microphthalmia

diagnosis

although orbital imaging is not routine in most centres, and is not included in the aium guidelines for obstetrical sonography, the bony orbits are reliably imaged on scanning the fetal face after the twelfth week. at 15 weeks the lenses and hyaloid arteries are recognized. care must be taken to avoid tangential cuts through the orbits. in general, microphthalmia is found in association with normal inner orbital distance, the outer orbital distance and the globe diameters are usually below the third centile. with the head in an occipitotransverse position, the transducer can be placed in two possible planes, firstly, in a coronal plane, approximately 2 cm posterior to the glabella-alveolar line, or secondly, in a transverse plane along the orbitomeatal line, approximately 2 to 3 cm below the level of the biparietal diameter. in both these views, the midline, orbital rings, nasal processes, and portions of the maxilla can be demonstrated. the inner and outer orbital diameters are measured from the leading edge to falling edge: outer diameters from the lateral border of one bony orbit to lateral border of the other bony orbit; inner diameters from medial border to medial border. occasionally, when the head is in this position, it is difficult to define accurately the distal orbital margin because of acoustic shadowing from the nose. with the head in an occipitoposterior position, a biparietal diameter cannot be obtained. however, the orbits can be identified and measured with the transducer placed in a plane that transects the occiput, orbits and nasal processes. measurements should only be obtained when the fetal face is perpendicular to the uterine wall, since measurements in an oblique plane may be unreliable. the optimal view of the fetal eyes on transvaginal sonography is on a transverse section of the fetal skull at the level of the orbit. the eyes within the bony orbits are detected as bilateral hypoechoic circles superolateral to the nasal bones. in addition to the bony orbital dimensions discussed above, direct measurement of the vitreous and lens circumferences can be performed, utilising transvaginal ultrasonography until 17 weeks gestation and a transabdominal approach thereafter. the lenses and the hyaloid arteries are easily recognized at 15 weeks. the eye is evaluated in a plane with the probe positioned lateral to the orbit. the probe orientation is adjusted until the lens appears as an annular structure within the anterior portion of the orbit, and a maximal diameter of the vitreous is found. the transverse and superoinferior diameters are measured from inner edge to inner edge, while the lens diameter is measured from outer edge to outer edge. microphthalmia may be associated with structural brain abnormalities, especially midline defects such as agenesis of the corpus callosum or absence of the septum pellucidum. accompanying hypophyseal/hypothalamic dysfunction may cause growth deficiency.