A 28-year-old man with acute left-sided headache and ptosis is found to have a dilated left pupil that does not react to light or accommodation. The right pupil is normal. Which single feature best distinguishes Weber syndrome from Horner syndrome?

Explanation

## Distinguishing Weber Syndrome from Horner Syndrome ### Comparative Table | Feature | Weber Syndrome | Horner Syndrome | | --- | --- | --- | | **Nerve involved** | Oculomotor nerve (CN III) | Sympathetic chain | | **Pupil size** | **Dilated (mydriatic)** | **Constricted (miotic)** | | **Pupil reaction to light** | **Non-reactive** | **Reactive but small** | | **Ptosis** | Ipsilateral, complete (CN III) | Ipsilateral, partial (Müller muscle) | | **Extraocular movements** | Impaired (CN III palsy) | Normal | | **Accommodation** | Lost | Preserved | | **Anhydrosis** | No | Yes (if preganglionic) | | **Location of lesion** | Midbrain (ventral) | Brainstem, neck, or orbit | | **Onset** | Acute | Acute or chronic | ### Pathophysiology **Weber Syndrome:** 1. Ventral midbrain lesion affecting oculomotor nerve fascicles 2. Ipsilateral CN III palsy → dilated pupil + ptosis + ophthalmoplegia 3. Pupil is dilated because parasympathetic fibers (which travel with CN III) are damaged 4. Pupil does NOT react to light or accommodation 5. Common causes: midbrain infarction, hemorrhage, tumor **Horner Syndrome:** 1. Interruption of sympathetic pathway (3-neuron chain) 2. Loss of sympathetic tone to pupil and Müller muscle 3. Ipsilateral miotic pupil (constricted) + partial ptosis + anhidrosis 4. Pupil STILL reacts to light (parasympathetic intact), but is smaller 5. Accommodation is preserved 6. Causes: brainstem (central), neck (preganglionic), orbit (postganglionic) **Key Point:** The **direction of pupillary change** is the single best discriminator: - **Dilated pupil** = Weber syndrome (CN III, parasympathetic loss) - **Constricted pupil** = Horner syndrome (sympathetic loss) ### Clinical Pearl **Mnemonic — "WEBER" (Weber Syndrome):** - **W**ide (dilated) pupil - **E**ye movements lost (ophthalmoplegia) - **B**rainstem (midbrain) lesion - **E**xcitation of CN III fibers - **R**eactivity lost (non-reactive pupil) **Mnemonic — "HORNER" (Horner Syndrome):** - **H**alf-closed eye (partial ptosis) - **O**scillations of iris (disdiadochokinesis) — actually, better: **O**scillations absent, pupil constricted - **R**eactive pupil (still reacts to light, just small) - **N**eck/brainstem/orbit involvement - **E**nhanced dilation lag (pupil dilates slowly) - **R**edden conjunctiva (loss of sympathetic tone) Alternative mnemonic: **"SAD" (Horner Syndrome):** - **S**mall pupil - **A**nhydrosis - **D**rooping eyelid (partial) ### High-Yield Distinction ```mermaid flowchart TD A["Acute Ipsilateral Ptosis + Pupil Change"]:::outcome --> B{"Pupil size?"}:::decision B -->|"Dilated (mydriatic)"| C["Weber Syndrome"]:::outcome C --> D["CN III palsy"]:::action D --> E["Midbrain lesion"]:::action B -->|"Constricted (miotic)"| F["Horner Syndrome"]:::outcome F --> G["Sympathetic loss"]:::action G --> H["Brainstem/neck/orbit lesion"]:::action ``` ### Why This Matters Both conditions present with ipsilateral ptosis, but the **pupil size and reactivity** tell you the underlying pathology: - **Dilated + non-reactive** = parasympathetic (CN III) damage = Weber - **Constricted + reactive** = sympathetic damage = Horner