During a neuroanatomy practical, a student is asked to identify the most common site of brainstem lesions that produce ipsilateral CN III palsy with contralateral hemiparesis. Which of the following is the most common location for this syndrome?

## Weber Syndrome — Most Common Midbrain Lesion **Key Point:** Weber syndrome is caused by a lesion in the **ventromedial midbrain** at the level where the oculomotor nerve (CN III) fascicles pass through the cerebral peduncle. This is the most common midbrain syndrome. ### Brainstem Syndromes at a Glance | Syndrome | Location | Ipsilateral Deficit | Contralateral Deficit | | --- | --- | --- | --- | | **Weber** | Ventromedial midbrain | CN III palsy (ptosis, mydriasis, down & out eye) | Hemiparesis (pyramidal tract) | | Benedikt | Ventromedial midbrain | CN III palsy | Tremor (red nucleus involvement) | | Claude | Dorsomedial midbrain | CN IV palsy | Ataxia (superior cerebellar peduncle) | | Foville | Ventrolateral pons | CN VI + VII palsy | Hemiparesis | | Wallenberg | Dorsolateral medulla | Horner + facial sensory loss | Body sensory loss | | Medial medullary | Ventromedial medulla | CN XII palsy (tongue) | Hemiparesis | **High-Yield:** Weber syndrome is the **most common brainstem syndrome** because: 1. The midbrain is a common site of vascular lesions (SCA territory) 2. The ventromedial location is frequently affected by small penetrating artery occlusions 3. The oculomotor nerve fascicles are concentrated in a small area, making CN III palsy a reliable sign ### Anatomical Basis of Weber Syndrome ```mermaid flowchart TD A[Ventromedial Midbrain Lesion]:::outcome --> B[Damage to CN III fascicles]:::action B --> C[Ipsilateral CN III palsy]:::outcome A --> D[Damage to cerebral peduncle]:::action D --> E[Contralateral hemiparesis]:::outcome C --> F[Ptosis, mydriasis, down and out eye]:::outcome E --> G[Weakness of face, arm, leg]:::outcome ``` **Clinical Pearl:** The **"down and out"** eye position (depression and abduction) is classic for CN III palsy because the unopposed action of the lateral rectus (CN VI) and superior oblique (CN IV) muscles causes the eye to look down and laterally. **Mnemonic:** **Weber = Ventromedial midbrain** — think "**W**eber in the **W**ay (ventromedial) of the **III**rd nerve." The oculomotor nerve passes through the cerebral peduncle in the ventromedial midbrain. ### Why Weber is Most Common - The midbrain is supplied by the **superior cerebellar artery (SCA)** and penetrating branches from the basilar artery - Small lacunar infarcts from hypertension frequently affect the ventromedial midbrain - The compact anatomy of the oculomotor nerve fascicles makes CN III palsy a sensitive sign of midbrain pathology - Accounts for approximately 50% of all brainstem syndromes in clinical practice