Ophthalmic Optics, Refraction and Contact Lenses
Core concepts
Understand the basics of refraction: emmetropia, myopia, hyperopia, astigmatism, and presbyopia. Know the principles of retinoscopy, subjective refraction, and the difference between spherical and cylindrical correction. Learn the optics of contact lenses: rigid gas permeable vs soft, oxygen permeability, and the tear film.
Know the complications of contact lens wear: microbial keratitis, corneal hypoxia, giant papillary conjunctivitis, and the importance of hygiene. Understand the optics of IOLs and the effect of vertex distance.
Traps
- Missing astigmatism on refraction.
- Overlooking contact lens-related keratitis in a red eye.
- Forgetting vertex distance in high refractive errors.
- Not considering presbyopia in a patient over 40.
Rule of thumb
Any red eye in a contact lens wearer is microbial keratitis until proven otherwise; and always check vertex distance for high prescriptions.
Worked example
A soft lens wearer with a painful red eye and a corneal infiltrate. Remove the lens, start topical antibiotics, and consider culture; avoid steroids until infection is controlled.