In a patient with acute Horner syndrome, which of the following pupillary findings would be expected?

Explanation

## Horner Syndrome: Pupillary Findings and Pathophysiology **Key Point:** Horner syndrome results from disruption of the **oculomotor sympathetic pathway**, causing a **miotic (constricted) pupil with preserved light and accommodation reflexes**. ### Anatomical Pathway The sympathetic pathway to the pupil consists of three neurons: 1. **First-order neuron:** Hypothalamus → brainstem → spinal cord (C8-T1) 2. **Second-order neuron:** Spinal cord → superior cervical ganglion (via sympathetic chain) 3. **Third-order neuron:** Superior cervical ganglion → internal carotid artery → eye Damage at ANY level results in Horner syndrome. ### Characteristic Pupillary Findings **High-Yield:** The pupil in Horner syndrome is: - **Miotic (1–2 mm smaller than normal)** — due to loss of sympathetic dilator tone - **Reactive to light** — parasympathetic pathway (CN II–CN III) is intact - **Reactive to accommodation** — accommodation pathway is intact - **Shows "reverse anisocoria"** — the affected pupil is smaller in both light and dark ### Associated Features (Classic Triad) | Feature | Mechanism | Notes | | --- | --- | --- | | **Miosis** | Loss of sympathetic dilator innervation | Pupil 1–2 mm smaller | | **Ptosis** | Loss of sympathetic innervation to Müller's muscle | Usually mild (1–2 mm) | | **Anhidrosis** | Loss of sympathetic sudomotor fibers | Distribution depends on lesion location | **Clinical Pearl:** In **acute Horner syndrome**, the ptosis may be more apparent and the anisocoria more pronounced. In **chronic Horner syndrome**, the pupil may appear less miotic due to "dilation lag" recovery. ### Pharmacological Testing to Confirm Horner Syndrome ```mermaid flowchart TD A[Suspected Horner Syndrome]:::outcome --> B[Cocaine test]:::action B --> C{Pupil dilates?}:::decision C -->|No dilation| D[Horner syndrome confirmed]:::outcome C -->|Normal dilation| E[Rule out Horner]:::outcome D --> F[Apraclonidine test]:::action F --> G{Reversal of anisocoria?}:::decision G -->|Yes| H[Confirms Horner]:::outcome ``` **High-Yield:** - **Cocaine test:** Horner pupil fails to dilate (no norepinephrine to release) - **Apraclonidine test:** Reverses anisocoria in Horner syndrome (α-2 agonist acts on denervated receptors) ### Comparison with Other Pupillary Disorders | Condition | Pupil Size | Light Reflex | Accommodation | Associated Features | | --- | --- | --- | --- | --- | | **Horner syndrome** | Miotic | Present | Present | Ptosis, anhidrosis | | **Argyll Robertson** | Miotic, irregular | Absent | Present | Neurosyphilis, diabetes | | **3rd nerve palsy** | Dilated | Absent | Absent | Complete ptosis, ophthalmoplegia | | **Adie tonic pupil** | Normal/dilated | Sluggish | Sluggish | Areflexia, young females | **Warning:** Do not confuse Horner syndrome with Argyll Robertson pupil — both have miotic pupils, but Horner preserves light reflex while Argyll Robertson loses it.