The challenges of diagnosing glaucoma in myopes

A consistent finding amongst population-based studies is that myopia is associated with an increased risk of developing open-angle glaucoma at a dose-dependent response.

1. In the Blue Mountains Eye Study, myopia was associated with glaucoma after adjusting for known glaucoma risk factors.¹ The odds ratio (OR) for low myopia was 2.3 and a stronger relationship was seen for moderate-high myopia (OR 3.3).
2. The Beijing Eye Study reported a stronger relationship of glaucomatous damage in high myopes of at least -6D (OR 2.28) compared to low and moderate myopes.²
3. A retrospective study in Tokyo found axial length >26.5mm associated with an increase in prevalence of glaucoma (28.1% in a medium to highly myopic study population).³
4. A recent meta-analysis of myopia as a risk factor for glaucoma concluded that for low myopia (myopia up to -3D) the odds ratio was 1.65 and for higher levels of myopia (in excess of -3D) the odds ratio was higher still at 2.46.⁴

Despite strong epidemiological evidence linking POAG with myopia, the pathogenesis of POAG itself is poorly understood.^1-4 Myopic eyes have numerous anatomical alterations as a consequence of axial elongation. These include:⁵

• Tilted optic nerves
• Peripapillary atrophy
• Thinner lamina cribrosa
• Enlarged and shifted Bruch’s membrane opening (BMO)
• Shifted superotemporal and inferotemporal RNFL bundles
• Deeper anterior chamber lengths

Tilted optic disc is a common morphological change of the optic nerve which presents as an ovoid and obliquely rotated optic nerve head, with an elevated disc rim on one side. The direction of tilt is usually inferonasal and it often presents bilaterally. 

Axial elongation can promote progressive disc tilt, where scleral remodelling causes Bruch's membrane opening (BMO) shifted temporally and posteriorly.⁶ Subsequently, the peak locations of retinal nerve fiber layer (RNFL) bundles and distribution of macular ganglion cell layer (GCL) may be shifted.

An optic nerve crescent (also known as peripapillary atrophy (PPA)) refers to a well-demarcated area usually on the temporal aspect of the optic disc where the inner scleral surface is directly observable. 

In myopic eyes, a crescent is usually formed from the lateral displacement of the choroid and RPE due to the elongation of the posterior pole. 

PPA can be classified into 3 different types - alpha, beta, and gamma-PPA. These are separated by variable atrophy to the retinal pigment epithelium, choriocapillaris, and Bruch's membrane, respectively.⁷ Beta-type PPA has been associated with a higher risk of glaucoma progression and is more pronounced in patients with high myopia-related glaucoma.^8,9

Below are some handy tips to assist in differentiating between the two in clinical practice.

1. Compare disc photos over time. Assessing disc photos over time can assist in identifying enlarged cupping, new notching or new RNFL defects, bayoneting of the vessels, increasing PPA, and other features characteristic of glaucoma.⁵

2. Check for asymmetry on the OCT-RNFL deviation map to identify localised defects. RNFL distribution bundles are often shifted in myopic eyes, causing OCT algorithms to flag false-positive ('red-disease') or even false-negative ('green-disease') nerve fiber layer thinning.

3. Consider repeat visual field testing, potentially with a different means of optical correction (e.g. contact lenses). Tilted discs with PPA can increase the likelihood of apparent visual field defects or show improvement with small changes in myopic correction.¹⁰ Repeating the test under the same conditions or with a different optical correction may confirm whether the defect is genuine.¹¹

4. Follow-up is needed to confirm progression in myopic eyes. As with any progressive condition, it is often not possible to distinguish glaucomatous from non-glaucomatous disease from a single examination.