The purpose of our study was to identify the incidence of falcine calcification shown on plain skull radiographs in people with basal cell naevus syndrome (BCNS) and in their relatives compared with a normal population. A population of people with BCNS and their relatives was identified on non-radiological grounds and the incidence of falcine calcification on skull radiographs in each of these two groups was compared with the incidence of falcine calcification in a control group of people and of a larger group who attended casualty departments. Falcine calcification was graded into dense, fine but definite, and faint. 85 people with BCNS had nearly 100% incidence of falcine calcification in adults. 83 first degree unaffected relatives showed no excess over "normal" incidence of falcine calcification. In the 970 casualty patients some falcine calcification was common (> 20%) in males over 30 and in females over 50 years of age. It occurred earlier and was more common in males than in females. Dense calcification occurred in 5-7% of females and males over 60 years of age. Dense calcification was rare (< 2%) under the age of 40 years. In conclusion, falcine calcification should be regarded as the fourth major feature and a very important diagnostic feature of BCNS. An adult labelled as a BCNS sufferer without falcine calcification probably has not got the syndrome. The genetic defect responsible for the metabolic defect resulting in dural calcification is probably the same as, or in close linkage disequilibrium to, that responsible for the major clinical features of the syndrome. The age and sex distribution of falcine calcification in a general hospital casualty population is described.

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