A 38-year-old woman from Mumbai presents to the emergency department with a 3-day history of severe left-sided headache, fever (38.5°C), and progressive left eye swelling. On examination, she has proptosis, chemosis, and ophthalmoplegia affecting the left eye. Her left pupil is dilated and non-reactive. She has a pustule on the left upper eyelid. Blood culture is pending. What is the most likely diagnosis and which cranial nerve is NOT typically affected in this condition?

Explanation

## Clinical Diagnosis: Cavernous Sinus Thrombosis **Key Point:** Cavernous sinus thrombosis (CST) is a life-threatening infection of the venous sinus within the middle cranial fossa. The source here is a left upper eyelid pustule (likely a furuncle or infected chalazion), which drains via ophthalmic veins directly into the cavernous sinus. ### Pathophysiology The "danger triangle" of the face (upper lip and nose) drains to the ophthalmic veins → cavernous sinus. Infection spreads rapidly, causing thrombophlebitis and sepsis. ### Clinical Features of CST 1. **Bilateral signs** (though may start unilaterally): proptosis, chemosis, ophthalmoplegia 2. **Fever and systemic toxicity**: high mortality without antibiotics 3. **Cranial nerve involvement**: CN III, IV, V₁, V₂, and VI all pass through the cavernous sinus 4. **Pupil changes**: CN III palsy → dilated, non-reactive pupil (as in this case) ### Cranial Nerve Involvement Pattern | CN | Location in Sinus | Typical Involvement | |---|---|---| | III | Medial wall, superior | Early and common | | IV | Medial wall, superior | Early and common | | V₁ (ophthalmic) | Lateral wall | Early and common | | V₂ (maxillary) | Lateral wall | Early and common | | VI | Central (free-floating) | **LAST to be affected** | **High-Yield:** CN VI is the ONLY cranial nerve that runs **free-floating through the center** of the cavernous sinus (not anchored to walls). It is therefore the **last to be affected** and the **first to recover** when treatment begins. In early CST, CN VI function may be preserved even when CN III, IV, and V are already involved. ### Clinical Pearl The patient's dilated left pupil indicates CN III involvement (parasympathetic fibers affected), confirming cavernous sinus pathology rather than simple orbital cellulitis. ### Management - **Immediate:** Blood cultures, broad-spectrum IV antibiotics (vancomycin + ceftriaxone + metronidazole) - **Imaging:** MRI with contrast (T1 post-gadolinium shows filling defect; T2 shows hyperintense thrombus) - **Source control:** Drain the eyelid pustule if fluctuant - **Anticoagulation:** Controversial but increasingly used in some centers [cite:Snell's Anatomy 9e Ch 3]