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Chapter 2: Surgical Anatomy :
Growth of eye from birth through childhood
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The eye undergoes significant growth between the neonatal period and adulthood (Figure 62). Study of this growth in vivo is made possible by the use of accurate, quick, and reliable A-scan biometry. The Ascan biometer, used principally for intraocular lens calculations in adults, has been applied to children to obtain measurements of the anteroposterior diameter. Gilles first used these measurements which he combined with corneal diameter measurements and measurements of the medial rectus insertion site to arrive at a more scientific formula for recession of the medial recti. Kushner found an inverse relationship between axial length and response (prism diopter change per millimeter of surgery) in esotropic patients. This finding is expected since the maximum ‘torque’ can be obtained with a smaller ‘gear’ or, in the case of strabismus surgery, with a smaller eye. In clinical practice, the most important advantages of using these measurements seem to be consistency and the ability to do the largest recession without crippling the medial rectus by placing the new insertion too far posteriorly.
Figure 62
A One-week-old child, anterior-posterior diameter 17+ mm
B Three-month-old child, anterior-posterior diameter 18+ mm
C Four-month-old child, anterior-posterior diameter 19.5+ mm
D One-year-old child, anterior-posterior diameter 20+ mm
E Two-year-old child,anterior-posterior diameter 21+ mm.
F Three-year-old child, anterior-posterior diameter 22+ mm
G Five-year-old child, anterior-posterior diameter 23+ mm
Studies of the globe in neonates and infants indicate that the posterior aspect of the globe is relatively hypodeveloped compared to the anterior aspect. This means that recession of the medial rectus in a newborn could put the new insertion site behind the equator, even in cases where as little as 3 mm of recession were done. Therefore, it has been advocated that surgery should not be done on very young infants. Surgery on infants as young as two or three months has been reported, but is not something that could be considered routine or advisable based on anatomic studies. In addition, there is good evidence that four months may be the earliest age that congenital esotropia can be diagnosed with confidence. Surgery on infants with infantile esotropia between the fourth and sixth month is now common and is safe. The axial length of the typical eye at this age is 19.5 mm. Such eyes are entirely suitable for surgery consisting of bimedial rectus recession to a point approximately 9.5 mm from the limbus. A safe lower age limit for surgery in cases of congenital infantile esotropia with no other contraindications is four months. Surgery for congenital esotropia by six months of age is now common and is shown in Figure 63.
Figure 63
A Six-month-old with 50 diopters congenital esotropia preoperatively.
B Same patient immediately after surgery in the operating room. Both medial recti were recessed to a point 10 mm from the limbus. The axial length was 19.5 mm in each eye.
C Same patient at age 1 year.
Nanophthalmos describes an otherwise normally functioning eye but with a shorter anteroposterior diameter. These eyes have increased scleral thickness but decreased rigidity and are subject to retinal detachment. These eyes also have hyperopia and an increased incidence of glaucoma. High myopia results in a significant increase in the anteroposterior diameter. Anteroposterior diameters as long as 27 mm in a six-year-old child with -11.00 D of myopia have been measured and some adults have axial length measurements greater than 30 mm. This can lead to intermuscular membrane rupture, pulley displacement, and muscle slip causing eso - hypotropia or ‘heavy eye.’
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