Milestones and normative data

Chapter 6 Milestones and normative data




At birth, eye size looks adult-like because the corneal diameter is only 1.7 mm smaller than in an adult, but volume increases threefold and weight doubles on maturity. In the full-term newborn eye volume is 3.25 cm3 and weight is 3.40 g. The weight increases 40% by the middle of the second year and 70% by the fifth year.


The development of the eye parts is meticulously sequenced, and understanding the milestones of development is needed to assess clinical observations.



Intercanthal distance and palpebra


Abnormalities in the distance between the inner canthi and the outer canthi and the size and shape of the palpebral fissure are important features in craniofacial malformations and fetal alcohol syndrome. A fast non-contact method of measuring facial components is provided in Fig. 6.1A,B.1



Palpebral fissure changes in early childhood have been studied by analyzing digital imaging:2 during the first 3 months of life the upper eyelid is at its lowest position, later rising to its maximum between the age of 3 to 6 months, and then declining until adulthood. The lower eyelid is close to the pupil center at birth, dropping until the age of 18 months when its position stabilizes. A single lower eyelid crease is common at birth, a double crease at the age of 36 months. Figure 6.2 shows the linear relationship between gestational age and orbital margin horizontal (OMH), as well as vertical (OMV) diameters in the unborn child.3 There is a linear relationship between gestational age and conjunctival fornix horizontal (CFH) and conjunctival fornix vertical (CFV) diameters (Fig. 6.3).3




The palpebral fissures are 15 ± 2 mm at 32 weeks of gestation, 17 ± 2 mm at birth, 24 ± 3 mm at 2 years of age, and 27 ± 3 mm at the age of 14.4,5 Inter-racial differences exist: the palpebral fissure is longer in Black Americans.6


Inner canthal distance and outer orbital distance are 16 and 59 mm, respectively, in premature infants; 20 ± 4 and 69 ± 8 mm in newborn babies; 26 ± 6 and 88 ± 10 mm at the age of 3; and 31 ± 5 and 111 ± 12 mm at the age of 14 (Fig. 6.4).7



A universal approach is the canthus index:



image



Normals, unrelated to age, lie between 28.4 and 38%.8 The canthus index of over 1000 children between 6 and 18 years old was determined as follows:9
















  Boys Girls
6 years 38.2% (SD 2.1%) 38.3% (SD 1.8%)
16 years 37.1% (SD 2.6%) 36.6% (SD 1.9%)



Cornea


The premature cornea lacks luster and clarity, making some diagnoses difficult. Shallow anterior chambers, miotic pupils, and bluish irides are features of prematurity. The corneal diameter in infants at 25–37 weeks postconceptional age increases by 0.5 mm every 15 days from 6.2 to 9.0 mm (Fig. 6.5).11,12 The horizontal and vertical diameters of the cornea in full-term boys are 9.8 ± 0.33 mm and 10.4 ± 0.35 mm and in girls 10.1 ± 0.33 mm and 10.7 ± 0.29 mm.13 Two millimeters of growth in corneal diameter (approximately 20%) occurs in early infancy and early childhood. An adult value of 11.7 mm is reached by 7 years.




Central corneal thickness


Abnormal thickness of the central cornea influences intraocular pressure, but also corneal hysteresis may play a role in children. Central corneal thickness (CCT) in a full-term baby is 0.54 mm greater than in a 1-year-old child. CCTs measured with optical pachymetry and corneal curvature are given for premature and full-term babies in Table 6.1.14



CCT in premature infants below 33 weeks gives a mean of 0.656 mm (SD ± 0.103 mm) 5 days postnatally and 0.566 (SD ± 0.064) at the age of 110 days.15 In full-term neonates,16 CCT is 0.573 ± 0.052 mm (range 0.450–0.691 mm) with a peripheral corneal thickness of 0.650 ± 0.062 mm (range 0.520–0.830 mm). Table 6.2 shows the decrease in thickness during the first few days of life.



Another study17 confirmed the above data and also measured peripheral corneal thickness: superior corneal thickness was 0.696 ± 0.055 mm, inferior was 0.744 ± 0.062 mm, nasal was 0.742 ± 0.058 mm, and temporal was 0.748 ± 0.055 mm. Adult values are reached at about 3 years of age. There is no significant difference of CCT among racial subgroups.18


Keratocyte density is around 60 000 cells per cubic millimeter in infancy with a decline of 0.3% per year through life.


Endothelial cell counts exceed 10 000 cells per square millimeter at 12 weeks of gestation, 50% of this at birth and 4000 cells per square millimeter in childhood.





Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Milestones and normative data
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