Which of the following is NOT a cause of a compressive lesion anterior to the optic chiasm that can cause optic disc edema?

• A. Myasthenia Gravis
• B. Thyroid eye disease
• C. Optic nerve glioma
• D. Optic nerve sheath meningioma

Answer: (A)

The concept here is compressive optic neuropathy: lesions that press on the optic nerve before the optic chiasm can cause optic disc edema. When a mass or inflammatory process crowd the orbital apex or nerve itself, axoplasmic flow is disrupted, leading to swelling of the optic disc and often vision changes.

Thyroid eye disease can cause optic neuropathy by enlarging extraocular muscles and orbital tissue that crowd the optic nerve at the orbital apex, producing disc edema. An optic nerve glioma grows within the nerve itself, directly compressing it and causing edema as well. An optic nerve sheath meningioma wraps around the nerve, slowly compressing it and producing similar edema and vision loss patterns.

Myasthenia gravis, on the other hand, is a neuromuscular junction disorder that causes fluctuating weakness of voluntary muscles, including the eyelids and extraocular muscles, but does not compress the optic nerve. It does not typically produce optic disc edema.