Adult Intradiploic Meningoencephalocele

Aims: Intradiploic meningoencephalocele (IM) is a rare calvarian lesion that presents a differential diagnostic challenge. Only four cases in adults were documented thus far. Here, the authors add a new illustrative case and review the characteristics of this exceptional pathological entity.

Methods: A 50-year old male patient presented with a slowly growing tender occipital mass he first noted 2.5 years ago. Neuroimaging showed an externally protruding cystic lesion in the occipital bone with discontinuous inner table and extreme thinning of the outer table. The content appeared to be isointense to cerebrospinal fluid (CSF). He underwent microsurgery to remove the lesion completely, which was found to distend the bone and eroded the dura so that brain tissue was prolapsed into its cavity. Histological examination confirmed an unexpected IM with locally expansive, intraosseous growth. The cyst wall consisted of a narrow band of brain parenchyma and a lamella of meningothelial cells of varying thickness. Adjacent to the cyst wall reactive, intraosseous meningothelial cells and fibrotic periosteum was found. No markers of malignancy were discovered. The clinical course was uneventful without signs of recurrence up to 6 months after surgery.

Results: We present a unique case of adult IM. This type of lesion is extremely rare but represents a differential diagnosis to be thought off in patients with cystic bony defects. They contain CSF and neural tissue, which is usually gliotic, and are located in the diploe of a skull bone with breach of the inner table and extreme thinning of the outer table. The adjacent dura is broken, which allows CSF to enter the intradiploic space. The cyst wall usually is made out of meningothelial cells, which can be fibrotic. They are mostly idiopathic or have a suspected traumatic origin. A developmental origin has also been proposed. Furthermore, similar lesions rarely occur in children and young adults after neurosurgical procedures.

Conclusions: When treating patients with a cystic intradiploic lesion one has various options for differential diagnosis, including simple and aneurysmatic bone cyst, epidermoid and arachnoid cyst. Adult intradiploic meningoencephalocele is a rare option to keep in mind if diagnostic criteria for the other lesions are not met. A cyst content of meningothelial and neural tissue is crucial for final diagnosis. IM is a benign disease for which surgery is potentially curative.