hypertelorism
description
orbital hypertelorism is defined as excessive separation of the medial walls of the bony orbit. this dimension should always be considered in relationship to overall head size, since excessive spacing of the orbits in a head greater than two standards above the mean may be considered normal. after birth, radiological assessment of the bony interorbital distance is the most accurate method to determine whether excessive eye spacing is present. however, interpupillary distance is the optimum dimension used in clinical determination of eye-spacing. this measurement may have ultrasonographic relevance in the future, particularly as a transvaginal approach allows visualisation of the fetal lens. during early embryonic life, the eyes grow laterally on optic stalks, and then move towards the midline. several mechanisms have been proposed to explain the development of orbital hypertelorism. firstly, there may be morphokinetic arrest that causes failure of migration of the orbits to the midline with some time-specific deficiency in differential growth. secondly, interruption of migration might occur because a primary midline lesion, such as an encephalocele, inhibits approximation of the orbits. thirdly, a pathological process affecting the cranial base, for example, secondary to craniosynostosis, could lead to separation of the orbits.
diagnosis
although orbital imaging is not routine in most centres, and is not included in the aium guidelines for obstetrical ultrasonography, the bony orbits are reliably imaged when scanning the fetal face from the 12th week onward. care must be taken to avoid tangential cuts through the orbits. the inner orbital measurement is most useful in the determination of hypertelorism, and is generally greater than two standard deviations from the mean. the outer orbital measurement often falls at the 95th percentile. with the head in the occipitotransverse position, the transducer can be placed in two possible planes, firstly, along the coronal plane, approximately 2 cm posterior to the glabella-alveolar line, or, secondly, 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 diameter from lateral border of one bony orbit to lateral border of the other bony orbit, inner diameter 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 the 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 ultrasonography is a transverse section of the fetal skull at the orbital plane. the eyes within the bony orbits are detected as bilateral hypoechoic circles superolateral to the nasal bones. a detailed search for intra- and extra-cranial anomalies should be performed since these are frequently associated with differences in eye spacing, and may provide evidence for a particular malformation syndrome, as detailed in the table of associated syndromes. prenatal diagnosis has been reported in opitz, crouzon , apert and median cleft face (frontonasal dysplasia) syndromes.
differential diagnosis
primary hypertelorism, due to lack of convergence of the bony orbits, should be differentiated from hypertelorism secondary to a midline lesion, such as a frontal encephalocele.
sonographic features
inner orbital distance greater than two standard deviations above the mean for gestational age.
often accompanied by outer orbital distance at the upper limits of normal
associated syndromes
- aarskog
- acrocallosal
- acrodysostosis
- aminopterin -like syndrome without aminopterin
- (assas)
- apert
- baraitser-winter
- chromosome anomaly
- coffin-lowry
- craniofrontonasal
- crouzon
- digeorge
- donnai
- frontofacionasal dysplasia
- frontonasal dysplasia
- greig
- hypertelorism-microtia clefting
- leopard
- neurofibromatosis
- noonan
- opitz [g, bbb]
- otopalatodigital
- robinow
- santos
- sotos
- stoll
- teebi
- teratogen exposure
references
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