How a macrodisc may lead to a misdiagnosis of glaucoma

May 01, 2010

Some observations on 'pseudoglaucomatous' discs and what to look out for

The size of the optic disc and the size of the cup vary physiologically in a large amount between different ethnic groups and even within a particular group. Fairly good estimations are possible by means of a slitlamp and lenses to measure the vertical diameter. Due to the normal values of the exact stereometric analysis of the optic nerve head, obtained by the Heidelberg Retinal Tomograph, the normal size of the disc area is between 1.69–2.82 mm2. Everything beyond the upper range can be classified as a macrodisc.

In several publications Jonas points out1–3 that a macrodisc may have a macrocup because of the direct relationship between disc area and cup area. Larger discs have more nerve fibers (axons) and a larger area of neuroretinal tissue. They have more and larger pores and a larger area of connective tissue in the lamina cribrosa and are more often combined with optic pits and may show a transition to the 'morning glory disc' (confluent pits).

The glaucomatous optic neuropathy includes changes to the prelaminar and laminar part of the disc. Atrophy of the nerve fibers, especially of those closer to the rim, is combined with changes to activated glial cells (remodelling), changing the hemostasis and blood flow in the lamina.4,5 The lamina itself shows a backward bowing.6 So, the location of the damage is in the lamina cribrosa.7

Taking into account that the distance from one side of the rim of Elschnig´s scleral spur to the other side of the rim is longer in a macrodisc than in 'normal' discs, that the collagen and elastic tissue of the lamina cribrosa has to withhold more stress and strain, resulting in higher sheer forces mainly close to the rim, it is - from the bio-mechanical point of view - no wonder, that these discs would be more susceptible to intraocular pressure. This situation could be compared to a rope bridge crossing a longer (or more critical) distance.

It is the visual field examination that helps to diagnose glaucoma in patients with macrodiscs. The databases of normal eyes of HRT and GDx for diagnosing glaucoma do not fit for macrodiscs. The 'number' will show higher values, influenced by the optic disc size, leading to false positive results.

Conclusion

A macrodisc may have a macrocup and should not misdiagnosed as glaucoma. Due to mechanical properties these eyes are probably more susceptible to the intraocular pressure, leading to glaucomatous neuropathy. So these patients should not be lost from follow-up. Glaucoma may occur in eyes with macrodiscs as well.

References

1. J. Jonas. Makropapillen mit physiologischer Makroexkavation (Pseudo-Glaukompapillen). Klin. Mbl. Augenheilkd., 1987; 191:452–7.

2. J. Jonas et al., Human optic nerve fiber count and optic disc size. Invest Ophthalmol. Vis. Sci., 1992; 33:2012–8.

3. J.B. Jonas. Papillenfotographie. In: A. Kampik and F. Grehn (eds), Augenärztliche Diagnostik, Thieme, 2003, pp 71–8.

4. M.R. Hernandez. Prog. Retin. Eye Res., 2000; 19:297–321.

5. E.C. Johnson and J.C. Morrison. J. Glaucoma, 2009; 18:341–53.

6. D.B. Yan et al., Br. J. Ophthalmol., 1994; 78:643–8.

7. H.A. Quigley et al., Arch. Ophthalmol., 1981; 99:635–49.

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