J Neurol Surg B Skull Base 2015; 76 - P053
DOI: 10.1055/s-0035-1546681

Heterotrophic Ossification of a Prepontine Mass: Idiopathic or Following Stroke?

Piyush Kalakoti 1, Osama Ahmed 1, Richard Menger 1, Richard Murray 1, Shyamal C. Bir 1, Connor David 1, Anil Nanda 1
  • 1Louisiana State University Health Sciences Center, Shreveport, Louisiana, United States

Objective: This study aims to report a rare case of heterotrophic ossification in a prepontine mass mimicking idiopathic intracranial dystrophic calcification, and to discuss the diagnostic and surgical challenges faced in the management of this case.

Background: Heterotopic ossification (HO) refers to ectopic formation of benign, mature lamellar bone in nonosseous tissues where bone normally does not exist. Direct trauma, elective surgery, spinal cord injury, traumatic brain injury or rare congenital diseases such as fibrodysplasia ossificans progressive have been implicated in its pathogenesis.

Material and Methods: Patient evaluation was performed by (1) clinical assessments and observations; (2) review of previous CT scans; (3) repeated brain MRIs; and (4) neuropathologically using histological technique.

Results: A 51-year-old male patient presented with painless, progressive, bilateral visual loss over the course of several months. His medical history was significant for an ischemic stroke 8 years ago, which involved the right middle cerebral artery and resulted in left hemiplegia. A review of CT studies performed at that time revealed a hyper density in the region of the pons in addition to the infarcted region. Repeated brain MRIs demonstrated a prepontine mass encroaching on the interpendicular cistern with extension to the midbrain without noticeable involvement of the clivus or the ventricles. The patient underwent right-sided pterional craniotomy aided by frameless stereotaxy for microsurgical resection of the mass. Dense adhesions were encountered between the mass and the surrounding arachnoid planes as well as between the mass and the brain stem. Postoperative imaging demonstrated complete resection of the mass. The resected mass consisted of an irregular, tan-gray to yellowish appearing bone-like structure, with two fragments of soft tissue. Excised fragments of benign bone and mature adipose tissue were noted. Sections demonstrated presence of a mature bony nodule composed of a thin rim of cortical bone with bony trabeculae and surrounding fibroadipose tissue. No histologic evidence of cartilaginous tissue, notochordal elements, or malignancy was seen. The pathologic diagnosis favored heterotopic ossification, probably developmental in origin.

Conclusion: Intracranial heterotrophic ossification within a prepontine mass is unusual and has not been reported earlier. Until histopathological evaluation, the mass was thought to be dystrophic calcification. Preoperative imaging could not help assess precise radiopathological diagnosis nor predict the density of adhesion of the mass to the brain stem. Retrospectively, we hypothesize the neurological insults following stroke as a possible cause of formation of bone-like tissue within the mass; however, the exact etiopathogenesis remains obscure. Futuristically, genetic and molecular research to further our current understanding of HO is warranted.