Studies of ocular malformations induced by teratogen exposure have been helpful in identifying sensitive periods during development. Microphthalmia and anophthalmia may result from insult at a number of developmental stages. Acute exposure to teratogens during early gastrulation stages results in an overall deficiency of the neural plate with subsequent reduction in size of the optic vesicle. This aberration results in microphthalmia, which may be associated with a spectrum of secondary malformations including anterior segment dysgenesis, cataract, and PHPV.24,27,28 Deficiency in size of the globe as a whole is often associated with a corresponding small palpebral fissure. Because the fissure size is determined by the size of the optic vesicle (most likely during its contact with the surface ectoderm), support is provided for a malformation sequence beginning at the time of formation of the optic sulcus or optic vesicle.
Failure or late closure of the optic fissure prevents the establishment of normal fetal IOP and can result in microphthalmia associated with colobomas, that is, colobomatous microph-thalmia (Fig. 1-27). This syndrome may be associated with orbital (or eyelid) cysts (Fig. 1-28). It is important to recognize that delay in closure of the fissure during a critical growth period may result in inadequate globe expansion. However, if the fissure eventually closes, it may be difficult to distinguish between colobomatous and noncolobomatous microphthalmia. In colobomatous microphthalmia, the optic vesicle size is initially normal and a normal-sized palpebral fissure would be expected, whereas with microphthalmia that results from a primary abnormality in the neural plate and optic sulci, the palpebral fissure would be small.
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