[Developmental mechanism of low myopia and therapeutic possibilities. A review]

We studied the developmental mechanism of low myopia and the possibility of its drug treatment. I. Prevalence of myopia. According to surveys by the Japanese Ministry of Education, Science, Sports and Culture, the frequency of myopia in school children has gradually increased. We examined office workers from 20 to 60 years of age over a 3-year period. Myopia progression was observed in the thirties and fourties. Late-onset myopia is an important problem around the world. II. Developmental mechanism of low myopia. 1. Reduction of refraction after cycloplegia was statistically significant in adults after their twenties. We measured accommodative hysteresis after a close visual task. Accommodative hysteresis persisted for a long time in late-onset and adult-onset myopia. Continuous ciliary contraction seems to be related to late-onset myopia. 2. Bovine ciliary muscle strips were suspended in a Magnus double tube. The changes in isometric tension of the strips, when chemicals were added, were measured with a force-displacement transducer. After the addition of the cholinergic agonist carbachol, the strips of ciliary muscle produced a tonic contraction. When the muscarinic receptor antagonist cyclopentolate was added, relaxation was produced. After the addition of ET-1, a dual response occurred which consisted of a moderate relaxation and a long-lasting contraction. The mean contraction caused by ET-1 was weak but continuous compared to carbachol. The contractile response was inhibited by an ETA receptor antagonist. Also, when takusha, one component of gorei-san, a Chinese drug, was added to the ciliary muscle strips, contraction with ET-1 was attenuated. Contraction of the ciliary muscle with ET-1 was attenuated after addition of sodium nitroprusside (SNP), an NO donor. This reaction suggests that NO causes relaxation of the bovine ciliary muscle through the activation of guanylyl cyclase and an increase in cyclic GMP. Currently, at least 5 muscarinic receptor subtypes are recognized; they are named M1 to M5. The effects of M1, M2, and M3 on the contractile response to transmural electrical stimulation of the bovine ciliary muscle were studied. The contractions produced by transmural electrical stimulation were greatly attenuated by 4-DAMP as an M3 antagonist. 3. We measured autofluorescence of the lens by fluorophotometry. A statistically significant relation was found between autofluorescence of the lens and refraction. 4. Possibility of axial elongation: The anterior and posterior suprachoroidal spaces are different anatomically and physiologically. When the choroid is stretched forward by accommodation, the intraocular pressure may exert more influence on the posterior part of the sclera, than on the anterior part. Using a fluorophotometer, fluorescence leakage at a site 3 mm in front of the retina was examined. Intensity of fluorescence 3 mm in front of the retina was strong in late-onset myopia. This may indicate disturbance of the barrier of the retinal pigment epithelium. When cultured fibroblasts of the sclera of the chick embryo was stretched by a stretching apparatus, proliferation of the cultured cells was inhibited. Therefore, some influence may involve the posterior part of the eyeball. III. Possibility of drug treatment of low myopia: From these results, muscarinic receptor antagonists (especially M3), ET receptor agtagonists, and NO donors are possible drugs for low myopia treatment. As there are many causative factors of low myopia, there are several treatment methods to be evaluated.