, known as sunsetting, due to unopposed downgaze innervation when upgaze is compromised in the dorsal midbrain syndrome.