Patients with myopia, and especially high myopia (more than -6.00 D), are at increased risk for numerous vision-threatening sequelae, including glaucoma. Detecting glaucoma in myopic eyes, however, presents us with unique challenges.
Myopic eyes often have optic nerves that are anomalous in appearance, confounding our clinical assessment. Myopia is also associated with findings on optical coherence tomography (OCT) of the retinal nerve fiber layer (RNFL) and visual field that may mimic glaucoma and further complicate accurate diagnosis. Structural changes in the sclera and lamina cribrosa are thought to predispose myopic eyes to glaucoma even within the “normal” range of IOP, requiring a higher degree of vigilance among clinicians. Overtreatment, accompanied by its associated costs and burdens, as well as undertreatment, along with its risk of disease progression and vision loss, often result from these overlapping and perplexing findings.
As the prevalence of myopia continues to increase worldwide, accurate and early identification of glaucoma in myopic eyes will become even more important. In this article, I will discuss some of the inherent challenges in detecting glaucoma in myopic eyes and offer pearls to navigate this diagnostic conundrum.
CLINICAL EXAMINATION
Clinical assessment of myopic eyes is often complicated by unique morphological characteristics of the optic disc. These include a large diameter, tilting, torsion and peripapillary atrophy. Cupping is often shallow and diffuse, leading to difficulty discerning the cup-to-disc ratio in eyes with high and pathologic myopia (Figure 1).
Still, documenting the optic nerve appearance with stereoscopic disc photographs remains valuable for longitudinal follow-up. I routinely order disc photos for myopic patients as part of their baseline glaucoma evaluations, keeping in mind that OCT RNFL technology will continue to evolve.
VISUAL FIELDS
Myopia can masquerade as glaucoma by producing classically glaucomatous patterns of visual field abnormalities, including paracentral scotomas, nasal steps and arcuate defects. In addition, superotemporal visual field defects are often seen with tilted discs, while enlarged blind spots are common in high myopes (Figure 2).
Although not a definite rule, myopic visual field defects can be distinguished from glaucomatous changes by their nonprogressive nature. A retrospective study of 16 young and middle-aged myopic Chinese men, about half of whom were treated with glaucoma medications, showed no progression of their glaucoma-like visual field defects over a 7-year period.1
Another retrospective study from Korea observed that myopic eyes with inferiorly tilted optic discs frequently developed extension of their visual field defects within the superior hemisphere, without involvement of the inferior hemisphere.2 This suggests that the myopic optic disc is predisposed to focal damage (and accompanying visual field defects) as a result of its morphology (ie, the direction of its tilt). In this scenario, if a new visual field defect were to arise in the inferior hemisphere, corresponding to a region of the superior neuroretinal rim that was previously intact, it might be more likely to represent glaucoma rather than myopia.
Longitudinal follow-up with serial perimetry is therefore a key strategy in distinguishing the cause of visual field defects. Whenever possible, attempt to obtain prior medical records for myopic patients. You may find that a stable visual field defect (or sectoral OCT RNFL thinning) has been present in an untreated eye for years. Visual field abnormalities may also be present in myopic eyes with a history of retinal detachment and would be expected to remain stable over time. If a new patient with myopia presents with a visual field defect, I repeat perimetry within a month or two to establish a baseline (keeping in mind that abnormalities on reliable visual fields were not confirmed in 85.9% of repeat visual fields in the Ocular Hypertension Treatment Study).3 In general, I obtain visual fields more frequently in glaucoma suspects with high and pathologic myopia given greater risk of glaucoma (every 6 months rather than annually, or more frequently if I am concerned for progression).
For patients with myopic maculopathy and poor fixation, Goldmann visual fields are a useful alternative. Also, serial perimetry is important for patients whose myopia produces significant artifact on OCT RNFL.
RNFL AND MACULAR IMAGING
Spectral-domain OCT of the RNFL and ganglion cell complex have become indispensable tools in glaucoma diagnosis and monitoring. However, their utility in myopic (and particularly highly myopic) eyes can be limited due to the exclusion of eyes with high myopia from normative databases.
Retinal thinning is common in myopic eyes, and currently available OCT RNFL machines may flag myopia-related global RNFL and macular thinning as “abnormal.” Since the machines color code “normal” as green and “abnormal” as red, these false positives are known as “red disease.” Red disease can also result from artifacts stemming from peripapillary atrophy if the scan circle is misaligned, or from erroneous segmentation due to abnormalities common in myopic eyes, such as peripapillary retinoschisis or vitreopapillary traction (Figure 3). In addition, the superotemporal and inferotemporal RNFL bundles tend to be oriented more temporally with higher degrees of myopia, leading to thicker, more robust-appearing temporal sectors and thinner, more abnormal-appearing nasal sectors.4 In an otherwise healthy myopic eye, serial imaging would likely demonstrate stability of this nasal thinning over time.
On the other hand, clinicians should not rely solely on color-coding of the green temporal sectors when assessing for progression. Rather, they should be aware of potential “progression in the green,” whereby the thickness of the temporal bundles may be declining over time while still being color-coded as green based on the normative database.
OCT RNFL reports provide a wealth of information. When analyzing OCT RNFL results for myopic patients, it’s helpful to look not only at the global and sectoral RNFL thickness but also the thickness and deviation maps, B-scan tomograms, temporal, superior, nasal, inferior, temporal (or TSNIT) plots and macular thickness to assess for “red disease.”
Given the limitations imposed by restrictive normative databases, compare patients’ scans to their baselines rather than relying only on color coding. If significant OCT RNFL artifact is present, follow patients with disc photos and visual fields instead. Interestingly, early studies show promise for the utility of a dedicated high myopia OCT RNFL normative database5 and the use of artificial intelligence to detect glaucoma in highly myopic eyes.6 Once validated adequately, such strategies will become game changers.
ADDITIONAL METRICS AND CLINICAL CONSIDERATIONS
Axial elongation results in scleral thinning and poor structural support for retinal nerve fibers, which is thought to contribute to the increased vulnerability of myopic eyes to glaucoma. The Singapore Epidemiology of Eye Diseases Study found that eyes with an axial length >25.5 mm and IOP ≥20 mm Hg were more than 16 times more likely to develop primary open-angle glaucoma when compared with eyes < 23.5 mm in length and with normal IOP.7 Axial length may therefore be an important clinical parameter to help risk stratify our patients.
In addition to structural changes in the sclera, highly myopic eyes may also have inherently different corneal biomechanical properties than emmetropic eyes. A meta-analysis demonstrated lower corneal hysteresis (but, interestingly, similar central corneal thickness [CCT]) in eyes with high vs low myopia.8 In addition, remember to ask your myopic patients about a history of corneal refractive surgery, as this can also alter corneal biomechanics.
Keep in mind that any method of IOP measurement may provide artifactually low readings in eyes with prior keratorefractive surgery. Rebound tonometry is especially vulnerable to IOP underestimation in eyes with thin CCT. Consider measuring the IOP using multiple different modalities (eg, dynamic contour tonometry or pneumotonometry) in patients who have undergone prior refractive surgery, especially if they seem to be progressing within a “normal” IOP range.
CONCLUSION
Longitudinal follow-up is critical in myopic eyes to distinguish progressive glaucomatous damage from nonprogressive abnormalities that may result from myopia alone. Try to obtain prior medical records to provide a historical perspective. Be on alert for artifacts on OCT RNFL and macular imaging, and exercise a low threshold to repeat suspicious visual field changes to confirm or refute them. In addition to standard ancillary testing, consider obtaining axial length and corneal hysteresis measurements to further risk stratify myopic patients. Be mindful of a history of prior corneal refractive surgery, which can contribute to erroneous IOP measurements. Consider other important risk factors, such as family history. Most importantly, talk to patients about the role their myopia plays in their lifetime risk of developing glaucoma.
Ultimately, more frequent follow-up and testing in myopic patients helps to identify glaucomatous conversion or progression early and help patients preserve their visual function. OM
References
1. Doshi A, Kreidl KO, Lombardi L, et al. Nonprogressive glaucomatous cupping and visual field abnormalities in young Chinese males. Ophthalmology. 2007;114:472-479.
2. Han JC, Lee EJ, Kim SH, Kee C. Visual field progression pattern associated with optic disc tilt morphology in myopic open-angle glaucoma. Am J Ophthalmol. 2016;169:33-45.
3. Keltner JL, Johnson CA, Quigg JM, Cello KE, Kass MA, Gordon MO. Confirmation of visual field abnormalities in the Ocular Hypertension Treatment Study. Ocular Hypertension Treatment Study Group. Arch Ophthalmol. 2000;118:1187-1194.
4. Leung CK, Yu M, Weinreb RN, et al. Retinal nerve fiber layer imaging with spectral-domain optical coherence tomography: interpreting the RNFL maps in healthy myopic eyes. Invest Ophthalmol Vis Sci. 2012;53:7194-7200.
5. Song Y, Li F, Chong RS, et al. High myopia normative database of peripapillary retinal nerve fiber layer thickness to detect myopic glaucoma in a Chinese population. Ophthalmology. 2023;130:1279-1289.
6. Lim WS, Ho HY, Ho HC, et al. Use of multimodal dataset in AI for detecting glaucoma based on fundus photographs assessed with OCT: focus group study on high prevalence of myopia. BMC Med Imaging. 2022;22:206.
7. Tham YC, Aung T, Fan Q, et al. Joint effects of intraocular pressure and myopia on risk of primary open-angle glaucoma: The Singapore Epidemiology of Eye Diseases Study. Sci Rep. 2016;6:19320.
8. Wu W, Dou R, Wang Y. Comparison of corneal biomechanics between low and high myopic eyes-A meta-analysis. Am J Ophthalmol. 2019;207:419-425.
