A patient presents with ptosis, mydriasis, and impaired adduction of the eye. Which cranial nerve is affected as it passes through the cavernous sinus?

Explanation

## Oculomotor Nerve Palsy and Cavernous Sinus Lesions ### Clinical Presentation of CN III Palsy **Key Point:** The oculomotor nerve (CN III) innervates four of the six extraocular muscles and the levator palpebrae superioris. Damage to CN III causes a characteristic triad of signs. ### Triad of CN III Palsy 1. **Ptosis** — Paralysis of levator palpebrae superioris (CN III innervated) 2. **Mydriasis** — Pupillary dilation due to unopposed sympathetic activity (parasympathetic fibers in CN III are damaged) 3. **Ophthalmoplegia** — Impaired eye movements, especially adduction (medial rectus innervated by CN III) **High-Yield:** The eye assumes the **"down and out"** position because: - Lateral rectus (CN VI) is unopposed → eye abducted (out) - Superior oblique (CN IV) is unopposed → eye depressed (down) ### Why CN III Passes Through Cavernous Sinus The oculomotor nerve: - Emerges from the midbrain at the interpeduncular fossa - Passes through the cavernous sinus within its lateral wall - Enters the orbit via the superior orbital fissure - Divides into superior and inferior branches **Clinical Pearl:** Cavernous sinus thrombosis, aneurysm of the internal carotid artery, or pituitary apoplexy can compress CN III as it traverses the sinus, producing this classic presentation. ### Differential: CN III vs. CN IV vs. CN VI Palsy | Feature | CN III | CN IV | CN VI | | --- | --- | --- | --- | | Ptosis | **Yes** | No | No | | Mydriasis | **Yes** | No | No | | Impaired adduction | **Yes** | No | No | | Impaired depression (down gaze) | Partial | **Yes** | No | | Impaired abduction | No | No | **Yes** | | Eye position | Down & out | Intorted | Abducted | **Mnemonic:** **"CN III = Ptosis, Mydriasis, Medial rectus palsy"** — remember the three P's and M.