Cloverleaf skull

Description

A cloverleaf configuration of the skull consists of trilobed protrusion of the skull with broadening of the temporal region and face. The skull protrusion is separated from focal bulges by furrows. Often the sagittal, coronal, and lambdoid sutures show premature synostosis, although other combinations of sutural involvement are possible, including coronal, lambdoid, and metopic closure, or sagittal and squamosal closure. Increased thickening of occipital bone squama prevents lengthening of the skull, producing the typical shape. Occasionally, atypical or unilateral sutural involvement may produce an asymmetric bulging of the calvarium. One individual has been described where cloverleaf skull occurred in the absence of craniosynostosis. In this latter case there was lissencephaly and hypoplasia of several cranial bones, allowing eventration of the brain and surrounding tissues into a trilobed configuration. The eyes usually protrude; occipital encephaloceles can occur, and hydrocephaly is common. 85% of individuals with cloverleaf skull will have other anomalies, and the pattern is often consistent with a syndrome diagnosis or specific genetic mutations. The skull develops from mesenchyme, with the portion surrounding the brain termed the neurocranium. The neurocranium consists of a cartilaginous portion (chondrocranium) and a membranous portion. The chondrocranium is the cartilaginous base of the skull, which develops by fusion of several cartilages. The membranous neurocranium ossifies to form the cranial vault, the bones of which are separated by connective tissue membranes known as sutures. Six fibrous areas, the fontanelles, are also present. The five major sutures are the frontal, or metopic, sagittal, coronal, lambdoid and squamosal. The sutures allow for moulding to occur during the delivery process, and enlargement to accommodate normal brain growth. When the cranial bones fuse prematurely there is decreased growth potential. Bone deposition occurs abnormally, with the majority being directed away from the fused bony plate. In turn, perimeter sutures adjacent to the prematurely fused suture compensate in growth to a greater extent than sutures distant to the fused area. The primary cause of synostosis may be a defect at the cranial base at the site of initial attachment of the dura; other studies suggest a vascular defect causing deficient osteoclasia. A recent morphologic, radiographic and histologic analysis of an 18 week fetus with cloverleaf skull suggests the calvarium may be the primary focus of abnormal events, with the cranial base involved as a secondary event. Cloverleaf skull historically has been classified into three groups: type 1, in which cloverleaf skull coexists with thanatophoric dysplasia; type 2, in which cloverleaf skull is associated with less severe skeletal lesions such as joint ankylosis and subluxation of the radial heads; and type 3, where the skull anomaly is isolated, and the prognosis is much improved.