The eyes develop from three sources: the neural ectoderm of the forebrain, the surface ectoderm of the head, and the mesoderm between the two aforementioned layers. Eye formation is first evident at about 22 days gestation, when a pair of grooves called optic sulci appear in the neural folds at the cranial end of the embryo. Soon these sulci form a pair of optic vesicles on each side of the forebrain. The optic vesicles contact the surface ectoderm and induce development of the lens placodes, the primordia of the lenses. As the lens placodes invaginate to form lens vesicles, the optic vesicles invaginate to form optic cups. The retina forms from the two layers of the optic cup.
The retina, the optic nerve fibers, the iris muscles, and the epithelium of the iris and ciliary body are derived from neuroectoderm. Surface ectoderm gives rise to the lens, the epithelium of the lacrimal glands and ducts, the eyelids, the conjunctiva, and the cornea. The mesoderm gives rise to the eye muscles (except those of the iris) and to all connective and vascular tissues of the cornea, iris, ciliary body, choroid, and sclera. The sphincter and dilator muscles of the iris develop from the ectoderm of the optic cup.
The critical period of human eye formation is during developmental stages 10-20 (22-50 days). Because of the complexity of eye development, many congenital abnormalities can occur.
Examination of the eye should include a review of periorbital structures. Are the supraorbital brows prominent, pugilistic, heavy, or hypoplastic? Is there periorbital edema? Is there excessive pigmentation? Are the eyes deepset or prominent? Are there deep creases under the eyes?
The spacing of the eyes is discussed in greater detail in the measurement section. Although one might assume that the clinical impression concerning whether the eyes are too near or too far apart might be consonant with actual measurement, experience does not bear this out. The clinician may be misled by the width of the face, the form of the glabella area, the presence of epicanthal folds, and the width and shape of the nasal bridge. In addition, spacing of the eyes must be compared to the head circumference for valid interpretation. True ocular hypertelorism or wide spacing of the eyes occurs with an increased interpupillary distance or increased bony interobital distance. With lateral displacement of the inner canthi and lacrimal punctae (primary telecanthus), there may well be a false impression of widely spaced eyes. A rough clinical impression of whether there is lateral displacement of the lacrimal puncta can be obtained by seating the individual directly in front of the observer and drawing an imaginary vertical line through the inferior lacrimal point. If this line cuts the iris, there is lateral displacement of the inner canthus. The differences between telecanthus and pure hypertelorism are documented in Fig. 7.31. Soft tissue measurements of eye spacing are less precise than bony measurements obtained from a standard radiograph. The bony interorbital distance is discussed, in detail, in the measurement section.
The eyelids develop from two ectodermal folds containing cores of mesenchyme. The eyelids meet and adhere by about the 10th week and remain adherent until about the 26th week of pregnancy. A defect of the eyelid (a palpebral coloboma) is characterized by a notch in the upper or lower eyelid.
Evaluation of the eyelid includes an assessment of the length of the palpebral fissure. There may be a discrepancy in the length of the palpebral fissure between the two eyes, although this is generally minor. Palpebral fissure length may be reduced in several dysmorphic syndromes such as Dubowitz syndrome and fetal alcohol spectrum disorder. The width of the palpebral fissure, that is, the degree of opening of the eye, may also vary
Figure 7.31 Comparison of telecanthus and ocular hypertelorism. From Pashayen (1973), by permission.
and may be asymmetric. Frank ptosis or drooping of the eyelid is generally caused by weakness of levator palpebrae superioris. Ptosis is frequently found in association with dysmorphic syndromes such as Smith-Lemli-Opitz syndrome, Noonan syndrome, and Freeman-Sheldon syndrome. Obliquity of the palpebral fissures is discussed in detail in the measurement section. One refers to upward-slanting palpebral fissures if the lateral margin is superior to the medial margin, and downward-slanting palpebral fissures if the lateral margin is inferior to the medial margin.
An epicanthal fold is a lateral extension of the skin of the nasal bridge down over the inner canthus, covering the inner angle of the orbital fissure. The upper end of the fold can begin at the eyebrow, at the skin of the upper lid, or at the tarsal fold (Fig. 7.32). Approximately 30 percent of Caucasians under six months of age have epicanthal folds in association with normal depression of the nasal root. Only 3 percent of Caucasians between 12 and 25 years of age have epicanthal folds, as one might expect, because the root of the nose is less depressed in adulthood and the nasal bridge becomes more prominent. Epicanthal folds are more frequent in Asians. Epicanthus inverses refers to an epicanthal fold originating from the lower lid.
The thickness and length of the eyelashes should be evaluated. Eyelashes are ingrown in entropion and everted in ectropion. Distichiasis is the term for double rows of eyelashes-one row in the normal position, the other behind it and located at the site of the openings of the meibo-mian glands. These glands are hypoplastic or absent in such cases. All four lids can be affected.
Sulcus orbitopalpebralis superior
Figure 7.32 Epicanthal variations. From Goodman and Gorlin (1977), by permission.
Figure 7.32 Epicanthal variations. From Goodman and Gorlin (1977), by permission.
The configuration of the eyebrows is useful in syndrome diagnosis. Evaluation should include position, shape, texture of hair, and distribution of the eyebrows. A medial flare is seen in association with Williams syndrome. A diamond shape with an arch laterally is seen in Noonan syndrome. Eyebrows which grow together over the nasal root are termed synophrys, and are seen in Cornelia de Lange syndrome. Fullness of the lateral aspect of the brow is seen in Noonan syndrome, Williams syndrome, and hypothyroidism.
Many anomalies of iris color and structure are found. Unilateral or patchy hypopigmentation can occur, producing heterochromia iridis. A prominent pattern of the iris stroma radiating out from the pupil is known as a stellate iris. Brushfield spots are elevated white, or light yellow, iris nodules. They are best seen in blue irides but are as frequent in brown irides. They are commonly present in the midzone of the iris in a ring, associated with peripheral iris hypoplasia in Down syndrome. They are seen more peripherally in normal individuals. Lisch nodules are small grey-tan hamartomata of the iris. Multiple Lisch nodules are seen in neurofibromatosis type 1. Careful slit-lamp examination is usually required to see them. Colobomas can also occur in the iris, giving the pupil a keyhole appearance. The gap or notch may be limited to the iris or may extend deeper and involve the ciliary body and retina.
Variation in size and shape of the pupil should be described. Pupil size varies under ordinary conditions in proportion to the amount of light let into the inner eye. Pupillary asymmetry or anisocoria may be congenital or may result from a disturbance either locally in the eye or in the neural pathways. Aniridia, congenital absence of the iris, may be an isolated finding or part of a syndrome (e.g., Wilms tumor-aniridia-genitourinary malformation [WAGR] syndrome).
The cornea may vary in size and shape. Microcornea consists of a reduction in size of the cornea to a diameter of 10 mm or less, in association with growth failure of the anterior part of the eye and incomplete development of the angle. Approximately 20 percent of patients with microcornea will develop glaucoma in later life. Megalocornea may occur as an isolated malformation, with a corneal diameter of 13-18 mm. If the anomaly is limited to the anterior portion of the globe, visual acuity can be normal. In association with megalocornea, there is frequently subluxation of the lens, cataract, enlargement of the ciliary ring, and noticeable iridodonesis. Iridodonesis is a shimmering of the iris when the eye is moved rapidly from side to side in the presence of a dislocated or absent lens. Keratoconus is an obtuse conical shape to the cornea, with the vertex in or near the corneal center. It produces abnormal refraction and is seen with an increased frequency in Down syndrome. It may be associated with retinitis pigmentosa and Alport syndrome. The cornea can appear cloudy in various storage disorders, particularly the mucopolysaccharidoses. Corneal clouding is best appreciated from the side of the eye or through a slit-lamp. Kayser-Fleischer rings are areas of greenish-yellow pigmentation in Descemet's membrane at the corneal periphery seen in Wilson's disease.
Abnormalities of the conjunctiva include telangiectasia, which are dilatations of capillary vessels and minute arteries forming a variety of angiomas, and pterygia, which are patches of thickened conjunctiva, usually fan-shaped, with the apex towards the pupil.
The sclera is generally white. It may be blue-grey in the neonate and in disorders such as osteogenesis imperfecta.
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