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KEY POINTS
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Key Points

  • Defined as a decreased distance between the medial aspects of the orbital walls.

  • Incidence is approximately 1 in 1220 livebirths.

  • Main concern is its strong association with brain anomalies (especially holoprosencephaly).

  • A fetal karyotype should be offered due to its association with aneuploidy, most commonly trisomy 13.

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CONDITION
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The term hypotelorism was used as early as 1960 to indicate a decrease in interorbital distance (Judisch et al., 1984). Ocular hypotelorism is defined as a decreased distance between the eyes or pupils, whereas orbital hypotelorism is defined as a shortened distance between the medial aspects of the orbital walls, with reduced inner and outer canthal distances (Converse et al., 1975; Judisch et al., 1984). The major concern regarding the finding of fetal hypotelorism is its association with midline craniofacial defects and major cerebral anomalies.

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Orbital hypotelorism results from developmental abnormalities of the telencephalon, which is a derivative of the forebrain. It is strongly associated with holoprosencephaly (see Chapter 14) (Converse et al., 1975; Achiron et al., 1995). Frequently, hypotelorism is more obvious radiographically or sonographically than clinically. In one study, the normal range of interorbital distances was documented in anteroposterior facial radiographs obtained in 250 normal children. The mean values ranged from 15 mm in infancy to 23 mm at 12 years of age. The interorbital distance was found to be narrower for girls than for boys. The interorbital distance normally remains small until 18 months of age, when distance gradually increases in both sexes. Interorbital growth levels off in females at age 13 years, but in males this distance continues to increase until 21 years of age (Converse et al., 1975). Interorbital distances are closely related to ethnic background. Individuals who trace their ancestry to Eskimos or Mongolians may have hypotelorism as a normal genetic variation (Awan, 1977).

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INCIDENCE
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Hypotelorism is uncommon. Most references list the incidence of hypotelorism as unknown, but one reference documented hypotelorism in 1 in 1220 deliveries in Taiwan (a frequency of 0.08% livebirths) (Kuo et al., 1990).

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SONOGRAPHIC FINDINGS
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Imaging of the fetal orbits and measurements of the interorbital distance are not routine in most centers that provide antenatal sonography. Measurement of the interorbital distance is not included in the American Institute of Ultrasound in Medicine (AIUM) or American College of Obstetrics and Gynecology (ACOG) guidelines for obstetrical sonography (American College of Obstetrics and Gynecology, 2008). Fetal orbital measurements should be taken in any pregnant woman with a previously affected child, or in the setting of a facial cleft, or with a central nervous system malformation such as hydrocephalus or holoprosencephaly. When orbital hypotelorism is discovered, the finding should be taken seriously, because of its strong association with cerebral abnormalities (Trout et ...

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