Posterior plagiocephaly is the term used to describe the shape that results from craniosynostosis of one of the lambdoid sutures. This is different from positional plagiocephaly, which is a deformation caused by outside pressure on the skull, without sutural fusion. The lambdoid sutures are located on the back of the skull, with one on the right side and one on the left side, making a shape like an upside down “V.” Usually, only one side fuses shut, but rarely both sides will be closed.  When viewed from above, the affected side of the back of the head is flatter than the opposite side. One sign that the lambdoid suture is fused is the presence of a low bump behind the ear, on the same side as the fused suture.  Another way to determine if the lambdoid suture is closed is to check if the ear on the same side as the skull flatness is pulled backwards and, sticks out more (although this is not always a reliable sign).  Another good sign of lambdoid synostosis is that the height of the back of the skull is lower on the flattened side. Finally, if the back of the skull seems to grow outwards on the opposite side of the flattened back of the head, a fused lambdoid suture is likely. However, if the ear is forward on the flat side (with respect to the opposite ear), and there is no reduction in skull height on the affected side, then a skull deformation should be suspected instead of a fused suture. It is critically important to determine whether or not a child truly has a fused suture because skull deformations almost never need to be surgically treated. There have been a number of articles in the news media about children who were treated surgically for flatness on the back of the head for presumed lambdoid synostosis when, in fact, the child had a deformation and could easily have been treated without surgery (either by doing nothing or by using a headband). Making the diagnosis even more difficult is the fact that on plain skull x-rays, the lambdoid suture can be misdiagnosed as being fused shut. Ideally, the diagnosis of lambdoid synostosis is made by either by an experienced craniofacial surgeon, who can usually tell by an examination alone, or by a CT scan performed by those with less experience with craniosynostosis. Children, who have lambdoid synostosis and significant flattening of the skull, typically require surgery to fix this condition. Experienced craniofacial surgeons will also discuss the potential for Chiari malformations (where a part of the brain, called cerebellar tonsils, can push on the spinal cord) that can occur with this condition, and what this means for treatment.

The incidence of posterior plagiocephaly from a fused lambdoid suture is currently unknown and is probably even rarer than one in 50,000 births. One reason for the difficulty in estimating how often this fusion occurs is that it has probably been over-diagnosed in the past. It is certainly one of the most uncommon types of craniosynostosis treated at craniofacial centers, and therefore, is probably best managed at one of the busier centers. The treatment for this condition is discussed in the Treatment section.