Prevalence:

• 1 in 20,000 births.

Ultrasound diagnosis:

• Decreased interorbital diameter <5^th percentile.
• Hypotelorism is part of the midline migration defects together with holophrosecephaly (which is almost always present). The degree of hypotelorism can be extreme as in cyclopia.

Associated abnormalities:

• Chromosomal abnormalities, mainly trisomy 13, are found in 50% of cases.
• Genetic syndromes are very frequent and the most common is Meckel-Gruber syndrome (autosomal recesive, lethal condition characterized by occipital encephalocele, multicystic kidneys and post-axial polydactyly).

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Invasive testing for karyotyping and array.

Follow up:

• Follow-up should be standard.

Delivery:

• Standard obstetric care and delivery.

Prognosis:

• Part of trisomy 13: lethal.
• Normal karyotype: high risk of neurodevelopmental delay depending on the degree of holoprosencephaly.

Recurrence:

• Isolated: no increased risk.
• Part of trisomy 13: 1%.
• Part of an autosomal recessive condition: 25%.