Enlarged parietal foramina are characteristic symmetric, paired radiolucencies of the parietal bones, located close to the intersection of the sagittal and lambdoid sutures, caused by deficient ossification around the parietal notch, which is normally obliterated by the fifth month of fetal development. Enlarged parietal foramina are usually asymptomatic. Meningeal, cortical, and vascular malformations of the posterior fossa occasionally accompany the bone defects and may predispose to epilepsy. In a minority of individuals, headaches, vomiting, or intense local pain are sometimes associated with the defects, especially on application of mild pressure to the unprotected cerebral cortex.
Typically oval or round, enlarged parietal foramina resemble a 'pair of spectacles' on postero-anterior skull radiographs. They may be less apparent on lateral skull radiographs because the lucencies are projected obliquely through normal bone. In young children, the disorder may present as a persistently enlarged posterior fontanelle caused by a single large central parietal bone defect (cranium bifidum). 3D CT scanning using bone windows clearly reveals the defect. MRI is useful in defining associated intracranial anatomic changes. MSX2 and ALX4 are the two genes in which mutations are known to cause enlarged parietal foramina.
Enlarged parietal foramina are inherited in an autosomal dominant manner with high, but not complete, penetrance. Most individuals diagnosed with enlarged parietal foramina have an affected parent. The proportion of cases caused by de novo mutations appears to be small. Each child of an individual with enlarged parietal foramina has a 50% chance of inheriting the mutation. Careful fetal ultrasound examination at 18 to 20 weeks' gestation can usually detect the defects in a fetus at risk. Fetal MRI is also an option. Prenatal diagnosis using molecular genetic testing is possible for families in which the disease-causing mutation has been identified in an affected family member.