A 28-year-old woman presents with sudden onset of horizontal diplopia and inward deviation of the right eye that worsens when looking to the right. On examination, the right eye cannot abduct beyond the midline, and the left eye shows compensatory abduction when the right eye fixates. Vertical movements are normal. The patient reports recent severe headache and fever 2 weeks ago, which resolved. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Sixth Cranial Nerve (Abducens) Palsy ### Key Clinical Features of CN VI Palsy **Key Point:** Sixth nerve palsy presents with: - Inability to abduct the affected eye (lateral rectus paralysis) - Esotropia (inward deviation) due to unopposed medial rectus action - Horizontal diplopia that worsens on looking toward the affected side - Normal vertical eye movements - Secondary deviation greater than primary deviation (hallmark of paralytic strabismus) ### Diagnostic Criteria Met in This Case | Feature | Finding | Significance | |---------|---------|---------------| | Onset | Sudden | Consistent with nerve palsy, not concomitant strabismus | | Abduction deficit | Right eye cannot abduct beyond midline | Pathognomonic for CN VI palsy | | Diplopia pattern | Horizontal, worse on rightward gaze | Lateral rectus weakness on right | | Vertical movements | Normal | Excludes CN III or CN IV involvement | | Recent history | Headache and fever 2 weeks prior | Suggests post-viral or inflammatory etiology | ### Pathophysiology **High-Yield:** The sixth cranial nerve has the longest intracranial course and the smallest cross-sectional area, making it vulnerable to: 1. Increased intracranial pressure 2. Viral or post-viral inflammation 3. Vascular ischemia (especially in diabetes, hypertension) 4. Neoplastic compression 5. Trauma ### Etiology in This Patient **Clinical Pearl:** The history of recent fever and headache suggests a post-viral inflammatory etiology. CN VI palsy can occur as part of viral meningitis or post-viral syndrome. ### Management Algorithm ```mermaid flowchart TD A[CN VI Palsy Diagnosed]:::outcome --> B{Age and Risk Factors?}:::decision B -->|Young, no vascular risk| C[Observe for 3 months]:::action B -->|Age > 50 or vascular risk| D[MRI brain to exclude structural lesion]:::action C --> E{Recovery by 3 months?}:::decision E -->|Yes| F[Continue observation]:::action E -->|No| G[Consider imaging and electrophysiology]:::action D --> H{Lesion found?}:::decision H -->|Yes| I[Treat underlying cause]:::action H -->|No| J[Supportive care, prism glasses for diplopia]:::action ``` **Key Point:** Most cases of CN VI palsy recover spontaneously within 3–6 months. Prism glasses can be prescribed to manage diplopia during recovery. ### Distinguishing Paralytic from Concomitant Strabismus | Feature | Paralytic (CN VI Palsy) | Concomitant Esotropia | |---------|------------------------|----------------------| | Onset | Sudden | Gradual (childhood) | | Eye movement | Restricted abduction | Full movements | | Secondary deviation | Greater than primary | Equal to primary | | Diplopia | Present | Absent (fusion suppressed) | | Head posture | Often adopted to minimize diplopia | Normal |