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Neuroradiology Case of the Week

Case 330

Balasubramanya Kolar, MD, Rajiv Mangla, MD
and P-L Westesson, MD, PhD, DDS

Clinical Presentation: The patient is a 3-month-old infant who presented with emesis, nystagmus and failure to thrive.

Imaging Findings: There is evidence of a large, lobulated, T1 hypointense, T2 and FLAIR hyperintense, homogeneously enhancing lesion noted within the suprasellar cistern extending into the third ventricle and bulging it with inferior extension into the prepontine cistern on the left side (Figs. 1&2). There is mass effect on the midbrain with dilatation of lateral ventricles. Diffusion-weighted images show free diffusion within the lesion (Figs. 3&4).There are also multiple enhancing nodules noted along the cerebellar hemispheres with leptomeningeal enhancement along the anterior and posterior surfaces of the cervical cord (Figs. 5&6)

Diagnosis: Pilomyxoid astrocytoma

Discussion: Pilomyxoid astrocytomas (PMA) are a rare variant of pilocytic astrocytomas (PA) which have recently been added to the nomenclature [1]. They have been variously described as an infantile form or atypical variant of PA.
     The characteristic location of these tumors is in the hypothalamic/chiasmatic region. PMA has also been described in the posterior fossa and in the spinal cord [2]. The mean age at diagnosis for patients with PMA is 18 months, it may present throughout childhood [1].
     The common presentation is with focal neurological symptoms, such as visual disturbances and endocrine dysfunction. The clinical manifestations include failure to thrive, developmental delay, altered level of consciousness, vomiting, feeding difficulties, and generalized weakness [3].
     MR findings of PMA as described previously include chiasmatic or hypothalamic enhancing solid tumor with hydrocephalus, highly homogeneous T2 signal intensity that extended into the deep white and gray matter, and CSF dissemination. These findings were hypothesized as diagnostic of PMA and all these findings are evident in our case [4].
     The differential diagnosis for other lesions in this region and age include hypothalamic /chiasmatic glioma (pilocytic astrocytoma), germinoma, craniopharyngioma.
     Pilocytic astrocytomas, in contrast, do not enhance homogeneously. A mural nodule may be seen or these tumors may show no enhancement.
     Germinomas show homogenous enhancement with CSF dissemination and may be found in this age group and location, however these tumors are usually not homogeneously hyperintense on T2 weighted images, they are cellular and may be isointense on T2 weighted sequences.
     Diffusion weighted images in our case show free diffusion within the lesion though the lesion enhances. Though the T2 hyperintensity and free diffusion indicates a cystic nature of the lesion, the homogenous enhancement suggests a solid component. This could partially be explained by the myxoid content of this rare tumor.
     The diagnosis however is made based on histological features. PMA is composed of piloid and highly monomorphous cells and classic PA is a compact neoplasm with biphasic architecture. Even though myxoid change is commonly encountered in PAs, PMAs display a much more uniform and extensive myxoid background. Rosenthal fibers are often not seen in PMA [5].
     A higher rate of local recurrence than PA and a substantial rate of dissemination in the cerebrospinal fluid confer a poorer prognosis with these tumors than pilocytic astrocytomas [5].

References:

1. Tihan T, Fisher PG, Kepner JL, et al. Pediatric astrocytomas with monomorphous pilomyxoid features and a less favorable outcome. J Neuropathol Exp Neurol. 1999 Oct;58(10):1061-8. [Medline]
2. Komotar RJ, Carson BS, Rao C, et al. Pilomyxoid astrocytoma of the spinal cord: report of three cases. Neurosurgery. 2005;56(1):191. [Medline]
3. Komotar RJ, Mocco J, Jones JE, et al. Pilomyxoid astrocytoma: diagnosis, prognosis, and management. Neurosurg Focus. 2005 Jun 15;18(6A):E7. [Medline]
4. Arslanoglu A, Cirak B, Horska A, et al. MR imaging characteristics of pilomyxoid astrocytomas. AJNR Am J Neuroradiol. 2003 Oct;24(9):1906-8. [Medline]
5. Fernandez C, Figarella-Branger D, Girard N, et al. Pilocytic astrocytomas in children: prognostic factors--a retrospective study of 80 cases. Neurosurgery. 2003 Sep;53(3):544-53. [Medline]