Anatomy of a baby’s eye

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In a baby born at term the eye has an anterior-posterior diameter of about 16mm and it continues to grow slowly over years to about 23 mm.

The eye is made up of three concentric tunics: outermost, middle and innermost.

The outermost tunic is made of the cornea [1] and sclera [2]. The sclera is a fibrous layer or the “white part of the eye”. The cornea is the transparent layer at the front of the eye and it helps focus light from an image on the retina.

The middle tunic is a vascular layer. It is made up of the choroid [3] at the back of the eye, the ciliary body [4] and the iris [5]. The iris is seen as a coloured ring around the pupil. Everyone has colour variations in their iris due to differences in the level of the pigment melanin. Melanin scatters light in the iris resulting in different shades of brown, blue and green. The iris contains a sphincter muscle which controls the amount of light entering the eye, by changing the size of the pupil in response to the intensity of light entering the eye. Mydriatic drops are used to dilate the pupil when examining the back of a premature baby’s eye for ROP. The health care team must be at hand during the administration of mydriatic drops and monitor the baby throughout the eye examination.

The innermost tunic is the retina [6], which is a nervous layer, made up of photoreceptors (rods and cones) which convert light impulses into electrical impulses that are then transmitted through the optic nerve [7] to the brain.

The macula [8] is a specialized area of the retina located two disc diameters from the optic nerve. It is a slightly pigmented area and has a small, central depression called the fovea. The fovea and macular have more photoreceptors than other parts of the retina which provide clear, central vision. Central vision can be measured, and this measure is called visual acuity.

Retinal artery and vein [9] and their branches supply blood to the retina. During fetal development these blood vessels grow out from the optic nerve towards the peripheral edge of the retina, the ora serrata [11]. This is a junction of transition between the different layers, especially the retina from non-photosensitive area of the retina to the multi-layered photoreceptor layer. In most term babies the blood vessels have already reached the ora serrata at the time of birth; in premature babies the blood vessels have not reached the ora serrata. ROP is a consequence of disruption of the normal development of these blood vessels.

The lens in the eye [10] is suspended behind the pupil. It is a transparent elastic tissue which has an ellipsoid biconvex shape. The lens can change shape to focus light entering the eye from near and far objects.

Vision in babies

All babies have poor vision due to immaturity of the retina, the brain and the pathways between them. Vision improves during the first few years of life, but does not reach adults levels until the age of 7-8 years. The same applies to infants born preterm.

Abnormalities of the eye linked to prematurity

Preterm infants who do not develop ROP are at greater risk of strabismus (misalignment of the eyes) and refractive errors (light entering the eyes is not focused on the retina). Preterm infants are also at greater risk of damage to different structures in the brain, which can affect the vision as well as lead to cerebral palsy. Preterm infants who do develop ROP are at even greater risk of these eye and vision complications than those who do not. This means that ophthalmic follow up of preterm babies is important, whether or not they developed ROP.

Visual development is a complex system that cannot be assessed simply by gestational or chronological age. In practice, no two children have identical visual development and there are great variations among preterms. This means that ophthalmic follow up of preterm babies is an important consideration, with or without ROP.

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