We see, because light bounces off whatever we’re looking at, & enters the eye through the pupil. It hits the retina, which picks up the reflection like camera film (old cameras). Then the optic nerve carries the image to the brain, which interprets it (see Diagram — Anatomy of the Eye). Inflammation of the Optic Nerve is called Optic Neuritis; it almost only happens in just one eye (although it can repeat itself another time in the other eye).
The eye hurts, but often does not look red. Pain may increase with eye movement. There is some blurry vision, and often sensitivity to light. The key to diagnosis is sluggish constriction of the pupil when light is shined in, compared to the other eye. This is the “swinging flashlight” test that unfortunately most clinicians don’t know about or don’t remember:
- In a very dark room, with patient focusing in the distance, we shine a light in the unaffected (“good”) eye. The pupil constricts, as is normal.
- Swing the light to the affected (“bad”) eye, and the pupil dilates [surprise!].
- Swing it back to the “good” eye, pupil constricts.
- Back to the “bad” eye, pupil dilates [surprise!]. Etc.
For an explanation of why this happens, see Afferent Pupillary Defect.
About 50% of Optic Neuritis cases are due to Multiple Sclerosis (M.S.), and the rest are “idiopathic,” meaning “just happen on their own” (there are some other rare causes). However, we can’t diagnose M.S. for sure the first time (there have to be “multiple” events affecting different “multiple” nerves). A brain MRI may show hints of M.S., and treating Optic Neuritis with special drugs may prevent or delay M.S. Unfortunately, these drugs have side effects, so the decision becomes very tricky. Ophthalmologists make the diagnosis of Optic Neuritis, but Neurologists (specialists in the nervous system) make the diagnosis of M.S. and manage treatment options.