Hyperostosis frontalis interna is a disorder marked by benign overgrowth of the inner table of the frontal bone. The pathological process may be bilateral and symmetrical with sessile or nodular thickened areas. The external lamina and the diploe of the bone are usually not affected, but the abnormality may extend to the parietal bones.
Hyperostosis frontalis interna usually affects postmenopausal women , but up to 12% of the whole female population may have this condition, including those of child-bearing age since the disease has also been associated with pregnancy. Men are seldom affected. The real prevalence of the disease may be underestimated because it is often asymptomatic and the diagnosis can only be established following a skull imaging study.
Rarely, the disease is associated with some of the numerous possible manifestations. The clinical examination may reveal certain physical traits: virilization, obesity, epilepsy, facial palsy, or headaches . Patients may be diabetic, therefore also exhibiting the characteristics of this condition: weight loss, polyuria, polydipsia, and polyphagia. Additional features for this condition include fatigability, vertigo, hypertrichosis, and acromegaly . Mental disturbances such as depression, paranoia, or aggressiveness are believed to be caused by intracranial volume decrease . Brain tissue compression, explaining epileptic seizures and dural irritation, may also be encountered . The disease may cause hormonal impairment, such as hyperprolactinemia or ovarian dysfunction.
In the past, hyperostosis frontalis interna was regarded as part of several syndromes, namely Morgagni (hypertrichosis, obesity, virilization, and headaches), Stewart-Morel (neuropsychiatric abnormalities and obesity), and Troell-Junet (diabetes mellitus, acromegaly, and toxic goiter) syndromes .
A conventional radiological evaluation may offer some information about the extent and localization of the abnormality. The bone deposition process is often confined to the frontal bone but sometimes extends to parietal bones. However, frontal bone areas related to venous channels and superior sagittal sinus are spared . Suture lines represent clear boundaries for the development of this pathological process. Lesions may be flat or nodular, isolated or confluent . The external contour of the skull remains normal, therefore the clinical examination could exclude other entities, such as Paget's disease or leontiasis ossea.
Magnetic resonance imaging (MRI) and computed tomography (CT) offer more detailed clues regarding both the bone proliferation and the status of the brain tissue and dura mater. The lesion border characteristics, along with its shape and location, are elements on which four disease types (A to D) have been defined .
If doubt persists, a histological evaluation is able to shed light on the case by describing thick lamellar bone deposition, predominantly on the inner table of the frontal bone . The pattern of bone deposition differs markedly when compared with patterns seen in Paget's disease, endosteal osteoma, or osteosarcoma. The bone changes are normally less than 1 cm thick in hyperostosis frontalis interna.