The red glass test 👀 is a simple way to check for diplopia (double vision) and figure out if it’s caused by a muscle weakness (like a cranial nerve palsy).
How it works
- Patient wears red glass over one eye (usually right eye → red lens OD).
- Look at a point light source (e.g., penlight) in front.
- Ask: “How many lights do you see? What color(s)?”
What it shows
- Normal: Sees one pink light (colors blend).
- Diplopia: Sees one red + one white light.
- Horizontal separation → lateral/medial rectus issue (CN VI or CN III).
- Vertical/oblique separation → other muscles (e.g., superior oblique CN IV, superior/inferior rectus CN III).
- Image location → helps localize the weak muscle (e.g., red image on right → right lateral rectus weakness).
Example
- Patient sees red light on right + white on left → right lateral rectus palsy (CN VI).
- Image is tilted/diagonal → likely a cyclovertical muscle issue.
Tips
- Control fixation: Make sure they’re looking straight at the light.
- Check gaze positions: Repeat in different directions → helps map out weakness.
- Cover-uncover test can help confirm tropia/phoria.
Limitations
- Subjective: Relies on patient description.
- Doesn’t pinpoint cause → need extra tests (e.g., cover test, motility).