relatively small. A pupil less than 1.8 mm or greater than 5.4 mm in diameter is suggestive of an abnormality. The pupillary light reflex is normally present after 31 weeks’ gestational age. At birth, the iris insertion is near the level of the scleral spur, but during the first year of life, the lens and ciliary body migrate posteriorly, resulting in formation of the angle recess.

Pediatric Eye Disease Investigator Group; Bradfield YS, Melia BM, Repka MX, et al. Central corneal thickness in children. Arch Ophthalmol. 2011;129(9):1132–1138.

Intraocular Pressure

Measurement of intraocular pressure (IOP) in infants can be difficult, and normal pressures vary depending on the method used to obtain them. Nevertheless, normal IOP is lower in infants than in adults, and a pressure of greater than 21 mm Hg should be considered abnormal. CCT influences the measurement of IOP, but this effect is not well understood in children. See Chapter 22 in this volume and BCSC Section 10, Glaucoma, for further discussion.

Extraocular Muscles

The rectus muscles of infants are smaller than those of adults; muscle insertions, on average, are 2.3– 3.0 mm narrower in infants than in adults; and the tendons are thinner in infants. In newborns, the distance from the rectus muscle insertion to the limbus is roughly 2 mm less than that in adults; by age 6 months, this distance is 1 mm less; and at 20 months, it is similar to that in adults. Enlargement of the posterior segment occurs during the first 2 years of life, resulting in a separation of 4–5 mm between the superior and inferior oblique insertions and migration of the inferior oblique insertion temporally.

Extraocular muscle function continues to develop after birth. Vestibular-driven eye movements are present as early as 34 weeks’ gestational age. Conjugate horizontal gaze is present at birth, but vertical gaze may not be fully functional until 6 months of age. Intermittent strabismus is present in approximately two-thirds of young infants but resolves in most by 2–3 months of age. Accommodation and fusional convergence are usually present by age 3 months.

Retina

The macula is poorly developed at birth but changes rapidly during the first 4 years of life. Most notable are changes in macular pigmentation, the annular ring, the foveal light reflex, and cone photoreceptor differentiation. Improvement in visual acuity with age is attributed to 3 processes: differentiation of cone photoreceptors, narrowing of the rod-free zone, and increase in foveal cone density (Fig 15-5). Retinal vascularization proceeds in a centrifugal manner, starting at the optic disc at 16 weeks’ gestational age and reaching the temporal ora serrata by 40 weeks’ gestational age.

Hendrickson A, Possin D, Lejla V, Toth CA. Histologic development of the human fovea from midgestation to maturity. Am J Ophthalmol. 2012;154(5):767–778.