Acute Angle-Closure Glaucoma is unlikely due to normal intraocular pressure, but:
- Optic Neuritis: This condition is characterized by inflammation of the optic nerve, leading to painful vision loss, and is often associated with a sluggish pupil reaction. It's a common cause of painful vision loss in young adults.

Migraine with Ocular Involvement: Migraines can cause sharp, unilateral headaches and may be accompanied by visual disturbances, including transient vision loss. The normal ocular exam and intraocular pressure support this diagnosis.
Cluster Headache: These are severe, unilateral headaches that can be accompanied by autonomic symptoms such as a sluggish pupil. While they are typically not directly associated with vision loss, the pain can be so severe that it might be perceived as affecting vision.
Cranial Arteritis (Giant Cell Arteritis): Although more common in older adults, this condition can cause headache, vision loss, and a normal ocular exam initially. The sharp pain and normal intraocular pressure do not rule out this diagnosis.

Central Retinal Artery Occlusion: This is an ocular emergency that can cause sudden, painless vision loss but might be perceived as painful due to associated headache. It requires immediate treatment to restore vision.
Pituitary Apoplexy: A condition where a pituitary tumor suddenly hemorrhages or infarcts, leading to severe headache, vision loss, and sometimes ophthalmoplegia. It's a medical emergency.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening condition that can cause headache, vision loss, and cranial nerve palsies. It often presents with bilateral signs but can start unilaterally.

Sarcoidosis: Can cause optic neuritis, uveitis, and other ocular manifestations, leading to painful vision loss. It's less common but should be considered in the differential, especially with systemic symptoms.
Tolosa-Hunt Syndrome: A rare condition characterized by headache, ophthalmoplegia, and vision loss due to inflammation in the cavernous sinus or superior orbital fissure. It's a diagnosis of exclusion.
Ocular Malignant Hypertension: Although the intraocular pressure is normal, malignant hypertension can cause vision loss and headache due to retinal and optic nerve damage. It's an unlikely diagnosis given the normal ocular exam and intraocular pressure but should be considered if systemic blood pressure is elevated.