This report is of a case of hyperostosis cranialis internal (HCI), a rare disease characterized by the abnormal thickening of cranial bones, usually in the frontal and parietal regions. It is an uncommon disease that has clinical significance because of its possible symptoms: neurologic, aesthetic concerns, or asymptomatic, which may incidentally be found on imaging. Detailed attention has been given to the case of a 32-year-old female patient who presented with a two-year history of right clear, salty nasal discharge, anosmia, and band-like headache. Computed tomography and magnetic resonance imaging findings helped in the diagnosis. The laboratory studies and treatment options have been described. A discussion on pathophysiology, differential diagnoses, and management is also done with the hope of adding to this limited literature on HCI. The implications for diagnosis and management in clinical practice are shown to be direct results of characterizing HCI as a unique bone disorder, whereby a better understanding of this condition is provided.

Hyperostosis of the skull is a rare bone dysplasia described in disorders such as hyperostosis cranialis interna (HCI) and hyperostosis frontalis interna (HFI). Other syndromes presenting with hyperostosis include Morgagni-Stewart-Morel (MSM) and Troell-Junet. HCI is an abnormal hyperostosis of most endosteal skull and calvarium surface regions. A more specific hyperostosis, HFI, is an unusual bone growth based on its volume and porosity; it is primarily located bilaterally on the frontal portions of the calvarium. However, the hyperossification does not cross the superior sagittal sinus. Upon cadaveric dissection, we found hyperossification beyond the frontal area, extending to the parietal and occipital bones with the significant characteristic of no midline interference. Hyperossification results in gross indentations on the corresponding frontal, parietal, and occipital hemispheric brain tissues. This report discusses possible differentials for this rare cadaveric finding of frontal, parietal, and occipital bone hyperostosis. This case report includes some major characteristic features indicative of HCI and HFI with some interesting variations and features suggestive of MSM and Troell-Junet syndromes. Due to the lack of patient history and medical records, no further conclusions about clinical differentials, symptoms, or causative syndromes could be drawn; further research needs to be conducted on HCI, HFI, and related syndromes to understand their presentations better.

Hyperostosis Cranialis Interna (HCI) is a rare bone disorder characterized by progressive intracranial bone overgrowth at the skull. Here we identified by whole-exome sequencing a dominant mutation (L441R) in SLC39A14 (ZIP14). We show that L441R ZIP14 is no longer trafficked towards the plasma membrane and excessively accumulates intracellular zinc, resulting in hyper-activation of cAMP-CREB and NFAT signaling. Conditional knock-in mice overexpressing L438R Zip14 in osteoblasts have a severe skeletal phenotype marked by a drastic increase in cortical thickness due to an enhanced endosteal bone formation, resembling the underlying pathology in HCI patients. Remarkably, L438R Zip14 also generates an osteoporotic trabecular bone phenotype. The effects of osteoblastic overexpression of L438R Zip14 therefore mimic the disparate actions of estrogen on cortical and trabecular bone through osteoblasts. Collectively, we reveal ZIP14 as a novel regulator of bone homeostasis, and that manipulating ZIP14 might be a therapeutic strategy for bone diseases.