However, in patients with high myopia and a suspected optic nerve and visual field defect, a diagnosis of glaucoma and the decision to initiate treatment must be made carefully, because glaucoma assessment is more challenging in these eyes, and abnormal structural and functional findings may be related to myopia and not glaucoma.
"Myopia itself can result in glaucomatous-like visual field defects, and, in [patients with myopia and] characteristic optic nerve tilting and peripapillary atrophy, it may not be appropriate to assume that the natural history of this condition will be similar to primary open-angle glaucoma," said Dr. Chang, assistant professor of ophthalmology, Byers Eye Institute, Stanford University, Palo Alto, CA.
"As initially reported in young Chinese [patients with myopia] by my Stanford colleague, Dr. Kuldev Singh, such defects may not always show the characteristic progression that defines glaucomatous optic nerve disease," he said. "Thus surgical glaucoma therapy, in particular, should be avoided in the absence of such progression. Longitudinal follow-up with regular structural and functional testing will often reveal whether the myopic optic nerve is also glaucomatous in an individual patient."
Depending on suspicion level and IOP range, Dr. Chang suggested that either close observation without treatment or medical IOP-lowering therapy with or without supplemental laser trabeculoplasty might be reasonable options for initial management. He cautioned against aggressive initial surgical intervention for such patients, many of whom are at risk of being labeled as having normal-tension glaucoma.
"One should not perform incisional glaucoma surgery in patients with visual field defects when myopia confuses the picture and there has not been enough evidence that the [glaucoma] is progressing toward visual disability without surgery," Dr. Chang said.
Assessment of the optic nerve for characteristic glaucomatous changes may be more difficult in high myopia because myopia can affect disc shape, cause severe disc tilt or rotation, and increase peripapillary atrophy, he explained. In addition, myopic defect patterns, including an enlarged blind spot and atypical temporal or cecocentral nerve fiber layer bundle defects, can mimic glaucomatous defect patterns.
"However, glaucomatous field defects usually respect horizontal midlines initially, followed by the appearance of a new arcuate, whereas myopic defects associated with tilted discs usually do not respect the vertical or horizontal midlines," Dr. Chang said.
To monitor for visual field progression patterns consistent with glaucoma, Dr. Chang recommended obtaining at least two reliable baseline visual field tests and then performing several repeat tests, at least annually thereafter.
Identification of progression based on changes in retinal nerve fiber layer thickness must also be considered carefully in highly myopic eyes given the propensity for more frequent algorithm errors and the lack of high myopia in spectral-domain optical coherence tomography (SD-OCT) normative databases, he noted.
"[Patients with myopia], with their abnormal optic nerve anatomy, need unique patient-specific baselines for SD-OCT to monitor for progression," Dr. Chang said. "When comparing two scans of similar signal strength before end-stage disease, thinning exceeding 8 to 10 µm, particularly in the superior and inferior quadrants, raises suspicion for progression."
In a patient with high myopia who presents with structural or functional findings that raise suspicion for glaucoma but whose IOP is normal or only borderline high, consideration may be given to checking diurnal IOP.
Additionally, one should remember to assess risk factors for secondary causes of glaucoma, such as ocular trauma or the use of steroids. Issues that must be weighed when deciding whether to initiate IOP-lowering therapy include how worried the patient is about his or her condition, the severity of the defect given the patient's approximate life expectancy, the likelihood that the patient will comply with follow-up visits, and the risk/benefit ratio of therapy, Dr. Chang said.
If prophylactic treatment is chosen right away to protect the eye against potential worsening, a prostaglandin analogue is a good option, not only because of its favorable safety profile, but also since this class of drugs is associated with excellent 24-hour IOP control and therefore may blunt unmeasured IOP spikes occurring outside clinic hours.