The sclera, choroid and ciliary bodies are internal layers within the eye. Since the eye becomes myopic by growing longer it is necessary for these structures to change or grow with increasing myopia. Research is aimed at better understanding the mechanisms involved with the goal of being able to eventually control the process.Nikolai M. Sergienko and Irina Shargorogska. (2012) The scleral rigidity of eyes with different refractions. (ABSTRACT) Graefe's Archive for Clinical and Experimental Ophthalmology VOL 250, Number 7 PG 1009-1012 doi 10.1007/s00417-012-1973-0
MyopiaPrevention.org comment: 176 eyes were tested for any increase in axial length while a 30 gram weight (approximately one ounce) was placed on the eye. A greater increase in length was considered a sign of decreased scleral rigidity. Hyperopic and emmetropic eyes did not lengthen much with the applied pressure, but myopic eyes did, with increasing amounts of change with increasing myopia. If it turns out that pre-myopic eyes show the same response, then this may be a test to predict who will become myopic. It may, however, turn out that the difference in rigidity is a result of the myopic changes, not the rigidity differences causing the myopia.Melissa D. Bailey.(2010) The Ciliary Body and Refractive Error: Unanswered Questions (FULL TEXT) Review of Optometry On-line 2/17/2010
MyopiaPrevention.org comment: "the ciliary body seems to be the only structure within the eye that thickens with myopia development." AS-OCT imaging of the ciliary body is possible. Children with myopia have higher AC/A ratios and greater accommodative lags. It is not clear if this is a cause or effect but the ciliary muscle is certainly involved. The working hypothesis for their further research is that the ciliary body "becomes hypertrophic as the eye is elongating during myopia progression." It may be that the thickening of the ciliary body is an indication of a disease process active during myopic progression.Rada JA, Shelton S, Norton TT.(2006) The sclera and myopia. (ABSTRACT) Exp Eye Res. 2006 Feb;82(2):185-200
MyopiaPrevention.org comment: A review showing that the sclera is a dynamic tissue that changes its composition and biomechanical properties in response to visual input in order to regulate ocular size and refraction. "Based on these studies, a strategy directed at reversing myopia-associated scleral extracellular matrix remodeling events would be warranted, particularly in cases of high myopia in humans."Tian Y, & Wildsoet CF. (2006). Diurnal fluctuations and developmental changes of ocular dimensions and optical aberrations in growing chick eyes: Significance for emmetropization. (FULL TEXT) Invest Ophthalmol Vis Sci. 47: 4168-78.
MyopiaPrevention.org comment: Chick eyes change cyclically each day. In this study various parts were measured several times a day for both normal chicks and chicks that had their accommodation (focusing) systems surgically paralyzed. Even without accommodation the eyes continued the emmetropization process. The eyes were longer in the evening compared to the morning but the choroid was thinner and the eyes less myopic in the evening when they were the longest.Rymer J, Wildsoet CF. (2005). The role of the retinal pigment epithelium in eye growth regulation and myopia: a review. (ABSTRACT) Vis Neurosci. 22: 251-261.
MyopiaPrevention.org comment: Since emmetropization (growth of the sclera) has been shown to be controlled by visual input (to the retina), the role of the retinal pigment epithelium (the layer between the sclera and retina) would appear to be an important pathway for chemical messages. This review describes such possible mechanisms, especially "dopamine, acetylcholine, vasoactive intestinal peptide (VIP), and glucagon".Phillips JR, McBrien NA. (2004). Pressure-induced changes in axial eye length of chick and tree shrew: significance of myofibroblasts in the sclera. (FULL TEXT) Invest Ophthalmol Vis Sci. 45: 758-763.
MyopiaPrevention.org comment: Elevation of IOP caused chick eyes to get longer and shrew eyes to get shorter. Shrew eyes (and other mammals) have myofibroblasts in the sclera (chick eyes don't) which may be a mechanism for control of emmetropization.McBrien NA, Gentle A.(2003) Role of the sclera in the development and pathological complications of myopia. (ABSTRACT) Prog Retin Eye Res. 2003 May;22(3):307-38.
MyopiaPrevention.org comment: A review of the mechanisms by which the sclera is involved in pathology related to high myopia.Hung LF, Wallman J, Smith EL 3rd. (2000) Vision-dependent changes in the choroidal thickness of macaque monkeys. (FULL TEXT) Invest Ophthalmol Vis Sci. 2000 May;41(6):1259-69
MyopiaPrevention.org comment: The choroid (the layer between the light receptors of the retina and the outer, tough layer of the eye called the sclera) is shown to increase or decrease rapidly in thickness in response to changes in the eye's refractive state, such as when focusing through a plus or minus lenses. The choroid, with its significant blood flow, may be a key to the process of emmetropization.