Unilateral Fusion of the Frontosphenoidal Suture: A Rare Cause of Synostotic Frontal Plagiocephaly

&NA; Unilateral coronal synostosis is the common appellation for premature, one‐sided fusion of the frontoparietal suture—the most common cause of synostotic frontal plagiocephaly. However, frontal asymmetry can also result from isolated fusion across the anterior cranial base without involvement of the frontoparietal suture. This article describes three patients with localized synostosis of the frontosphenoidal suture, the medial extension of the coronal ring. Two patients were initially misdiagnosed as having unilateral coronal synostosis and the other as having deformational frontal plagiocephaly. The patients had variable frontal flattening, with depression and recession of the ipsilateral orbital rim. The nasal root was midline or slightly deviated to the contralateral side. The sagittal position of the ipsilateral malar eminence was slightly retruded in one patient and symmetric in the other two. The auricular position was symmetric in the sagittal plane for all patients. In all three patients, computed tomography examination demonstrated a patent frontoparietal suture and fusion of the frontosphenoidal suture (basilar hemicoronal ring). Two patients had involvement of contiguous sutures: one had fusion extending to the sphenoethmoidal suture and the other's involved part of the sphenozygomatic suture. The sagittal suture was midline in all patients. In summary, synostotic frontal plagiocephaly denotes a relatively broad phenotypic spectrum that includes unilateral coronal synostosis and more isolated fusions in the basilar coronal ring. The physical findings resulting from frontosphenoidal synostosis are unique, yet careful evaluation will minimize confusion with other causes of asymmetric frontal flattening. Proper diagnosis necessitates awareness of this uncommon entity and requires focused computed tomographic assessment.