Occipital Bone Defect With Meningoencephalocele and Plexiform Neurofibroma in Neurofibromatosis-1

Neurofibromatosis type 1 (NF-1) is the most common phakomatosis with autosomal dominant inheritance, caused by mutations in the NF-1 gene on chromosome 17q11.2, which codes for neurofibromin protein, a negative regulator of the RAS/MAPK pathway. A defect in this can lead to altered cellular growth and tumor development. NF-1 is a complex multi-system neurocutaneous disorder with some of the common manifestations being café au lait macules, axillary/inguinal freckling, and neurofibromas. Musculoskeletal manifestations include scoliosis, sphenoid wing dysplasia, tibial pseudoarthrosis, and, rarely, calvarial defects resulting from dysplastic bone. Central nervous system manifestations range from focal areas of signal intensities (FASIs), optic pathway gliomas, dural ectasia, and meningoceles. We report the case of a 24-year-old male who presented with a progressively enlarging swelling over the posterior scalp and left side of the neck since childhood, which on examination led to suspicion of underlying calvarial defect, and along with general examination findings of multiple cafe-au-lait macules and cutaneous neurofibromas fulfilled the diagnostic criteria for NF-1. Examination revealed a palpable calvarial defect. Cross-sectional imaging (MRI and CT) of the occipital swelling revealed a large plexiform neurofibroma, associated with a bony defect involving the occipital bone and lambdoid suture with herniation of dysplastic posterior fossa structures into the neurofibroma in the form of meningoencephalocele. Our case not only highlights the rare occurrence of neurofibroma associated with rare location of calvarial defect and underlying meningoencephalocele, it identifies the role of imaging in evaluating the extent and characterization of the neurofibromas for management and surveillance.