EVALUATION
Sonographic assessment of the fetal eyes is not a requirement for routine prenatal sonography according to the amalgamated guidelines from the (American Institute of Ultrasound in Medicine, American Congress of Obstetricians and Gynecologists,American College of Radiology) AIUM, ACOG, and ACR.Assessment of orbital biometry is a reasonable expectation as part of a detailed anatomy scan, particularly in the setting of suspected central nervous system (CNS) malformation. However, evaluation of the globe itself can be far more challenging.
Complete ocular evaluation comprises ocular biometry, assessment for the presence of the globes, and examination of the morphology of the lens, vitreous, and optic nerves.There is a necessity for a working knowledge of ocular pathologies because syndromes involving the eyes can go unrecognized without a systematic approach.
Embryology
At 22 days’ gestation, the optic vesicles start to develop as bilateral outpouchings from the inner wall of the forebrain. When the vesicles contact the surface ectoderm, this induces the lens placode, which then starts to invaginate into the optic vesicles that become the optic cups. By 5 to 7 weeks, the lens placode loses contact with the surface ectoderm to form the lens vesicle. Elongation of the cells within the lens vesicle fills the lumen of the lens vesicle to form the solid lens. With growth, the optic cups envelop the lens circumferentially to form the globe. The hyaloid artery, the primary supply of nutrients to the developing lens, is contained within the optic stalk, which extends from the optic disc through the vitreous humor to the lens. After the tenth week, the lens grows independent of blood supply and the hyaloid artery regresses leaving a clear central zone, known as the hyaloid canal or Cloquet canal.
Imaging/Biometry
Growth charts for the fetal lens and orbit,as well as the measurements of binocular (BOD) and interocular (IOD)distances have been determined sonographically (see Table 25A in Appendix A1). The latter are measured according to the bony landmarks of the medial and lateral orbital walls. The orbital measurements are ideally made in the axial plane, with both orbits of equal and largest possible
diameter.The binocular distance is measured between the two malar margins, and the interocular distance between the two ethmoidal margins of the bony orbits (Fig. 13.1-1).These bony landmarks are difficult to see by fetal magnetic resonance imaging (MRI), and therefore standard sonographic growth charts cannot accurately be applied to fetal MR studies.
FIGURE 13.1-1: Orbital Measurements by US. Transorbital view of the fetal face shows the measurement of the BOD with the calipers on the malar margins of the orbit. The IOD is measured between the two ethmoidal margins (arrows).
On T2-weighted MRI, the whole lens is low signal compared with the high signal of the vitreous. The BOD and IOD measurements can be made in any plane from true axial to true coronal, provided both eyes can be seen in the same image and have the most equal and largest possible transverse diameters. The binocular and interocular distances are, respectively, measured between the two malar or ethmoidal margins of each vitreous (Fig. 13.1-2). These measurements can be plotted against gestational age (see Table 26 in Appendix A1). Other available nomograms additionally include measurements of the transverse diameter of the lens9and anteroposterior diameter of the globe.10 It has been noted that the globe is ellipsoid earlier in gestation. This is thought to be attributable to the presence of the hyaloid artery initially tethering the lens to the optic disc. With involution of the hyaloid artery, the globe is increased in anteroposterior diameter.
