Disorders of bone and cartilage
External restriction of skull growth in utero reported, with at least one case of documented microcephaly. 22] Patients who appear to have a primary form of significant microcephaly, meaning that it is not associated with other malformative features, are sometimes referred to as having microcephalia vera. This heterogeneous condition (autosomal recessive and X-linked forms have been identified) shows a striking reduction of head circumference up to 5 standard deviations below the mean. No distinct facial dysmorphism is associated with this condition. These individuals are severely mentally retarded and have no recognizable speech but are often described as having a relatively preserved personality. Although patients often have a somewhat lumbering gait, there is a paucity of other neurological findings. Skull films at birth show that the sutures are present. The brain is usually small, less than 300 g (normal 1200 to 1500 g) and shows a primitive gyral pattern. The cortex is thickened and disorganized without clear lamination. Isolated microcephaly can also be transmitted as an autosomal dominant trait. In this circumstance, the degree of microcephaly usually is not as pronounced, and intelligence is relatively preserved. Given the preponderance of genetic conditions presenting with isolated microcephaly, Tolmie and colleagues calculated an empirical recurrence risk of 19 percent after a single affected proband.
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