Foramen Magnum Stenosis (FMS): Neuroradiological Aspects before and after Cervical Decompression in Paediatric Patients with Achondroplasia (ACH).The 'Achondroplasia Multidisciplinary Gaslini's Group' (AMGG) Istituto Giannina Gaslini, Genova, Italy: Child Neuropsychiatry Unit, Neuroradiology Unit, Department of Paediatrics, Neurosurgery Unit, Orthopedic Unit, Rehabilitation Unit, Pulmonary Disease and Allergy Unit.