Differential Diagnosis of Unilateral Third Nerve Palsy
Single Most Likely Diagnosis
- Ischemic third nerve palsy: This is the most common cause of unilateral third nerve palsy, often due to diabetes or hypertension, which can cause microvascular ischemia to the nerve. It typically presents with sudden onset of ptosis, diplopia, and ophthalmoplegia, but usually spares the pupil.
Other Likely Diagnoses
- Aneurysmal compression: An aneurysm, especially of the posterior communicating artery, can compress the third nerve, leading to palsy. This often involves the pupil and can be accompanied by pain.
- Trauma: Head trauma can cause a third nerve palsy due to direct injury to the nerve or increased intracranial pressure.
- Tumor: Various tumors, such as meningiomas or schwannomas, can compress the third nerve, leading to palsy. These cases might have a more gradual onset.
Do Not Miss Diagnoses
- Aneurysmal rupture or impending rupture: Although less common than ischemic causes, aneurysmal compression is critical to identify due to the high risk of rupture and subsequent subarachnoid hemorrhage, which is life-threatening.
- Pituitary apoplexy: Sudden hemorrhage or infarction of a pituitary adenoma can compress the third nerve, leading to acute palsy. This condition requires urgent medical and possibly surgical intervention.
- Cavernous sinus thrombosis: This is a rare but potentially life-threatening condition that can cause third nerve palsy among other cranial nerve deficits. It often presents with proptosis, chemosis, and decreased vision.
Rare Diagnoses
- Inflammatory or infectious causes: Conditions like sarcoidosis, tuberculosis, or fungal infections can cause third nerve palsy, though these are less common.
- Genetic disorders: Certain genetic conditions can predispose to third nerve palsy, such as congenital third nerve palsy or those associated with other systemic neurologic diseases.
- Migraine: Though rare, some migraines can cause transient third nerve palsy, often with pupillary involvement.