Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Neuro-Ophthalmology_Wright, Spiegel, Thompson_2006

therapy of the sound eye to treat the amblyopia, as good visual outcome can be achieved even when there is macular involvement.103

EYELIDS

The eyelids develop from surface ectoderm that gives rise to the epidermis, cilia, and conjunctival epithelium. Neural crest cell mesenchyme gives rise to deeper structures including the dermis and tarsus. The eyelid muscles, orbicularis and levator, are derived from mesoderm. By 6 weeks of gestation, the upper and lower eyelid buds are visible (see Fig. 1-15). They come from mesenchymal accumulations called frontonasal (upper lid) and maxillary (lower lid) processes. The lid folds grow together and elongate to cover the developing eye. Upper and lower lids fuse together at approximately 10 weeks. By 6 months gestation, glandular structures and cilia develop, and the lids gradually separate.

EXTRAOCULAR MUSCLES

The extraocular muscles arise from mesoderm in somitomeres (preotic mesodermal segments), with the primitive muscle cone first appearing at 5 to 7 weeks gestation (Fig. 1-13). The oculomotor innervated muscles originate from the first and second somitomeres, the superior oblique muscle from the third somitomere, and the lateral rectus muscle from the fifth somitomere.77 The motor nerves to the extraocular muscles grow from the brain to the muscle and innervate the mesodermal condensation at approximately 1 month.

The extraocular muscles develop from local mesenchyme (mesodermal origin) in situ within the orbit, rather than from anterior growth of surrounding mesoderm, as had been an earlier hypothesis. Additionally, muscles do not grow from the orbital apex anteriorly; rather, the insertion, belly, and origin develop simultaneously.93,94 The orbital axes rotate from the early stages of optic cup development to adulthood.

OCULAR DYSGENESIS

Syndromes of ocular dysgenesis are summarized in Table 1-2.

Microphthalmia

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 microphthalmia (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.

Optic Fissure Closure Anomalies (Coloboma)

Colobomas represent an absence of tissue that may occur through abnormal fusion of the optic fissure, which normally closes at 4 to 5 weeks gestation. Colobomas may occur anywhere along the optic fissure and can affect the iris, choroid,

Source: NIH Online Mendelian Inheritance in Man: www3.ncbi.nlm.nih.gov/

macula, and optic nerve (Figs. 1-21, 1-22, 1-23). Colobomas are often associated with microphthalmia (colobomatous microphthalmia) or, less frequently, orbital or eyelid cysts (Fig. 1-22). Because the optic fissure closes first at the equator of the eye, and then in a posterior and anterior direction, colobomas are most frequently found at the two ends of the optic fissure, that is, iris and optic nerve. When the optic nerve is involved in the coloboma, vision is usually affected, in some cases causing blindness. Optic nerve colobomas may be associated with basal encephaloceles, which also represent a failure of fissure closure.59,85 Large choroidal colobomas may be associated with posterior pole staphylomas, causing macular disruption and poor vision. Occasionally, a line of choroidal colobomas occur along the fetal fissure area with skip areas (Fig. 1-23). Isolated iris colobomas usually do not affect visual acuity unless there is an associated refractive error. Typical iris colobomas occur infer-

onasally along the location of the optic fissure whereas atypical iris colobomas are not associated with abnormal fissure closure and can occur elsewhere. Atypical iris colobomas usually have an intact iris root (Fig. 1-24).

Differentiation of choroidal and iris stroma is determined by the adjacent structures of the optic cup: the iris epithelium, anteriorly, and the future retinal pigment epithelium, posteriorly. In animals exhibiting primary abnormalities in differentiation of the outer layer of the optic cup, anterior and posterior segment colobomas are seen in a very specific distribution associated with the iris epithelium or RPE defects,25,26 and this is the most likely explanation for atypical uveal colobomas. The term lens coloboma is actually a misnomer, as this defect results from a lack of the zonular pull in the region of the coloboma rather than regional hypoplasia of the lens. Ciliary body colobomas are often associated with abnormal lens shape or subluxation or both.

Colobomatous microphthalmia with eyelid cyst syndrome may be unilateral or bilateral (see Fig. 1-28). Colobomatous cysts form from the inner layer (neuroectoderm) of the optic cup as it grows out of the persistent opening of the optic fissure. The lower lid cyst contains primitive vitreous contents that were not enclosed within the eye because the optic fissure did not close. The cyst has a stalk that connects to the microphthalmic eye. For those who are unaware of the syndrome, the lid cyst is often mistaken as an abnormal eye located in the lid.

Dermoids and Dermolipomas

Dermoids are choristomas (histologically normal tissue in an abnormal location) and are thought to represent arrest or inclusions of epidermal and connective tissues (surface ectoderm and neural crest cells). They may be associated with abnormal closure of the optic fissure. This collection of epidermal and connective tissue can occur at the limbus (limbal dermoid), in the conjunctiva (dermolipoma), and subcutaneously in and around the orbit. The most common location of subcutaneous periorbital dermoid cysts is the superotemporal and superonasal quadrants of the orbital rim. These dermoids are usually found attached to bone, associated with a cranial suture.

Limbal dermoids are similar to subcutaneous dermoid cysts and consist of epidermal tissue and, frequently, hair (Fig. 1-25). Corneal astigmatism is common in patients with limbal dermoids. Astigmatisms greater than 1.50 are usually associated with meridional and anisometropic amblyopia. Removal of limbal dermoids is often indicated for functional and cosmetic reasons, but the patient should be warned that a secondary scar can recur over this area. Limbal dermoids can involve deep corneal stroma, so the surgeon must take care to avoid perforation into the anterior chamber.

Dermolipomas (lipodermoids) are usually located in the lateral canthal area and consist of fatty fibrous tissue (Fig. 1-26). They are almost never a functional or cosmetic problem and are best left alone. If removal is necessary, only a limited dissection should be performed to avoid symblepharon and scarring of the lateral rectus. Unfortunately, restrictive strabismus with limited adduction frequently occurs after removal of temporal dermolipomas.

A

B

FIGURE 1-22A–B. (A) Photograph of 6-month-old with colobomatous microphthalmia and orbital cyst anomaly. Note the left lower eyelid cyst causing a mass in the lower lid, left blepharophimosis (small lids and narrow lid fissure), and apparently normal right eye. (B) Desmarres retractors open the eyelids in an attempt to expose the microphthalmic left eye. The only remnant of eye that could be seen externally was a small dimple just nasal to the lid retractors.