What are the implications of inferior and temporal RNFL thinning in both eyes for a patient with high myopia and normal intraocular pressures?

Inferior and Temporal RNFL Thinning in High Myopia with Normal IOP

Your pattern of inferior and temporal RNFL thinning in both eyes, combined with high myopia and normal IOP, places you in the glaucoma suspect category and requires close monitoring with baseline testing, as this pattern—particularly the inferior thinning—is highly suspicious for early glaucomatous damage that may precede visual field loss. 1

Why This Pattern Matters

The inferior RNFL thinning is the critical finding here. While high myopia physiologically causes generalized RNFL thinning (particularly in moderate myopia), focal inferior or superior thinning remains highly suspicious for glaucoma regardless of myopia status 1. The American Academy of Ophthalmology guidelines explicitly state that in approximately 80% of glaucomatous cupping, both inferior and superior rims are thinned, violating the normal ISNT rule 2.

The Myopia Complication

High myopia creates diagnostic complexity:

• Moderate myopia (-4 to -8D) shows true RNFL thinning on OCT measurements, with significantly lower TSNIT average, superior, and inferior measurements compared to emmetropes 3
• High myopia (>-8D) paradoxically shows supranormal RNFL values due to peripapillary chorioretinal atrophy and scleral birefringence, which can mask true glaucomatous damage 3, 4
• Nontemporal RNFL parameters (superior, inferior, nasal) thin more significantly than temporal regions in high myopic glaucoma suspects 5
• Your temporal thinning may represent the physiologic effect of high myopia, but the inferior thinning cannot be dismissed as purely myopic change 1, 5

Required Immediate Actions

You need comprehensive baseline documentation beyond just OCT 1:

• Visual field testing with standard automated perimetry (SAP) to detect functional deficits, as structural damage often precedes detectable field loss 2, 1, 6
• Multiple IOP measurements at different times of day, since nearly 40% of glaucoma patients show normal office IOP, and unrecognized IOP fluctuations increase risk 2, 6
• Gonioscopy to exclude angle-closure or secondary causes 2
• Central corneal thickness (pachymetry) measurement 1
• Stereoscopic optic nerve examination through dilated pupils using slit-lamp biomicroscopy with red-free illumination to assess for disc hemorrhages, rim notching, and parapapillary atrophy 2, 1

Monitoring Protocol

You require monitoring every 3-6 months initially with repeat OCT, visual field testing, and IOP measurements 1. This frequency is critical because:

• Recent evidence shows that 22.5% of treatment-naïve myopic NTG patients showed progression within 2 years, even with low baseline IOP 7
• Structural optic nerve damage precedes visual field defects in early glaucoma 2, 6
• Disc hemorrhages herald focal damage and visual field loss, occurring in 13.6% of patients who developed POAG versus 5.2% without hemorrhages 2

Treatment Decision Framework

Treatment becomes indicated if any of the following develop 1:

1. Evidence of progression on serial OCT or visual field testing indicating conversion to POAG
2. Visual field defects consistent with glaucomatous damage (arcuate scotomas, nasal steps, paracentral defects respecting horizontal midline) 6
3. High-risk profile development including:
   • Family history of glaucoma (significant risk factor for progression in myopic NTG) 7
   • Mean IOP >14.2 mm Hg (identified as significant breakpoint for accelerated RNFL thinning) 7
   • Maximum IOP elevations during follow-up 7

Treatment Target When Initiated

If treatment becomes necessary, aim for 20-30% IOP reduction from baseline, as clinical trials demonstrate this reduces risk of developing POAG and slows progression 1, 6. Notably, no patients with mean IOP <11 mm Hg showed progression in recent myopic NTG studies, though this doesn't mean prophylactic treatment to this level is indicated without other risk factors 7.

Critical Pitfall to Avoid

Do not assume all RNFL thinning in high myopia is physiologic. While high myopia affects OCT measurements, the inferior quadrant thinning pattern you describe matches the preferential damage pattern of glaucoma at the superior and inferior optic disc poles 6, 5. The combination of high myopia as an established POAG risk factor and your specific thinning pattern mandates glaucoma suspect classification and close surveillance 1.