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Answer: A low-level E/M code (99212 or 99213) would be appropriate for a stable patient with no complaints.
If the patient has no complaints and the physician performs a simple pressure check and finds that the medication is working and the pressure is normal, 99212 is probably the most appropriate code. The physician probably sees the patient two or three times a year to make sure the medication is working and may alternate the glaucoma diagnosis system (92135, Scanning computerized ophthalmic diagnostic imaging [e.g., scanning laser] with interpretation and report, unilateral) or visual fields (92081-92083) at these visits, which are billable with 99212 (no modifier is needed on the E/M). An eye examination code (92012) would not be appropriate because this code "describes an evaluation of a new or existing condition complicated with a new diagnostic or management problem," according to the CPT introduction to ophthalmology.
If a full anterior segment evaluation is also done, use 99213 instead of 99212. If a full anterior segment is not done, there probably aren't enough elements for 99213, and 99212 is the appropriate code.
Code 99213 does not necessarily include a dilated exam. A glaucoma check requires a complete anterior segment exam, with documentation of all anterior segment elements. If the ophthalmologist performs an anterior plus posterior exam, the line into 99214 is crossed. The criteria for 99213 could be fulfilled if visual fields, visual acuity, intraocular pressure, and a full anterior segment exam are documented but no posterior segment evaluation is done.
The answer to your second question is still 99212 or 99213 if the patient is controlled on medication and has no other problems. Performing a dilated examination doesn't necessarily justify billing for it. If there were new reasons for performing the dilated examination, such as vision decrease that is unexplained by the glaucoma, 92012 or 92014 would be justified, with an appropriate diagnosis code.
If the patient comes in three times a year for a pressure check and the glaucoma is always well controlled, the physician may perform a dilated examination once a year. It's appropriate to do a dilated examination annually even on a stable glaucoma patient. Although most of the visits are for checking the anterior chamber and intraocular pressure, some patients develop low-tension glaucoma. They will have normal pressure readings and are taking medication, and visual fields may show no progressive damage. But when a detailed evaluation of the ocular disk is performed, it may show changes in the rims or the pallor or the cupping of the discs. So a comprehensive visit 92014 once a year is reasonable, with the 99212 or 99213 checks every four months or so.