The level of IOP and the biomechanical properties of the sclera primarily determine the development of hypotony maculopathy. The biomechanical properties of the sclera are mainly regulated by the specialized dense irregular extracellular matrix (ECM) composed of collagen fibrils embedded in a matrix of proteoglycans. In human eyes, scleral tissue contains approximately 90% collagen by dry weight.13 The collagen plays structural roles and contributes to mechanical properties, organization, and shape of tissues. Collagen fibrillogenesis, fibril orientation, size, and arrangement are mainly influenced by proteoglycans. The proteoglycans are composed of core proteins with at least one end attached to a glycosaminoglycan (GAG) side chain and this side chain supplies negative charge to mediate interaction with collagen.14 The core proteins of the proteoglycans are classified into the large core protein with resilience, aggrecan, and the small core protein, including biglycan and decorin.15
With aging, the sclera undergoes a progressive degeneration of collagen fiber, a loss of GAG, and scleral dehydration, which are associated with an increase in scleral density and an increase in scleral rigidity.16 On the contrary, the scleral rigidity is relatively less in young people. Myopic eyes have been found to be associated with thinner sclera, particularly at the posterior pole of the eye due to physical loss of scleral ECM and altered ECM content. Previous studies have shown that scleral thinning is related to a general loss of collagen and proteoglycan (especially aggrecan), a narrowing and dissociation of the collagen fiber bundles, and a reduction in collagen fibril diameter.17 The reduction of scleral collagen fibril diameter in highly myopic eyes can result in reduced rigidity of the sclera.18 Therefore, in young myopic eyes, the sclera is thinner with less rigidity at the posterior pole and the changes are associated with weaker biochemical properties, which might make the sclera vulnerable to collapse during hyopotony.19
Although the axial length (AL) of the eye was found to reach adult length by the age of 13 years, AL might continue to increase after adolescence in highly myopic patients, leading to the stretching and thinning of the posterior ocular tissues and further pathologic changes.20,21 Besides the continuing elongation of the eyeball with aging, the morphology of posterior staphyloma also changes as the patient ages.22 Both structural changes might indicate that the sclera in highly myopic eyes is mechanically weakened with aging.18 Other than the anatomic changes, the scleral ECM remodeling in aging myopic eyes has also been documented, including progressive reduction in scleral collagen and GAG content and reduction of fibril diameter. All these changes in aging myopic eyes contribute to scleral thinning with less rigidity.15,21 In summary, the progressive thinning of sclera and reduction of scleral rigidity are observed in aging myopic eyes and these changes might result in progressively weakened biomechanical properties of sclera, which make sclera more likely to collapse under the condition of hypotony.18,19
Hyphema, flat anterior chamber, and bleb leak are the most frequent early-onset complications within the first 3 months after trabeculectomy.23,24 Late bleb leak, which is noted >3 months after trabeculectomy, contributes to the most late-onset hypotony maculopathy. Young age and application of antifibrotic agent are risk factors for bleb leak and our patient had both these risk factors.12,25 However, there was no bleb leak or IOP change in our patient during the period of hypotony maculopathy, which implies that other factors other than the level of IOP contribute to the development of late-onset hypotony maculopathy. Furthermore, early hypotony of RE was noted after trabeculectomy, but hypontony maculopathy did not develop until 14 years later. The postulated mechanism is that the sclera of RE was biomechanically strong enough not to collapse during hypotony in the early period after trabeculectomy. After 14 years, the biomechanical properties of the aging myopic sclera had became too weak to maintain the shape of the eyeball during hypotony and therefore subsequently developed hypotony maculopathy.
In conclusion, hypotony maculopathy can happen 14 years after trabeculectomy in a highly myopic patient with juvenile open-angle glaucoma, even without bleb leak. The weakened biomechanical properties of the sclera, which are related to progressive scleral thinning, reduced scleral rigidity, and scleral morphology change, are hypothesized as main factors in the pathogenesis of late-onset hypotony maculopathy in aging highly myopic eyes. In highly myopic patients with juvenile open-angle glaucoma, hypotony should be avoided after trabeculectomy.
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Key Word: hypotony maculopathy; trabeculectomy; high myopia; sclera