Diplopia
Summary
Diplopia is double vision, caused by misalignment of the visual axes so that an image falls on non-corresponding retinal points. Binocular diplopia resolves when either eye is covered and implies an ocular motor problem; monocular diplopia persists and implies a problem within the eye itself.
Detail
The first step in evaluating diplopia is to determine whether it is monocular or binocular, because that single question separates two entirely different differential diagnoses. Monocular diplopia persists with one eye closed and points to refractive error, cataract, corneal irregularity, or lens subluxation, as in Marfan syndrome or homocystinuria. Binocular diplopia disappears on covering either eye and reflects failure of conjugate gaze: cranial nerve III palsy (eye down and out, ptosis, and in compressive lesions such as posterior communicating artery aneurysm a blown pupil), CN IV palsy (vertical diplopia worse on downgaze, head tilt away from the lesion, trouble descending stairs), CN VI palsy (horizontal diplopia, impaired abduction, a classic false localizing sign of raised intracranial pressure), internuclear ophthalmoplegia from a medial longitudinal fasciculus lesion in multiple sclerosis, myasthenia gravis with fatigable ptosis, thyroid eye disease, or orbital blowout fracture with inferior rectus entrapment.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.