3 and 9 o'clock staining
staining, 3 and 9 o'clock
Punctate corneal staining located just inside the limbus usually on the horizontal meridian on both sides, hence called 3 and 9 o'clock staining (or 4 and 8 o'clock). It may appear only on one side. It is observed with the biomicroscope after instillation of fluorescein. It is very useful to have the patient look nasally to inspect the temporal cornea and then temporally for the nasal portion. This staining is associated with corneal contact lens wear, although in some rare cases mild staining is observed without contact lens wear. It is due to inadequate spreading of the tear film over these areas of the cornea as a result of incomplete and/or infrequent blinking. Another cause for this type of staining is a small contact lens that is too thick. This condition is a mild form of exposure keratitis. See exposure keratopathy; Burton lamp; fluorescein test.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann