MEDULLOMYOBLASTOMA. Given the rapid growth of the lesion and the regions of enhancement on the spinal MRI compatible with drop metastasis, craniospinal radiation was initiated 4 weeks after surgery, with boost radiation delivered at the surgical focus and at the level of the spinal enhancing foci. After intense neurorehabilitation, the patient was discharged home six weeks after surgery.
Presentation with an isolated abducens nerve paresis progressing to complete palsy in the absence of other cranial nerve dysfunction or increased intracranial pressure should trigger a complete work-up, including infectious, inflammatory/degenerative, and tumoral etiologies. Demonstration of an area of enhancement at the pontomedullary junction in a patient with an isolated abducens nerve paresis should evoke a tumor arising from the abducens nerve (2,5). The rapid radiological progression documented on follow-up raised the possibility of an aggressive nerve sheath tumor. In the present case, the intra-operative frozen analysis was critical suggestive of an aggressive intraaxial lesion with an exophytic component. Final pathology assessment was suggestive of a medullomyoblastoma (3).
Cerebellar medulloblastomas are very rare lesions. Although they are more commonly encountered in the pediatric population as midline intraaxial posterior fossa lesions, in adults they are most often laterally located(4). The typical clinical manifestations of cerebellar medulloblastomas are headaches secondary to raised intracranial pressure as well as cranial nerves and cerebellar symptomatology(4,6). The MRI appearance of cerebellar medulloblastomas and medullomyoblastomas is variable and non-specific(1,3). Their origin is thought to be from the lateral surface of the cerebellum with possible exophytic components extending in the cerebellopontine angle, or even with a primary extra-axial location(1).
To the author's knowledge, this is the second reported case of medullomyoblastoma of the prepontine and premedullary cisterns and/or cerebellopontine angle, and the first report of a well-documented paramedian site of origin(6). The histogenesis of the myogenic component of medullomyoblastomas remains not well understood. The muscle fibers in medullomyoblastoma are thought to arise either through the induction of pluripotential ectomesenchymal cells within or near the tumor, such as within the leptomeninges, or directly from primitive neuroepithelial cells themselves(8). The clinical behavior and prognosis of the myogenic component of medullomyoblastoma remains uncertain given the small number of reported cases(7). Although it has been suggested that the presence of foci of large cell and anaplastic changes, similar to classic medulloblastomas or de-novo LC/A subtypes, confers a more aggressive clinical course to medullomyoblastomas, the significance of these findings in medullomyoblastomas is still under investigation(3).
In summary, this case describes a unique case of medullomyoblastoma that presented with an isolated abducens cranial nerve palsy. Early imaging documented a paramedian site of tumor origin, which later progressed to occupy the prepontine/premedullary cisterns. Its initial clinicoradiological features mislead us to suspect an abducens cranial nerve schwannoma with potential malignant features given its rapid growth and possible spinal associated lesions. Histopathological assessment was critical, documenting a rare medullomyoblastoma. Detailed documentation of such cases is important as their clinical significance is still under study.
Contributed by Nancy McLaughlin, MD, PhD, FRCSC, Justin Dye, MD, Négar Khanlou, MD, William H. Yong, MD, Neil A. Martin, MD